Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/655822
www.machinerylubrication.com | March - April 2016 | 23 valve is normally mounted on or near the charge pump. The outlet flow of this relief valve is usually ported into the pump case where it returns to the tank through the main pump's case drain line. In the system shown in Figure 2, the relief valve setting determines the pressure in the system when in idle mode. This pressure is typically 200-300 PSI. On systems that utilize a hot oil shuttle valve, a shuttle relief valve determines the pressure on the low side of the loop when driving the motor. Makeup Check Valves Makeup check valves permit 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 oppo- site check valve. The check valves are normally accessed by removing the charge pump. Crossport Relief Valves Crossport relief valves limit the maximum pressure in the system. If the motor should mechanically stall, the relief valve on the high-pressure side would open and dump fluid back to the low-pres- sure side of the loop, protecting the motor from overpressurizing. The valves also absorb shock spikes in the system. To best absorb the pressure spikes, the valves are generally 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. The valves typically are preset to 200 to 400 PSI above the maximum operating pressure. Some drives may have a maximum pressure override, which operates similarly to a pump compen- sator. When the pressure override setting is reached, the pump volume is reduced to an output of nearly zero GPM. The pump will only deliver enough oil to maintain the pressure override setting. On these systems, the pressure override is set below the crossport relief valve settings. Hydraulic Motor The speed and direction of the motor is determined by the vari- able displacement hydraulic pump. Maximum pressure to the motor is controlled by the crossport relief valve settings. The motor case drain flow should be checked and recorded for future trouble- shooting purposes. On systems with hot oil shuttle valves, the tank port of the shuttle relief valve is sometimes ported into the hydraulic motor case drain line. With these systems, checking the case flow would not provide an accurate indication of bypassing. This occurs because excess flow in the system would combine with the bypassing in the hydraulic motor. Pump Control The most common method of varying the pump volume is either by a mechanical connection or a servo valve. The mechanical control is done with a cable or other mechanical linkage. In some instances, the mechanical connection shifts a valve on the pump, which ports oil to the spring-loaded cylinders inside the pump. In other cases, Regular Maintenance Checks To effectively troubleshoot a hydrostatic drive, some preliminary checks should be made when the system is operating properly in order to establish a reference. • Record the charge pump relief valve setting. When the main pump is idle, the charge pump relief valve setting will be indicated on all gauges in the system. The exception is when a two-position hot oil shuttle valve is being used. • Record the shuttle relief valve setting. Check this pressure on the low- pressure side of the loop when driving the hydraulic motor. • Record the maximum operating pressure. Check when the drive has the heaviest load on the machine. Check in both forward and reverse directions. • Check the command voltage to the amplifier and the current to the servo valve. The motor's revolutions per minute should be recorded for a specific DC signal to the servo valve. Speed problems in hydrostatic drives are usually related to either the incoming DC signal or the servo valve. Some pumps have a displacement indicator. The indicator position should also be recorded for a specific command voltage to the amplifier. • If the motor is a piston type, check the case drain flow. As the motor wears, more oil will bypass. Be sure to check when driving the motor, as excessive bypassing occurs when pressure is at the maximum level. This will not be an effective check if the shuttle relief tank line is ported back through the motor case. • Check the filter indicators. Filters typically 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 on a regularly scheduled basis.