What happens to the fluid in the neutral position on an open center valve?

[J_J]

I know in the neutral position the pump fluid is returned to tank, but what happens to the fluid in a motor circuit. I believe it locks up and goes nowhere. What say you.

I am thinking the relief valve is on the input side, and that would not do anything for the pressure on the other side. You all know that a cylinder, or motor will lock in place when the handle is returned to neutral, blocking the output flow.

Now, on this same circuit, you are turning a hydraulic motor at max, and the handle on the valve is pulled back to neutral. What happens to the fluid in the output circuit, considering the fact that the hyd motor is still spinning with all that inertia. Will the hydraulic motor try and act like a pump. The flow is now blocked, so what happens.

I know one should use a motor spool valve when using a hydraulic motor, which will let the fluid circulate in the neutral position.

Sometimes we think that we understand, but don't. You know, that fuzzy gray area.

 

[KML]

"Will the hydraulic motor try and act like a pump. The flow is now blocked, so what happens."

You're dead on...

The motor will BE a pump. Many hydraulic motors and pumps are interchangable. IE one part number can be used as a motor or pump.

When you place the valve in neutral the work ports will be blocked and the fluid trapped. The inertia of the load will continue to try to turn the motor ( now a pump), pressure will increase because the fluid has no where to go. Things will come to a violent stop. Depending on the load and the strength of the mechanical and hydraulic components something will probably give, if not the first time, eventually. The more mass the motor was moving the more energy to deal with, the more violent the event.

There are things you can do to allow the use of a cylinder spool valve for a motor. I've been told that a relief valve between the two work ports will stop the pressures from getting out of control but haven't actually done this. Hopefully someone who has will post a work around for you.

 

[Farmerford]

JJ:

For simplicity, in one case I have a Charr-lyn 4.9 cu/in motor driving a Bush Hog that is coupled directly to a Prince 5.7 cu/in PTO pump; there is no need for a valve between the pump and the motor because the PTO/clutch starts and stops the entire mechanism, just as if there were a solid PTO shaft between the tractor PTO and the mower. However, since I only care about the motor (and mower) turning in one direction, I placed a check valve across the motor ports that allows flow from the motor out port back to the motor in port (when the in port is not under pressure). If for some reason the PTO stops turning the pump while the mower is still turning (out of fuel, stall, stupid operator, etc) the substantial inertia of the Bush Hog gearbox, stump jumper, and blades keeps the motor turning and acting like a pump, drawing fluid in the in port, and sending pressurized fluid out of the motor out port. However, the check valve allows that fluid to flow back to the motor in port and recirculate through the motor. THis lets the motor continue to turn freely. It takes it a minute or more to wind down from full speed.

An overrunning coupler on the PTO would accomplish the same thing, except the pump itself would continue turn with the motor/mower.

A spool valve designed for a cylinder, with the work ports blocked in neutral, creates even more flow blockage when closed than does the stalled PTO pump, because in the case of the direct coupled PTO pump the motor exhaust has a direct path to the tank through the return line filter. So a check valve across the motor ports solves that problem as well, at the cost of a non-reversible motor (because fluid moving in reverse through the circuit goes through the checkvalve and bypasses the motor.

 

[J_J]

Let's say that the output of the motor is directed to tank. It will not build up pressure, but do you think the motor acting as a pump would try and draw a vacuum on the motor input line with the handle in neutral and what do you think would happen?

 

[KML]

J.J.

I think with it set up like that you would still want a check valve between the motor inlet and outlet. The motor will try to pull fluid on the inlet side and the closed work port will make this hard on the system. Most seals use the hydraulic pressure to help them seal, the more pressure the harder they "seal". When you make negative pressure in the system many seals will fail quickly. At best you are asking for air and all of the dirt that has collected around the seal to get pulled past the seal and into the system, not good!

Farmerford's answer would be the cheapest/easiest way to run a motor from a DA cylinder valve. Again, I've never done this but it sounds like he has and it makes sense to me.

Good Luck!

 

[Farmerford]

See attached for how I did it.

 

[john_bud]

I know that spools for motors are different that spools for cylinders. If you are using a cylinder spool for a motor, you need to put a "double relief cushion valve" after the spool and before the motor.

 

[J_J]

I know that spools for motors are different that spools for cylinders. If you are using a cylinder spool for a motor, you need to put a "double relief cushion valve" after the spool and before the motor.

Your reply showed a red x , was a picture supposed to be there.

It seems like that would work quite well. Someone even mentioned a valve with float, which would let the fluid go back to tank.

This should work, below

http://www.surpluscenter.com/item.asp?UID=2008091909121077&item=9-4019-A&catname=hydraulic

 

[3RRL]

J.J.
A little different application than yours, but that is exactly what I used for my hydraulic winch set up.

 

[john_bud]

Your reply showed a red x , was a picture supposed to be there.

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