Bucket lifting capacity (math)

[LD1]

The lifting force of most all FEL is given at the bucket pivot at the end of the lift arm, so many thousand pounds. This is the lifting force not including the weight of the bucket. The net lifting force is lifting force at the pivot (? the weight of the bucket or any other attachment.

This than becomes a problem in High School Physics, The Sum of the moments around a fixed point, or Torque.

Were the torque = the distance from the reference pivot point X the weight.

What does this all mean?

Pick the reference pivot point. Lets say, we use the vertical post at the cab. Now this point does not have to be a POINT, it may be a vertical line through the reference point, but always use the same reference [now called the reference plain]. The way you measure this distance is from the ref plain is at a right angle to the plain and the distance to the load.

The first calculation:

With the bucket on the ground, measure the distance from the ref plain to the bucket pivot.

Lets say that distance is 56 inches and the lift force is 2,500 pounds. There fore the torque is 56 X 2,500 inch-pounds.

Calculation 2:

With the pallet arms in place, measure the distance from the ref plain to the center of the pallet arms, say 72 inches

Now divide the torque in calculation 1 by 72 to obtain the new lift weight.

56 x 2,500 / 72, this is the lifting force not counting the weight of the bucket and pallet arms.

This is incorrect. You cannot pick ANY point. You have to pick the common point at which both points are rotating round, which is the rear loader pivot.

With your fomula, all you woudl have to do would be to choose a point farther away and come up with a larger number Which cant be.

In your own example, you are giving distances of 56 and 72. What you are saying is that the lift force is 56/72 of the origional capacity (77.7%)

The difference in distance between the pivot @ 56 and the forks @ 72 is 16". This distance does not change no matter what point you choose.

So now lets choose a "point" farther back. Say we are now 90" from the pivot. This would make the forks @ 106" away because they are 16" appart and that cannot change. So 90/106 is 84.9 %.

Nothing else changed except the refrence point.

So you have to use the point at which both are pivoting aroung and NOT ANY point.

Unless I am misunderstanding you on something.

 

[LD1]

You all can pencil whip those figures all day, but the trial and error method will get it real close.

Why not just use 55 gal drums of water. Each 55 gal of water weighs about 440 lbs. Two full drums of water would weigh about 880 lbs. Add another drum if you can lift the two, and add gal of water until you can not lift the load. Each gal of water weighs about 8 lbs.

Or cement blocks.

Now where's the fun in that

 

[apease]

LD1 - good point to emphasize safety, and easy to calculate now that I have everything set up - loader should be able to generate 308lbs at the tip of 4' pallet forks (4' from the edge of the bucket that is). But I think there was a flaw in my previous bucket force calculation. Using the same equations as for the loader, and assuming 12" from cylinder to pivot I get roughly the same force for the bucket cylinder of 1600lbs (when using the BX24 manual figure of 992lbs breakout force at bucket edge). That give us a slightly different figure of 447lbs breakout force for a load centered on a 4' pallet, and 288lbs for a load at the tip of the forks.

So, the bucket does become the weak link in the chain at the tip of the forks, although only by 20lbs.

 

[apease]

J_J - theory and practice work together. It's very important to test the theory in practice, because the calculations or assumptions might be wrong. But I think it's also a good idea to know the theory for cases where you need to extrapolate to a case you can't test, and to allow for explaining things to others. It's always good to know *why* something is the way it is.

 

[LD1]

LD1 - good point to emphasize safety, and easy to calculate now that I have everything set up - loader should be able to generate 308lbs at the tip of 4' pallet forks (4' from the edge of the bucket that is). But I think there was a flaw in my previous bucket force calculation. Using the same equations as for the loader, and assuming 12" from cylinder to pivot I get roughly the same force for the bucket cylinder of 1600lbs (when using the BX24 manual figure of 992lbs breakout force at bucket edge). That give us a slightly different figure of 447lbs breakout force for a load centered on a 4' pallet, and 288lbs for a load at the tip of the forks.

So, the bucket does become the weak link in the chain at the tip of the forks, although only by 20lbs.

I knew it would, I just did not know how far it would on your tractor.

A real world example, we set the trusses on my polebarn with my little L3400 and they were 24' to the peak.

We strapped a 16' section of 3-leg TV antenna tower to the bucket. The trusses were only ~150-200lbs each and the bucket would NOT curl them. But it would lift them with pleanty of power to spare.

So by lifting them, I was exerting more than 2500psi on the curl cylinders. Since my tractor makes 2500PSI and that wasnt enough to curl. Fortunatally I did not blown any hoses or damage anything, but this was the point I was getting at.

 

[Grandad4]

Interesting and informative discussion... thanks!

Once you guys get the math sorted out for a load that's perfectly centered on the forks, you might have a go at figuring out how far off center that load can be (i.e. unbalanced side to side) before the tractor lifts a wheel. The calculations start getting more complex, but "seat of the pants" math tells me there's very little lateral tolerance with a load hanging way out in front like that.

 

[apease]

Grandad4 - If you treat the wheels as fixed, and assume a static situation, I don't see how you'd get sideways tipping. Since the bucket is only as wide as the tractor, the load can't be beyond the fulcrum (the wheel). In reality the situation would be a little more complex I think since the wheel is going to compress under force so maybe there's something going on that I don't know how to model.

However, as another poster mentioned from experience, you can certainly get tipping over the length of the tractor. So, I'd tried to figure that out. I'll start with the worst case of no backhoe and nothing on the three-point. Tractor alone (BX24) weighs 1520lbs. The wheelbase is 55.1" so we can assume a point mass (W) of 1520lbs at the midpoint 27.5" (X) from the fulcrum. Reach of the bucket is 54.7" and we assume a point mass in the middle of the 48" forks so 78.7" (L). F=W*X/L which gives us 531.13lbs which is greater than the 383.89lbs the loader can lift. The Kubota engineers are smart - they gave us a margin of safety so that it's not possible to tip the tractor even in a worst case scenario. However, a dynamic load (i.e. dropping and stopping the load, dropping the front wheels into a depression while driving) is another matter, which could easily raise the force to the tipping point.

Anyway, that's my amateur analysis.

 

[apease]

oops, two error - forgot to include the weight of the loader, and you probably also meant a load the extends beyond the bucket, like moving a long tree laid the long way on the bucket. Analysis coming right up...

 

[dennisgw]

Granddad/Appease:
I can appreciate Granddad's query on stability and things being off center. I had my BXpanded forks on and was carrying a load of branches (probably overhanging 6-7' each side of the bucket forks) across a part of my property that had a "slight incline". I did have the bucket up a bit so the hanging branches would clear the ground and dodge some flowers coming up. At a certain point Grandad's concerns occurred, I could feel the tractor lift slowly lifting on the back...........not a good feeling! I leaned over to the lifting side and dropped the bucket, the tractor immediately sat flat again. I kept the bucket very low for the rest of my journey. So center of gravity of the load in combination with angle is a powerful combination. Lesson learned.

 

[apease]

ok, revised lengthwise tipping calcs - assume the center of mass of the loader+bucket is X=14" forward of the wheel. LA240 loader and bucket together weigh W=507lbs. So, the effect on our forklift load is L=78" is again F=W*X/L giving us 92lbs, or a tipping point weight on the forks of only 438lbs vs lifting capacity of the loader at that point of 383lbs. That's cutting it a bit close, so theory matches practice and says to have the backhoe or something on the three point when using forks.

Sideways tipping calcs next...