Hello All,

I am working on a tracked backhoe/fel project, and I need to estimate the size motors needed to drive the tracks (my preference is to drive the sprockets directly with the motors - ie. without gear reduction). I am a beginnier at hydraulics, so any help you can give would be greatly appreciated. Here is some information about the project: I think it will weigh in around 3k to 4k lbs, I am using steel tracks (I think from a compact excavator) that I picked up at the scrap yard, the engine is a 2.8 litre V6 Chevy. I do not have the pump yet, but I am thinking of buying a dual (1 circuit for the motors, and another for the auxillary). Also, how do I plumb these things, so that the motors can drive together or independently? Many thanks in advance!

Skid loaders use 2 variable displacement piston pumps. Sometimes ganged together in one assembly. As to size, the hydraulic system needs to be able to kill the engine without walking through the pump.

Thanks E718, but unfortunately I have to use a fixed displacement pump due to cost.

Pardon my ignorance but what the hey does "the hydraulic system needs to be able to kill the engine without walking through the pump." mean ?

flow and pressure demands should be able to stall the driving motor or engine without destroying the pump.In the case of a motor,overcurrent protection should intervene after the time constant

Gee, I woulda thought that flow and pressure demands should be MET by the motor or else you're underpowered. And where does the "current" get involved in this application?

Bunkclimbers suggestions make sense as far as the system should not be breakable, but there are also other ways to do it. In a small system it is not too hard to have the pump strong enough to stop the motor without damage. In larger installations (or any installation) you put a relief valve at the output of the pump to bypass oil so the pressure never exceeds the rating of the pump. I was on a ship once where a valve was turned at the wrong time and broke the housing on a large pump driven by one of the main engines, so even professional designs miss things. I have always put a relief valve up stream of a hydraulic quick connect in case a hose falls off. I had never actually tested it but two years ago it finally happened and the relief valve worked as planned.

You also need to make sure you don't exceed the pressure rating of the hoses even when operating outside of the design parameters. In otherwords, if generating enough back pressure to stall the prime mover means you exceed the rating on a hose, then you need to protect the hose with a relief valve. People have been badly hurt by hydraulic hose failures.

The main thing is to consider the what if's and plan for them so you don't break an expensive piece of machinery or hurt anyone.

Chris

Thanks for the comments. Can you guys give me some direction on my original question? Thanks.

Todd, does your project machine have reduction ratios or are your track drives direct? you can build a lot of torque with gear reduction and use a smaller hyd motor.If you see on this site,'gator'is building a articulated loader drivetrain with a hydraulic motor driving a transmission so he has selections of drive ratios..bein's your stated design is tracks,you'll want two hyd motors(1 each side)for track drives,so you can independently drive each side in either direction to steer or counterrotate the chassis.How fast of a travel speed do you want? Off the cuff without any design data it's a crap shoot as to what to use size wise,but based on my experience with motors,if you direct drive a track sprocket you'll need at least a 16cu/in motor or better on each track,with at least 2000psi.You can use a regular 4-way valve,one for each motor(track)to control your hyd motors.I'd try to build in some gear reduction and make a lot of torque,which is what really moves a machine.I've bought a lot of used pull-off hyd motors and had very good luck with them.If you look on eBay you can find a ton of used stuff.If you can give us some specs-like your intended track sprocket size(OD)? this spec makes a big difference on how much power you'll need to turn a track.

sent you a PM..look at top of page under 'my stuff'

It pays to look at other machines. Try to learn whatever you can about the machine the tracks came off. Do you have the sprockets that go with the tracks? Using a gear reduction, planetary or chain or whatever, gives your hydraulic motor an advantage as far as braking the machine. It's nice if it doesn't want to creep back down the hill while you are trying to dig. Another consideration if you want to direct drive the sprockets is the side load capability of the hydraulic motor shaft. If you have to add more bearings it all takes up space and you may want to look at a chain drive.

If you can find the specs on the machine the tracks came off or a machine that is a close match, look at the advertised walking speed and the weight and horsepower.

The little excavator I am playing with right now can spin the tracks on dirt with 13 hp and vehicle weight of 3200# but without meaningful coefficient of friction numbers I can't translate the into a torque number.

But then there is the internet

data on dozers

So wet sand and gravel gives a coeeficient of traction of .35 so for a 3500# machine the tracks would have a thousand pounds of tractive force before they would spin. (I assume steel tracks). You want a bit more force than this. The number for dry clay is .90 so the tractive force would be 3150# but we don't know if the tracks can slip on this machine if it is on dry clay. We also don't know how much tension we can put on the tracks. The tension is the sum of the tension from tightening the track tensioners and the tension from the drive.

