This is a picture of a differential pressure limiter from the main valve assembly on a Pettibone 10044. (2). I am trying to track down the cause of my engine lugging when actuating functions, particularly when multi-functioning. Notice the 2 springs. They are nested, and the smaller one is shorter, contrary to the visual rendering in parts book. Also notice that it is bent. I can buy the entire limiter assembly for no less than $700. My questions: 1. Is the parts book visual description of the springs enough to go by to conclude that the one has been permanently compressed and that they should both be the same length? 2. Could a hydraulic repair shop source or possibly manufacture a spring of length equal to the main one, if I provide the gauge and number of coils? Thanks.
The parts book drawing is not dimensionally accurate, so it is not enough to conclude that both springs should have the same free length. Nested springs can have different lengths intentionally to produce a two-stage spring rate. However, the smaller spring being bent may indicate that it was not seated correctly or has buckled. I would measure the differential/margin pressure before replacing anything. Engine bogging during multifunction can also be caused by excessive margin pressure, a sticking pump compensator, load-sense problem or main relief pressure set too high. A spring manufacturer could reproduce it, but wire diameter and number of coils are not enough. They also need the free length, mean coil diameter, active coils, material, end type and preferably the original load-versus-deflection curve. I would not make the smaller spring the same length without confirming the specification.
Thank you for the reply. I have connected a gauge and yes the differential pressure is not to spec. When adjustment is complete, system pressure should be 500 psi when in standby. I can't get it below 1000 if I follow the adjustment procedure, which is: 1. Bottom out differential pressure adjustment on the main valve. 2. Adjust the D.P on the pump, down to 625 initially. 3. Adjust the D.P on the valve, down to 600. 4. Adjust the D.P on the pump, down to 500. The only way I can get a system pressure value under 1000 psi is if I screw the D.P valve adjustment out, which I took to be the opposite of what is specified in the manual as "bottoming out". However this is perhaps asked and answered. I will see if I can make the adjustments with the D.P screw on the valve backed out, so I can proceed to step 2.
Update: Proceeding with the D.P screw turned out instead of in yields a system pressure value of around 500 psi. However, as in the above-mentioned case, the pressure reading remains unchanged when trying to adjust the D.P on the pump. So I will either purchase the D.P limiter kit for an arm and a leg, or humor the local hydraulic shop about coming up with a cheap alternative (spring), even if it may not be entirely completely all the way to spec. I have next to no technical information for this machine, much less specs for individual springs. In addition to this, any parts that I order seem to be special-order or obsolete.
Before buying the D.P. limiter kit, I would check the pump margin compensator and the load-sense circuit. If the pump D.P. adjustment produces no pressure change, the valve spring alone may not be the problem. Measure pump outlet pressure and LS pressure at the same time. The actual differential is P minus LS. A trapped LS signal, blocked orifice, sticking compensator spool or incorrect adjustment point could keep standby pressure above 1000 psi. Also inspect the limiter spool/poppet, bore, spring guides, shims and O-rings. The bent spring may be the result of a missing guide or spool binding. If a replacement spring is made, the shop should test and reproduce its load-versus-deflection curve, not only its dimensions.