Allowing for slip between two hydraulic wheel motors...

[Greasy Hands]

Hello all:
I'm tinkering with how to allow slip between two hydraulic wheel motors used to propel a small tractor. Originally, I was going to control each motor with a dedicated double acting spool valve to provide both motive power and steering. However, the design evolved to use dual wheel motors for propulsion only and to provide steerable front wheels instead of casters. Ordinarily, I would drive two hydraulic motors using a flow divider, however, in a turn the outside wheel will needs to rotate faster in order to keep up with the turn. Of course, I don't want to be dragging the outside wheel. How can I allow for a higher oil flow to one wheel when turning? I am certain that this is not a unique idea, but I have never seen it applied.

Greasy Hands,
Austin, Tx

 

[BrianPetersen]

Connect them in parallel rather than in series and use a plain ordinary T-fitting at the split between supply and the two motors, without a "flow divider" control valve. It will act just like an open diff if you do this. If one wheel feels like spinning more, it will let it.

 

[Greasy Hands]

Well, yes, but then I lose two things. One, the benefit of having two driven wheels. If either one spins, your stuck. Also, I had intended to use them as brakes by having a blocked center on my spool valve. Something just occurred to me. If I use a double ended cylinder for the steering actuator, I wonder if there is a way that I could link the position of that cylinder to effect the flow to the two drive motors. Or, I wonder if there is away that the differential flow to the motors could actuate the the steering cylinder.

Greasy Hands,
Austin, TX

 

[BrianPetersen]

The steering should be the primary control; actuating the steering based on secondary effects from some other system sounds like asking for trouble.

Driving the wheels in lock-step with the calculated speed relationship from the steering is getting into servo-control. Can it be done; sure. At a certain point you have to ask whether it is worth bothering with the hydraulics and simply use electric drives. VFDs and servo drives are common technology.

If you insist on hydraulics ... If you have wheel speed sensors you can implement a primitive traction control strategy by switching flow restrictors in and out. Or you can just give the driver a manual control to switch in a flow restrictor on the free-spinning side.

Or perhaps you can implement that flow-divider, but have controlled leak paths around it so that it is not 100% lock-step equal flow to each side.

 

[Greasy Hands]

Yeah, no doubt, steering has to remain the priority control. I was going to put a small front-end loader on this thing and I don't think that castering front wheels would work with a bucket. Still, complexity caused by front steering is making it look like a 'bridge too far'. Proportional flow control is too expensive and beyond where I'm willing to go. Maybe, I could have the steering action actuate a float valve to freewheel the motor on the inside of a turn while allowing the outside motor to pull the tractor through the turn. I don't know. Maybe that I need to rethink this whole thing.

Greasy Hands,
Austin, TX

 

[hydtools]

Use separate motor circuits and dual hand controls to the drive/steer wheels like a skidsteer. How do zero-turn mowers steer?

Ted

 

[Greasy Hands]

Ted:
That was exactly my initial approach. Basically steering with the drive wheels. However, to do that, the front wheels must "caster". When I decided to add a small front end loader to the tractor's design, it meant that I couldn't use the simple castering front wheels. And that has brought up all sorts of problems. Just adding front wheel steering drives up the cost by over $1K. If I am to solve this it looks like I will have to think completely outside the box. Maybe turn the whole thing around and make the hydraulic motor driven wheels the front wheels and have castering trail wheels. Let the driven wheels carry the weight of the bucket. ??? Ignore me. I'm starting to hallucinate.

Greasy Hands,
Austin, TX

 

[Jacc]

A needle valve between the two circuits could be a cheap and dirty solution.
Fully open = open diff
Semi closed = LSD
Fully closed = Locked diff


Assuming you are running compensated LS-valves to start with.
If you are running simple open center valves it will be open diff all the way regardless.

 

[Hurtan_Albertan]

If you had anti-cavitation checks on the working ports of your motors, then the outside wheel when turning would be able to pull the extra flow required. The outside wheel motor would have to be in an overdriven condition and would temporarily turn into a pump, while the inside wheel motor would be doing the work (positive pressure).