Hydraulics is a major topic on the Red Seal Millwright exam. This guide covers Pascal's Law, system components, pump types, valve functions, cylinders, fluid maintenance, and troubleshooting principles.
Pascal's Law
"Pressure applied to a confined, incompressible fluid is transmitted equally in all directions throughout the fluid."
This principle allows a small force applied over a small area to generate a much larger force over a larger area — the core concept behind hydraulic cylinders and jacks.
Pascal's Law — Force Multiplication
Small force in, large force out
Larger piston area = greater output force. This is how a hydraulic jack multiplies force.
Key Formulas
Basic Hydraulic Circuit
Fluid flow through the system
Fluid flows: Reservoir → Pump → Valve → Cylinder → back to Reservoir.
Hydraulic Pump
Converts mechanical energy into hydraulic energy by moving fluid.
Hydraulic Motor
Converts hydraulic energy back into mechanical (rotational) energy.
Hydraulic Cylinder
Converts hydraulic pressure into linear force and motion.
Control Valves
Direct, regulate, and control fluid flow and pressure.
Reservoir (Tank)
Stores hydraulic fluid and helps dissipate heat.
Accumulator
Stores hydraulic energy under pressure for later use.
Filter / Strainer
Removes contaminants from hydraulic fluid.
Pressure Relief Valve
Protects the system from overpressure by diverting excess fluid.
Ready to test Pascal's Law in practice?
Try Red Seal–style hydraulics questions with instant feedback.
The pump is the heart of the hydraulic system — it does not create pressure, it creates flow. Pressure is created by resistance to flow.
TIP
Remember: pumps create flow, not pressure. Pressure is a result of resistance. This distinction appears frequently on exams.
Hydraulic cylinders convert hydraulic pressure into linear mechanical force and movement.
Single-Acting
Pressure extends the rod. Spring or gravity returns it.
Double-Acting
Pressure used for both extension and retraction.
Cylinder Force: Extend vs Retract
Tap to see how the area changes
Extend
Pressure is applied to the full bore side of the piston. The rod extends outward.
F = P × A_bore
Full piston face receives pressure — maximum force.
Controls the direction of hydraulic fluid flow to actuators. Example: 4/3 directional valve.
Limits maximum system pressure. Opens to divert flow back to tank when pressure exceeds the set limit.
Regulates the rate of fluid flow to control actuator speed.
Allows fluid to flow in one direction only. Prevents backflow.
Hydraulic fluid serves multiple roles: it transmits power, lubricates components, dissipates heat, and prevents corrosion. Contamination is the #1 cause of hydraulic system failure.
Particle Contamination
Metal wear, dirt, rust entering the system.
Water Contamination
Causes corrosion and reduces lubrication.
Air Contamination
Causes spongy operation and cavitation.
Thermal Degradation
Overheating breaks down fluid viscosity.
Prevention is key.
Change filters regularly, use breather caps on reservoirs, use proper fluid storage, and keep system connections clean and tight.
Slow or Weak Operation
Overheating
Noisy Operation
External Leaks
Always relieve system pressure before disconnecting lines or components.
Never use your hand to check for hydraulic leaks — use cardboard or a detector. Hydraulic injection injuries are life-threatening.
Lockout/Tagout (LOTO) must be applied before any maintenance work on hydraulic systems.
Hydraulic hoses have pressure ratings — never exceed them.
Hot hydraulic fluid can cause severe burns.
According to Pascal's Law, pressure applied to a confined fluid is transmitted in which direction?
What is the primary function of a hydraulic pressure relief valve?
What is the most common cause of hydraulic system failure?
Which type of hydraulic pump is capable of the highest operating pressures?
What causes cavitation in a hydraulic pump?
When studying hydraulics, focus on:
Hydraulics is just one part of the Red Seal exam. Keep building your knowledge with study guides on bearings, rigging, safety, power transmission, and alignment.
Continue studying:
Last updated: August 2026 · MW Red Seal Millwright Prep is built by a millwright, for millwrights. Content is aligned with the National Occupational Analysis (NOA) for Industrial Mechanic (Millwright) — the same document that structures the Red Seal exam.