Machinery Lubrication

Machinery Lubrication March April 2016

Machinery Lubrication magazine published by Noria Corporation

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24 March - April 2016 | www.machinerylubrication.com HYDRAULICS the mechanical control is connected directly to the swashplate. An operator will move a joystick or foot pedal to stroke the pump. The gallons per minute the pump delivers are directly proportional to the amount the joystick or pedal is moved. The direction of pump flow and thus the rotation of the hydraulic motor are determined by which direction the pedal or joystick is moved. If the pump is deliv- ering fluid when the joystick or pedal is centered, then the mechanical linkage may need to be adjusted. Most hydrostatic drives in industrial applications use a servo or proportional valve to control the main pump. The specific valve is usually mounted on the pump housing. The valve is controlled by an input signal into the valve amplifier (normally a positive and negative direct current voltage). The input signal can come from a potentiom- eter, joystick or programmable logic controller (PLC). A positive voltage generally will shift the valve into the "A" (straight arrows) position, while a negative voltage will shift it into the "B" (crossed arrows) position. In Figure 1, the servo valve is shifted into the "A" position to port oil from the charge pump to the spring-loaded cylinder for stroking the pump swashplate. Once the swashplate moves proportionally to the amount the servo valve spool shifts, a mechanical feedback will block the oil flow out the servo valve. The pump swashplate will then stop moving and maintain the selected volume. To reverse the flow direction out of the pump, a nega - tive direct current (DC) voltage is applied to the amplifier. The valve will then shift proportionally into the "B" position and deliver fluid out the opposite port to reverse the motor. When there is no electrical signal to the valve, the pump volume output should be zero GPM. If the hydraulic motor is drifting, either the centering springs on the cylinders need adjusting or the valve needs to be nulled. The oil flow to the valve is filtered by a non-bypassing 3- to 10-micron element. Most servo valves also contain a small pilot filter that has a 100- to 200-micron rating. If either filter plugs, the pump will stroke very slowly or not at all. Hot Oil Shuttle Valve and Shuttle Valve Relief One of the disadvantages of hydrostatic drives is that the majority of the oil stays in the loop and doesn't return to the reser- voir for cooling. One way to return some of the oil back to the tank is by using a hot oil shuttle valve. The purpose of this valve is to direct a portion of the flow exiting the motor through a cooler before returning to the tank. When the motor is driven in the forward direction, the shuttle valve is shifted so the oil in the suction side of the loop is ported to the shuttle valve relief. The charge pump will deliver more oil to the pump suction side than is needed to make up for the bypassing inside the main pump and motor. This causes the pressure to build up to the shuttle valve relief setting (150-220 PSI). The shuttle relief valve will then open and port a small amount of the oil that flows out of the motor through the cooler and back to the tank. The setting of the shuttle relief valve spring determines the pressure on the low-pressure side of the loop. Although not all systems utilize shuttle valves, they are highly recommended to reduce heat in the system. Troubleshooting Hydrostatic Drives • If the neutra l position is diff icult or impossible to f ind, check the control va lve and linkage. Null the va lve if possible. • If the system is overheating, check the oil level in the tank, inspect the heat exchanger, check the inline pressure filters, inspect the crossport relief valves, and check the pump and motor case drains for excessive bypassing. • If the drive only operates in one direction, check the crossport relief valves, the command voltage, the control valve and linkage, and the makeup check valves. Also, inspect the hot oil shuttle valve. • If there is a sluggish response, check the charge pump pressure, charge pump suction filter, charge pump relief valve, hot oil shuttle relief valve, control valve, crossport relief valves, charge pump suction filter and charge pump. • If the drive will not operate in either direction, check the oil in the tank, the control valve and linkage, the command and power supply voltages, the crossport relief valves, the charge pump pressure, the charge pump relief valve, the hot oil shuttle relief valve, the pressure override, and the pump and motor case drain lines for excessive bypassing.

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