Parasitic drag of hydraulic motor question

[ParkerFly]

Got an unusual question I'm hoping you all can help with. While in float, does anyone know if a hydraulic motor (char-lynn) would have more parasitic drag on it if the hydraulic pump is turned on or off? Currently I have a setup like this where I put the valve controlling the motor in float and turn the hydraulic pump off (has a clutch on the pulley). I'm considering going to a constant flow pump so I don't have to worry about turning the pump on and off all the time. I don't need the motor creating any more parasitic drag though, it needs to be as free as possible. Would having the pump on/off affect this? Thanks,

Justin

[kennyd]

I assume you have a spool valve in between the motor and the pump?

[J_J]

How is the hyd motor doing anything if the pump is off. If the valve you are using to control the hyd motor has float, it is used when you turn the hyd motor off to free wheel.

After the motor has stopped, you put the valve lever to neutral.

Float connects all the ports.

Perhaps you should be using a motor spool valve.

Explain what you mean by parasitic drag.

The hyd motor is moving or not moving. You can however use external forces to turn a hyd motor with no pump flow.

http://en.wikipedia.org/wiki/Parasitic_drag

[ParkerFly]

Yes I am using a spool valve between the motor and pump. This motor is either working, or I have the valve locked in float letting it free wheel. When its in float it is constantly being turned by the device its attached to until I stop that device and use the motor to reposition it. Then back in float. So while in float, the motor is constantly turning. With the hydraulic pump off, there is no fluid flowing through the system, only the fluid that is left in the lines that may get pumped out as the motor gets turned. If I leave the pump on though there will be plenty of fluid in the lines, and I'm wondering if that makes it harder for the motor to freewheel. A hydraulic guy told me to beware of that once before.

[oldnslo]

If the motor is being driven by some external mechanical force you MUST keep both ports of the motor connected. When you are mechanically driving the motor it is acting like a pump and with one of the ports blocked the mechanical driving device will cause pressure to build in one of the motor ports until either:

A) The pressure creates enough load to stall the motor against it's mechanical load.

B) Some thing breaks.. I.e. hose, fitting, drive shaft, etc..

c) Motor cavitates from no inlet flow and slowly self destructs. (A good reason to use a float style spool)

Excess parasitic losses could be caused if the control valve is sized to small to handle full hydraulic pump flow as well as the flow being generated by the motor being mechanically driven. The sum of these two flow rates must pass through the spool valve.

Roy

[ParkerFly]

Yes I think I'm on the up and up about needing to keep the ports on the motor connected and will always be leaving my valve in the float position. I'm just trying to determine if it makes any difference whether the pump is pumping fluid through the lines or not while the motor is "floating". While in float it can be spun very fast (well beyond what the motor itself will turn when doing work) and I'm not sure if having fluid pumped through the lines will create the parasitic drag.

[J_J]

If you put that same valve back to neutral, the flow to the motor is blocked, however forces can perhaps turn the motor. My Power-trac has hyd motors on each wheel. If I park on a hill, the weight of the machine will cause the hyd motors to act as a pump and push fluid around the hyd circuit. or bypass the motors internally. How much pressure can be generated, I don't know.

They also tell us not to tow the machine very fast or any great distance as the fluid is passing over the relief valves, in a closed loop circuit, and will heat the fluid.

When in neutral, a relief valve across the hyd motor would relieve any pressure build up as the fluid is flowing from the OUT port, to the IN port of the hyd motor.

Just a little curious why you want the hyd motor turning when not in use.

[kennyd]

I would change the valve to one made for use with motors, then the work ports are isolated from the IN and OUT ports completey when the valve is in neutral. At that point it will not matter if the pump is on or not.

[ParkerFly]

I guess I'll show my ignorance, I didn't know there were different valves available for use with motors. Can you point me in the direction of one of those? And if there is an electric one it would be great too, but mechanical is fine also. Thanks.

[kennyd]

I guess I'll show my ignorance, I didn't know there were different valves available for use with motors. Can you point me in the direction of one of those? And if there is an electric one it would be great too, but mechanical is fine also. Thanks.

I can't link one now, but you want one with a "motor spool". They can be had in electric and manual versions. The two work ports are joined together when the valve is in neutral allowing the motor to coast or freewheel. Check the Surplus Center