Pumps and Valves

The primary function of a hydraulic pump is to:

Create pressure directly Convert mechanical energy into hydraulic energy by producing flow Store hydraulic fluid Convert hydraulic energy into mechanical rotation

System pressure in a hydraulic circuit is actually developed by:

The pump alone at all times Resistance to flow at actuators and valves The reservoir size The colour of the hoses

A fixed-displacement gear pump:

Varies displacement automatically with load Delivers a constant volume per revolution regardless of pressure (within its rating) Provides flow only at zero pressure Is not used on mobile equipment

External gear pumps consist of:

Pistons and swash plate Two meshing gears rotating inside a closely fitted housing Vanes sliding in a rotor Radial pistons in a cam ring

Vane pumps use:

Intermeshing gears only A rotor with sliding vanes running in an eccentric cam ring Helical screws Centrifugal impellers

Axial piston pumps commonly control displacement by:

Changing gear size Varying the angle of a swash plate that pistons ride against Changing hose diameter Heating the fluid

Pump volumetric efficiency is reduced by:

Tight clearances and clean fluid Internal leakage due to wear or high clearances Operating within rated pressure Cool fluid temperatures

The main function of a pressure relief valve is to:

Increase flow to actuators Limit maximum system pressure by diverting excess flow to tank Directly operate cylinders Measure flow rate

A direct-acting relief valve opens:

Using pilot pressure and remote control only When spring force is overcome by system pressure on the poppet or spool Only when manually lifted Only at zero flow

Pilot-operated relief valves are often used because they:

Are simpler than direct-acting valves Provide more stable pressure control at higher flows Cannot be adjusted Operate only at very low pressures

Directional control valves (DCVs) are used to:

Change fluid viscosity Control the path of flow to actuators, determining direction of motion Filter contaminated oil Cool the fluid

A 4/3 directional control valve has:

Four positions and three ports Four ports and three positions Four ports and four positions Three ports and four positions

In an open-centre hydraulic system, the centre condition of the main directional valve:

Blocks all flow with pump deadheaded Allows pump flow to return to tank with minimal restriction when actuators are not being operated Always directs flow to one actuator Sends flow to the cooler only

In a closed-centre system, the pump is usually:

A fixed-displacement gear pump with constant flow A variable-displacement pump that destrokes to maintain system pressure Off whenever valves are centred Mounted in the reservoir

Flow control valves are used primarily to:

Limit maximum pressure Control actuator speed by regulating flow rate Direct flow to tank under all conditions Change oil temperature

Pressure-compensated flow control valves:

Provide a nearly constant flow over a range of pressure drops Change flow rate only with temperature Cannot be used in mobile equipment Always bleed flow to tank

Check valves in hydraulic circuits:

Allow flow in both directions Allow flow in one direction and block it in the opposite direction Act as relief valves Control flow regardless of direction

Counterbalance (holding) valves are used to:

Bleed air from the system Prevent a load from falling in case of hose failure and control overrunning loads Increase pump displacement Cool the oil

Sequence valves are used where:

Flow must be directed to all actuators simultaneously One actuator must complete its movement and reach a set pressure before another circuit operates Filters must be changed Coolers are installed

Load-sensing hydraulic systems:

Operate pump at full flow and pressure at all times Adjust pump output to match flow and pressure requirements of the load Do not use relief valves Require only gear pumps

A typical symbol for a pressure relief valve includes:

A square with a directional arrow A spring-loaded poppet symbol connecting pressure line to tank when setpoint is reached A circle with two arrows A triangle pointing into a reservoir

Spool-type directional valves may be actuated by:

Only manual levers Manual, mechanical, hydraulic, pneumatic or solenoid operators Electrical relays only Fluid contamination

Centre conditions of directional valves (such as open centre, closed centre, tandem centre) determine:

Filter size How pump flow and actuator ports are connected when the valve is in neutral Cylinder bore size Reservoir capacity

Cushion valves in cylinders are used to:

Increase cylinder speed at end of stroke Slow and dampen piston movement near the end of stroke to reduce impact Increase system pressure rating Vent air into the cylinder

Pump case drain lines are used to:

Feed fluid from reservoir to pump inlet Return leakage oil from pump housing to reservoir at low back pressure Direct high-pressure flow to actuators Vent air from reservoir

Direction of rotation of a hydraulic pump must:

Be ignored, as pumps are all bidirectional Match the engine or drive direction specified on the pump to ensure correct inlet and outlet Be reversed periodically Always be counterclockwise

When replacing a pump, the parts technician must match:

Paint colour and approximate size only Displacement, speed rating, rotation, mounting, shaft type and port locations Only shaft diameter Only manufacturer name

A tandem centre directional valve centre condition:

Blocks all ports Connects pump to tank while blocking actuator ports Connects pump to actuator ports Connects both actuator ports to tank while blocking pump

Pressure-reducing valves are used where:

Higher pressure is required in a branch circuit A lower controlled pressure is needed downstream for a specific function No pressure control is required Filters are located

Unloading valves allow the pump to:

Pump at maximum pressure constantly Deliver flow at low pressure to tank when high pressure is not required Stop turning when pressure drops Reverse rotation automatically

A closed-centre directional valve used with a fixed-displacement pump will:

Allow free flow to tank Deadhead the pump and rapidly build pressure against the relief valve Reduce system pressure Increase reservoir capacity

Pilot lines in valve circuits are used to:

Cool the fluid Transmit control pressure to operate other valves or pump controls Measure oil level Support machine structure

A proportional valve differs from a simple on/off solenoid valve because it:

Is only manual Can vary flow or pressure in proportion to an electrical control signal Cannot be used with electronics Is always fully open or closed

When troubleshooting low system pressure, a common first check is:

Replace all hoses Verify relief valve setting and operation with a pressure gauge Change reservoir size Increase engine idle speed only

Load checks or anti-cavitation checks on directional valves help to:

Increase temperature Prevent cavitation by allowing oil into a cylinder port when external forces retract the rod Reduce flow to tank Bypass filters

In a closed-centre load-sensing system, the load-sense line:

Drains directly to tank Feeds pressure information from the highest load to the pump control Supplies oil to the reservoir Operates the steering only

Creeping movement of a cylinder at neutral may indicate:

Perfect valve sealing Internal leakage past valve spools or cylinder seals Correct relief setting Low oil temperature