Fork to Lift the Weight

What materials are you considering? Look at the specs of steel for starters.

steel of 4140 grade

21 ton is a lot of mass to lift.

I presume that your comment that the shaft has to just lift the 21 ton mass means two things here.

First, that you've already counted and summed all of the forces, drags and torques on this shaft to lift, not just statically hold, this mass. Hopefully, your number of 21 ton includes a safety margin. There is one thing that bothers me from your sketch. The implied lifting direction (direction of gravity?) of either vertically or horizontally in this sketch is nowhere near inline with the center of mass of this assembly of fork and load. So along with a lifting force a torsional force will be required to maintain orientation. I see no provision for this torque in your sketch, nor a clear indication of any applied forces.

Second, you should have read the CR4 FAQ about homework problems. We do not do your academic homework. I hope that this is an academic problem because 21 ton is an enormous amount of weight to support. You are not qualified to do this engineering. You freely admit to being unqualified by asking your question to figure out how to calculate the forces.

As Adam Savage of the TV show Mythbusters so frequently says, "Failure is always an option".

Hire a mechanical engineer, unless you are willing to risk the lives of other people on the advice of strangers.

Drawing not clear (color weak at least on my screen) some dimensions not indicated. As concept bad. Welds can be overloaded. Sections are not totally characterized for resistance estimation.

Better hire a professional since you do not appear to be one.

Consider as well that CR4 is NOT a free engineering consulting office.

Regardless of your design, you're attempting to weld AISI 4140 which is a bad idea to begin with. You've selected the material based on assumed strength characteristics in heat treated condition but you cannot weld it in such condition (highly not recommended). The fact that you don't know this means you should consult engineers who do. I hope this was just a homework question and not something you actually intend to build.

I think that if RedFred is successfully answered, in addition to the advice posted here, we may have a solution.

I think there is a problem in your design. 1.) When the load is carried (im not sure but I suspect the load is a plate coil) there is a possibility that the whole structures will swing backwards since it is hinged on the top. 2.) Then the main shaft that carries the load will be inclined pointing downwards and thus slips the load going down. 3.) The 130mm thick support which is hinged on the top, will probably have cantilever effect and thus bends it backwards since there are no other member to support the direction of the load. 3.) If you are using welds, i'm not sure if it's practical to this kind of load. Since weld can change the properties of the metal. 4.) Solid or Hollow Shaft? it could be either but for calculations are made for the desired load, the result hollow shaft diameter would be greater than the solid shaft.

My suspicion is that this fork is going to be mounted on a forklift. The OP is not showing the forklift structure on the right which will act as the anti rotation feature. If you look at forklift tines, you can see they are designed in a similar way. Support at the top and reaction feature at the bottom. Look at image on the right of this page. I'm on my phone so I can't link to it........http://www.modullift.com/forklift_attachments.php

thx...good info...

Ralish, am I correct in assuming you are attempting to design an adapter for a forklift?

yes....i am doing the same.