Moment Arm Application

[thejackrabbit]

Greetings and Felicitations to You All.
This is my first visit to your very useful and interesting resource. I will be lifting solid wood beams and glulams using a tractor forklift with an extension in order to set beams on posts. It would be great if I could calulate the force exerted on the forklift by the moment arm of the extended load so as to not overload the tractor and implement. I recall the formula for Moment is M = F X D. I am having a little trouble converting that into a usable load number.

If the beam lifted is 400 pounds and the distance of the forks plus extension is say 12 feet, how would I calculate the effective load of the beam? That is to say what kind of force is exerted on the tractor?


Thanks in advance.

[Richard A Hetzel]

It would depend on how far out from the front wheels of the tractor the beam is located. and then whether the tractor would remain stable would depend on the mass of the tractor and counterweights if any, and the distance from the front wheels. You would also have to factor in the weights of any part of the tractor that are also extended out from the front wheels. All those measurements are taken frm the centerline of the wheels, and by "front wheels" I mean the ones nearest to the lifting mechanism.

It's a complicated problem.

[Don_P]

But to plug in to your problem as a quick example, 400 lbs (force) X 12'(distance) = 4800 ft-lbs

[thejackrabbit]

But to plug in to your problem as a quick example, 400 lbs (force) X 12'(distance) = 4800 ft-lbs

Does this mean that if the beam is being lifted with an arm of 12 feet from the fulcrum point on the tractor forks the load would be equivalent to 4,800 pounds? Is there a way to convert that into just dead weight pounds in order to determine is I am exceeding the tractor's load capacity?

Richard's factors are important in determining the tractors ability to move safely. It seems this problem starts with the equivalent load exerted on the forks by the moment arm force. IF that is within capacity then I can figure out the other factors Richard identified. For example:

the tractor would not be moving more than about ten feet as the beam has bee dropped where it is to be lifted.

The tractor has wheel ballast, front and rear (4 wheel drive utility category 2) and significant ballast can be added to the rear end. Moment arm is also relevent here as the ballast would be attached to the rear three point hitch.

[Don_P]

You could duplicate the force on the forks by placing a 4800 lb weight 1 foot out on them... 4800 lbs (force) x 1ft (distance) = 4800 ft-lbs

Since I had beams on the computer I was going the udder way from the forks in my head first. This shows the technique used to figure it. The beam equation is below, they use the terms M=P/l, Moment equals point load times distance, same thing.

4800 ft-lbs X 12=57600 inch-lbs
Lets try to find a size for the beam, Assuming its of "somewhat above average wood" (that's a big assumption, use good stuff) I'm going to give it Fb=1000 psi

S=M/Fb
Section modulus required equals max Moment divided by allowable Fiberstress in bending.
Divide 57600 in lbs by 1000 psi= 57.6 in3

S... Section Modulus for a rectangular beam is bd2/6
I'm going to guess a full cut 6x8 will handle the load so (6*8*8 )/6=64 in3 we pass for a static load.

You're rolling across the ground, I'd double the moment and try again, for impact it doesn't hurt to triple it.

I can't help you too much on the tractor itself except to say that mine has a broken lift cover and I could have had a 3 pt enema with a few more pounds on it ;)

[Richard A Hetzel]

When in doubt, attach as much ballast to the other end of the tractor as possible. Too much can't hurt you, but not enough sure could.

[Don_P]

Actually that was my point, too much can break things. Unsprung weight (tire ballast) is a whole lot easier on things than sprung weight on the 3pt. for ballast. Look the machine, arms and connections over carefully. The weak spot on mine was where the hydraulic cylinder connects to the lift cover directly below the seat. When my load shifted, it cracked and wanted to shove the lift cylinder up. Make sure you think things like this through.