Running a Hydraulic Motor - Valve Question

[thirdroc17]

Thanks for the advice guys.

I can't heat the barn this time of year, and the old joints don't allow working in the cold, so............ I'm asking around.

I agree form follows function. What good is pretty if it doesn't work? That said, price always plays a bigger issue than I care to admit to.

SO, my current thought is... since I can't do the actual assembly work until spring, when I won't be needing the blower for a while, I'm going to plumb the motor in with small, (which also happens to be cheaper) but identical, fittings and hoses on both sides of the motor, plug it in, see how fast it goes. If it's too fast, I'll install some sort of restriction on both sides. The one way flow valves being the best, but also, the costliest way. But cheaper than a motor with blown seals. AAARRRRGGHHHH, I hate being poor!

 

[J_J]

GPM's and displacement determine the speed of the motor, and the control valve supply fluid to the motor.

Engine rpm also determines the pumps speed, and therefore the pumps GPM.

 

[thirdroc17]

Correct.

Restrictive losses from hose/line length, diameter, size of fittings, number of fittings, etc. also come into play. The biggest pump and valve in the world won't flow any more than what the various lines and fittings allow.

Small lines and fittings will flow less than large ones. I'm certain large lines and fittings would spin the motor too fast. Smaller ones probably will too, but not to as big of a degree. Since I'm not made of money, I'm not buying flow restrictors until I know I need them.

Alternatively, I could do a mechanical speed reduction between the motor and rotor shaft. Cross that bridge when I come to it.

 

[thirdroc17]

Another question guys. I'm just getting more and more confused. The motor I was looking at from Surplus Center says, "three-zone spool valve, integral check valves, high pressure shaft seal and a provision for a case drain to reduce pressure on internal seals to improve product life." The main part I'm beginning to wonder about is the "provision for a case drain". Ok, it improves life, but just how necessary is it? It's claiming a high pressure seal. Since this Kubota is new to me, I'm not sure where I'd plumb a drain line back into the transmission as it shares oil with the hydraulic system. It's rated for 2250 continuous, 3000 intermittent. The tractor maxes at 1850. Seems like a fairly good safety zone on the pressure.

The next size pump they have, (No, they are not the only source, just an easy and fairly inexpensive source) is 4.5 times as large. The ones in between don't have an adequate pressure rating, some weird manifold ports, or other undesirable traits.

The tractor hyrdaulic pump is rated at 6 GPM, but what's the actual flow through the valves? Darn good question, huh? I have to assume it's being shared with the hydrostatic drive and power steering, so there may not be all that much flow left. Hate to spend twice as much on a pump with no drain, only to have it barely move.

 

[J_J]

Case drain can go to any return hose. The fluid that flows from the case drain also lubricates the motor.

GPM determines the speed of the motor, so just reduce the speed of the motor by using a flow control or reduce the engine rpm.

You can also use belt and pulley, or chain/sprockets.

 

[Farmerford]

The integral check valves will connect the motor case to the motor work port that has the lowest presssure. If when the motor is operating the outflow from the motor flows through the directional control valve and ultimately to the tank, then the pressure at the motor out port (and also the motor case via the check valve) will be virtually that same as the tank. As a practical matter the integral check valves eliminate the need for a case drain line as long as the flow from the motor has an unobstructed path to the tank.

If there is an obstruction to the return flow from the motor, so that the pressure in the motor out port exceeds tank pressure, then the case pressure will increase to the out port pressure unless the case drain is connected to provide a low pressure, unobstructed path to tank. In some applications two motors may be connected in series; that is, out flow from one motor becomes in flow to the second motor; in that case out flow does not have an unobstructed path to tank and motor case pressure rises to the out flow pressure.

It seems to me that you plan to have the motor out flow pass back to the directional control valve and then to tank via the common return line. In that case, the integral check valves will work satisfactorily to keep motor cases pressures virtually that same as return line pressures.

 

[J_J]

You might also consider this.

Quote from Sauer Danfoss:

A case drain line must be connected to one of the case outlets to return internal leakage
to the system reservoir.

 

[RickB]

Not every orbit motor has a dedicated case drain. In fact most small displacement, low torque motors such as discussed here, do not. Certainly one sized to turn the chute on a small 3PT hitch snowblower would not have a separate case drain.