Machinery Lubrication

Machinery Lubrication September October

Machinery Lubrication magazine published by Noria Corporation

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16 | September - October 2016 | HYDRAULICS oil at the pump outlet port will bypass through these tolerances and flow into the pump case. The oil is then ported back to the reservoir through the case drain line. This case drain flow does no useful work and is therefore converted into heat. The normal flow rate out of the case drain line is 1 to 3 percent of the maximum pump volume. For example, a 30-gallon- per-minute (GPM) pump should have approximately 0.3 to 0.9 GPM of oil returning to the tank through the case drain. A severe increase in this flow rate will cause the oil temperature to rise considerably. To check the flow, the line can be ported into a container of a known size and timed (Figure 2). Unless you have verified that the pressure in the hose is near 0 pounds per square inch (PSI), do not hold the line during this test. Instead, secure it to the container. A f low meter can also be permanently installed in the case drain line to monitor the f low rate. This visual check can be made regularly to determine the amount of bypassing. When the oil f low reaches 10 percent of the pump volume, the pump should be changed. A typical variable-displacement, pres- sure-compensating pump is shown in Figure 3. During normal operation when the system pressure is below the compen- sator setting (1,200 PSI), the internal swashplate is held at the maximum angle by the spring. This enables the pistons to fully stroke in and out, permitting the pump to deliver the maximum volume. Flow from the pump's outlet port is blocked through the compensator spool. Once the pressure builds to 1,200 PSI (Figure 4), the compensator spool shif ts, directing oil to the internal cylinder. As the cylinder extends, the angle of the swashplate moves to a near vertical posi- tion. The pump will only deliver enough oil to maintain the 1,200 PSI spring setting. The only heat generated by the pump at this time is the oil that f lows past the pistons and through the case drain line. To determine the amount of heat the pump is generating when compensating, the following formula can be used: horse- power (HP) = GPM x PSI x 0.000583. Assuming the pump is bypassing 0.9 GPM and the compensator is set to 1,200 PSI, the amount of heat generation is: HP = 0.9 x 1,200 x 0.000583 or 0.6296. As long as the system cooler and reser- voir can remove at least 0.6296 horsepower of heat, the oil temperature should not increase. If the bypassing increases to 5 GPM, the heat load increases to 3.5 horsepower (HP = 5 x 1,200 x 0.000583 or 3.5). If the cooler and reser voir are not capable of removing at least 3.5 horsepower of heat, the oil temperature will increase. Relief Valves Many pressure-compensating pumps utilize a relief valve as a safety backup in Figure 1. The tolerances between the pistons and barrel on a pressure- compensating, piston-type pump are approximately 0.0004 inch. Figure 2. Check oil f low by porting the case drain line into a container of a known size and timing the f low rate. Figure 3. This illustration shows a variable-displacement, pressure-compensating pump in normal operation. Figure 4. As pressure increases to 1,200 PSI, note the changes in the pump's compen- sator spool, internal cylinder and swashplate.

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