In a closed loop hydrostatic transmission circuit, a low side hot oil shuttle valve is used to discharge supercharge oil from the low side of the loop for cooling. But in some fast reversal applications, high pressure and flow transients and cavitation can cause severe damage to downstream components.
This shuttle valve is piloted from the pump work ports, with the highest pressure shifting the valve so that the opposite (low pressure) side of the loop is connected to tank (typically through a hot oil relief valve, into the motor case – see Figure 1). When the pump is commanded to neutral, both sides of the loop see equal charge pressure, and the shuttle valve centres, causing all charge oil to exit the loop through the charge pump relief.
With pump reversals, the shuttle will quickly shift from one side of the loop to the other, following the highest loop-side pressure. In some fast reversal applications, such as drill or auger drive circuits, high pressure and flow transients can be transmitted outside the hydrostatic loop and can cause severe damage to components downstream of the shuttle valve, such as the heat exchanger, filter (if installed), and the motor case. Also, sudden loop flow losses can cause cavitation damage when instantaneous losses are greater than charge pump flow.
Delaying the shift
To address this circuit problem, Sun developed the DS*D-XEN hot oil shuttle with a delayed shift feature. When work port pressures reverse, this valve is designed to centre quickly, but will remain centred as long as the two work ports are at a value equal to or greater than 20 bar (300 psi) plus the hot oil relief setting (eg, with a 14 bar [200 psi] hot oil relief setting, the shuttle valve would not shift until one side drops below 34 bar [500 psi]). This valve eliminates damaging pressure reversal transients in two ways:
* It remains centred until one side of the loop truly becomes the low side. Once the low side has been established, it will shift in a controlled fashion after an approximate two second delay.
* An additional unique feature, standard on Sun’s DS*H low side hot oil shuttles as well as the DS*D delayed shift shuttles, is an internal hydraulic spool stop which allows a small pilot flow from the work ports to the outlet. This feature allows the hot oil relief pressure setting to be verified and adjusted while the transmission is in neutral.
Although there is a slight price premium for the delayed shift version of the low side hot oil shuttle, specifying it is the ‘safe way to go’ for any hydrostatic transmission application. The delay in shifting time has virtually no impact on overall loop cooling.
Operating principle
Low-side (hot oil) shuttle cartridges allow hot oil to be diverted from the low pressure side of a closed loop system. When both work ports (ports 2 and 4) are at equal pressures the valve is spring-centred to an all-ports-blocked position. When one of the work ports (port 2 or 4) sees a higher pressure the opposite work port is connected to the common port (port 3). The delay shift shuttle prevents flow transients downstream of the hot oil circuit.
For more information contact Fritz Kern, Axiom Hydraulics, +27 (0)11 334 3068, [email protected], www.axiom.org.za
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| www: | www.axiom.org.za |
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