Often when an internal combustion engine is used on mobile hydraulic equipment a problem occurs where an hydraulic pump is asked to deliver full system pressure while the engine speed is low.
If the engine speed is below the speed range of the pump, then the internal slip may be greater than the discharge and the pump will fail to deliver hydraulic oil. This is not an unusual occurrence and the problem worsens as the pump wears. Because there is no pump output, rapid localised heating occurs within the pump and external temperature safety devices, which are usually located in the reservoir, do not detect this overheating. Pumps subjected to these conditions are soon destroyed.
Sun Hydraulics has come up with a solution to this problem: a valve that 'senses' engine speed and acts as a relief valve on an unloading valve.
In its basic form (see Figure 1) the valve works by responding to differential pressure across a fixed orifice. This pressure differential is proportional to flow, which is in turn related to engine speed.
Figure 1. Low idle valve for pump protection
At low pump flow the unloading valve remains open and hydraulic fluid is returned to the reservoir. With fixed displacement pumps as flow (engine speed) increases the pressure across the orifice increases, until, at a predetermined pressure, the unloading valve closes (stops by-passing) and hydraulic fluid flows to the load. Should speed drop below the setpoint for any reason then the valve will cut out (start by-passing) giving free circulation for the oil and preventing damage to the pump.
The same technique can be used in conjunction with a differential pressure sensing valve for accumulator unloading (see Figure 2).
Figure 2. Low idle valve with differential pressure sensing valve for accumulator unloading
In many cases a cheaper gear or vane pump can be used in place of a piston pump. No matter which pump is chosen this valve will prolong its life. There are four sizes of low idle valve available covering flows from 0 to 280 litres per minute.
This valve with its unique features saves users maintenance costs and enhances the life of hydraulic pumps.
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