Calculating FEL Lift Capacity

[npalen]

I was curious what the relief valve setting should be on my Kubota BF350 front end loader (FEL). So I did the attached layout based on measurements taken.

I come up with a theoretical lift force at the bucket pins (excluding friction and weight of loader itself) of 2,720 lbs.

The owners manual for the Kubota B9200 shows a loader lift capacity of 838 pounds at 1636 psi relief setting. (Discovered this data after using estimated 1500 psi)

So I'm needing help understanding where the discrepancy is occurring between the 2,720 pounds and 838 pounds.

Here are my calculations: 2" cylinder bore equals 3.14 sq. in. piston area times two cylinders = 6.28
1500 psi times 6.28 =9420 pounds force
9420 lbs. times 15.524" lever arm = 146,236 in. lbs torque about loader arm pivot point.
146,236 divided by 53.750 lever arm from bucket pins to loader arm pivot pins = 2720 lift force at bucket pins

Anyone care to check the math for me?

 

[gladehound]

Not sure I'm following exactly what you did....

What is 15.514"? Is that the distance from the loader arm pin to the loader cylinder attachment? If so, I don't see where you accounted for the direction of the force. The cylinder isn't at the tangent to the loader arm arc. To calculate torque on the loader arm, you need to know what portion of the cylinder force is acting at the tangent to the loader arm arc. This will change for every loader position. So check it at full height to match with your spec. But then you will only have torque and not lift so it will only be right when the loader arc tangent is perpendicular to the ground.

An easier way to get a rough estimate of the average loader force is take the loader cylinder stroke divided by the loader lift height (adding in any loader travel below ground level). For example, if you're cylinder has a 1 foot stroke and bucket pins can travel six vertical feet you have 1/6. Multiply 1/6 by the 9420 pounds of force, subtract out estimated loader / bucket weight at the pins and it will give you an estimated average lift. The estimated average will be less than your lift at the bottom of the range and more than your full height lift. It might approximate what you can load from a trailer / truck but not really because remember it is at the bucket pins and the load will be out further. But you can do the math for that too.

If you want to know lift fairly accurately at a given position without doing all the geometry, Just measure loader vertical movement over a short distance and measure cylinder movement over same distance and multiply cylinder force by that ratio.

coincidentally, last night I just discovered that the curl spec in the Kioti literature for my KL402 loader isn't even close to being right. Then I noticed other specs in other places, some that seemed closer to reality. I haven't taken all the measurements and done all the math yet but will post if I ever get around to finishing it.

 

[npalen]

Thanks for the feedback! Some interesting bits of information to ponder. Using the formula of cylinder travel divided by lift height times cylinder force gives 2,088 lbs in my case. ((1.33/6)*9420)

The 15.514" that you ask about is the distance from the loader arm pivot point perpendicular to the cylinder(s) line of force. I used that distance to calculate the torque about the loader arm pivot which is the 146,236 in. lb.

The 53.750" dimension is the distance from the loader arm pivot to the vertical line above the bucket pins. This 53.750" divided into 146,236 in. lbs. gives the theoretical vertical lift force at ground level of 2,720 lbs. at the bucket pins. This is obviously not taking into account friction and the weight of the loader etc but it's much more than Kubota's spec of roughly 800 lbs. Incidentally the loader model of BF350 would suggest a lift capacity of 350 KG which is 770 LBS.

Incidentally, the main relief valve for this tractor is set at approximately 2500 PSI but with the loader relief set at about 1600 PSI, the rear remote valve has much less PSI output than if the remote valve was "in line" ahead of the loader valve. (Using power beyond to feed the 3PH position control) Not sure I need more PSI at the remote (three spools) but might consider changing the plumbing.

 

[roadhunter]

Or just stack sacks of concrete in the bucket to get a real world number.

 

[Rock knocker]

I come up with about 800 psi on the cylinder at 832 pound load, giving the actual FEL a weight of zero.

But you are assuming that the limitation is purely hydraulic. It is likely due to lifting the rear end off the ground

 

[oldnslo]

Effective force of a cylinder working at an angle to direction of the load travel :
F = T x sine A
T = total cylinder force in pounds
F = Force acting to move the load
A = angle between the cylinder axis and load direction.

Like Gladehound stated with FEL's the angle is constantly changing but try and pick worst case for a FEL.

Example: 2" bore with 25 degree angle @2500 PSI = 3319 lbs force trying to move the load or approximate 42% of the total cylinder force.

Also have to figure in the lever ratio of distance from FEL arm pivot to cylinder connection Vs bucket pivot to cylinder mount on FEL arm.

 

[npalen]

I come up with about 800 psi on the cylinder at 832 pound load, giving the actual FEL a weight of zero.

But you are assuming that the limitation is purely hydraulic. It is likely due to lifting the rear end off the ground

How did you come up with the psi and load? Just curious.

 

[LD1]

Forgot to account for the angle of the cylinder. If the cylinder were mounted 90 degrees to the loader arm, then 100% of the cylinders force would be used for lift. But it is not. its mounted on a 29 degree angle. You have to take the Sine of the angle, and multiply that by the cylinders force. Whats left is the vertical component.

Then you have to account for the changing angle. The largest angle is with the loader at its lowest. Thus a loader has the most lift force at ground level usually called breakout force when reading specs. ITs usually twice as high as when fully raised. +

So, Raise the FEL to max height.

Re-measure the angle of the hydraulic cylinder. Re-calculate the math. The new number would be the lift to max height.

 

[rustyshakelford]

Or just stack sacks of concrete in the bucket to get a real world number.

Yup. Less brain, more brawn

Brett

 

[gladehound]

I was curious what the relief valve setting should be on my Kubota BF350 front end loader (FEL). So I did the attached layout based on measurements taken.

I come up with a theoretical lift force at the bucket pins (excluding friction and weight of loader itself) of 2,720 lbs.

The owners manual for the Kubota B9200 shows a loader lift capacity of 838 pounds at 1636 psi relief setting. (Discovered this data after using estimated 1500 psi)

So I'm needing help understanding where the discrepancy is occurring between the 2,720 pounds and 838 pounds.

Here are my calculations: 2" cylinder bore equals 3.14 sq. in. piston area times two cylinders = 6.28
1500 psi times 6.28 =9420 pounds force
9420 lbs. times 15.524" lever arm = 146,236 in. lbs torque about loader arm pivot point.
146,236 divided by 53.750 lever arm from bucket pins to loader arm pivot pins = 2720 lift force at bucket pins

Anyone care to check the math for me?

This is probably a dumb question - but do you actually have the spec on the I.D. of the lift cylinder? Or does it just look like ~2" from the outside?