Hydrostatic Drive The Hydrostatic drive is used to drive a hydraulic motor at variable speed. A bi-directional, variable displacement pump controls the direction and speed of the hydraulic motor. This type of drive is commonly called a closed loop drive. The two ports of the hydraulic pump are hydraulically connected to the two ports on the hydraulic motor forming the closed loop. 1. Main Pump A piston pump is used in a hydrostatic drive. The pump volume can be varied from 0 to maximum. In the illustration shown, the pump swashplate is in the vertical position, which means that the pump output is now 0 GPM. To drive the hydraulic motor forward, the swashplate will angle and deliver fluid out of the A port. Port B will act as the suction port. To drive the motor in reverse, the swashplate will angle in the opposite direction. The B port will then be the pressure port, and the A port will be the suction. The amount the swashplate angles in each direction determines the flow from the pump. Basic Hydrostatic Drive Page 6-20
2. Charge Pump This is also sometimes known as the replenishing pump. The pump is usually mounted on the back end of the main pump. The charge pump volume is normally 10-15% of the main pump. The purpose of the charge pump is to provide make up fluid to the system. On systems that use hydraulic cylinders to stroke the main pump, the charge pump supplies the fluid to the cylinders. Because of leakage at the pump and motor case drains, less oil flows out of the motor than what the main pump is actually calling for. Let s look at a typical example. Page 6-21
3. Charge Pump Relief Valve The charge pump relief valve provides a flow path for the excess charge pump volume to return to tank. The relief valve is normally mounted on or near the charge pump. The outlet flow of this relief valve is usually ported into the pump case. In the system shown, the charge pump relief valve setting determines the pressure on the low pressure side of the loop. This pressure is usually 180-300 PSI. On systems with shuttle valves, the shuttle relief valve determines the pressure on the low side of the loop. The shuttle valve is discussed later in this section. 4. Make Up Check Valves The specific check valve permits free flow from the charge pump to the low pressure side of the loop. At the same time oil in the high pressure side is blocked to the low pressure side by the opposite check valve. The check valves are usually accessed by removing the charge pump. Hydrostatic Pump Assembly Idle Mode 1 2 4 4 3 Page 6-22
Hydrostatic Pump Forward Operation Hydrostatic Pump Reverse Operation Page 6-23
Servo Case Drain Stroking Cylinder Charge Pump Relief Charge Pump 5. Crossport Relief Valves The crossport relief valves limit the maximum pressure in the system. If the motor should stall, the relief valve on the high pressure side would open and dump the fluid back to the suction side of the loop. The valves also absorb shock spikes in the hydraulic system. To best absorb the pressure spikes, the valve should be mounted as close to the motor as possible. Depending on the system, the valves may be located on the pump, mounted in a separate block or on the hydraulic motor. Set the valves 200 400 PSI above the maximum operating pressure. Some drives may have a pressure override, which operates similarly to a pump compensator. On those systems the pressure override should be set below the crossport relief valve settings. Page 6-24
6. Hydraulic Motor The speed and direction of the motor is determined by the hydraulic pump. Maximum pressure to the motor is determined by the crossport relief valve settings. The motor case drain flow should be checked (if a piston type motor) and recorded for future troubleshooting purposes. On systems with shuttle valves, the tank port of the shuttle relief valve is sometimes ported into the motor case. On those systems, checking the case flow would not give an accurate indication of bypassing. Motor Stalled Page 6-25
Pump Control Mechanical Control The two most common methods of varying the hydrostatic pump volume is either by a mechanical connection or a servo valve. The mechanical control is by a cable or other mechanical linkage. An operator moves a joystick or foot pedal to stroke the pump. The GPM the pump delivers is directly proportional to the amount the joystick or pedal is moved. If the pump is delivering fluid when the joystick or pedal is centered, then the mechanical linkage may need to be adjusted. The mechanical control is normally found on mobile equipment and knucklebooms. Page 6-26
