An intermediate course for future industrial machinery mechanics and millwrights. You will identify bearing types and read failure signs; tension belts and check chain slack and coupling alignment; calculate gear ratios, speeds and torque; read hydraulic schematics and trace pneumatic circuits; diagnose pump problems such as cavitation; choose a lubricant and a relubrication interval; write a preventive maintenance work order the way a CMMS stores it; plan a lift on paper with sling angles and rated capacities; and name the main welding processes and their hazards. Prerequisites are Workplace Safety & Tools (hazards, PPE, lockout/tagout) and Technical Drawing & CAD Basics (reading drawings). Hands-on tasks use paper, spreadsheets, household models and free tools only. This course is awareness and knowledge: working on real machinery, lifting loads or welding requires employer training, a qualified supervisor and, for most mechanic roles, an apprenticeship or an associate degree in industrial maintenance.

Learn how bearings, belts, chains, couplings, gears, hydraulics, pneumatics and pumps work, how they fail, and how maintenance teams lubricate, plan PM work, rig loads and weld safely.

Mechanical Systems & Maintenance

## Why it matters

Almost every rotating machine in a plant, from a conveyor pulley to a pump to a fan, spins on bearings. When a bearing fails, the machine stops, and a seized bearing can wreck a shaft, a housing or a motor along with it. Mechanics who can name the bearing in front of them, read its part number and spot early warning signs catch problems during a planned shutdown instead of at 2 a.m.

## What a bearing does

A bearing supports a load while letting one part move relative to another with little friction. Loads come in two directions:

- **Radial load** pushes at right angles to the shaft, like the weight of a pulley hanging on it.
- **Axial (thrust) load** pushes along the shaft, like the push from a helical gear or a vertical pump impeller.

Choosing a bearing is mostly about matching its design to those loads, the speed and the space available.

## The main types

**Rolling-element bearings** have an inner ring, an outer ring, rolling elements and a cage that keeps the elements spaced.

- **Deep groove ball bearings** are the most common. They take moderate radial loads, some thrust in both directions, and high speeds. Electric motors usually use them.
- **Angular contact ball bearings** have raceways set at an angle so they carry heavy thrust in one direction. They are often mounted in pairs.
- **Cylindrical roller bearings** carry heavy radial loads but little or no thrust.
- **Tapered roller bearings** carry combined radial and thrust loads. Wheel hubs and gearbox shafts use them, usually in opposed pairs that must be set with the right end play.
- **Spherical roller bearings** have two rows of barrel-shaped rollers and tolerate some shaft misalignment. They are common on heavy conveyors and fans.
- **Needle bearings** are thin cylindrical rollers for tight spaces.
- **Thrust bearings** (ball or roller) carry axial load only.

**Plain bearings** (bushings and sleeve bearings) have no rolling elements. The shaft slides on a lubricated surface of bronze, babbitt or polymer. Large turbines and compressors run on oil-film plain bearings.

**Mounted bearings** such as pillow blocks and flange units put a bearing in a housing that bolts to the frame, which is what you will replace most often on conveyors.

## Reading a bearing number

Most manufacturers follow a similar numbering pattern for common ball bearings. Take **6205-2RS**:

- `6` is the type: single-row deep groove ball bearing.
- `2` is the dimension series (a light series).
- `05` is the bore code. For codes 04 and above, multiply by 5: 05 × 5 = **25 mm bore**. Codes 00, 01, 02 and 03 mean 10, 12, 15 and 17 mm.
- `2RS` means rubber seals on both sides (`2Z` or `ZZ` would mean metal shields).

So a 6308 is a deep groove ball bearing with a 40 mm bore in a medium series. Suffixes for internal clearance (such as C3) and cage material vary between brands, so always confirm with the manufacturer's catalog or interchange tool before ordering.

## How bearings fail

A bearing that is clean, lubricated, aligned and properly loaded usually outlives its rated life. Most early failures trace back to a few causes:

- **Lubrication problems**: too little, too much, the wrong grade, or old grease that has broken down.
- **Contamination**: dirt or water past a damaged seal scratches and corrodes the raceways.
- **Misalignment and poor fits**: a shaft or housing out of tolerance, or a bearing hammered on through the rolling elements.
- **Overload and imbalance**: a belt tensioned far too tight is a classic one.
- **Electrical damage**: stray current, often from a variable frequency drive, arcs across the bearing and leaves a washboard pattern called fluting.
- **False brinelling**: vibration while the machine is stopped dents the races, common in standby equipment.

## Warning signs

You usually get warnings before a bearing seizes:

1. **Vibration** rises first, often detected by a route-based vibration program long before you can feel it.
2. **Noise** changes: a rumble, growl or rhythmic clicking.
3. **Heat** rises at the housing. Compare it with an identical bearing nearby rather than guessing.
4. **Grease changes**: discolored, burnt-smelling or gritty grease, or grease purging past the seals.
5. Finally, **visible damage**: looseness, smoke or a locked shaft.

When a failed bearing comes out, keep it. Spalling (flaking metal), discoloration from heat, corrosion and fluting each point to a different root cause, and fixing the cause is what stops the next failure.

## Common mistakes

- Pressing a bearing on by pushing on the outer ring when the inner ring has the tight fit. Force must go through the ring being fitted.
- Spinning a bearing with compressed air. It can overspeed and burst, and it damages the raceways.
- Ordering by bore size alone. Width, outside diameter, seals and clearance all matter.
- Replacing the bearing without asking why it failed.

## Safety

Bearing work happens on stopped, locked-out equipment. Bearing heaters, pullers and presses store a lot of force and heat. Follow your site's lockout/tagout procedure and get hands-on training from your employer before doing this work. OSHA's lockout/tagout page is the place to start: https://www.osha.gov/control-hazardous-energy (Checked on 2026-10-23.)

## Recap

- Radial loads push across the shaft; thrust loads push along it.
- Ball bearings suit speed, roller bearings suit heavy loads, tapered and angular contact bearings handle combined loads.
- In a number like 6205, the last two digits × 5 give the bore in millimeters.
- Vibration, noise, heat and grease condition warn you before failure.

## Try it

Open the **Do** tab and take the quiz on bearing types, numbers and failure signs.


Identify the main bearing types, read a bearing number and recognize the early signs of bearing failure.

Bearings

What is Bearing? Types of Bearings and How they Work?

Vibration Analysis - Bearing Failure Analysis by Mobius Institute

Tape two round jars or cans upright on a table, about 30 cm (12 in) apart, center to center. Stretch a wide rubber band around both like a belt.

Measure the span: the straight run of rubber band between where it leaves one jar and meets the other. Write it down in inches or millimeters.

Calculate the target deflection with the 1/64 in per inch rule (about 1.6 mm per 100 mm of span). Write the result next to the span.

Lay a ruler across the two jars next to the span as a reference. Push the middle of the span sideways with a pencil eraser until it moves the target distance, and note how hard you had to push (light, medium or firm).

Move one jar to make the band tighter, then looser, and repeat the push each time. Notice how the same deflection needs more force as tension rises.

