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.

 

[Mace Canute]

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.

Another Zombie thread!

There is no one answer to that question because there are so many variables involved...different categories of three point hitches, different manufactures of tractors, different configurations for how the 3ph is constructed are just a few of the differences that make a one-answer-fits-all impossible . Each case has to be taken individually and the physics/geometry worked out for that particular 3PH.

 

[LD1]

Zombie thread full of bad info to. Nothing I saw skimming the old.posts is even remotely correct for a 3ph

 

[jmc]

Zombie thread full of bad info to. Nothing I saw skimming the old.posts is even remotely correct for a 3ph

Tell us what is incorrect about post #7.

 

[bcp]

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.

Much more is involved than pressure.

Diameter of cylinder
Length of arm from cylinder ram to pivot of lift arm
Length of lift arm
Length of draft arm before and after lift link attachment
Angles through which all these links operate
Friction

And probably some I forgot.

Bruce

 

[LD1]

Tell us what is incorrect about post #7.

Everything.

3PH is not a simple lever. Its a parallelogram linkage.

Depending on how the toplink is set if (both length and if you have multiple mounting locations), and how the implement is configured (pin spacing between the lower links and toplink) changes the toplink angle in relation to the lower link angles, Will all yield different lift capacity results.

The lower you mount the toplink on the tractor side, the less you can lift as you are lifting the tail end of the implement higher (lifting and rotating) than if you choose the highest point on the tractor that font lift as high.

Lifting something a lesser distance takes less force. Or in otherwords, you can lift more.

 

[Egon]

Old thread. First post for member reviving it!

 

[LD1]

Old thread. First post for member reviving it!

Certainly a zombie thread. And the member that revived it has not even logged on to the site since.

Normally I would have just left it alone, but its full of bad info, and brought right back to the top of the board.

 

[jmc]

Everything.

3PH is not a simple lever. Its a parallelogram linkage.

Depending on how the toplink is set if (both length and if you have multiple mounting locations), and how the implement is configured (pin spacing between the lower links and toplink) changes the toplink angle in relation to the lower link angles, Will all yield different lift capacity results.

The lower you mount the toplink on the tractor side, the less you can lift as you are lifting the tail end of the implement higher (lifting and rotating) than if you choose the highest point on the tractor that font lift as high.

Lifting something a lesser distance takes less force. Or in otherwords, you can lift more.

We have to assume the tractor company knows where the sweet spot linkage position is when they give us maximum lifting force at two different points along that position.

From those two provided points, we can calculate the maximum lifting TORQUE applied to the linkage.

From the lifting TORQUE, we can calculate the lifting FORCE at any point along the linkage, in addition to the two points the company provided.

Before you start insulting everyone, you should do more than "skim"(your term) the thread.

 

[LD1]

Its not an insult to say a statement is wrong when it is.

Your figuring of the 3PH would be as if the implement were just an extension of the lower arms. IE: simple lever. Which the 3PH is NOT.

The 3PH is a parallelogram type linkage. And to further complicate things, it is an ADJUSTABLE one. Most modern tractors have at least 3 places where you can hook the toplink to the tractor. The highest hole is gonna have significantly MORE lift capacity than the lowest hole. Cause the lowest hole causes the implement to lift AND rotate much more.

Picture a bushhog attached to your tractor. When I hook mine up and use the top hole, it lifts nearly level. IE: at max height, the tailwheel is only lifted about 12". Doing nothing but putting the TL in the bottom hole, makes the tailwheel lift about 4' in the air with the same range of motion from the 3PH linkages. Lifting something HIGHER takes more input force. Since input force is fixed, it will lift less weight in this position. (or more weight with the TL in the highest hole on the tractor)

The fact is the 3PH is NOT a simple lever and there is NO simple way of calculating how much it can lift at a given distance. Too many variables that can change.

There are a bunch of threads on this very topic already.

Here is a good start if you are not too stubborn and open to learning something:

http://www.tractorbynet.com/forums/...ion-factor-3ph-load-capacity.html#post1682683
http://www.tractorbynet.com/forums/attachments/333848-lift-capacity-3-pt-hitch.html#post4089382

I dont know why, but for whatever reason people seem obsessed trying to simplify calculations for lift capacity for FEL and 3PH. there IS NO WAY to simplify things. They are what they are and thats not gonna change. There is no way to make simple lever calculations and come up with any kind of accurate result. But everyone keeps trying.

 

[Jerry/MT]

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

the relief valve may not be the limiting factor. My TO-30 will lift till the front wheels come off the ground and the relief valve doesn't blow.