Options for DIY Skid-Steer hydraulics - advice/knowledge needed!

[SensibleNick]

Hi all.

A little Background first?
I'm one of those people who gets an idea in his head, and has to play with it. Lack of budget and time don't mean I can't work out how to do it... right?`

So... At some point in the future, (next year?) I want to make a smallish tracked vehicle using a pair of tracks from a Volvo BV 202. I guess somewhere between 1250lb and 1750lb.

The main aim would be playing in the snow, being able to get to the shops in extreme snow, and the ability to pop a 3pt snowblower or flail mower on the front to clear the driveway or tame the grass. (we have soft watery land in some seasons), and absolute simplicity to operate is really important: The gearbox-fwd/Rev-high/low on the tractor daunts my missus: I want her to be able to use this.

This won't be a bulldozer - the most traction work it'll have to do is maybe pulling a car out of a snowdrift, so instead of torque, A healthy turn of speed (35mph?) would be nice to have. Also worth noting is that while blowing snow for example, I'm not going to need full speed/power to drive me forwards.

The Powerplant is likely to be something massively available here: like a VW 1.9TDI unit - producing the most power at 3000rpm. (just shy of 100bhp)

This is where I put on my novice hat and need some sagely advice.
I have ways of raising money for this project, but don't want to waste money on over-complicating things.
Drive-Options.
in it's Hydrostatic pumps section, Surpluscenter.com has a $2000 Tandem variable pump. I understand this to be essentially two "swash-plate" pumps that will drive a pair of motors one way or the other - controllable individually. Seems a bit expensive though.
They also have "Double Pumps", and I'd like to know if people have successfully used these in combination with double-acting valves...

I'd also need basic auxiliary hydraulics for top/tilt and lifting the 3pt etc... and then a fairly hefty pump/motor combination to power a 1.6 or 1.5meter snow-blower or flail-mower.

So... 4 pumps in all? 3 of them high capacity with valves to suit, one of them low capacity with a bank of say... 4 spool valves.

Is there a a recognised way of doing this, or is it a free art-form? I've seen pictures of pumps stacked together: is there a range of stackable ones at surpluscenter?
I'm a mechanical engineer, and would have no trouble designing a way of giving drive to a whole set of pumps, but if there's a neater easier solution around, I'll be a happy fella

I know this is less of a question, and more of a rambling thought, but how would *you* do it?

 

[J_J]

The Power-Trac that I have, uses one hydrostatic closed loop hyd pump to power 4 hyd wheel motors. It uses a 45 HP engine to run the three pumps.

The hydrostatic drive pump also has a gear pump piggybacked on it for the PTO hyd's. The other pump is for loader and steering.

The wheel motors are in a series-parallel combination.

Machine weighs about 3900 lbs.

Although the machine is not fast, larger wheels will give faster speed at a loss of torque.

Are you going to use wheels or tracks? My machine steers by articulation using hyd cyl.

I also have a Struck mini-dozer that steers by braking one side.

My hydrostatic zero turn does just that, it has a hyd pump and hyd wheel motor for each side, and can turn a circle in it's own length.

 

[SensibleNick]

Hi there JJ

I'll be running tracks (actual BV202 tracks - so the picture in my first post should help to give an idea of the machine's size. I plan on a single drive sprocket per track (about 13" diameter I think) and it'd be true "skid steer".

A assume that your PowerTrac has the LHS wheels in series, and the RHS wheels in series with the LHS and RHS couple in parallel to allow a bit of differential as you steer?

You mention a closed-center pump. Is that one that supplies at full pressure, and varies it's own flow dependent upon demand?

My reasoning for thinking that I should have 2 pumps was to ensure the same flow to each main drive motor when the control valves are fully open.... and I was thinking about an open center system for simplicity.

If any of my assumptions are wrong, I'd LOVE to hear it!

 

[J_J]

I corrected my closed center mistake, it should be closed loop, whereby the fluid from the
wheel motors are directed back to the input of the hydro pump. The charge pump provides any makeup fluid to keeps the closed loop full.

Hydrostatic tractors use open loop hyd system, whereby thet draw the fluid from the reservoir.

My fluid is cooled by the flow out of the case drains going through the pump case and then to a reservoir. The charge pump is always pumping and when not supplying makeup fluid, the charge pump fluid is routed through the hydro pump case to help with cooling.

My hyd tank is a 20 gal tank, and we use 10W-40 motor oil as the hyd fluid. The motor oil in our system has been used since the early 90's. So you might also consider the motor oil. maybe with an additive pkg. I believe some have also been using synthetic motor oil.

Your control valve levers will dictate the direction the vehicle takes, similar to the zero steering. You might have to push one lever further than the other.

If you are looking for something fast, and with good torque, you will need a large cu in pump with high pressure, and enough engine HP to power the pump, plus run the other pumps and electrics.

Some of the newer wheel motors have the spring assisted brake system. The spring assisted brakes keep the motor stopped until the pressure on the motor is sufficient to release the brakes.

I really don't have a good brake system on mine, as I use the foot pedal for fwd and rev.

Pic 1 hydrostatic pump plug the red PTO pump.

Pic 2 steering pump driven by the fan belt.

Pic 3 input filter assembly which goes to the charge pump.

Pic 4 Gas engine 4 cyl, 45 HP Ford industrial eng.