Now check alignment: lay a straightedge (a long ruler or a book edge) across the faces of both jars at the same height. Note whether it touches both jars at two points.

Shift one jar slightly forward to create offset, then twist its tape so it leans to create an angle. Describe what the straightedge shows for each case.

Write a two-line note as if for a work order: the span, the target deflection, and the corrections you made.

## Why it matters

Belts, chains and couplings carry power from a motor to the machine it drives. They are also among the parts you will adjust most often. A belt that is too loose slips, squeals and glazes; one that is too tight overloads bearings and shafts. A coupling that is a fraction of a millimeter out of line can halve the life of the bearings on both sides. Getting tension and alignment right is everyday mechanic work.

## Belt drives

**V-belts** wedge into the grooves of pulleys (called sheaves). The wedging creates grip, so a V-belt does not need to be extremely tight. **Synchronous (timing) belts** have teeth that mesh with toothed pulleys, so they never slip and keep shafts in time. **Flat belts** and **multi-rib belts** are also common.

Speed through a belt drive follows the pulley diameters:

```text
driven rpm = driver rpm × (driver diameter / driven diameter)
```

A 1,750 rpm motor with a 4 in sheave driving a 10 in sheave turns the fan at 1,750 × 4 / 10 = 700 rpm.

## Tensioning a V-belt: the deflection method

The manufacturer's tables give the exact force, but the common field method is:

1. Measure the **span**: the distance between the points where the belt leaves one sheave and meets the other.
2. The target deflection is about **1/64 in per inch of span** (about 1.6 mm per 100 mm). For a 32 in span, that is 0.5 in.
3. Use a belt tension gauge to push the middle of the span until it deflects that distance, and read the force.
4. Compare with the manufacturer's force range for that belt section and sheave size. Adjust the motor base and re-check.
5. New belts stretch and seat. Re-check tension after the first run-in period (often 24 hours of running).

Signs of wrong tension: squealing on start-up, a shiny glazed sidewall, belts flapping, or excess heat at bearings.

## Sheave alignment

Even perfectly tensioned belts wear fast if the sheaves are out of line. Lay a straightedge (or a laser alignment tool) across both sheave faces. It should touch each face at two points. Check for:

- **Offset**: sheaves parallel but not in the same plane.
- **Angular misalignment**: shafts not parallel, so the straightedge touches only one side.

Replace multi-belt drives as a **matched set**. Mixing one new belt with old ones puts all the load on the tightest belt.

## Chain drives

Roller chains are used where loads are high and slip is not allowed. Key checks:

- **Slack**: on a horizontal drive, the typical target is about 2 to 3 percent of the center distance, measured as up-and-down movement at the middle of the slack span. A 20 in center distance gives roughly 0.4 to 0.6 in.
- **Wear (elongation)**: chain pins and bushings wear, so the chain gets longer. Measure a known number of pitches and compare with the manufacturer's wear limit, often around 1.5 to 3 percent. Replace a stretched chain together with worn sprockets.
- **Lubrication**: chains need oil in the pin joints, not just on the outside.

## Couplings and shaft alignment

A coupling joins two shafts end to end, such as a motor and a pump. **Rigid couplings** need near-perfect alignment. **Flexible couplings** (jaw, grid, gear, disc and tire types) tolerate small misalignment, but tolerating it is not the same as liking it: misalignment shows up as heat, vibration and worn bearings and seals.

There are two kinds of shaft misalignment, measured in both the vertical and horizontal planes:

- **Offset (parallel)**: shaft centerlines are parallel but displaced.
- **Angular**: shaft centerlines meet at an angle.

The usual sequence is:

1. Lock out and verify zero energy.
2. Check and correct **soft foot**: a motor foot that does not sit flat. Loosen one foot bolt at a time with a dial indicator on it; movement means shims are needed.
3. Do a rough alignment with a straightedge.
4. Do a precision alignment with **dial indicators** (rim-and-face or reverse-indicator methods) or a **laser alignment system**.
5. Move the motor, not the pump: the driven machine is usually piped in place. Add or remove shims for vertical corrections, then move it sideways for horizontal corrections.
6. Torque the bolts in sequence and take a final reading.

Alignment tolerances depend on speed: the faster the shaft, the tighter the tolerance. Use the coupling or laser manufacturer's table.

## Common mistakes

- Prying a belt over a sheave instead of moving the motor in. It damages the belt cords.
- Tensioning by feel only. Use a gauge.
- Aligning a hot machine to cold readings without allowing for thermal growth.
- Skipping the soft foot check, which makes alignment readings change every time you tighten a bolt.

## Safety

Belt, chain and coupling work is done only on locked-out equipment with guards removed under your site's procedure and refitted before restart. Nip points on belts and sprockets cause serious hand injuries. Your employer must train and authorize you first. See OSHA's machine guarding page: https://www.osha.gov/machine-guarding (Checked on 2026-10-23.)

## Recap

- V-belt deflection target: about 1/64 in per inch of span, at the force in the manufacturer's table.
- Check sheaves with a straightedge for offset and angle.
- Chain slack about 2 to 3 percent of center distance; replace stretched chain and worn sprockets together.
- Coupling alignment: soft foot first, then offset and angular corrections in both planes.

## Try it

Open the **Do** tab to practise the deflection and straightedge methods on a safe tabletop model.


Tension a V-belt by the deflection method, check chain slack and describe how a coupling is aligned.

Belts, chains, and couplings

How to Check if your V-Belt is Properly Tensioned

Shaft Alignment 101: The Basics | ACOEM

## Why it matters

Motors like to run fast, typically 1,200 to 3,600 rpm, but conveyors, mixers and hoists need slow, strong motion. Gearboxes (also called speed reducers) bridge that gap. A mechanic needs to know what kind of gears are inside, how to work out the speed and torque at the output, and what a healthy gearbox looks, sounds and smells like.

## Gear types

- **Spur gears** have straight teeth parallel to the shaft. Simple and efficient, but noisy at speed. They produce no thrust load.
- **Helical gears** have angled teeth, so they mesh gradually and run quieter. The angle produces thrust along the shaft, which the bearings must carry.
- **Bevel gears** connect shafts at an angle, usually 90 degrees. Spiral bevel gears are the quieter, curved-tooth version.
- **Worm gears** use a screw-like worm driving a wheel. They give large reductions in one stage (for example 40:1) and can resist being back-driven, but they are less efficient and run hotter.
- **Planetary gears** have a central sun gear, planet gears around it and an outer ring gear. They pack a high ratio into a small space.
- **Rack and pinion** turns rotation into straight-line motion.

## Gear ratio

The gear ratio of one pair is the number of teeth on the driven gear divided by the teeth on the driver:

```text
ratio = driven teeth / driver teeth
```

A 20-tooth pinion driving a 60-tooth gear has a ratio of 60 / 20 = 3, written 3:1. The output turns once for every three turns of the input.

