Hydraulic Systems and Components

Hydraulic Systems and Components

Introduction to Fundamental Principles

Hydraulics is the technology of transmitting power through a pressurized fluid. For the heavy equipment mechanic, mastering hydraulic systems is essential, since the majority of a machine's functions — steering, braking, bucket, boom, travel — depend on this system. The Red Seal exam requires a thorough understanding of the physical principles, components, and diagnostic procedures.

Pascal's Law

The foundation of any hydraulic system is Pascal's Law: pressure applied to a confined fluid is transmitted fully and equally in all directions. Mathematically, pressure (P) equals force (F) divided by area (A): P = F / A. The SI unit is the pascal (Pa), but in industry, the kilopascal (kPa) or megapascal (MPa) is commonly used. The conversion: 1 MPa = 1,000 kPa = 145 psi.

Force multiplication principle: If a force of 100 N is applied to a piston of 10 cm², the pressure generated is 10 N/cm². If this pressure acts on a second piston of 50 cm², the resulting force is 500 N. The displacement is inversely proportional: the small piston moves 5 times more than the large one. This is the principle of the hydraulic multiplier.

Flow and Speed

Flow rate (Q) is the volume of fluid displaced per unit of time, measured in litres per minute (L/min) or gallons per minute (GPM). The speed of a cylinder is directly proportional to the flow rate: the higher the flow, the faster the cylinder moves. The relationship between flow, piston area, and speed (v) is: Q = A × v.

Practical calculation: A cylinder with a 50 cm² piston must move at 10 cm/s. The required flow is: Q = 50 cm² × 10 cm/s = 500 cm³/s = 30 L/min. This calculation is fundamental for sizing a pump.

Pressure, Force, and Work

Pressure in a hydraulic system is created by resistance to flow. If a cylinder pushes a load, pressure rises until the generated force equals the resistance. Maximum pressure is limited by the relief valve. Hydraulic power (in kW) is calculated: P (kW) = Q (L/min) × p (MPa) / 60.

Example: A pump delivers 100 L/min at 20 MPa. The hydraulic power is: 100 × 20 / 60 = 33.3 kW. The mechanical input power will be higher due to losses (volumetric and mechanical efficiency).

Hydraulic Fluids

Types and Specifications

Hydraulic fluid serves as a power transmitter, lubricant, cooling agent, and sealant. The main types are:

Viscosity is the most critical property. A fluid that is too viscous increases pressure losses and heat generation; a fluid that is too thin reduces lubrication and increases internal leakage. The viscosity index (VI) indicates the stability of viscosity with temperature. A high VI (above 100) is preferable for Canadian climates.

Contamination and Filtration

Contamination causes more than 70% of hydraulic failures. Contaminants include: solid particles (dust, metal), water, air, and chemicals. ISO 4406 and NAS 1638 standards classify fluid cleanliness. A typical heavy equipment system must maintain an ISO 18/16/13 class or better.

Filtration rules:

Water in the fluid causes corrosion, reduced lubrication, and emulsion formation. Water content must be below 0.1% (1000 ppm). Air, dissolved or free, causes cavitation, noise, and fluid oxidation. Signs of aeration are foamy fluid and erratic operation.

Fluid Analysis

Regular fluid analysis is an essential preventive procedure. The parameters measured are:

A sample must be taken hot, after circulation, using a sterile sampling kit. The sampling point should be upstream of the return filter, in a turbulent zone.

Hydraulic Pumps

Classification and Principles

Pumps convert mechanical energy into hydraulic energy. They are classified into two categories: fixed displacement pumps and variable displacement pumps. Displacement is the volume of fluid delivered per revolution.

External Gear Pump

The most common on heavy equipment. Two identical gears rotate in opposite directions inside a housing. Fluid is carried between the teeth and the housing. Clearances are critical: tooth-to-tooth clearance (0.05-0.15 mm), clearance between gear faces and housing (0.05-0.13 mm), side clearance (0.02-0.10 mm). Excessive clearance causes a drop in volumetric efficiency.

Characteristic wear: cavitation erosion on the housing, scoring on the gear faces, bearing wear. Cavitation occurs when suction pressure is too low (clogged filter, fluid too viscous, suction lift too high).

Axial Piston Pump

Used for high-pressure and variable-flow applications. Pistons are arranged parallel to the axis of rotation and bear against a swashplate. The swashplate angle determines the piston stroke and therefore the flow. A 0° angle gives zero flow; a reversed angle reverses the flow direction.

Critical components:

Pressure compensation is a mechanism that automatically reduces flow when pressure reaches a preset threshold. The system is said to be pressure-compensated: the pump maintains constant pressure while reducing flow to zero in the hold position. This reduces energy consumption and heat generation.

Vane Pump

Vanes slide in rotor slots and bear against an eccentric stator. The volume between two vanes increases at intake and decreases at discharge. Vane pumps are quieter than gear pumps but less tolerant of contamination.

Efficiency and Losses

Volumetric efficiency (ηv) is the ratio of actual flow to theoretical flow. It decreases with wear and pressure. Mechanical efficiency (ηm) is the ratio of theoretical power to absorbed power. Overall efficiency is the product of the two. For a new pump, overall efficiency is typically 85 to 92%.

Theoretical flow formula: Q (L/min) = Displacement (cm³/rev) × Speed (rpm) / 1000.

Example: Gear pump of 50 cm³/rev at 2200 rpm. Q = 50 × 2200 / 1000 = 110 L/min. If volumetric efficiency is 90%, the actual flow is 99 L/min.

