Hydraulic motor and pump help

Metalburner · Nov 26, 2016

So I'm in the works of building a mini dozer similar to the Struck Corp MD750 and the MT1800/2000, with independent hydraulic wheel motors vs doing a hydrostatic transaxle and putting brakes on each axle. Expected weight is going to be around 800lbs, possibly higher. What this will be used for is pulling a plow for gardening or other ground engagement, as well as having a front loader for moving snow, dirt, gravel, or sand. Would like to use a Honda GX390 or possibly a Briggs 16hp V-twin, depending how much power I'd need.

Where my first problem is, I'm not sure what size hydraulic motor I would need for the drive. I'm either going to use dual #80 roller chain or maybe ca550 chain, if I can get 9-10" diameter sprockets. I would also like to do a gear reduction, in that, from the wheel motor there would be a 10 tooth #40 sprocket to a 35 tooth sprocket on an axle to the track sprockets. I've looked at using zero turn mower wheel motors on surplus center from Hydrogear, but I don't want to go much faster than 2-3 mph; and most of them were rated for 5-10mph, depending on model. So I'm not sure what way to go.

Next part is hydraulic pump size for everything. I'm not sure what size I would need to use the motors and using the front loader at the same time. The front loader will utilize 2" bore and 18" stroke welded cylinders for the arms and a 2" bore 10" stroke cylinder for the curling of the bucket. Frame of the mini dozer and front loader will be made from 2"x4x1/8" tubing, as it'll be more than adequate for the whole thing.

Another reason I wanted to stay away from the zero turn wheel motors is, I'm not sure how long they would last for ground engagement. This isn't some toy, it will be used and I'd like it to last for years. When I looked at the MD750 in the manual, they don't mention the specs of the wheel motors or the size of the hydro pump.

Main inspirations was the model from the mini dozer near complete build on here from a few years ago.

oldnslo · Nov 26, 2016

Looking at the parts used for the MD750 i appears they had a single gear pump feeding everything. Motors looked like the were probably low speed high torque geroter or geroler style but not sure.

Looked like a three section directional valve, feeding a second three section valve. First valve was for drive and second one one for implements. What I am not sure of is if there was some form of flow splitting device in the steering valve that always supplies some flow to the implement valve. If not traveling and using implements could pose a problem.

Simple but effective lower cost system.

Going to a full scale Hydrostatic drive like used in skid loaders is the other end of the cost spectrum. Gives better performance but requires significantly more room for components.

Size of components will depend on several factors.

The engine size selected dictates the pump size and pressure at which it will operate at.

Pump size and gearing will help select the motor size. etc.

Agree that zero turn transaxles probably would not survive ground engagement higher power use.

Metalburner · Nov 26, 2016

I like the idea of full hydraulic drive for the sake of a lot of torque. See from the very little I know, when I look at hydraulic motors, they are rated at max gpm, say about 15.8gpm, would that mean you would need a 30+ gpm pump to run two motors? I know what you are talking about, I believe they call it a priority valve or something like that, where it splits off to run a few things.

At work we have a snowrator that has 4 wheel motors and a 48" plow with 30* of angle adjustment to the left and right, powered by a GX390 engine. If they would give complete specs of the hydro system, I'd have a better idea of what to look for. I hadn't tried adjusting the plow while moving, since the drive controls are like zero turn walk behind levers and it would be very difficult, if not dangerous to try to adjust the plow while on the go. The wheel motors from a zero turn I've looked at on surplus center are rated for equipment weighing over 1200lbs, but like I've said, I don't know if they'll hold up.

We have a driveway that's just under 100' long and about 16' wide all flat, if that helps.

oldnslo · Nov 27, 2016

Metal burner,
Just because a motor is rated for 15 GPM doesn't mean you have to use 15 GPM to operate it. They will turn and develop full torque at 2 0r 3 GPM also. This is the feature that makes HST's nice for loader work. Infinite speed control with basically full torque capability at all speeds.

Zero turn motors: Depends on the style of wheel motor. Eaton check ball style will die an early death under heavy usage. LSHT (Low Speed High Torque) Geroter or Geroler style motors will survive longer under heavy duty use. Weight rating may be based on bearing capacity if these are direct wheel drive motors.

Axial or radial piston will probably provide the longest life under heavy use but they are also the most expensive. Some ZTR trans axles use a axial piston design but they are limited in duty cycle due to heat dissipation capabilities.

I would suggest starting by doing some design calculations based on the information you provided.

Use your track sprocket size to determine what RPM it has to spin at to give desired travel speed. Then use you intended gear reduction to figure out what hydraulic motor RPM is required to provide this travel speed.

Then you have to estimate required drive torque or tractive effort required to pull implements, turn etc.

Using this info size your hydraulic motors.

Hydraulic pump size will be determined by engine HP, and desired operating pressure. Note: General rule of thumb subtract 10% or engine HP for running alternator, cooling fan / flywheel blower, etc.

Example: You could drive a 9 GPM pump @ 2,500 PSI with 15 HP.

Metalburner · Nov 27, 2016

So with a Dynamic low speed high torque motor with a displacement of 9.5 cubic inches, rpm of ,300, gear ratio of 3.5 and track diameter of 12", I'll get 3.05mph and a torque out put of over 9000in/lbs or almost 800ft/lbs.

oldnslo · Nov 27, 2016

Metal,
I didn't check the torque values but I believe the flow required is off. 300 RPM x 9.5 CIR = 2850 cubic inches/min divided by 231 = 12.34 GPM.

NOTE: You will only have half of your total pump flow for driving each track.

Not sure if you know this but for figuring travel speeds with track you use the sprocket diameter not the outside of track. Reason for this that you are laying the track and driving over it.

Metalburner · Nov 27, 2016

Oh okay. I had a feeling that was what I needed to input. So then it would be a 10" sprocket. So going by that I would need a 12gpm pump? This is where I get confused. Sorry if I am causing any issues.

Metalburner · Nov 27, 2016

To help with how I got the 9000in/lbs, I started with 2750 as a baseline, since that particular wheel motor is maxed rated 3200. I figured 2750 would be a good safety margin.

This is the motor I'm thinking on using if it helps. Sorry the link is in mobile, I'm mainly using my phone for posting. http://m.northerntool.com/products/shop~tools~product_200319051_200319051?hotline=false

oldnslo · Nov 28, 2016

Below are some example sizing calculations:

Using the 10" diameter I get 101 RPM drive sprocket speed required for 3 MPH travel speed.

101 x 3.5 gear reduction = 354 hydraulic motor RPM

Based on 15 HP available from engine you would have capabilities of 9 GPM @ 2,500 PSI

ignoring implement hydraulics this would provide 4.5 GPM per drive motor.

At approx 85% efficient a 2.5 CIR motor would provide 354 RPM.

Hydraulic motor torque: 2.5 CIR @ 2,250 PSI differential across the motor equals 74.6 ft-lbs @ 100% efficient. that would be approximately 260 ft-lbs at drive axle. I have no clue if this would be an adequate amount of torque or not.

Drop your speed to 2 MPH and you could use a 3.75 CIR motor which would develop around 110 ft-lbs torque at 2,250 PSI.

Big unknowns: What is the required torque / drawbar pull required for your machine to operate?

System efficiencies: Components vary greatly. A good system is 85 - 90% lesser quality components can drop this to 50%. I would suggest doing your best to find actual documentation on expected performance for the components you are looking at. Some companies provide good documentation and others don't provide much at all.

I would also look into the travel control valves. Not sure if companies valves designed for this but finding valves rated for the flow rate with good metering characteristics will making changing speeds, turns, slow maneuvers much easier.