**Speed** goes down by the ratio:

```text
output rpm = input rpm × driver teeth / driven teeth
```

**Worked example 1.** A 1,750 rpm motor drives a 20-tooth pinion meshing with a 60-tooth gear.

```text
output rpm = 1,750 × 20 / 60 = 583.3 rpm
```

**Torque** goes up by the ratio, minus losses:

```text
output torque = input torque × ratio × efficiency
```

If the motor delivers 10 N·m and the gear pair is 98 percent efficient, the output torque is 10 × 3 × 0.98 = 29.4 N·m. Power is conserved apart from losses: speed down, torque up.

## Multi-stage trains

Gearboxes often have two or three stages. Multiply the stage ratios to get the overall ratio.

**Worked example 2.** Stage 1 is 18:54 (ratio 3) and stage 2 is 20:80 (ratio 4).

```text
overall ratio = 3 × 4 = 12
output rpm    = 1,800 / 12 = 150 rpm
```

An **idler** gear between two others changes direction but not the ratio, because its teeth cancel out.

## Worm gears

For a worm, the ratio is the wheel teeth divided by the number of **starts** (threads) on the worm. A 40-tooth wheel with a single-start worm gives 40:1. A two-start worm with the same wheel gives 20:1. Worm efficiency can be well under 90 percent, so check the nameplate before assuming output torque.

## Reading a gearbox nameplate

A reducer nameplate usually lists the ratio, input and output speed, rated input power or output torque, service factor, mounting position and lubricant. The **mounting position** matters: the oil level and breather location are set for a specific orientation, and mounting it differently can starve upper bearings of oil.

## Inspecting a gearbox

During a PM or a fault call, look and listen for:

- **Oil level and condition**: check the sight glass at the correct mounting position. Milky oil means water; dark, burnt-smelling oil means overheating; metallic glitter means wear.
- **Leaks** at shaft seals and gaskets.
- **Temperature**: compare with the same unit on a sister machine or with past readings.
- **Noise**: a steady whine is normal for some gears; a knock once per revolution may mean a damaged tooth.
- **Backlash**: the small free play between teeth. Too much suggests wear.
- **Breather**: a blocked breather builds pressure and pushes oil past seals.

## Common mistakes

- Dividing the wrong way. If the output should be slower, the answer must be smaller than the input speed.
- Forgetting that torque rises as speed falls, so a slower output shaft, key and coupling must be sized for much higher torque.
- Topping up with the wrong oil. Gear oils differ in viscosity and additives; use the one on the nameplate or lubrication schedule.
- Ignoring the mounting position when refitting a gearbox.

## Safety

Gear meshes are severe nip points. Inspect running gearboxes only from outside the guards, and open covers only after lockout. Hot oil can burn. Your employer must train you on their equipment before you service a gearbox.

## Recap

- Ratio = driven teeth / driver teeth; multiply stage ratios for a gear train.
- Output speed = input speed × driver / driven; output torque = input torque × ratio × efficiency.
- Worm ratio = wheel teeth / worm starts.
- Check oil, leaks, temperature, noise, backlash and the breather.

## Try it

Open the **Do** tab and finish a gear train calculator by writing the output speed formula.


Name the common gear types and calculate gear ratios, output speeds and torque through a gearbox.

Gears and gearboxes

How To Calculate Gear Ratios: Torque, RPM, & Mechanical Advantage

What Are Gearboxes? || THORS Gearbox Basics Course Preview

Bearings, belts, chains, couplings and gears: how machines carry motion and load from a motor to the work, and how those parts fail.

Power Transmission

## Why it matters

Presses, injection molding machines, lifts and heavy mobile equipment use hydraulics because oil under pressure can produce enormous force from a compact cylinder. When a hydraulic machine is slow, weak or leaking, the schematic is your map. Being able to read it, and to respect the stored energy in the system, is a core mechanic skill.

## The core idea: Pascal's law

Pressure applied to a confined liquid acts equally in all directions. Pressure is force per area, so:

```text
force = pressure × area
```

**Worked example.** A cylinder has a 4 in bore. The piston area is π × (4 / 2)² = 12.57 in². At 2,000 psi:

```text
force = 2,000 × 12.57 ≈ 25,100 lbf
```

That is over 12 tons from a cylinder you can hold in two hands. On the return stroke, oil pushes on the rod side, where the rod takes up some of the area, so the retract force is smaller and the retract speed is faster for the same flow.

Two ideas keep you straight:

- **Pressure** decides force. The pump does not "make" pressure; resistance to flow does. Pressure rises until the load moves or the relief valve opens.
- **Flow** decides speed. Cylinder speed = flow / piston area, so more gallons per minute means faster movement.

## Main components

A basic circuit has:

- **Reservoir (tank)**: holds the oil, lets air and heat escape and contaminants settle.
- **Pump**: moves oil. Gear, vane and piston pumps are positive displacement: each turn moves a fixed volume.
- **Relief valve**: limits maximum pressure by dumping oil back to tank. It protects the system and is one of the most important safety parts.
- **Directional control valve (DCV)**: sends oil to one side of the actuator or the other.
- **Actuators**: cylinders for straight-line motion, hydraulic motors for rotation.
- **Check valves, flow controls and filters**, plus hoses, tubing and fittings.
- **Accumulator** (on some systems): a vessel that stores oil under gas pressure, which means stored energy even after the pump stops.

## Reading the schematic

Hydraulic schematics use standard symbols (ISO 1219 is the common international standard). They show function, not physical layout.

- **Lines**: solid lines are working (pressure and return) lines; dashed lines are pilot or drain lines.
- **Pump**: a circle with a solid triangle pointing outward (fluid leaves). A hydraulic motor has the triangle pointing inward.
- **Tank**: an open-topped rectangle, often drawn at several places to save lines.
- **Valves**: each square is one **position** of the valve. A 4/3 valve has 4 ports and 3 positions. Arrows inside a square show flow paths in that position; a T shows a blocked port. The actuators drawn at the ends (a solenoid, a lever, a spring) show what shifts the valve.
- **Center position**: on a 3-position valve, the middle square shows what happens in neutral. A tandem center unloads the pump to tank; a closed center blocks everything.
- **Relief valve**: a square with an arrow offset from the flow path and a spring, connected to tank.
- **Check valve**: a ball on a seat; flow goes only one way.
- **Cylinder**: a rectangle with a piston and rod.

To read one, start at the pump, follow flow through the DCV in the position you care about, out to the actuator, and back to tank. Then ask what limits pressure (the relief) and what controls speed (flow controls).

## Troubleshooting basics

- **Slow movement**: low flow, often a worn pump, internal leakage across a cylinder seal, or a partly open relief.
- **Weak force**: low pressure, often a relief valve set too low or leaking.
- **Noisy pump**: cavitation (starved inlet, clogged suction strainer, cold thick oil) or aeration (air drawn in at a loose inlet fitting).
- **Hot oil**: energy wasted, usually by oil dumping over the relief or leaking internally.
- **Contamination** causes most hydraulic failures, so filter changes and clean practices matter.

