Hydraulic Pump question

"Andy H " wrote in news: snipped-for-privacy@4ax.com:

Wrong type of valve for the application. You should have a 'open center' valve for this application. When the valve returns to center, the flow from the pump goes to tank, not to any device port. The device ports are blocked from flow in both directions. If you have a pressure relief before the valve, after the pump, you can use a 'closed center' valve. This will also block device ports but it blocks all flow from the pump when the spool is centered. "Open Center" valves are typically used with positive displacement pumps. "Closed Center" valves are typically used with variable displacement pumps. Using a "closed center" valve with a positive displacement pump that runs continuously will cause all flow to be diverted through the pressure relief valve when the spool is centered, this is very inefficient and generates a lot of heat since the pump will be working at the relief valve pressure setting the entire time the spool is closed. This is the reason they are typically used only with variable displacement pumps.

"Andy H " wrote in message news: snipped-for-privacy@4ax.com...

What is the application? Mobile? Lift gate or hoist? AC or DC?

If it is a double acting system, when you release the button a couple of things happen. Both coils on the spool valve should be de-energized, and the power is cut to the pump.

Sounds like the spool valve is not centering properly, or has some damage. In the relaxed state the valve should be open from P to T, with the A & B work ports blocked. It's letting oil go from T to a work port as the pump winds down. Easy to check. Disconnect the solenoids on the spool valve and fire up the pump. If the cylinder moves then it's time to look at the spool valve. Keep is CLEAN. Disassemble. Look for physical damage to the spool or bore, trashed o-rings or seals, broken springs, and crud in itty bitty ports and passages.

Is it possible you have air trapped in the system? This would allow residual pressure to extend the cylinder slightly after the pump had stopped.

Does your cylinder have built-in bumpers to cushion the end of the stroke? Could the bumpers be pushing the piston outward after the retract pressure is released?

dennis in nvca

Common on the cheaper pumps that just use one solenoid. No real solution that I know of. My use for the pumps is in hay lifts so I just make sure that I put the right side of the cylinder to the valve of the pump to make sure it goes up and stays up properly. A slight raising back up when all the way down doesn't hurt.

The only suggestions I can think of is to get a different pump (make sure it has two valve solenoids) or to put add a external solenoid valve assembly. Both are expensive options.

Or perhaps add a small time delay relay to the coil keeping the retract solinoid energized for a second (or less?) or so after the original off cycle to the pump is initiated? This could be relativly inexpensive and easy to install.

dennis in nca

Definitely possible though finding 12V time delay relays rugged enough might take a little looking.

Check out the electronic ones. Adjustable time delay from much less than one second to 10 hours (going by the example which I picked up at a hamfest). There is a tiny thumbwheel switch behind the dial plate (which pops out), and that allows selecting one of ten ranges.

This one was made by "idec" (yes -- all lower case).

Contacts are DPDT, rated at 10A 120 VAC (which should be plenty for controlling the solenoid), and the "coil" on this one is also 120 VAC, though I'm sure that they have DC voltage ones as well.

It has an octal base, and a knob on the top to select time delay within the range selected by the hidden thumbwheel switch. The cover panel has one side marked in seconds and the other side marked in minutes, and there are slide-ins to change the actual digits displayed through holes, so the panel can match the range of the selection. The

And the octal socket which came with it snaps onto a DIN rail to join other relays, terminal strips and similar devices.

I have no idea what the new price is (but Mouser probably carried them). I paid $5.00 at the hamfest, IIRC. Maybe only $3.00.

Enjoy, DoN.

I imagine there's a timer available for almost every duration and voltage. In a pinch, however, you might consider just changing the solinoid coil on your valve (if interchangable) for one with a higher voltage.

In addition to the octal plug-in style you can also find "hockey puck" styles which mount directly to your board.

dennis in nca

It's a DC unit, used with a cylinder on an H-frame press (cylinder is at the bottom of the frame instead of the top). On this pump, there is only one valve solenoid. When the up button is pressed on the remote, the motor solenoid is activated and fluid goes to the work port. When the down button is pressed, both the motor solenoid and valve solenoid are activated.

Because the valve solenoid is deactivated almost instantaneously when the button is released, a bit of fluid is sent out to the work port as the motor comes to a stop.

-- Andy H.

I think the answer to your problem is to switch the hoses going from the pump to the cylinder and then change the switch to make it right again. This will prevent the backing off of the cylinder when pressing (just make it travel a little bit back down when raising but that shouldn't hurt much).

The simplest solution is to split out the switches. One switch runs the motor, the other controls the valve. When you want to extend, all is normal. When you want to retract, hit the switch for the valve first, then hit the motor switch. When you get it retracted, hold the valve switch, and release the motor switch. When the motor is fully stopped, release the valve switch.

Jon