formula fou figuring lift capacity

[wvhillbilly]

Do any of you physics guru's know of a formula for figuring lift capacity at various distances from the 3-point hitch?? For example.. A tractor that has a lift capacity of 2000 pounds at the lift point and 1500 pounds at 24 inches behind the lift point. That is two pieces of information. Basically feet and pounds. So would you use the equation for torque (ft-lbs) to figure lift capacity at let's say 4, 5 or even 6 feet?
Bear with me here.... At 24 inches (2feet) 1500*2=3000 ft-lbs. So now that we know that the "torque" is 3000ft-lbs for this tractor, could we say that at 6 feet the lift would be 500 pounds. Solving for pounds the equation would be lbs=torque/ft. I think this is the correct equation to use since Torque is a measure of rotational force. And if you look at a 3-point lift, the unit that lifts the lift arms "rotates" at the tractor. Less of course larger tractors that have external lift cylinders. Does anybody know if I am on the right track here?? Please help me out. Thanks

 

[Henro]

Well, I'll do a little talking without thinking and maybe get the ball rolling for you. Unless a reply came while I was typing this... /forums/images/graemlins/blush.gif

The lower lift arms pivot back at the tractor housing, and this is an important reference point for analysis in my mind's eye.

Now the hydraulic cylinders connect some distance out on the lower arms, and exert a lifting force at this point. If at this same point weights were added, a point would be reached at which the hydraulics could not overcome the added weight. THis would tell one the lifting ability of the hydraulics at the point where the hydraulic lifting force is applied.

Drill a hole in the lower arm, at a point double the distance from the pivot point from where the hydraulic lift is applied. Hang half the amount of weight that stalled the hydraulics in the first case from that new hole, and the hydraulics will be stalled again.

Each time you double the distance from where the weights stall the hydraulics, you can halve the weight in question and stall the hydraulics.

So there is a ratio relationship between the the pivot point, the lift point where the hydraulics actually act on the lower arms, and the point where the lifting force is specified.

It looks like if you know the pivot point, the distance from the pivot point, and the rated weight that can be lifted at that distance from the pivot point, then you could come up with a formula to give you lifting capacity at any distance from the pivot point at the tractor end of the lower arms.

Now...I am not sure where the lift points of a 3PH are. I would like to know for sure. Are the lifting points that are referenced in manufacter's specs the ball ends of the lower arms, or are they the points where the lifting force is applied to the lower arms?

Hard to calculate anything without a good definition of where the specified lift points are...

Well, that's the way it looks to me, anyway... /forums/images/graemlins/smile.gif

 

[wvhillbilly]

Thanks Henro. Iwasn't even thinking between lift arms and lower arms. That definately throws my math out the window.

 

[JerryG]

Depending on which brochure you are looking at they may be using the ball end figure or 24" behind them. Everything that Bill said effects it as well as the piston size and the ratios of the mechanisms under the lift top and the length of the rockshaft arms. There are many variables that can effect the lift capacity.

 

[bebster]

That sure sounds like a clever and simple way to analyze the problem. You might be able to get the lifting specs on the hydraulic cylinders themselves. Since there are 2 of them, you should be able to lift double what the analysis approach indicates.

Do you think the tractor's hydraulic pump will be the limiting factor (vs the cylinders themselves)?

 

[rambler]

The relief valve pressure setting is always the limiting factor.

You can increase the valve setting, until either the pump shears something off (or stalls the engine if the pump is way overbuilt), or the lift cylinder blows something out. I suspect on a static lift-until-it-breaks, the pump would fail first, because the cylinder is built a little heavier to withstand shock loads from traveling over bumpy ground.

--->Paul

 

[jmc]

wvhillbilly,

Yes you can without much difficulty. I hope you made those two specs up though because it tells me your lower link is 6 ft long! Either that or the factory underrated one of them for some other reason. Here's what I did:

Let x= lower arm length and the loads are spec'ed when the arms are near horizontal.

stall torque at first position = stall torque at 2nd position.

2000x=1500(x+2)

Solving for x: x=6 ft.

Therefore, stall torque = 2000lb times 6 ft = 12000 ft-lb

If you want to calculate max force 4 ft beyond the lower link's implement attachment point:

12000 ft-lb = F(6+4)

F = 1200 lb

John

 

[wvhillbilly]

Thanks a million John. That helps me out tremendously...And yes, I did just pull those numbers out of a hat.
Thanks again,
Dave

 

[satyjeet]

I have just startd working in at tractor manufacturing industry and lil confuse about the hydraulic calculation for three point linkage , how much pressure is required in addc to lift 500kg.

 

[4570Man]

The relief valve pressure setting is always the limiting factor. You can increase the valve setting, until either the pump shears something off (or stalls the engine if the pump is way overbuilt), or the lift cylinder blows something out. I suspect on a static lift-until-it-breaks, the pump would fail first, because the cylinder is built a little heavier to withstand shock loads from traveling over bumpy ground. --->Paul

Your didn't consider a hose busting or the lift arms bending.