## Common mistakes

- Feeling for a pinhole leak with your hand. A fine jet of oil at high pressure can inject oil under the skin, a medical emergency that needs urgent surgery.
- Loosening a fitting on a "stopped" machine without bleeding pressure, including the accumulator and any trapped pressure under a raised load.
- Turning up the relief valve to "fix" a weak machine instead of finding the cause.

## Safety

Hydraulic systems store energy in pressurized oil, accumulators and raised loads. Lockout/tagout for hydraulics includes blocking or lowering loads and bleeding pressure to zero, verified with a gauge. Only do this after your employer has trained and authorized you. See OSHA's lockout/tagout page: https://www.osha.gov/control-hazardous-energy (Checked on 2026-10-23.)

## Recap

- Force = pressure × area; speed = flow / area.
- Pump moves oil; resistance creates pressure; the relief valve caps it.
- On a valve symbol, each square is a position; read flow from pump to actuator to tank.
- Never feel for leaks by hand; always bleed stored pressure before opening a system.

## Try it

Open the **Do** tab and take the quiz on hydraulic force, symbols and safety.


Explain how hydraulic systems multiply force and read the main symbols on a hydraulic schematic.

Hydraulics

Hydraulic System for Absolute Beginner: What you need to know

How to Read a Hydraulic Schematic: The Basics

## Why it matters

Compressed air runs grippers, clamps, sorters, packaging lines and air tools in almost every plant. Pneumatic systems are cleaner and simpler than hydraulics, but they leak, they need dry, clean air, and they store energy too. A mechanic who can trace a pneumatic circuit finds the stuck valve or the cut tube quickly.

## Pneumatics vs hydraulics

Air is compressible; oil is nearly not. That difference drives everything:

- Pneumatic systems run at lower pressure, typically around 80 to 100 psi (about 5.5 to 7 bar), so forces are smaller.
- Air cushions motion, so cylinders are fast but less precise, and they can bounce or creep under load.
- Exhaust air vents to atmosphere, so there is no return line to a tank.
- Leaks waste energy (compressed air is expensive to make) but do not make a mess.

Force still follows force = pressure × area. A 2 in bore cylinder (area 3.14 in²) at 90 psi pushes about 280 lbf.

## The supply side

1. **Compressor** makes the air, usually a rotary screw or piston machine.
2. **Receiver tank** stores air and smooths demand.
3. **Dryer** removes moisture. Compressed air holds a lot of water, which condenses in pipes and ruins valves and tools.
4. **Distribution piping** with drops taken from the top of the main so water stays in the main.
5. **FRL unit** at each machine: **Filter** (removes water and particles, with a drain), **Regulator** (sets the machine's working pressure, with a gauge) and **Lubricator** (adds a fine oil mist, though many modern components are pre-lubricated and must not be lubricated).
6. A **lockout valve** (shut-off and dump valve) that isolates and exhausts the machine's air.

## Actuators

- **Single-acting cylinder**: air extends it, a spring returns it (or the reverse).
- **Double-acting cylinder**: air drives it both ways through two ports.
- **Rodless cylinders, rotary actuators and grippers** for other motions.
- **Vacuum generators** (venturi) and suction cups for picking parts.

Cylinders often carry **reed switches** or magnetic sensors that tell the PLC whether the cylinder is extended or retracted.

## Valves and symbols

Pneumatic symbols follow the same ISO 1219 style as hydraulics:

- Each square on a valve is one position. A **5/2 valve** has 5 ports and 2 positions, the standard way to drive a double-acting cylinder. A **3/2 valve** drives a single-acting cylinder.
- Port numbers: **1** = supply, **2 and 4** = outlets to the actuator, **3 and 5** = exhausts. Pilot or control ports are 12 and 14.
- An exhaust is a triangle (open) at the port, often with a silencer.
- The ends of the valve show how it is operated: solenoid, spring return, push button, roller lever or air pilot.
- **Flow control valves** (a throttle with a bypass check) set cylinder speed. They are usually fitted to meter **out**, restricting exhaust air, which gives smoother motion.
- **Quick exhaust valves, shuttle valves (OR) and two-pressure valves (AND)** build logic in pure-air circuits.

## Tracing a circuit: worked example

A pick-and-place arm has a double-acting cylinder driven by a 5/2 single-solenoid, spring-return valve, with meter-out flow controls on both cylinder ports.

1. Start at the FRL: regulator at 80 psi. Air goes to port 1 of the valve.
2. **Solenoid off** (spring position): port 1 connects to port 2, which feeds the rod end. The cylinder is retracted. The cap end exhausts from port 4 to port 5.
3. **Solenoid on**: port 1 connects to port 4, which feeds the cap end. The cylinder extends. Rod-end air exhausts through its flow control, then from 2 to 3.
4. The flow control on the rod-end line therefore sets **extend speed**; the one on the cap-end line sets **retract speed**.

If the cylinder extends but will not retract, suspect a stuck valve spool, a failed spring, a solenoid that is still energized (check the PLC output), or a kinked rod-end line.

## Troubleshooting tips

- Listen for leaks with the machine idle; use soapy water or an ultrasonic leak detector.
- Check the regulator gauge first. Low supply pressure causes many "weak" or "slow" faults.
- Drain filter bowls; water in the lines causes sticky valves.
- Most solenoid valves have a **manual override** button, but pressing it moves the machine. Use it only when the area is clear and your procedure allows it.

## Common mistakes

- Using compressed air to blow off clothes or skin. Air can enter the body through the skin and cause serious injury. OSHA limits air used for cleaning to under 30 psi with chip guarding and PPE.
- Disconnecting a line without dumping pressure. Hoses whip and cylinders move.
- Forgetting that a cylinder can still move after the supply is shut off if air is trapped between valve and cylinder.

## Safety

Compressed air is stored energy: lockout includes closing and locking the dump valve and verifying zero pressure at the gauge, plus blocking anything that could fall. Get your employer's training before working on air systems. OSHA's compressed gas and equipment page: https://www.osha.gov/compressed-gas-equipment (Checked on 2026-10-23.)

## Recap

- Air is compressible: lower force, faster, cushioned motion, exhaust to atmosphere.
- Supply: compressor, receiver, dryer, FRL, lockout dump valve.
- 5/2 valve ports: 1 supply, 2 and 4 outlets, 3 and 5 exhausts.
- Meter-out flow controls set cylinder speed.

## Try it

Open the **Do** tab and take the quiz on tracing pneumatic circuits.


Trace a pneumatic circuit from the compressor and FRL through a valve to a cylinder, and name its main symbols.

Pneumatics

Basics of Pneumatics and Pneumatic Systems: Part 1 (Animation / Sub)

Simple pneumatic circuit - double acting actuator

Draw the system on paper: tank, suction line with strainer and valve, pump, discharge valve, cooling tower. Label where each gauge sits.

Write what changed in the readings. Which side of the pump (suction or discharge) changed first?

List at least four possible causes from the reading and the symptom table, and mark each as system-side or pump-side.

Cross out any cause the facts rule out (for example, the tank level and temperature are normal). Explain each one in a few words.

Rank the remaining causes from most to least likely. Explain why a higher suction vacuum plus gravel noise points to an inlet restriction.

