Hydraulic driver for garden tractor

[HDon]

Planning to build a replacement yard tractor with independent hydraulic drive to both rear wheels for steering. Would appreciate comments and a simple hydraulic design providing description/layout/specification from: pump-to-control-valve-to-motors-to-return tank. I plan to use the following components based on initial design assumptions:
- driver: indrustrial-type 22hp Wisconsin aircooled engine
- full load rpm 2700 (engine is old but in good condtion)
- wheels: - standard car type snow tires - dia: 22"
- drive train to each wheel: hydraulic motor 930rpm to 15:1 reduction gear providing 10-12 km/h speed range
- hydraulic motor: Sauer/Danfoss 930rpm -700ins-lb @ 3-cu/in/rev displacement and 1800 psi max-pressure
- hydraulic pump: gear pump - appr. 6.5 cu/in/rev
- control valve 1: forward/reverse - not selected
- control valve(s) 2: steering - not selected
My objective is to keep the system simple by stopping prior to selecting reverse. I would like to control steering with a standard steeringwheel having a right & left lobe on the bottom of the shaft connected to the control valve(s) that would modulate flow to the right and left motor respectively. Finally, I estimate from basic hydraulic formulae that the drive would need about 10 hp maximum and the remaining 10 hp I plan to use for mechanically driving the mower with a standard belt configuration. Your comments would be much appreciated.
Henry Don

 

[JStephen]

Sounds like a lot of bother for something you can just buy outright.

 

[DesignerMike]

For simplicity, I would low-speed, high torque motors instead of the motor/reducer combination (much cheaper).

It may be simpler from a control standpoint to use a dual pump (seperate circuit for each wheel)

How are you going to control speed with a gear pump? Once the engine is running the pump will put out the full displacement, and you can't change that. You would need to divert flow back to the tank to keep the mower from going full speed ahead. Doing this constantly (at pressure) will create a lot of heat and waste a ton of energy. Controling this will be tricky at best. Proportioning and divert valves will cost more than a new mower.

The easiest and most efficient way would be to use a dual, variable displacement pump. You would have almost no valves, just stroke the pump forward or reverse. Skid steers use this priciple. They are incredibly simple, and very robust. The pump controls are spring-loaded to return to a neutral state. Pushing the "swash-plate" one direction or the other will create flow to the motor depending on how far the control is pushed. I'm sure that the commercial mower units use this priciple.

As Jstephen said, buy a new or used mower, or get drive parts for a mower from a junk-yard. I would guess it will be almost impossible to put something together cheaper than you could buy it (and it may not work very well).

 

[aviat]

The same question is at thread1083-118755