Hydraulic Cylinders

Types and Configurations

Cylinders convert hydraulic energy into linear motion. The two main types are:

The annular area (rod side) is smaller than the piston area. Consequently, at equal pressure, the retraction force is less than the extension force, but the retraction speed is greater. The area ratio is typically 1.2 to 1.5.

Force and Speed Calculations

Extension force: F = P × A (where A is the piston area)

Retraction force: F = P × (A - a) (where a is the rod cross-section)

Extension speed: v = Q / A

Retraction speed: v = Q / (A - a)

Example: Cylinder of 100 mm diameter, rod of 50 mm, pressure of 20 MPa.

End-of-Stroke Cushioning

Large cylinders are equipped with end-of-stroke cushioning. A plunger enters a reduced orifice at the end of stroke, which increases flow resistance and slows the movement. This prevents mechanical shocks and pressure spikes. Cushioning is adjusted using a restriction screw.

Telescopic Cylinder

Used for truck dump bodies and lifting equipment. Several stages nest inside one another. Total stroke is long for a compact retracted length. Speed is maximum in the first stage (small area) and decreases with each stage. Force is maximum in the first stage and decreases afterward.

Hydraulic Motors

Principles and Types

Hydraulic motors convert hydraulic energy into rotary mechanical energy. They are the mirror image of pumps: most pumps can operate as motors. Types include: gear, vane, axial piston, and radial piston.

The torque (T) developed by a motor is: T = ΔP × V / (2π × ηm), where ΔP is the pressure drop, V is the volumetric displacement, and ηm is the mechanical efficiency. The speed is: N = Q × ηv / V.

Example: Piston motor of 100 cm³/rev, flow of 120 L/min, ΔP of 25 MPa, ηv = 95%, ηm = 92%.

Fixed and Variable Speed Motors

Fixed displacement motors rotate at a speed determined by the flow. Variable displacement motors allow speed and torque to be varied. A variable displacement motor with a power regulator maintains constant power: displacement increases when pressure decreases, and vice versa.

Control Valves

Pressure Valves

The relief valve limits the maximum system pressure. It is normally closed and opens when pressure exceeds the set threshold. Fluid is then diverted to the reservoir. Adjustment is made using a spring compression screw. Setting accuracy is ±5%.

The sequence valve activates a secondary circuit when the pressure of the first circuit reaches a threshold. The pressure reducing valve maintains constant downstream pressure, lower than the supply pressure. The back pressure valve maintains a minimum pressure in a circuit.

Directional Valves

Directional valves control the direction of flow. They are designated by the number of ways (ports) and positions. A 4/3 valve (four ways, three positions) is the most common for double-acting cylinders.

The centre position of a 4/3 valve can be:

Open centre is used on mobile equipment: in neutral position, fluid flows from the pump to the reservoir at low pressure. Closed centre maintains pressure in the circuit.

Flow Control Valves

Restrictions (orifices) limit flow. The relationship is: Q = C × A × √ΔP, where C is a coefficient, A is the flow area, and ΔP is the pressure drop. Flow therefore varies with the square root of pressure.

The pressure-compensated flow control maintains constant flow regardless of pressure. It combines a variable restriction and a compensation valve that maintains a constant pressure drop across the restriction.

Check Valves

The check valve allows fluid flow in one direction only. The pilot-operated check valve allows reverse flow when a pilot pressure is applied. It is used to hydraulically lock a cylinder in position.

Accumulators

Types and Functions

An accumulator stores hydraulic energy in the form of pressurized fluid. The three types are:

The gas used is nitrogen (N₂), which is inert and non-flammable. The precharge (gas pressure) must be set to approximately 80-90% of the minimum operating pressure.

Applications

Gas Law

The relationship between gas pressure and volume is governed by Boyle's Law: P₁ × V₁ = P₂ × V₂ (at constant temperature, isothermal process) or P₁ × V₁^γ = P₂ × V₂^γ (adiabatic process, γ = 1.4 for nitrogen). In practice, rapid cycles are adiabatic, slow cycles are isothermal.

Precharge calculation: A 10 L accumulator must maintain pressure between 15 and 20 MPa. The precharge must be 80% of 15 MPa = 12 MPa. The available fluid volume is calculated using Boyle's Law.

Hoses and Fittings

Standards and Specifications

Hydraulic hoses are classified according to SAE J517 and ISO 18752 standards. Common types are:

The minimum bend radius must be respected to avoid pinching and cross-section reduction. The burst pressure is typically 4 times the working pressure.

Fittings and Assemblies

Fittings are crimp-type or reusable-type. Crimping must be done with a machine calibrated according to manufacturer specifications. Poor crimping causes leakage or hose ejection under pressure.

Installation rules:

Diagnostics and Troubleshooting

Symptoms and Causes

Testing Procedures

The flow meter is the primary diagnostic tool. It measures flow, pressure, and temperature. The full-load flow test consists of measuring flow at working pressure. A drop of more than 10% from the rated value indicates pump wear.

The internal leakage test for a cylinder: maintain pressure on the cap side, measure rod drift over a given period. Excessive drift indicates worn piston seals.

The back pressure test: measure pressure in the return line to the reservoir. Excessive back pressure (> 1 MPa) indicates a clogged filter or restriction.

Safety

Before any work on a hydraulic system:

A high-pressure hydraulic fluid jet can penetrate the skin and cause serious injury. In case of an injection injury, seek medical attention immediately.

Preventive Maintenance

Maintenance Program

Oil Change Procedure

Air Bleeding

Air in the system causes spongy operation and cavitation damage. Bleeding is done by:

Pitfalls to Avoid

Summary

This chapter covers the essential knowledge for the Red Seal exam on hydraulic systems. Master the calculations, principles, and procedures. Practice identifying components on schematics and solving diagnostic problems. Good luck with your preparation.