Snowcat hydrostatic conversion

[trackhead]

I have a DIY snowcat I built ground up with 1 ton differential/brake steer configuration that is somewhat spartan that I'd like to convert to hydrostatic drive. I'm a newb in this compartment, so bear with me. Looking for initial build ideas/concepts before possibly venturing down this path this summer.

Currrent engine:
Yamaha RX1 4 stroke/4 cylinder snowmobile motor 140hp/85ft pounds torque
Ideal RPM for this motor is 6-7,000
Internal gear reduction 1.19:1 at shaft

Idea:
-Seems like optimal input RPM for tandem pumps is around 2000-3000rpm.
-Utilize chain drive gear reduction from output shaft of current motor to reduce RPM 2:1 or so to meet RPM input requirement/increase torque.
-Utilize bearing to avoid axial loading of pump.
-I'd like to stick with this motor because it's already mounted, functional, reliable but I realize a 50-60hp diesel might be more suitable, but finances are a concern.
-Seek expert advice on appropriate GPM, motors, etc.

Specs:
Weight 2,000lbs wet
Width 76"
Ground PSI 0.5

Thanks for any input.

 

[oldnslo]

Trackhead,
Is your desire to completely remove the existing differential and brake steering or just make input into the differential a hydrostatic drive.

Either way some of the things that will have be taken into consideration are:
1) machine max loaded weight
2) desired tractive effort - drawbar pull.
3) desired slope capabilities.
4) desired operating speeds
5) track drive sprocket pitch diameter

The above information is then used to start calculating the required track drive torque and input RPM

Then you can decide are you going to use a gearbox with small motor to drive tracks or a larger hydraulic motor to direct drive tracks.

Once hydraulic motor size and RPM is determined you then start figuring the pump size based on flow requirements and input RPM from the engine.

Then typically you start looking at I can get this size motor that operates at this pressure will it work? If yes now what pump option do I have.

General rule of thumb is that a well designed hydrostatic system is at best 85% efficient so that needs to be factored in also.

Hopefully this information provides you with some idea of some of the steps required to obtain a machine that operates close to the way you expect vs it won’t pull its own shadow downhill with a tailwind..

 

[trackhead]

Thx, looking for a starting point of discussion.

1. Remove all drive components, replace with hydrostatic
2. Drive accomplished via hydraulic motor per track
3. Desired speed max 12mph, typically 6-8mph

GVW would be 3k, but typically closer to 2500lbs.

Thx again for the insight.

 

[oldnslo]

I did some digging in my old reference handbooks and found a table showing drawbar pull per 1000 lbs of vehicle weight on level ground. The chart shows 25 - 37 lbs for snow

Formula to figure axle torque is: T = F x R

T = torque in-lbs
F = drawbar pull in lbs
R = wheel or drive sprocket pitch radius

grade resistance formula: F = GR x W

GR = Grade resistance in percent (20% is .20)
W = gross vehicle weight

Using the above info should allow to calculate required torque at the axles.


Travel Speed: RPM = (336 x MPH) / D

RPM = wheel or drive sprocket RPM
D = wheel or drive sprocket diameter

The above can be used to calculate required hydraulic motor speed.

 

[MossRoad]

I have a DIY snowcat I built ground up with 1 ton differential/brake steer configuration that is somewhat spartan that I'd like to convert to hydrostatic drive. I'm a newb in this compartment, so bear with me. Looking for initial build ideas/concepts before possibly venturing down this path this summer.

Currrent engine:
Yamaha RX1 4 stroke/4 cylinder snowmobile motor 140hp/85ft pounds torque
Ideal RPM for this motor is 6-7,000
Internal gear reduction 1.19:1 at shaft

Idea:
-Seems like optimal input RPM for tandem pumps is around 2000-3000rpm.
-Utilize chain drive gear reduction from output shaft of current motor to reduce RPM 2:1 or so to meet RPM input requirement/increase torque.
-Utilize bearing to avoid axial loading of pump.
-I'd like to stick with this motor because it's already mounted, functional, reliable but I realize a 50-60hp diesel might be more suitable, but finances are a concern.
-Seek expert advice on appropriate GPM, motors, etc.

Specs:
Weight 2,000lbs wet
Width 76"
Ground PSI 0.5

Thanks for any input.
View attachment 2397829

Neat project.

You'll need 2 wheel motors as well as variable volume pumps, won't you?

 

[trackhead]

Neat project.

You'll need 2 wheel motors as well as variable volume pumps, won't you?

Yes, tandem piston pump, motors, reservoir, lines, etc.

 

[MossRoad]

I'd suggest a way to start the motor with the pumps disengaged. Sounds obvious, but the machine I have has a 24hp kohler gas engine with three hydraulic pumps permanently affixed to the engine with shaft couplings. No way to disengage. I don't think my variable volume drive pump puts a tremendous load on the engine when starting since the swash plate is in a neutral position, but it's charge pump is said to take about 3-4 gallons per minute and that's constant. The other two pumps are 4 and 8 GPM respectively to operate steering, front end loader, quick attach from the 4 and PTO from the 8. It puts quite the load on the starter in cold weather.

So just something to keep in the back of your mind.

Sounds like an interesting project. I'll be following. Good luck!

 

[jonsstihl]

following just because this sounds really interesting as a DIY project. Other than having driven a real snowcat grooming ski trails years ago I don't much useful input.

 

[ArlyA]

Bully's use planetary drives. If that helps... When you are building your own, the more you copy what the industry is already using, the less grief you'll have.

 

[jonsstihl]

it takes alot of energy to drive tracks. I notice the difference on my honda 450 atv . the sprocket is roughly half the diameter of the tire so it is geared down. 5th gear is practically useless unless on hard packed snow going downhill. and top speed is half of the top speed with tires.