433A hydraulics and pneumatics practice questions.

Five free practice questions on hydraulics and pneumatics: block E of the Red Seal 433A Industrial Mechanic Millwright exam, about 21 of its 135 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Maintains hydraulic systems

   A mine hoist power unit has a suction strainer, a pressure filter ahead of the servo valves and a return-line filter. The PM sheet says the pressure filter is changed on the bypass indicator, not on hours. Why?

   1. The indicator shows real element loading, so it is changed when needed.
   2. Because pressure filters never plug within a calendar interval.
   3. The indicator counts the litres that have passed through.
   4. Changing on hours would void the pump warranty.
   Show the answer

   A. Correct. A differential-pressure indicator reads the real condition of the element. Changing on the indicator (checked at operating temperature) avoids both running in bypass, which sends unfiltered oil to the servo valves, and throwing away clean elements. Many sites still set a maximum interval as a backstop.

   Why not the others

   B. They do plug, and how fast depends on ingress, not the calendar. That is exactly why hours alone are a poor guide.

   C. Bypass indicators sense differential pressure, not volume. A high differential means the element is loading up.

   D. Warranty terms do not drive the practice. The indicator is used because it reflects the element's loading.

2. Critical · Diagnoses pneumatic and vacuum systems

   A pneumatic cylinder on a sawmill sorter has started banging hard at the end of its extend stroke. The retract stroke is still quiet. Speed controls have not been touched. What should be checked first?

   1. The regulator setting for the whole machine.
   2. The lubricator drip rate.
   3. The main line for a leak.
   4. The cushion adjustment and cushion seal at the rod end.
   Show the answer

   D. Correct. Built-in cushions trap a pocket of air in the last part of the stroke and bleed it through an adjustable needle to decelerate the piston. Banging on one stroke only points to that end's cushion: the needle has vibrated open, or the cushion seal has worn so the air escapes instead of being trapped. Adjust the needle; if it will not hold, replace the cushion seal.

   Why not the others

   A. Regulator pressure affects both strokes. One quiet stroke and one banging stroke means an end-specific fault.

   B. Lack of lubrication makes the cylinder stick and squeal, not bang at the end of one stroke.

   C. A supply leak lowers pressure and slows the cylinder; it does not cause a hard stop on one stroke.

3. Critical · Diagnoses hydraulic systems

   A pulp-mill press circuit is drawn with the relief set at 13 800 kPa (2 000 psi). Today the gauge peaks at 8 300 kPa (1 200 psi), the press stalls under load and the reservoir has climbed to 75 °C (167 °F). What is the most likely fault?

   1. The pump is running backwards.
   2. The relief valve is opening below its setting.
   3. The cylinder piston seals are worn.
   4. The heat exchanger is fouled.
   Show the answer

   B. Correct. Pressure that tops out well below the setting while oil temperature climbs is the signature of a relief valve passing flow at the wrong pressure: a poppet held open by contamination, a broken spring or an adjustment that has drifted. Flow dumped over the relief at 8 300 kPa becomes heat. Check and reset the relief with the actuator stalled before suspecting the pump.

   Why not the others

   A. A reversed pump makes no pressure at all and cavitates loudly; it does not hold a steady 8 300 kPa.

   C. Seal bypass would lower stall force and add heat, but the pressure gauge at the pump would still reach the relief setting because the relief, not the cylinder, limits it. A ceiling at 8 300 kPa points to the relief.

   D. A fouled cooler explains rising temperature but not a pressure ceiling below the relief setting. The heat here is being generated, not just retained.

4. Recall · Maintains pneumatic and vacuum systems

   A plant runs a refrigerated dryer on its main air supply but a heatless desiccant dryer on the drop to an outdoor conveyor control panel. Why the second dryer?

   1. Desiccant dryers are cheaper to run.
   2. A desiccant dryer reaches a far lower dew point, so outdoor lines will not freeze.
   3. The refrigerated dryer is too small for the plant.
   4. The desiccant dryer removes oil, which the refrigerated dryer cannot.
   Show the answer

   B. Correct. A refrigerated dryer bottoms out at a pressure dew point of about 3 °C (37 °F) because it cannot go below freezing without icing up. Air lines outdoors in a Canadian winter need a dew point of -40 °C (-40 °F) or lower, which only a desiccant (adsorption) dryer delivers. Desiccant dryers use purge air to regenerate, so they are applied where needed rather than plant-wide.

   Why not the others

   A. They cost more to run because they consume purge air or heat to regenerate. They are chosen for the low dew point, not the operating cost.

   C. Capacity would be solved by a larger refrigerated dryer. The reason for a different technology is the dew point required outdoors.

   D. Neither dryer type removes oil; that is the job of coalescing filters. Dryers remove water vapour.

5. Recall · Installs hydraulic systems

   A press circuit spends most of its cycle holding pressure with no motion. Why is a pressure-compensated variable-displacement pump specified instead of a fixed-displacement pump with a relief valve?

   1. It does not need a relief valve at all.
   2. It produces higher pressure than a fixed pump can.
   3. It destrokes at set pressure, so little flow becomes heat.
   4. It can run with a partly blocked suction line.
   Show the answer

   C. Correct. When the compensator setting is reached the swash plate strokes back and the pump delivers only what leaks past the components, at pressure but with almost no flow. A fixed pump would push its full flow over the relief valve during the hold, and that flow at full pressure becomes heat in the oil.

   Why not the others

   A. A relief valve is still fitted as a backup, set somewhat above the compensator, because a compensator can stick or respond slowly to a shock.

   B. Pressure is set by the load and the compensator or relief, not by pump type. Many fixed pumps run at higher pressures than a given variable pump.

   D. No pump tolerates a restricted inlet; a piston pump is the least forgiving because inlet starvation causes cavitation. The advantage is about heat, not inlet conditions.

02Answer key

03What block E covers

This block covers hydraulic, pneumatic and vacuum systems. It includes pumps, cylinders, motors and accumulators, relief and directional control valves, hoses and fittings, reservoirs, filters and ISO cleanliness codes, overheating, cavitation and aeration, compressed air mains, FRL units, regulators, dryers and leak surveys, vacuum generators and cups, and releasing stored fluid energy before repair.

The Red Seal Occupational Standard names this block “Services fluid power systems”. It carries 15.6% of the exam, about 21 of the 135 questions, split across two tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Force equals pressure times area

Cylinder force is pressure x area, with area equal to pi x radius squared. Working in kPa and square metres gives kilonewtons, so 10 000 kPa on a 100 mm bore gives about 78.5 kN. Pressure sets force; flow sets speed.

Where the heat comes from

Flow passing a relief valve at pressure turns into heat. A fixed pump behind a closed-centre valve dumps its full flow over the relief during dwell. A gauge peaking well below the relief setting while the oil runs hot points to a relief opening early.

Pressure is no test of wear

A badly worn pump can still reach relief pressure at stall. Measure its delivery with a flowmeter at operating temperature and compare it with rated flow. On a piston motor, case-drain flow above the manufacturer's limit shows internal leakage.

Pneumatic set-up rules

Air passes the filter, then the regulator, then the lubricator. Control cylinder speed by metering the exhaust, which keeps back-pressure on the piston. Pressure that is normal at rest but sags while a cylinder moves means flow starvation.

05The other blocks

The 433A exam has six blocks. Each has its own page of free questions:

The 433A Industrial Mechanic Millwright practice exam page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.