Tricky Orifices That are One Way Restrictor Valves

     Orifices are commonly used in hydraulic pump displacement controls to meter servo-cylinder flow. In some cases, pump manufacturers replace the orifices with very compact one-way restrictor valves, small enough to fit the standard orifice places. Normally, these valves consist of a small hardened pin, which has differently shaped cuts on each "nose", and a body, that contains the pin and "provides the seats". Alternatively, instead of placing the pin inside a single-piece body, it can be mounted between two separated seats, that are tightened one against the other in a threaded oil passage of the pump's body.

    We all know that an obstructed orifice is one of the most common reasons for a hydraulic pump control to malfunction. However, one-way restrictors can bring about another failure pattern, because apart form the natural clogging, these extremely small flow-control valves are subject to mechanical wear, as the small but very hard internal pin will hit the seats every time a servo-cylinder movement is made. When the wear reaches "critical stage", the pin can break the seats and "go with the flow", causing all sorts of mishaps.

   An example of such special orifices, that were mounted on a Rexroth A4VG closed loop pump, can be seen on the pictures to the left. One of the valves is lacking the pin - over the many hours of service it smashed its way through the body of the valve and finally fell into the servo-cylinder, where it managed to mash-and-cut the plastic cap, that was retaining the two piece lock ring of the servo-cylinder spring plate. In the end the lock ring halves fell out of the shaft groove and jammed the servo-cylinder.

   In oil hydraulics small things can cause big damage, therefore no "small" things should be neglected during an overhaul of a hydraulic pump, like checking whether the pump control is using a simple orifice or a one-way restrictor valve, and evaluating its condition. Here you can see that one of the valves is broken, and the other one is about to break. Most likely during the previous pump overhaul this damage could have been discovered and this failure could have been prevented.

Attention to detail, my good hydraulic friends, it's all about attention to detail....