Write the checks you would ask for, in order, starting with the ones that need no tools or lockout (valve positions, strainer differential gauge, work order history).

Write the step where lockout is needed (for example, cleaning the strainer) and note that it must follow the site procedure and be done by an authorized person.

Write a short corrective work order: the fault, the probable cause, the fix, and one PM task that would prevent it happening again.

## Why it matters

Pumps move water, chemicals, coolant and product; compressors make the plant's air. Together they are some of the most maintained machines in industry. Many pump "failures" are really system problems: a clogged strainer, a closed valve, a tank run low. A good mechanic diagnoses the system before tearing the pump apart.

## Two families of pumps

**Centrifugal pumps** spin an impeller that flings liquid outward into a casing (the volute), converting speed into pressure. They are the most common type. Their flow depends on the system: as the discharge is throttled, flow falls and pressure (head) rises along the **pump curve**. They should not run for long against a closed discharge valve, because the trapped liquid heats up.

**Positive displacement (PD) pumps** trap a fixed volume and push it out: gear, lobe, vane, diaphragm, piston and progressive cavity pumps. Flow stays nearly constant whatever the pressure, so a PD pump against a closed valve keeps building pressure until something breaks. That is why PD pumps need a **relief valve**.

Compressors split the same way: **dynamic** (centrifugal) and **positive displacement** (reciprocating piston, rotary screw, scroll).

## Key terms

- **Head**: pressure expressed as the height of liquid the pump could lift, in feet or meters. Pump curves use head because it does not depend on the liquid's density.
- **Best efficiency point (BEP)**: the flow where the pump runs most efficiently. Running far from BEP raises vibration, heat and bearing and seal wear.
- **NPSH** (net positive suction head): the margin of pressure at the pump inlet above the liquid's vapor pressure. The system provides NPSHa (available); the pump needs NPSHr (required). If NPSHa falls below NPSHr, the pump **cavitates**.
- **Affinity laws** for centrifugal pumps: flow is proportional to speed, head to speed squared, and power to speed cubed. Slowing a pump by 20 percent cuts power to about 0.8³ ≈ 51 percent, which is why variable speed drives save so much energy.

## Cavitation

When inlet pressure drops too low, vapor bubbles form in the liquid and then collapse violently as they reach higher pressure inside the impeller. It sounds like the pump is pumping gravel. Over time it pits and erodes the impeller. Common causes:

- Clogged suction strainer or filter.
- Partly closed suction valve (suction valves should be fully open; throttle on the discharge).
- Low level in the supply tank, or the tank too far below the pump.
- Hot liquid (higher vapor pressure).
- Running far to the right of the pump curve (too much flow).

## Seals

Shaft seals keep liquid in where the shaft leaves the casing. **Packing** is meant to drip slightly to cool and lubricate itself. **Mechanical seals** should not visibly leak. Running a mechanical seal dry, even for seconds, can destroy it, and misalignment and vibration shorten seal life.

## Worked example: diagnosing a pump

A coolant pump's flow has dropped and it sounds rough. Work from the outside in:

1. **Check the system first.** Is the tank level normal? Are suction and discharge valves in the right positions? Is the suction strainer clean?
2. **Read the gauges.** Low suction pressure points to an inlet restriction. Normal suction with low discharge pressure points to a worn impeller, wrong rotation or air in the pump.
3. **Check rotation.** A centrifugal pump running backward (after motor wiring work) still pumps, but poorly. Compare with the arrow on the casing.
4. **Listen and feel safely.** Gravel noise suggests cavitation; a steady rumble suggests bearing trouble.
5. **Look for leaks** at the seal and for heat at bearings.
6. **Compare with records.** The CMMS history may show the same fault six months ago.

Only after the system checks out do you lock out and open the pump.

## Common symptom table

```text
Symptom                     Likely causes
No flow                     Not primed, closed valve, wrong rotation, blocked suction
Low flow / low pressure     Worn impeller or wear rings, air leak at suction, speed low
Gravel noise                Cavitation: starved suction, hot liquid, strainer blocked
High vibration              Misalignment, cavitation, bent shaft, worn bearings
Seal leaking                Seal run dry, misalignment, worn seal faces
Motor overloads             Too much flow (running off the curve), thick liquid, binding
```

## Common mistakes

- Throttling the suction valve to reduce flow, which causes cavitation.
- Running a pump dry to "check rotation".
- Starting a PD pump against a closed discharge without a working relief.
- Replacing the impeller without fixing the low tank level that caused the cavitation.

## Safety

Pumps can hold hot, pressurized or hazardous liquids even when stopped. Lockout includes isolating and draining the pump and following the chemical's safety data sheet. Compressors restart automatically and store energy in receivers. Your employer's training and permits are required before you work on either.

## Recap

- Centrifugal pumps follow a curve; PD pumps give nearly constant flow and need relief protection.
- Cavitation happens when NPSH available drops below NPSH required.
- Diagnose the system (levels, valves, strainers, gauges, rotation) before the pump.

## Try it

Open the **Do** tab to work through a pump diagnosis on paper.


Compare centrifugal and positive displacement machines and diagnose common pump problems such as cavitation.

Pumps and compressors

Centrifugal Pump Basics - How centrifugal pumps work working principle hvacr

Troubleshooting Centrifugal Pumps: Avoid These 3 Common Issues

Hydraulic and pneumatic systems, their schematic symbols, and the pumps and compressors that drive them.

Fluid Power

## Why it matters

Lubrication is the cheapest maintenance there is and one of the biggest causes of failure when done badly. Too little lubricant and metal touches metal; too much and bearings overheat; the wrong type, or the right type contaminated with dirt and water, and parts wear fast. Many plants have a lubrication technician role for exactly this reason.

## What lubricants do

A lubricant separates moving surfaces with a film, reduces friction and wear, carries away heat, protects against rust and, in the case of grease, helps seal out dirt. In a rolling bearing the oil film is thinner than a human hair, so a few particles of grit can break through it.

## Oil

Oil is used where speeds or temperatures are high, where heat must be carried away, or where a circulating system already exists: gearboxes, hydraulic systems, compressors and large plain bearings.

**Viscosity** is the oil's resistance to flow, the single most important property. Industrial oils are graded by the **ISO VG** system: the number is the viscosity in centistokes at 40 °C. ISO VG 32 and 46 are common hydraulic oils; VG 220 and 320 are common gear oils. Higher loads and slower speeds need thicker oil; higher speeds need thinner oil. The **viscosity index** tells you how much viscosity changes with temperature.

Additives matter too: anti-wear, extreme pressure (EP) additives for gears, rust and oxidation inhibitors, anti-foam. Oils are not interchangeable just because the viscosity matches.

## Grease

Grease is oil held in a thickener, like a sponge holding water. It stays in place, so it suits bearings that cannot have an oil bath, open gears and parts that are hard to reach.

