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Circuit problem

Circuit problem

Circuit problem

(OP)
Please have a look at this circuit and see if you can give me some advice. The device labeled 2 is the one in question. This is a valve supplying fluid to 2 small cylinders. cylinders extend and allow fingers to locate on a machine tool holder. When valve is off cylinder retracts and locks onto tool. These cylinders are tiny and require 1/2 as much pressure as the other 3 devices on the circuit. Well when this circuit is working the cylinders on device 2 are extending and we have heavy tools falling on the ground. We think it might be related to a drain issue. Thanks in advance..John
http://i933.photobucket.com/albums/ad179/jnmaney/img788.jpg

RE: Circuit problem

Functions 1, 3 and 4 are listed as 928-1000psi.

The two sandwich plates on #2 appear to be relief valves set to 425psi (425psi=30kg/cm2=3.0MPa), which dump to tank.

According to this schematic pressure from the P port can not be supplied to 2A and 2B at the same time.

Any back pressure in the tank line will reduce the holding force of the springs inside of the clamping actuators. I assume that there is some over-design, so lets assume that you only need 300-350 to extend the actuators. If you have 50psi of backpressure you have already lost ~15% of your clamping force.

Since you need hydraulic force to overcome the springs and extend the clamping actuators, the other explanation would be a sticking or miss-wired actuation valve.

ISZ

RE: Circuit problem

(OP)
Thank you very much. I'm following you so far. I will try and see what can be done on the back pressure issue.  

RE: Circuit problem

(OP)
oh and yes 2a and 2b do not function at the same time.

RE: Circuit problem

Is there leakage through the #2 valve?  Do you have any pressure gauges/transducers to connect downstream of the valve and see if pressure is building slowly when it should not be?

RE: Circuit problem

You might add a check valve to the #2 valve return line so that fluid cannot feed back when the other functions are operated.
Or increase the return line size to reduce pressure required to return flow to the reservoir.

Ted

RE: Circuit problem

(OP)
hydromech- leakage thru the #2 valve...for sure because tools are falling out of the arm that is supposed to be locked when a or b  are supposed to be turned off (cylinder retracts when pressure off ). I could hook up a gage but i know there would be some pressure. Hydtools- adding a check to the return line hmmm. not quite sure where to put it, i think this is a slightly different application for us as the tank is farther away than normal so increasing the size of the line back to tank might be the easiest way to see if makes a difference. Many thanks....

RE: Circuit problem

Or change valve 2 to a cylinder valve, one that blocks all ports(P, T, A, B) in the center position.

Ted

RE: Circuit problem

If you change tools infrequently just add some ball valves in the line and close them during normal operation.

ISZ

RE: Circuit problem

(OP)
I cracked the fittings at the cylinders at device 2 and found there is pressure at one side. at idle should this be or no ? Also followed the drain line back to tank and it's a long distance, plus connects to other tank lines converging and going back to tank. at one point it takes a 90 and goes up against a larger line coming down from top of the machine.  

RE: Circuit problem

What do you mean 'at idle'?  Did fluid spray out and you concluded that pressure was present?

Probably fluid in the return line simply drained back out the cracked open fittings.

Ted

RE: Circuit problem

(OP)
Just checking, boss said if there was backpressure causing the cyl to open you could confirm by cracking loose the line. Idle = no call for a or b to open.  

RE: Circuit problem

It might be of value to install pressure gages at the cylinders to see what pressure is there through a normal operating sequence.

Ted

RE: Circuit problem

(OP)
I did add gages to both sides of the cylinders in question and do not see any back pressure at the gages.  

RE: Circuit problem

If no pressure, then the cylinders are not moving because of a hydraulic problem.  There is no force to overcome the cylinder return springs.

Therefore, no hydraulic problem.

The springs are not applying enough force to hold the loads.

Ted

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