How much power?

[danwest]

First a confession. There was no "backyard engineer" to choose in the sign-up section so I lied. But hey, I'll be glad to help you with medical advice!

Now the problem. I'm trying to design a vehicle that will move a total load of 3-4 tons up a 13% grade. It will be on soft ground some of the time, hard ground some of the time, and in water some of the time. I plan on using six hydraulic motors moving three ATV tires (12" wide 18" diameter) on each side. Kind of like a giant skid steer. I don't need any great speed since it will only need to move about 300'at a time. I've looked up hydraulic motors and they're all rated in touque. Is there a web site, program, formula or any way to figure out how much power I'll need to make this thing move?

 

[Metalguy]

Ah, another easy one. Power will tell you how fast it's theoretically possible to move it, but not if it will move at all (you could waste all your power via heat, etc).

First you need to figure how much axial force you'll need. The soft ground could be a killer. Then you need to use some simple math to arrive at a wheel-torque requirement. Once you're sure the load will actually move up the grade, you can figure out how much HP you need to move it at the speed you want.

If you had a 1 HP motor and it was geared low enough, it could do the job if you had all day.

 

[danwest]

...er...yeah. Axial force... that's what I meant. Now,how do you figure that again? (An educated guess would be OK, but err on the high side.)

 

[patprimmer]

As you say some of the time is in water, I guess you mean average slope, unless you found a way to incline water.

Speed, traction and friction are the 3 unknown variables in your problem.

Regards
pat

 

[danwest]

Yes, the bottom of the incline is under water.
3-6 mph is fine. How do I determine traction and friction?

 

[patprimmer]

I don't know any way other than field tests, making sure to include worst case scenerio, ie lowest traction, highest friction conditions, like tyre on wet grass, load on sandstone, or in mud.

Will the water tend to float the load and make the start easier, or will it tend to float your tractor and reduce traction.

You need to know the force needed to start motion, ie overcome friction, then you need to calculate the mechanical advantage that the output shaft of the motor has over the wheel, then you need to calculate the turning force required on that shaft in ft pounds of torque or whatever units you are working in

Regards
pat

 

[danwest]

When the rear sets are floating, there would be less load on the whole thing, due to the boat/dock combination also floating. I’m HOPEING that when the rear most tires are floating, the front most ones will provide enough pull to get the rear ones to a point that they can get more traction.

 

[sbi]

Sound like you are trying to get your boat out behind your cabin with out a ramp??


Had a simmilar problem a few years back for my folks cabin, they also had riprap to go over, we ended up building a rail line out of 3" steel tub and using the trailer on the rims on the steel tube ( minus the tires) with a elect winch at the top, still works fine. had a cross piece hooked to the tunge

sbi@alltel.net

 

[danwest]

That's what I would like to do, but the Corp. has prohibited any further development of the shoreline. Unfortunately, they consider a tramway the same as a dock, even though you can hardly see a tramway

 

[strokersix]

First you need to figure how much axial force you'll need.

I'm sure you actually mean tangential force, not axial.

We use traction formulas in the ag industry to calculate power, tractive force, braking force, slippage, etc. of tires on soil. If you really want to calculate this let me know and I'll post the formulas. Beware that the biggest unknown is the soil characteristics, which is a major contribution in the in the calculation.

 

[Metalguy]

Strokersix- No, I meant axial. Tangent to what? I wasn't referring to the wheels-just the load itself.

Danwest-Not to attempt a redesign, but this thing sure would be easier if you could use a long steel cable.

Are you driving the wheels with reduction gears after the hyd. motors? I hope you have a pretty low gear!

 

[patprimmer]

Also, if traction or power is a problem, try puting that cable around a pully and driving down hill, then the weight of the tractor acts as a counter ballance instead of extra load. Heck, you could even load it up with ballast.

I guess the boat is on some sort of cradle with wheels.

Make sure it has big enough tyres, and prefferably at least 2 on each side, so as not to get a wheel stuck in a pothole, and also to help it roll easily over soft surfaces like sand or wet ground.

Also, although you are not allowed any further foreshore development, I am sure you could get away with makeing a bit of a track by knocking the tops off any bumps and filling holes and soft spots with a bit of gravel, otherwise your 13% slope might have localised spots with much steeper slope.

Regards
pat

 

[danwest]

The cable thing is a little problematic for a couple of reasons. The first is that I would need a way to get the entire thing down to the water ? not just up. The second it the straight line problem. There is a path to the water that is plenty wide, but it isn?t straight. With the new rules, you can?t even cut grass/underbrush, so making a path in a straight line is out of the question.

Strokersix, I would like to see the formula. Remember to keep it simple, please.

Mikebull, how much would it be reasonable to add to your numbers to cover a variety of soil types? (Not an exact number, but are we talking 10% to be safe or 100%?)

Metalguy, I would like to make a direct connection from the pump to the wheel if I can find one with high enough torque. I?ve seen a lot of them in the 3000 in/lb range and around 300 rpm. Six of those would exceed Mikebull?s number by a good bit.

THANKS to ALL for the great input so far!

 

[Metalguy]

Yes, it sounds like you'd have enough wheel torque IF all 6 wheels are pulling-you'd have ~1,800 lb. of "pull". Those are pretty big motors. How constant is the 13%?

 

[danwest]

13% is the most it ever is and most of it is considerably less than that.

 

[Metalguy]

Good. So if those are the hyd. motors you can/are getting, the next thing is to figure out how much HP you need to drive a hyd. pump.

Here's the deal, and I'm sure others here will supply the equations. You start with a motor/engine of X HP at the RPM that best suits both the engine and the pump. You can go direct-drive for max. efficiency or use a belt drive for speed-matching. I like toothed belts-no slippage, low belt tension for the HP delivered. Now you must start subtracting all the drive losses, including the hyd. fluid losses thru the hoses, valves and the hyd. motors themselves, etc.

Probably safe to assume you'll lose 50% of what you start with. No problem, after buying all that hyd. equip. the cost of a bigger engine won't mean much!

So, back to the basics, at 100% eff. 1 HP will lift 550 lb. 1 ft. in 1 sec. Now you need to figure/measure the total vertical distance your boat/cabin/whatever will travel. Then crank out the math.

 

[strokersix]

OK I'll try the formulas here. These are taken from ASAE paper 78-1538. I'll present the traction formulas, braking are similar.

The formula for Pull/Weight ratio is:

P/W=0.75[1-e^(-0.3CnS)][1.2/Cn+0.04]

The formula for tractive efficiency TE is:

TE=[1-(1.2/Cn+0.04)/(0.75(1-e^(-0.3CnS)))](1-S)

Cn=wheel numeric=CI*b*d
CI=cone index in psi (soil property)
b=tire width
d=tire diameter
S=slip which is typically 0.10-0.15 for maximum TE, higher for maximum pull.

Hope this helps, Mike