- **NLGI grade** measures stiffness. NLGI 2 is the most common general-purpose bearing grease. NLGI 0 or 00 are soft greases for centralized systems; 3 is stiffer.
- **Thickener type** (lithium complex, polyurea, calcium sulfonate, aluminum complex and others) affects water resistance, temperature range and compatibility.
- **Base oil viscosity** still matters, just as it does for oil.

**Compatibility**: mixing greases with incompatible thickeners can make the mixture soften and run out or harden. When changing a grease type, purge the old grease out and check the manufacturer's compatibility chart.

## How much grease, how often?

Bearing makers publish formulas and charts. A widely used starting point for the amount when regreasing a rolling bearing from the side is:

```text
grease (grams) = 0.005 × D × B
```

where D is the bearing outside diameter and B its width, both in millimeters.

**Worked example.** A 6310 bearing is 110 mm OD and 27 mm wide.

```text
grease = 0.005 × 110 × 27 ≈ 15 g
```

A typical grease gun gives roughly 1 to 1.5 g per stroke, so weigh a few strokes of your own gun. Here that is around 10 to 15 strokes, not "until it comes out the seal".

**Intervals** depend on bearing size and speed, temperature, load and contamination. Manufacturer charts give a base interval, which you shorten for heat (a common rule halves it for every 15 °C above about 70 °C), vertical shafts, vibration, dust or water. Then you adjust with experience: condition monitoring such as ultrasound can tell you when a bearing actually needs grease.

## Good lubrication practice

- **Clean before and after**: wipe the grease fitting and the gun tip. Fit caps to fittings.
- **Label everything**: color-coded tags on fittings and matching containers stop mix-ups.
- **Store properly**: sealed containers, indoors, first in first out. Use filtered transfer containers for oil, not open buckets.
- **Check, do not just add**: sight glasses on gearboxes, oil condition (color, smell, water), breathers and seals.
- **Oil analysis** on important machines finds wear metals, water and degraded additives long before failure.
- **Record it in the CMMS**: what, where, how much, when.

## Common mistakes

- **Over-greasing**: the most common error. Excess grease churns, heats the bearing and pushes past seals. In electric motors it can get into the windings.
- Mixing incompatible greases.
- Using a general grease where a high-temperature or food-grade (NSF H1) one is specified.
- Topping up a gearbox with whatever oil is nearby.
- Greasing a sealed-for-life bearing that has no fitting by drilling it.

## Safety

Lubrication routes often take you close to moving machinery. Many plants let trained staff grease some points while running, through extended lines outside the guards, but only where the site procedure says so; everything else needs lockout. Grease guns develop very high pressure: never point one at anyone, and treat injection injuries as emergencies. Read the safety data sheet for each lubricant.

## Recap

- Oil for speed, heat and circulating systems; grease where the lubricant must stay put.
- ISO VG = viscosity at 40 °C; NLGI grade = grease stiffness.
- Regrease amount ≈ 0.005 × D × B grams; intervals shorten with heat and contamination.
- Clean, labeled, recorded and not overdone.

## Try it

Open the **Do** tab and take the quiz on choosing a lubricant and interval.


Choose between oil and grease, read viscosity and NLGI grades, and set a sensible relubrication amount and interval.

Lubrication

What are the main differences between grease and oil?

Lubrication Basics

Create an Assets sheet with columns: asset ID, description, make/model, location, criticality (1-3). Add your chosen equipment.

Look up the owner's manual (most are free online) and list the maintenance tasks and intervals the manufacturer recommends.

Create a Work Orders sheet with the fields from the reading: WO number, asset, type, frequency, priority, estimated time, safety, parts/tools.

Write the safety line first: how the equipment is made safe (unplugged and plug kept with you, engine off and cooled, wheels chocked). Keep it specific.

Write 4 to 8 numbered tasks. Each task must be specific and measurable, with a spec where possible (for example, tire pressure 50-60 psi from the sidewall).

Add a Readings section with blank cells for at least two measurements you would record each time.

If it is safe and simple (for example, checking bicycle tire pressure and chain wear with a ruler), carry out the inspection tasks only and record the readings.

Fill in Findings and write one follow-up corrective work order for anything you would fix later.

Ask someone else to read your work order and tell you one task that was unclear. Rewrite it.

## Why it matters

A plant that only fixes things when they break spends nights and weekends on emergency repairs, buys parts at rush prices and loses production. Planned maintenance flips that around. Mechanics do not just do the work: they write and improve the PM tasks, record what they find and close work orders in the computerized maintenance management system (CMMS). Good records are how a team learns.

## Maintenance strategies

- **Reactive (run to failure)**: fix it when it breaks. Fine for cheap, non-critical items such as a light bulb in an office. Expensive for anything that stops production.
- **Preventive (PM)**: tasks on a fixed schedule, by calendar (every month) or by usage (every 500 operating hours). Examples: change filters, grease bearings, inspect belts.
- **Predictive / condition-based (PdM)**: measure the machine's condition (vibration, temperature, oil analysis, ultrasound) and act when the data shows a problem developing. You will see more of this in the PLC course's reliability lesson.
- **Reliability-centered maintenance (RCM)**: a structured method that asks how each asset can fail and what the consequences are, then picks the right strategy for each failure mode.

Most plants mix all of these. The skill is matching the strategy to the asset's criticality.

## Key metrics

- **MTBF** (mean time between failures) = operating time / number of failures. Higher is better.
- **MTTR** (mean time to repair) = total repair time / number of repairs. Lower is better.
- **PM compliance** = PMs completed on time / PMs scheduled. Many teams target 90 percent or more.
- **Planned vs reactive ratio**: the share of maintenance hours that were planned.

**Worked example.** A conveyor ran 2,000 hours last quarter and failed 4 times, with repairs of 3, 5, 2 and 6 hours.

```text
MTBF = 2,000 / 4 = 500 hours
MTTR = (3 + 5 + 2 + 6) / 4 = 4 hours
```

## What a CMMS does

A CMMS is a database of the plant's maintenance. Typical records:

- **Assets** in a hierarchy (site, area, line, machine, component), each with an ID, make, model, serial number, criticality and manuals.
- **Work orders**: requests, corrective repairs and PMs, with status (requested, approved, scheduled, in progress, complete, closed).
- **PM schedules** that generate work orders automatically.
- **Parts inventory** with stock levels, bin locations and reorder points.
- **Labor and history**: who did what, how long it took, what was found.

Common systems include IBM Maximo, SAP PM, Fiix, Limble and UpKeep, and many small shops use spreadsheets. The concepts are the same.

