Need some math help!!!!!

[Nat]

I picked up an old C Allis Chalmers tractor last week. It seems to be a pretty good old treactor. It has been converted to a 3PH. The hitch works good but the arms on the rock shaft are to long. The rockshaft arms stick out 14 1/2 " drom the center of the shaft to the center of the swivel eyes. The verticle adjustment rods hinge on the swivel eyes at the top and are pinned to the lift arms. From the eye that is hooked to the tractor to the verticle adjustment rods is 28". What I want to do is move the rockshaft swivel eyes in toward the rockshaft X number of inches so that I can move the lower end in 10 " to a set of holes that are already there.
I doubt if I'm explaining this very well, but the porpose of this is to make it a stronger setup. Not stronger in the sense that it will lift more, but in being an alltogether stronger lift as the arms that are on the rockshaft are only 1/2 x 1 1/2" bar. At 14 1/2" from where the eyes are to the shaft it seems to me like they will bend if the lift is loaded to near capicity, where-as if the arms were only 8" or so they would be much stronger. I can double them up if I could figure out how short to make them and be able to move the lower end in to the exsisting holes which are 18" from the current mount.
I know there are a lot of people on here that are smarter than me so maybe you can help. I hope so. Thanks, Nat.
On edit I noticed that I didn't say that the amount of travel the current lift arms is fine. I don't need to change the ratio, just don't know how to figure the proper ratio to keep the current amount of travel of the lift arms while shorting the travel of the rockshaft swivel eyes. Thanks again, Nat

 

[bobodu]

If it were me...I'd keep the geometry the way it is and strengthen the arms. But ya have to remember the little C is gonna get light real quick anyway.

I built the whole shebang (including fitting the tractor with hydraulics..) for my WC and with the 6' Bush Hog hanging off the back....you can tell it's light up front. But this is an easy way to beef the arms up.

 

[Nat]

Bob, your WC is done exactly like my C except fot thr fact is my rockshaft arms aren't doubled like on your WC. I would like to get them in closer so the verticle arms went in the existing holes then the arms would be stronger and not stick so far out the back.
Trying to do the math it seems if I shortned the rockshaft arms to where they would work in the existing holes would be----
9/14ths of the current 14 1/2 shoul be the equivelent to moving the arms from their current 28" in to the desired 18" 18/28= 9/14 right? so what is 9/14ths of 14 1/2?
I am one of those guys that can make most anything, but ciphering ain't my strong suit. neither is spelling. Later, Nat

 

[Farmerford]

Nat:

The maximum stress in the rockshaft arms is at the top of where they attach to the rockshaft. Shortening both the rockshaft arms and the lift arm attachment points proportionally will not reduce the stress at the attachment point. If, for example, you shortened both by 1/2, the load on the end of the rockshaft arms would be twice the original load. That's because by halving the distance to the lift point on the lift arms you double the upward force needed on the lift arm to lift the same load. The torque at the rockshaft arm attachment point is what causes the stress. And torque is force times distance. So if you half the length of the rockshaft arm, but double the load, the torque is unchanged, and so is the stress.


Note that bobodu's upper arms have a plate connecting the top to the rockshaft in addition to the double arms themselves. That is because the designer recognized that the greatest stress occurs where the plate is located. If you sister your arms for the first few inches from the rockshaft with the same size material and put a similar plate on the top, you will greatly increase the capacity with very little work.

 

[bobodu]

Go on....ask me how I knew I needed to add that there plate!!!

Despite my advanced engineering degree ( high school dropout) and the latest in CAD software ( felt tipped marker) and the highest quality measuring devices (six soybeans and bits of string)....I busted the arm off the first time I used the box blade.

 

[Scotty370]

9/14 = .642 Therefore .642 x 14.5" = 9.30" = about 9-3/8"

Just doing the 'Math', not your engineering!

 

[MJPetersen]

It is 9.32 or a pinch more than 9 21/64ths--slightly less than 9 11/32--between 9 5/16 and 9 3/8. As if that kind of accuracy is necessary in something like this.

Again just the math not the engineering.

Mike

 

[Nat]

My tape doesn't measure less than 1/8ths so 9 3/8 it is. I knew that shortening the arms wouldn't make the stress less at the connection point, but would make beefing it up easier.
I can shoe\rten the rockshaft arms and then double them up for strength and still have less weight hanging 2' behind the rear wheels and take less of my precious metal to do it. Who would have ever though of cold rolled as precious metal? Thanks for the replies, later, nat

 

[Scotty370]

Gee Mike- I was figuring he'll use 'wide' chalk!!

 

[Eagleview]

I wish you guys would go metric , it's just 1+1 =2 . No fractions to confuse , even a tough one , 1.25 + 2.50= 3.75 .