There is no magic answer. The more power you put into the tracks and gear the more stress and the stronger everything has to be. I also depends on what you want to do.

To climb a 30 degree slope the force would be mg*sin(30) which would be the weight of the machine times .5 so you need 1750# of tractive force to climb a 3500# machine up a 30 degree slope (very steep). With 8" sprockets (effective diameter) then you have 4" radius times 1750# equals 7000 in- lb of torque. There are two motors so half the torque (3500) for each motor.

You need to be it clay or the tracks will slip.

Hope this helps a little but people with way more knowledge than me have already done the engineering. The best thing is to find a similar machine and see what they used. Manufacturers publish a fair amount of stuff about their machines. Look for displacement of final drive motors and whether there are planentary gears etc.

The other approach is to take your best guess, get some motors that are a good deal and in the end if you need more torque, get bigger motors or add a chain reduction.

Chris

Thanks, guys. I wish I knew what machine these tracks came from. I have some rollers, but I am not looking forward to making the drive sprockets. You've convinced me that I should include some sort of reduction. I will probably have to do this with chains, because I don't have much room to work with. I ran some numbers (based on formulas on the Hyd Supermarket site), and I think I'm comfortable with motors generating about 3k in lbs of torque.

Bunkclimber, when you say a "4 way valve," are you just referring to a 2 section motor spool valve? If so, will it automatically divide the flow to the two motors if I open both sides at the same time?

Also, is there some sort of "rule of thumb" for the optimum cylinder speed on the backhoe and fel? Thanks.

Todd
A four way valve refers to the number of ports on a double acting valve. This is the type valve that you would use to control a double acting cylinder or a reversible hydraulic motor. This one valve has four ports, fluid in, fluid out, and two pressuer ports. In an open center valve the fluid will flow from the fluid in port to the fluid out port when the valve is centered. When moved to one side the fluid will go from the fluid in port to the pressure port that cooresponds to the position of the valve, the fluid will then return to the valve through the other pressure port and exit the valve through the fluid exit port. You'll need a valve for each moror. The flow should be split prior to entering the two steering 4 way valves.

Hope this helps

todd the best way to do this would be with a closed loop pump and drive motor setup,like in a zeroturn lawnmower.You can do it with open loop(center)pump(s)and 4-way valves..you should use a 1flow-in,2-out rotary flow divider to keep the fluid flow balanced side to side for track drives..if you open one track valve the pressure builds into the motor and then turns the motor..if you just tee the inlet flow between the other drive valve,when you open the second drive valve then pressure will drop until the second motor begins to turn under pressure-the pressure backs up thru the valves and the motor with the LEAST resistance will turn first WITHOUT the rotary divider.In your case,the rotary divider evens the flow out between valves and limits pressure interactions between the track motors. Alternatively,you could solve the problem by using two pumps(or one 2-section pump)and feeding each side with its own pump section,valve and motor.I used a 2-section pump for my loader and it solved a lot of flow control problems.Both pump sections would have to be the same size(displacement)for this to work properly.

Just read Docs message on how to do this.

Sent two files one is zipped. Sometimes we cannot down load files with certain extensions such as .xls

Swap the x and s if this file even downloads.

I don't know if the above files are my latest files or not. Feel free to modify. The sheet is protected so you will need to unprotect to make changes. I think it auto protects via Visual Basic at start up. Someone wanted cylinder speeds added for retract some months back but I never got a round toit. Most of the math is theoretical but will give you an idea what size pump, motor, engine ect you will need.

I have modified the ratio thing "RPM vs Speed" a time or two. Just put 1 in the place where you don't want a ratio.

I should work on this thing some more but I love it the way it is.

Hate to be the bearer of bad tidings, Clint, but it seems the files didn't take for some reason. (Download looks like it starts and then just sits there with no bytes transfered).

That the names showed up means you probably went about it right, but that the system just missed something on the upload.

Could you perhaps try editing your message and re-uploading the files?

As a test I usually download anything I upload at least once to see that things are working right.
6:40 PM Mountain: Still no worky.
Doc

OK. I seem to have uploaded the file after changing .xls to .slx however I cannot download it. So I will remove it if no one else can download and try again. I does show for me that all 120K is downloading it just never finishes.

Does anyone have the spreadsheets that were attached? I cannor download them and am very interesred in looking at them.

I have that hydrualic and torque.xls on other post that does work. Try searching for it. If you don't find let me know and I will post here.