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Skid-Steer Problem

Skid-Steer Problem

Skid-Steer Problem

I am new to the industry, thus I have minimal experience with applying fluid power concepts to design applications. The project I have in my hands now is an interesting one, as in I don't think it's even possible to use an efficient solution. If anyone has direct experience with a similar application, any helpful information would be appreciated.

I am working on the drive system for an OEM that builds railroad maintenance equipment. Usually their machines are built for use on railroad tracks. This machine, however, will use two CAT track systems for ground mobility. The GVW empty is 120,000 lb, and full load could be upwards of 250,000 lb, depending on the average density of the iron ore it will be loading and transporting. This means possibly 125,000 lb supported by each set of tracks, assuming even distribution of the weight (not the case, but assume true for sake of simplicity at this point). The largest problem, I think, is the dimensions of this machine. It is roughly the shape of a railcar - long and narrow. To be specific, the dimensions from center to center of the tracks and width are 30 ft x 10 ft. Not the ideal ratio of dims to apply a skid steer.

Two design directions at the moment: steer both sets of tracks with differential flow, or steer one set (front) and fix the other (rear). What I would like to know is how to roughly spec a system like this (pumps, hyd motors, torque specs). I assume something needs to be known about friction coefficients and associated forces on the tracks. Ultimately, can this machine be steered in this way?

RE: Skid-Steer Problem

Look at large tracked excavators.


RE: Skid-Steer Problem

If this is the task you've been given and you consider yourself to have minimal experience applying fluid power concepts then prepare youself for an extremely steep learning curve. You might be as well to team up with an experienced supplier of hydraulic transmissions... but you're right to want to have a feel for the numbers before you start ringing round.

Go to the Brevini website (other gearbox manufacturers are available) and download their catalogue for track drive gearboxes. There are a bunch of equations and tables which you can work through to size the gearbox you will almost certainly need to drive the tracks. Specific questions you will need to ask yourself are: how fast does the thing have to move, what is the steepest hill it has to climb, what sort of ground does it have to traverse.

Just as a rough guess, if you use a closed circuit transmission you could realistically expect a maximum of 70% efficiency (in terms of power) between the flywheel of the engine and the input to the track drive gearboxes. There will be losses in the gearboxes (allow 3% per stage in an epicyclic box). Then you make a further allowance for losses between the big sprocket and the track and even more allowances for power wasted in the track itself and all the loaded rollers that are carrying the weight of the machine. Only then will you start to add on the extra power you need for acceleration, going up hills, the nature of the ground etc.

For the pumps and motors (and brakes and pilot controls): you need to decide how the machine will be controlled - consider where in the world it will be used and choose a technology that is appropriate for the skills/resources available to maintain it. The machine you design for California will be very different from the one you design for Ethiopia. You can reasonably expect that the increasing sophistication brings with it some energy savings/efficiency - you are right to expect a rather inefficient machine.


RE: Skid-Steer Problem

Oldhydroman -

I appreciate the heads up. I know that I have much research to do. Thank you for the initial thoughts to get me started.

This machine will be used in a railyard in Brazil. The earth it is navigating is relatively compacted and shouldn't exceed a grade of 3 percent. It needs enough maneuverability to pull up parallel to a line of railcars on a set of railroad tracks. I can't imagine a very good turning radius for this machine, making this sort of 'parallel parking' maneuver quite difficult.

Here is a primitive sketch of the scale of the machine:

RE: Skid-Steer Problem

We will most likely be using Sauer H1 pumps and motors, PVG proportional valves, and dual joystick controls for both propel functions and other utilities.

RE: Skid-Steer Problem

Remote control?

You could turn both sets of tracks 90 degrees to parallel park.


RE: Skid-Steer Problem

The 'swivel' function of the tracks will not be used in this application - they're both fixed in the position shown. The tracks are not meant to be 'force-steered' in that way. The swivel action is normally used, for example, for an excavator to rotate once the vehicle has stopped travel.

RE: Skid-Steer Problem

Not sure how you would steer that rig if the two sets of treads don't swivel independently.

RE: Skid-Steer Problem

Based on my preliminary calculations, at least 150,000 lbf total tractive effort by each of the two sets of tracks. I agree that it would make sense to swivel the tracks independently, but the problem is that it may not be possible with the equipment considering the GVW.

RE: Skid-Steer Problem

I think the distance between track sets is so long compared to track-to-track distance that the car may not turn even with counter-rotating tracks. There will be a lot of side pushing at the ground contact.

Excavators can skid steer while traveling. The excavator upper structure can rotate while traveling.


RE: Skid-Steer Problem

Compare moment generated by counter rotating tracks against the tracks resistance to slide sideways on the compacted gravel. Choose the center of the car as the center of rotation.


RE: Skid-Steer Problem

You said you are working for an OEM? See if you can talk to the hydraulic supplier's application engineers. They would probably be able to help you design about the best possible system, but I don't know if they would be able to donate too much time if it is a 1-off machine but it's worth a shot.

I know Sauer-Danfoss has application engineers who do nothing but help people design hydraulic circuits, so it would be worth trying to contact them since you mentioned you were looking at our/their products (Sauer-Danfoss Engineer here). That said, I'm sure Bosch-Rexroth, Parker, Eaton, etc... have similar people on hand to help you with your questions as well.

RE: Skid-Steer Problem

Use two 400 size excavator track frames with slewing ring bearings. This will provide the power, ability to support the load a turn.

Ed Danzer

RE: Skid-Steer Problem

In order to steer this vehicle you will need to be able to rotate your tracks to some extent against the box. Using a counter steer type design in this application will be less than optimal due to the great length between your two sets of tracks. If you could make the entire length of the box have track, you would have a better chance at skid steering - your biggest concern then would be the sheer force exerted on the track centering lugs (but there are design methods to help with that).

Chris Brunner
Brunner Equipment LLC

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