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Troubleshooting Tips

For your Hydraulics system

Hydrostatic Transmission Circuit and Pumps

Most planer hydraulic drives are hydrostatic transmission systems. A hydrostatic transmission consists of a variable-displacement pump and a fixed or variable displacement motor, operating together in a closed circuit. Fluid from the motor outlet flows directly to the pump inlet, without returning to the tank.   A hydrostatic closed loop pump is not only variable but can be reversed. From within the pump both the direction and speed of motor rotation are controlled. This eliminates the need for directional and flow control valves.
Mounted on the rear of the main pump is a small fixed-displacement pump called a charge pump. This is used to ensure that the main loop between the pump and the motor remains full of fluid during normal operation. It pressurizes the “low pressure” side of the loop to between 110 and 360 PSI, depending on the transmission manufacturer. A simple charge pressure circuit comprises the charge pump, a relief valve and two check valves, through which the charge pump can replenish the transmission loop. Once the loop is charged to the pressure setting of the relief valve, the flow from the charge pump passes over the relief valve, through the case of the pump or motor or both, and back to tank providing flushing/cooling.

Pump or Motor Problems

When a pump or motor is worn or damaged, internal leakage increases and therefore the flow available to do useful work decreases. The condition of a pump or motor can be determined by measuring the flow from its case drain line.  The pump manufacturer will specify “normal” ranges of “friendly leakage”.   Besides measuring the flow of the case drain you can feel the line with your hand and other lines similar to it function to see if it seems hotter.  This also hold true for motors.  Excessive heat generally means a problem is brewing with the pump or motor.  One sure sign you might have a problem with a hydrostatic transmission pump is loss of charge pump pressure. This would generally be caused by high pressure leakage that exceeds the charge pump flow capacity.  This should be the first thing to check on. If you don’t know what it should be, check the schematic if you have one, or look up the manufacturer’s information online or email us.

Valve Testing

For testing a 4- way valve, it is necessary to obtain access to the tank return ports so that the amount of leakage can be observed. To make the test, disconnect both cylinder lines and plug these ports on the valve. Start up the system and shift the valve to one working position. Any flow out the tank port line while the valve is under pressure is the amount of leakage.

Is the cylinder okay?

Run the piston to one end of its stroke and leave it stalled in the position under pressure. Crack the fitting on the same end of the cylinder to check for fluid leakage. After checking, tighten the fitting and run the piston to the opposite end of the barrel and repeat the test. Occasionally a cylinder will leak at one point in its stroke due to a scratch or dent in the barrel. Check suspected position and run the piston rod against it for testing. Once in a great while a piston seal may leak intermittently. This is usually caused by soft packing or O-ring moving slightly or rolling into different positions on the piston, and is more likely to happen on cylinders of large bore. When making a test on a hydraulic cylinder the line should be completely removed from the cylinder port during the test, and open line from the valve should be plugged or capped since a slight back pressure in the tank return line would spill oil from the line it not plugged. Pistons with metal rings, and seals can be expected to have a small amount of leakage across the rings and even those ”leak tight” soft seals may have a small bypass during break-in of new seals or after the seals are worn in.

Why do Hydraulic systems overheat?

Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat. An example would be high pressure leakage across a valve.  A hydraulic system’s heat load is equal to the total power lost (PL) through inefficiencies and can be expressed as: PL(Horsepower) = psi x gpm leakage/1714.

Hydraulic Fluid Temperature. How hot is too hot?

Hydraulic fluid temperatures above 180°F (82°C) damages most seal compounds and accelerate degradation of the oil. While the operation of any hydraulic system at temperatures above 180°F should be avoided, fluid temperature is too high when viscosity falls below the optimum value for the hydraulic system’s components. This can occur well below 180°F, depending on the fluid’s viscosity grade.  Your oil supplier can provide you with this information.

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