Evaluating a Customer’s Circuit Before Trouble Shooting
By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training.
I have been teaching hydraulics since 1980 and found that many good maintenance technicians cannot determine what pressures should be on the attached circuit. How well will you do?
Note: The actual drawing shown does not show the counterbalance bypass check that was built into the valve used. (Errors and omissions like this are common with drawings we are given to trouble shoot a problem.)
First: What would you expect the approximate pressure to be on gage “A,” if only the electric motor and Sol. 1B were energized?
Second: If the system were turned off with the ram fully retracted, would the ram drift down?
Third: If the ram were held in the fully retracted up position with the motor running and Solenoid 2B energized, what would gage “E” read?
See Solution
The first question: gage “A” would be somewhere around 0.35 to 0.7 MPa (50 to 100 PSI). The pilot operated relief’s pilot flow through an orifice is vented back to tank allowing the valve to unload at the bias spring pressure.
The second question: Will the ram drift down from the fully retracted up position? Yes, it will. Most Counterbalance valves use sliding spools that require clearances in the bore of the valve. Sun hydraulics show their valves leak several drops per minute at 21 MPa (3000 PSI). The directional valve spool leaks more. Over a weekend this press drifted three feet.
The third question: Everyone seems to say, it would be 10.3 MPa (1500 PSI). It would be system pressure at 17.2 MPa (2500 PSI). Any pressure in the drain line of the pressure reducing valve is additive to the spring setting. In this case, the valve would only reduce extension pressure and have no control on retraction pressure.
Robert Sheaf has more than 45 years troubleshooting, training, and consulting in the fluid power field. Email rjsheaf@cfc-solar.com or visit his website at www.cfcindustrialtraining.com. Visit fluidpowerjournal.com/figure-it-out to view previous problems.
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