Bucket lifting capacity (math)

[apease]

Prompted by the thread "Pallet Forks for BX or B-Series", I wanted to figure out how much my BX24 FEL can lift with forks. I've had to make some assumptions on figures since I'm not near my tractor. If anyone can make this more precise, I'd be grateful.
For a start, to keep the math manageable, I assume that the FEL is at level lifting height. I'm also ignoring any change in power the hydraulic cylinder may have from minimum to maximum extension.
I'm guessing on the distance from the FEL pivot point to the cylinder attachment, from the diagrams on 5-6 of the operators manual, since that's not given. It looks to be the same as distance "F", which is 54.7". The "bucket bottom mid point" also isn't precisely specified - is it the midpoint of the 500mm bucket width, or the mid-point of the actual bucket bottom, which might be 100mm farther from the FEL pivot? I made two guesses at 63.8" and 65.8" from FEL pivot.
I use the basic lever arm equation of

F = W*X/L
or
payload capacity = lifting force * payload to pivot distance / lifting force to pivot distance

(sorry for all the dots in the diagram and table below - the TBN editor seems to remove spaces and tabs so I'm not sure how else to show this)
rough diagram:
..........-X-...........F
.........................|
o-------------------v
.........^
...-L-...|
.........W

................................................................F=WxX/L.....W = FxL/X
lever arm (L).................................54.00in...750.00lbs.....1620.00lbs*
arm at mid-bucket (L) (guess #1).....63.84......529.00.........1350.91*
arm at mid-bucket (L) (guess #2).....65.81......529.00.........1392.56*
arm at bucket edge (L)...................73.69.......518.00.........1526.75*
arm at 4' pallet midpoint (L)............97.69.......383.89*

Numbers with an '*' have been calculated. I get some widely disparate estimates of cylinder upward force (W). I'd love to know why. For the calculation of F I just assumed 1500lbs for W.

All comments welcome.

 

[dennisgw]

Not sure where you got a value of F=1500lbs. Here are some bucket lift capacities for BX and B that I found on a TBN post. You are not going to be any higher than these. You would need to subtract the weight of the forks from the lift capacity and if the center of gravity of the pallet load is out from the bucket edge then you REALLY have to lower what you think you can lift. I was trying to unload my BX42 chipper from a trailer with my Bxpanded Forks. If the lifting strap was a foot out from the bucket edge, I could not life (chipper is somewhere around 400 lbs). I could only lift it if the sling slid down the fork arms to the edge of the bucket. I would love to lift like a fork lift but I think you will not exceed what your current lift capacity is and more than likely less.

BX 2200 LA 211 - 460 LB
B 7400 LA 272 - 495 LB
B 7500 LA 302 - 660 LB
B 2410 LA 352 - 770 LB
B 2710 LA 402 - 882 LB
B 2910 LA 402 - 900 LB
B 21 TL 421 - 926 LB

 

[LD1]

For the nature of trying to keep things simple, your 1500lb estimate/average which will result in a 389lb capacity is fairly close give or take a few lbs.

No need to try to hash out and get exact figures because the pressure will vary a little form tractor to tractor and the MFG's numbers are just calculations as well.

But as far as the varying #'s, it could be a cew things. First, when measuring from the cylinder attachment point on the boom to the bucket pivot, were you measuring in a straight line, or were you following the curve of the boom.

If you were measuring in a straight line, that # is correct.

But what isn't accouted for in the other measurments is the bucket curl. No matter what, the measurment from the pivot pin to the cylinder point CANNOT change. BUT, the bucket mid point and bucket edge CAN change depending on where in the curl cycle the measurments were taken. The bucket edge is going to be closest to the cylinder point when curled all the way back. But as you dump, it will move farther away. Thus changing the length and the figures all together.

Another think you should account for is the bucket curl force. The curl moves in a different arc than the lift. So usually at the bucket edge, your curl cylinders will have more force than the lift cylinders. But with an attachent on like forks, there comes a point somewhere out in front that the curl cylinders will have LESS force than the lift cylinders.

The reason fo this is that 4' in front of the bucket as opposed to the edge is a larger percentage of change from the curl cylinders than it is from the lift cylinder.

