Hydraulic Fundamentals: Pascal's Law, Units, and System Overview

What Is Hydraulics?

Hydraulics is the branch of engineering that uses pressurized liquid to transmit force and motion. In heavy trucks and diesel equipment, hydraulic systems operate dump-body hoists, steering gear, brake boosters, fifth-wheel sliders, and implement circuits on vocational trucks. Understanding the physics behind these systems is the foundation of every diagnostic and repair task you will perform in this trade.

Pascal's Law

The entire science of hydraulics rests on one principle stated by French mathematician Blaise Pascal in the seventeenth century:

Pressure applied to a confined, incompressible fluid is transmitted equally in all directions throughout the fluid and acts with equal force on all equal areas of the containing vessel.

In plain shop language: if you push on oil in a sealed circuit, that push travels instantly to every point in the circuit. A small force applied over a small area can create a large force at a larger area, which is the mechanical advantage that lets a compact hydraulic cylinder lift a loaded dump body.

The Core Equation

Pascal's Law gives us one simple relationship between three variables:

F = P × A

• F = Force (pounds-force, lbf)
• P = Pressure (pounds per square inch, psi)
• A = Area (square inches, in²)

Rearranging for the other variables:

• P = F ÷ A
• A = F ÷ P

You must be comfortable moving between all three forms. On the job, you will use these formulas to size cylinders, verify relief-valve settings, and calculate whether a pump can generate enough pressure to lift a given load.

Units Used in Hydraulic Work

| Quantity | US Customary | Metric / SI |

|---|---|---|

| Pressure | psi (lb/in²) | bar (1 bar ≈ 14.5 psi); kPa |

| Force | lbf | N, kN |

| Area | in² | cm², mm² |

| Flow rate | GPM (gallons per minute) | L/min |

| Displacement | in³/rev | cc/rev |

OEM service manuals and Parker or Bosch Rexroth references may use either system; you must convert fluently. The conversion you will use most often is 1 bar = 14.504 psi.

Overview of a Heavy-Truck Hydraulic System

A dump-body circuit is a good teaching example because every major component category appears in it.

Reservoir — stores and conditions the fluid; it allows air to escape, heat to dissipate, and contamination to settle.

Pump — converts mechanical energy (from the engine via a power take-off, or PTO) into hydraulic flow. Pump output is measured in GPM. Pump displacement is rated in in³/rev (cubic inches per revolution), so at a given shaft speed you can calculate flow: GPM = (displacement × RPM) ÷ 231.

Relief Valve — the circuit's safety device. It opens at a set maximum pressure and routes oil back to the reservoir, preventing hose burst, cylinder damage, and structural failure. Never defeat or cap a relief valve.

Directional Control Valve (DCV) — routes flow to extend or retract the cylinder, or holds it in neutral. Operator lever position determines which port receives flow.

Actuator (Cylinder) — converts hydraulic pressure back into mechanical force and motion. Force out = pressure × piston area.

Lines and Fittings — hoses, tubes, and adapters carry fluid between components. They are rated for maximum working pressure and must match the system's relief-valve setting with an adequate safety margin.

Filter — removes particles that cause wear. Many systems use both a suction strainer and a return-line filter.

Hydraulic vs. Pneumatic vs. Mechanical Power Transmission

Three technologies compete for the same jobs on heavy equipment:

• Hydraulic — high force density, smooth variable control, self-lubricating; fluid leaks are a fire and slip hazard, systems require clean oil and quality seals.
• Pneumatic — uses compressed air; lighter lines, no spill hazard, but lower working pressures mean lower force for the same actuator size. Common in braking.
• Mechanical — direct, efficient, predictable; limited by geometry—linkages and shafts cannot route around obstacles the way hoses can.

Safety First

Hydraulic systems on heavy trucks operate at pressures that can exceed 3,000 psi. At those pressures, fluid injection injuries are life-threatening and can occur without visible puncture wounds. Follow these rules every time:

1. Relieve pressure before opening any fitting. Lower the body or implement to the ground, cycle the valve to neutral, and shut off the PTO before loosening any connection.

2. Never use your hand to check for leaks. Hold a piece of cardboard near the suspected leak and look for a wet spot. Even a pinhole stream at high pressure will penetrate skin.

3. Use proper PPE — safety glasses, chemical-resistant gloves, and face shield when opening hydraulic lines.

4. Lock out / tag out the PTO and engine before working inside the circuit. Follow your shop's LOTO procedure per OSHA standards.

5. Inspect hoses and fittings for abrasion, cracking, or corrosion before pressurizing. Replace any hose showing wire braid or outer jacket damage.

6. Hydraulic fluid is flammable. Keep it away from exhaust manifolds and hot surfaces. Have an appropriate fire extinguisher nearby.