## Anatomy of a good PM work order

```text
WO number:     PM-0412          Asset: CNV-03 Conveyor 3 drive
Type:          Preventive       Frequency: every 4 weeks / 500 h
Priority:      3 (routine)      Estimated time: 45 min, 1 mechanic
Safety:        LOTO per procedure LOTO-CNV-03; gloves, safety glasses
Parts/tools:   Belt tension gauge, grease gun (NLGI 2), straightedge
Tasks:
  1. Lock out and verify zero energy at the drive disconnect.
  2. Inspect V-belts for cracks, glazing, wear. Record condition.
  3. Measure belt deflection force. Spec: 4-6 lbf at 0.5 in. Record.
  4. Check sheave alignment with straightedge. Record OK / adjusted.
  5. Grease both pillow block bearings: 6 strokes each. Record.
  6. Refit guard, remove locks per procedure, test run, listen.
Readings:      Tension ____ lbf   Bearing temp after run ____ °C
Findings:      _______________________________________________
Follow-up WO:  _______________________________________________
Completed by / date / actual time: ___________________________
```

Notice what makes it good:

- **Specific and measurable**: "measure deflection force, spec 4 to 6 lbf", not "check belts".
- **Safety first**: the lockout procedure is referenced by number.
- **Parts and tools listed** so the mechanic does not walk back to the crib.
- **Readings recorded**, so trends show up over months.
- **A place for findings and follow-up work**, so a worn belt becomes a planned corrective order, not a breakdown next week.

## Closing the loop

The work order is not done when the wrench goes down. Record actual time, parts used, readings and what you found, then close it. Failure codes (problem, cause, remedy) on corrective orders let the team spot repeat failures. If a PM never finds anything, it may be too frequent; if failures happen between PMs, it may be too infrequent or checking the wrong things.

## Common mistakes

- Vague tasks such as "inspect pump" that two mechanics would do differently.
- Signing off a PM without recording readings.
- Pencil-whipping: marking tasks done that were not done. It hides real problems and can be a safety issue.
- Never reviewing PMs after they are written.

## Beyond the platform

Industrial machinery mechanic roles usually require an apprenticeship or an associate degree in industrial maintenance with supervised hours, plus site lockout and qualified-person training. Optional credentials include NIMS (https://www.nims-skills.org/) and SMRP's CMRT (https://smrp.org/). Registered apprenticeships are listed at https://www.apprenticeship.gov/ (Checked on 2026-10-23.)

## Recap

- Match strategy to criticality: reactive, preventive, predictive, RCM.
- MTBF = operating time / failures; MTTR = repair time / repairs.
- A good PM work order is specific, measurable, safety-first and records readings.

## Try it

Open the **Do** tab to write your own PM work order.


Compare maintenance strategies and write a preventive maintenance work order the way a CMMS stores it.

Preventive maintenance and CMMS

What is Preventative Maintenance?

What is a CMMS?

Write the load weight and add an allowance for rigging hardware (assume 30 lb). Write the total.

Sketch the lift: hook above the motor's center, two sling legs down to the lugs. Label the sling length (6 ft = 72 in) and half the lug spacing (15 in).

Find the vertical height of each leg with Pythagoras: height = square root of (72² − 15²). Then find the angle from horizontal: sin(angle) = height / 72.

Calculate the tension per leg: (total load / 2) / sin(angle). Compare it with each sling's WLL. Is it OK?

Check the shackles (3/4 ton = 1,500 lb) and the hoist (2 tons = 4,000 lb) against the loads they will see. Note which item has the smallest margin.

Now imagine the only slings available are 18 in long. Recalculate the angle and tension. Write whether the lift is still acceptable and why.

Write the area controls: barricade, who is the single signal person, the travel path, and where the cart is chocked.

Write the pre-lift inspection checklist for the slings, shackles and hoist from the reading.

## Why it matters

Millwrights and machinery mechanics install, move and remove heavy equipment: motors, gearboxes, pumps, whole machines. Lifting is where small mistakes become fatal ones: a sling cut on a sharp edge, a hook loaded on its tip, a load that weighs twice what someone guessed. This lesson is awareness only. Before you rig or signal a lift for real, your employer must train you and, for many lifts, certify you as a **qualified rigger** under OSHA rules.

## The lift plan

Every lift, even a routine one, follows the same thinking. Complex or critical lifts get a written plan signed by a competent person.

1. **Know the weight.** Use the nameplate, drawings or manufacturer data. If you must estimate, calculate volume × density and round up. Add the weight of rigging hardware.
2. **Find the center of gravity (CG).** The hook must be directly above the CG or the load will tilt or swing when it lifts.
3. **Choose the hitch and slings**, accounting for the sling angle.
4. **Check every capacity**: hoist or crane, hook, slings, shackles, eyebolts, beam or gantry.
5. **Plan the path and landing**: overhead obstructions, floor loading, blocking at the landing spot.
6. **Control the area**: barricades, a single designated signal person, nobody under the load.
7. **Inspect** all rigging before use and remove anything damaged from service.

## Weight estimates

Steel weighs about 490 lb per cubic foot (about 7,850 kg per cubic meter). A solid steel plate 4 ft × 2 ft × 2 in is 4 × 2 × (2/12) = 1.33 ft³, so about 650 lb. Machines are mostly air and castings, so use the nameplate or shipping weight when you can.

## Hitches

- **Vertical**: one sling straight down. Full rated capacity, but the load can spin.
- **Choker**: the sling wraps around the load and through itself. Capacity drops, typically to about 75 to 80 percent of vertical for a standard choke.
- **Basket**: the sling goes under the load with both eyes on the hook. Capacity can be up to twice vertical when the legs are vertical, but only if the load is balanced and cannot slip out.

Always use the manufacturer's tag on the sling, which lists capacity for each hitch.

## Sling angle: the critical calculation

When two sling legs spread out from the hook, each leg carries more than half the load. The flatter the angle, the higher the tension.

```text
tension per leg = (load / number of legs) / sin(angle from horizontal)
```

```text
Angle from horizontal   Load multiplier per leg
90 degrees (vertical)   1.000
60 degrees              1.155
45 degrees              1.414
30 degrees              2.000
```

**Worked example.** A 2,000 lb gearbox is lifted with a two-leg bridle at 45 degrees.

```text
tension per leg = (2,000 / 2) / sin(45°) = 1,000 / 0.707 ≈ 1,414 lb
```

Each leg needs a working load limit (WLL) of at least 1,414 lb, not 1,000 lb. At 30 degrees each leg would see 2,000 lb, the full load. Many sites forbid angles below 30 degrees; aim for 60 degrees or steeper. You can estimate the angle from the sling length and the vertical height: sin(angle) = height / sling length.

## Hardware

- **Slings**: wire rope, alloy chain (only grade 80 or higher for overhead lifting), synthetic web and round slings. Every sling must carry a legible capacity tag.
- **Shackles**: the pin must be fully seated; load the bow in line, not sideways.
- **Eyebolts**: shouldered eyebolts lose much of their capacity when loaded at an angle; use them in line with the shank or use swivel hoist rings.
- **Hooks**: load in the bowl, never on the tip; the latch must work.

## Inspecting rigging

Remove from service: slings without a tag; web slings with cuts, burns, holes or exposed red warning yarns; wire rope with broken wires, kinks, birdcaging or crushing; chain with stretched, nicked or bent links; hooks with a throat opening that has grown or a twisted tip. Protect slings from sharp edges with softeners.

## Moving machinery without a crane

Mechanics also use machinery skates, toe jacks, pry bars, come-alongs and gantries. The same rules apply: know the weight, keep below rated capacity, keep hands and feet out from under the load, and crib (block) a raised load before anyone reaches under it.

