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.