Servo Control Most hydrostatic drives operating in plants today use a servo valve to control the pump. Servo valves are explained in detail earlier in this manual. A variable D.C. voltage controls the direction and amount of flow the pump delivers. Normally a positive D.C. voltage will shift the servo spool into the A position. The amount the spool shifts is directly proportional to the strength of the voltage. The charge pump fluid is then ported out the servo valve A port and to the A cylinder. As the A cylinder extends to stroke the pump a mechanical feedback from the swashplate exerts a force on the servo valve torque motor. When the feedback force is slightly higher than the electrical force on the torque motor, the servo valve spool returns back toward center, allowing only enough oil to hold the piston in position. The fluid in the cylinder holds the pump on stroke. Oil is now delivered out of the A port of the pump. Page 6-27
The higher the voltage to the servo valve amplifier, the more volume the pump delivers. To reverse the flow direction out the pump, a negative voltage is applied to the amplifier. The pump will again stroke proportionally to the voltage and deliver fluid out of the B port. When there is no electrical signal to the servo valve, the pump volume output should be 0 GPM. If the hydraulic motor is drifting either the centering springs on the cylinders need adjusting or the servo valve needs to be nulled. Nulling the servo valve is discussed in detail in the servo valve section. The oil flow to the servo valve is filtered by a non-bypassing element. Most servo valves also contain a small pilot filter that has a 100-200 micron rating. If either filter plugs then the pump will stroke very slowly or not at all. Shuttle Valve and Relief Valve Although all systems do not have shuttle valves, it provides for cooling part of the inline fluid. The purpose of the shuttle valve is to direct a portion of the flow exhausting out of the motor back to tank. The oil is then ported back through a cooler prior to returning to the reservoir. This permits a small volume of oil in the main system to be cooled. In the previous example, the excess charge pump oil was dumped to tank over the charge pump relief valve. On systems with shuttle valves, the shuttle relief valve is set at a lower pressure than the charge pump relief valve. The setting of the shuttle relief valve determines the pressure on the low pressure side of the loop. It is important that the pressure of the shuttle relief valve is set below the charge pump relief valve. If set above, then the excess charge pump fluid will dump through the charge pump relief valve. Page 6-28
The shuttle valve and relief valve are many times bolted onto the hydraulic motor. It may also be in the same block as the crossport relief valves. Forward Operation Page 6-29
Filtration The fluid in the hydrostatic loop constantly re-circulates except for the oil flow through the shuttle relief valve. The best filter arrangement is to filter the fluid in both directions on each side of the loop. If filtering is not done in both directions then when the pump fails, the contamination from the pump can go directly into the motor or vice-versa. The filters should have visual or electrical indicators to indicate when the elements are contaminated. Page 6-30
Checking the Hydrostatic Drive To effectively troubleshoot the drive some preliminary checks should be made when operating properly to establish a reference. Charge pump relief valve setting. When the main pump is off stroke, the charge pump relief valve setting will be indicated on all gauges in the system. (The exception is when a two position shuttle valve is used). Shuttle relief valve setting. Check on the low pressure side of the loop when driving the hydraulic motor. Operating pressure. Check when the drive has the heaviest load on the machine. Check in both the forward and reverse direction. Crossport relief valve settings. The lines to the hydraulic motor will have to be plugged or the motor stalled. If a pressure override is used, it s pressure setting should also be recorded. The crossport relief valves should be set 200-400 PSI above the pressure override setting. Be careful not to exceed the maximum pressure of the system components when setting the crossport relief valves and pressure override. Check the command voltage to the amplifier and current to the servo valve (if used). The RPM of the hydraulic motor should be recorded for a specific D.C. signal to the servo valve. Speed problems in hydrostatic drives are usually related to either the incoming D.C. signal or the servo valve itself. Some pumps have a displacement indicator. The indicator position should also be recorded for a specific current to the servo valve. Motor case drain flow (if the motor is a piston type). As the motor wears, more oil will bypass. Be sure to check when driving the motor as excessive bypassing occurs when the pressure is at maximum. This will not be an effective check if the shuttle relief tank line is ported back through the motor case. Page 6-31
Filter indicators. Filters usually have a color coded or other visual indicator to show the element condition. If the elements are partially plugged on non bypassing type filters, the drive will slow down. The filters should be checked and changed as necessary on a regularly scheduled basis. Page 6-32