## Common mistakes

- Guessing the weight.
- Ignoring sling angle, so each leg is overloaded.
- Standing under or beside a suspended load, or riding a load.
- Using a sling with a missing tag "because it looks fine".
- More than one person giving signals.

## Rules and training

OSHA covers slings in 1910.184 (general industry) and rigging equipment in 1926.251 (construction), with crane rules in 1926 Subpart CC: https://www.osha.gov/cranes-derricks. ASME B30.9 covers slings: https://www.asme.org/codes-standards/find-codes-standards/b30-9-slings. Rigger and signal person certifications are offered by bodies such as NCCCO: https://www.nccco.org/ (Checked on 2026-10-23.)

## Recap

- Know the weight and CG, choose the hitch, then check every capacity.
- Tension per leg = (load / legs) / sin(angle); 30 degrees doubles the load on each leg.
- Inspect and tag; protect from edges; one signal person; nobody under the load.
- Real lifts require employer training and, for many, a qualified rigger.

## Try it

Open the **Do** tab and plan a lift on paper.


Plan a safe lift on paper: estimate the load, account for sling angle, check rated capacities and control the area.

Rigging and moving machinery

Basic Rigging Terms You Should Know

Do's and Don'ts for Loading a Hook | Tips for Safe Rigging with Cranes & Overhead Lifting

## Why it matters

Maintenance mechanics fabricate brackets and guards, repair cracked frames, build up worn shafts and cut out seized parts. Even mechanics who are not certified welders need to know the processes, read a weld symbol on a drawing, and set up hot work so it does not start a fire. This lesson is awareness only: welding and cutting must be learned hands-on in a supervised shop or class, and hot work in a plant requires a permit.

## Arc welding processes

All of these melt the base metal and a filler with an electric arc, and shield the molten pool from air.

- **SMAW (shielded metal arc, "stick")**: a flux-coated rod. The flux burns to make shielding gas and slag. Portable, tolerant of dirty steel and wind, good for structural repairs outdoors. Slower, with slag to chip off.
- **GMAW (gas metal arc, "MIG")**: a wire fed continuously through a gun, shielded by gas (often argon and CO₂ mix for steel). Fast and easy to learn, common in fabrication shops. Needs clean metal and still air.
- **FCAW (flux-cored arc)**: like MIG with a flux-filled wire, with or without extra gas. Good penetration and usable outdoors.
- **GTAW (gas tungsten arc, "TIG")**: a non-consumable tungsten electrode with a separate filler rod, shielded by argon. Slow but very precise and clean; used for stainless steel, aluminum, thin material and critical welds.

A quick rule: stick for outdoor and dirty work, MIG for speed in the shop, TIG for precision and non-ferrous metals.

## Cutting and other processes

- **Oxy-fuel cutting**: an oxygen and fuel-gas (acetylene, propane) flame preheats steel, then a jet of oxygen burns through it. Also used for heating and brazing.
- **Plasma cutting**: an electric arc and compressed gas cut any conductive metal, including stainless and aluminum, quickly and cleanly.
- **Brazing and soldering**: join parts with a filler metal that melts below the base metal, without melting the base metal.
- **Grinding** prepares joints and finishes welds. Abrasive wheels have maximum speeds and must be guarded.

## Joints and weld symbols

Common joints are butt, tee, lap, corner and edge. On drawings, the American Welding Society (AWS A2.4) symbol tells you what to make:

- A horizontal **reference line** with an **arrow** pointing to the joint.
- The weld symbol **below** the line means the weld goes on the arrow side; **above** means the other side.
- A triangle is a **fillet** weld; the number to its left is the leg size (for example, 1/4 in).
- A flag at the bend means weld in the **field** (on site); a circle means weld **all around**.

## Weld quality

Good welds have consistent width and ripple, proper fusion into both pieces and no cracks. Defects include **porosity** (gas holes, often from dirty metal or lost shielding gas), **undercut** (a groove melted into the base metal), **lack of fusion**, **cracks** and **slag inclusions**. Structural and pressure welds are made to a written procedure by a certified welder and inspected.

## Hazards and controls

- **Arc radiation**: ultraviolet and infrared light burn eyes ("arc eye") and skin within seconds. Use a welding helmet with the correct shade, flame-resistant clothing covering all skin and welding screens to protect people nearby.
- **Fumes and gases**: welding fume contains metal oxides; stainless steel releases hexavalent chromium, galvanized steel releases zinc oxide, and some coatings release toxic gases. Use local exhaust ventilation and, where required, respirators. Never weld on degreased parts still wet with chlorinated solvent.
- **Electric shock**: keep cables, gloves and clothing dry and in good repair; never stand in water.
- **Fire and explosion**: sparks travel up to 35 ft (about 11 m) and fall through cracks. Never weld or cut containers that held flammables unless cleaned and certified safe.
- **Compressed gas cylinders**: secure upright, caps on when not in use, oxygen stored away from fuel gases, regulators and hoses with flashback arrestors.
- **Burns and hot metal**: mark hot parts "HOT".

## Hot work permits

In a plant, welding, cutting and grinding outside a designated shop needs a **hot work permit** (NFPA 51B). The permit confirms that combustibles within 35 ft are removed or covered, a **fire watch** with an extinguisher is in place during the work and for a set time afterwards (often at least 30 to 60 minutes), sprinklers and detectors are handled correctly, and the area is checked before the permit closes.

## Beyond the platform

Learn welding in a supervised school, community college or apprenticeship shop. Certification is commonly through AWS (https://www.aws.org/) or employer procedure tests. OSHA's welding, cutting and brazing rules: https://www.osha.gov/welding-cutting-brazing. NFPA 51B for hot work: https://www.nfpa.org/codes-and-standards/nfpa-51b-standard-development/51b (Checked on 2026-10-23.)

## Common mistakes

- Watching an arc without a helmet, even for a moment.
- Welding near combustibles without a permit or fire watch.
- Welding galvanized or coated parts without removing the coating and ventilating.
- Leaving oxygen and acetylene cylinders unsecured or with regulators loose.

## Recap

- Stick for outdoor and dirty work, MIG for speed, flux core for penetration outdoors, TIG for precision and non-ferrous metals.
- Weld symbol below the reference line = arrow side; triangle = fillet weld.
- Control arc light, fumes, shock, fire and cylinders.
- Hot work outside the shop needs a permit and a fire watch; real welding needs supervised training.

## Try it

Open the **Do** tab and take the quiz on welding processes and safety rules.


Name the main welding and cutting processes, what each is used for, and the hazards and controls that come with them.

Welding and fabrication basics

Welding Basics for Beginners

4 Types of Welding Explained: MIG vs TIG vs Stick vs Flux Core

Lubrication, preventive maintenance and CMMS work orders, rigging and moving machinery, and welding and fabrication basics.

Mechanical Systems & Maintenance

About this course

Course content

Power Transmission

Fluid Power

Maintenance Practice

Where this course leads

Next course on the path

Careers that use it