Wooden Screw Press Calculations....help!

[kckuhns]

I am a bit out of my depth here as I am a retired telecoms systems engineer, so I am hoping that one of the mechanical gurus here might be able to nudge me in the right direction. So, I am attempting to design a number of wooden screw configurations for various wood-based applications. Consider, for illustration, a cider press. Suppose that I am using a wooden screw through a wooden nut to drive the plate of the press. I need to calculate the amount of force that I can apply to the plate based upon the following factors:


1 - wood species used
2 - shear strength parallel to grain of wood
3 - compression strength perpendicular to grain of wood
4 - diameter of screw
5 - threads per inch of screw
6 - thread type, pitch, depth


As an experienced woodworker, I know that most of the North American hardwoods that I would consider [Beech, Hickory, Hard Maple] for this application have shear and compression values of around 1,500 to 2,500 psi. But I just can't seem to come to a calculation for the screw failure point analysis. For example, I can use the ISO calculator for thread stress area to show that if I have a screw of 3.5 inches in diameter and 2 TPI then the tensile stress area is about 7.1 in2. Okay, this is telling me something, I suppose that I have 7.1 in2 of thread surface to play with. Does this mean that the thread failure point for this example wooden screw/nut may be as high as, say, 10,000 psi [7.1 x 1500]? Seems a bit high to me.


Another approach to the same set of issues is, suppose I know that I need to apply 1 ton to that cider press plate. So I can say that the tensile stress on the screw thread is only about 280 psi [2000/7.1]? So that I can confidently assume to be well within the working stress range of the screw?


Of course I haven't even gotten to the finer details of thread type [Acme, et al] and coefficient of friction to get to torque. I am just trying to arrive at some basic understanding of the minimum diameter dowel of which wood type I might consider for each application.


Any screw gurus out there?

[JAlberts]

To answer your first basic question, the key stress for a screw used in a device such as a press is the shearing stress on the screw and nut threads not tensile stress.

While there are many references for calculating this stress, for a V thread the stress is the calculated for the screw thread at the minimum diameter of the nut thread; and, for the nut thread at the maximum diameter of the the screw thread.

For a V thread joint, the screw shear stress = screw load / (nut thread minimum diameter X 1/2 of the thickness the nut)

For the nut thread shear stress the formula is the same except using maximum screw thread diameter instead of the minmum nut thread diameter.

In reality, assuming the bolt and nut are made of the same material (and in the case of wood, that the grain directions are the same for both items), the actual restricting stress is the screw shear stress.

This formula is also reasonably accurate for a true acme thread with tapered thread sides.

[Kelly_Bramble]

A design which does NOT rely on applying loading to a wood screw thread might be an overall better approach.

[kckuhns]

Thank you JAlberts, this is very helpful input.

[kckuhns]

Yes, that's correct, but the parameter of my problem includes using an all wood design. Thanks

A design which does NOT rely on applying loading to a wood screw thread might be an overall better approach.