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

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.