But the reason I bring this up is, pallet forks can cause a great deal of pressure and possible rupture hoses (especially on smaller tractors) in the curl curcuit. The curl cylinders may only be capable of "curling" 150lbs at the tip of the forks. BUT if the lift cylinders are ABLE to lift 300lbs, with that kind of load on the tips, you are in a sense pressurizing the curl curcit to double the PSI the tractor pump is putting out. And there is NO pressure relief for this cause the valve is blocking the flow. So if you have a heavy load in the tips of the forks, and a little bouncing, it is very easy to damage the curl cylinders or hoses. (Ask me how I know)

That is why pallet forks that actuall replace the bucket are better IMO if you are doing a lot of pallet moving. Getting rid of the bucket keeps the load much closer to the pivots. Which results in greater capacity and less chance for a blown hose.

 

[LD1]

Not sure where you got a value of F=1500lbs. Here are some bucket lift capacities for BX and B that I found on a TBN post. You are not going to be any higher than these. You would need to subtract the weight of the forks from the lift capacity and if the center of gravity of the pallet load is out from the bucket edge then you REALLY have to lower what you think you can lift. I was trying to unload my BX42 chipper from a trailer with my Bxpanded Forks. If the lifting strap was a foot out from the bucket edge, I could not life (chipper is somewhere around 400 lbs). I could only lift it if the sling slid down the fork arms to the edge of the bucket. I would love to lift like a fork lift but I think you will not exceed what your current lift capacity is and more than likely less.

BX 2200 LA 211 - 460 LB
B 7400 LA 272 - 495 LB
B 7500 LA 302 - 660 LB
B 2410 LA 352 - 770 LB
B 2710 LA 402 - 882 LB
B 2910 LA 402 - 900 LB
B 21 TL 421 - 926 LB

I think he was reverse figuring how much vertical force is applied at the cylinder point by using lift specs that kubota provided.

That is how he came up with a much greater # that you listed. Because it takes more force because the lift point is so far behind the actual load.

 

[dennisgw]

LD1, Yep. Thanks for reminding me. Wasn't aware of the curl issue either.

 

[apease]

dennisgw - LD1 is right - in my message I gave a calculated figure of 1500lbs for W - which is the force applied by the hydraulic cylinder. Since its closer to the pivot than the load, it has to be a much greater force than the weight of the load.

LD1 - good point about the bucket being a potential point of failure. We have two "links in the chain" and the actual lifting force the tractor can generate is the minimum of what the two joints can handle. So, I tried to calculate the force on the bucket for the same pallet fork load.

breakout force at pivot......................1415.00lbs
breakout force at lip...........................992.00

423.00lbs less per 19.69in
21.49 lbs less per inch

breakout force at 4' pallet midpoint.......476.28lbs > 383.89 for loader

It looks to me that in this case the loader pivot is still the limiting factor and we have a margin of safety on the allowable forces on the bucket hydraulics.

 

[Transit]

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.

 

[LD1]

dennisgw - LD1 is right - in my message I gave a calculated figure of 1500lbs for W - which is the force applied by the hydraulic cylinder. Since its closer to the pivot than the load, it has to be a much greater force than the weight of the load.

LD1 - good point about the bucket being a potential point of failure. We have two "links in the chain" and the actual lifting force the tractor can generate is the minimum of what the two joints can handle. So, I tried to calculate the force on the bucket for the same pallet fork load.

breakout force at pivot......................1415.00lbs
breakout force at lip...........................992.00

423.00lbs less per 19.69in
21.49 lbs less per inch

breakout force at 4' pallet midpoint.......476.28lbs > 383.89 for loader

It looks to me that in this case the loader pivot is still the limiting factor and we have a margin of safety on the allowable forces on the bucket hydraulics.

What would your breakout force be at the tip of those 4' forks as opposed to the loader lift??

There may be just that one time when you have a load right on the tip tht I am concerned with.

Like when you need to push that pallet just a little farther, and you back up and lift with just the tip, or when you cant quite reach what you are trying to lift and use just the tip, etc.

 

[apease]

Transit - aren't we saying the same thing? Torque of the load and of the cylinder must be equal. I've calculated an approximate force from the cylinder, and so knowing the distance of that force from the pivot and the force from gravity on the load at a known distance from the pivot I solve for the load allowed.

 

[J_J]

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.