2 orthogonal screws

[baumisf]

Hey,

I just fnished med school and I have been conduction biomechanical research for the past 3 years now. I am currently working on a project plan and I am encountering the following problem:

We would like to measure compression across a joint, using partially threaded screws. For the standard procedure, 3 screws, more or less orthogonal to each other, are used. From a mechanical point of view, a second screw, orthogonal t the first one, should not add any any compression as the energy is absorbed by the first screw.

I would like to ask you 2 questions:
1) which law / mechanical rule has to be applied in the case mentioned above
2) i would like to illustrate this in a presentation ... any ideas?

Thank you for your help in advance!

Kind regards,

Sebastian

 

[Ron]

Please post a sketch of your issue. It will make it easier to understand.

In general, if you have one screw in compression, the orthogonal screw will be in shear if sharing the same load source.

 

[baumisf]

hey, pls find attached a"scetch" of the setup. the red lines are the screws and the 2 black bodies ae the bones which are supposed to be fused (compression between the two spherical surfaces)

I hope that helps. Thank you very much for your time and patients.

Sebastian

 

[MintJulep]

We would like to measure compression across a joint, using partially threaded screws

How? Screws are fasteners. Not load measurement instruments.

 

[Ron]

Assuming you want to measure compression of the joint in the vertical downward direction, your screws are not fully in compression. There is a compression component, but they are in bending as well. If you measure the movement between the two bones, you can compute the stresses in the screws. Easier said than done, though. It would be easier to fill the space with a compressible material and measure the "bulge".

It appears from the geometry that your are trying to immobilize the joint. If that's the case, the stress on the screws might be quite large. Further, you have to consider fatigue in the screws.

As MintJulep noted, I'm not sure how you intend to "measure" the compression of the joint using the screws.

But then...I know nothing about medicine or anatomy...just trying to toss in a bit of common sense.

 

[electricpete]

There are various means to attempt to establish "preload" level of a threaded fastener.

Most common is nut factor or similar calculation as here:
faq725-536

I'm sure many will point out that is not exact. Better to use a specifically calibrated nut factor based on measurements of change in dimension of your type fastener with your type lub under your conditions.

There is also turn of nut method. Some fraction of a turn past initial contact.

There are also a miriad of fasteners that have provisions to indicate or assure preload or stud tensioning equipment.

Perhaps you can tell us what method you will use. Ideally you tighten in a manner which assures centering in proper location. When you're all done if you have a preload in one bolt, Newton's law says you have an equal/opposite preload in the bolt 180 opposite (the bolt 90 opposite cannot provide that reaction force).

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[baumisf]

Hey, thanks for your comments ...

regarding the illustration, their will be no space between the two parts. the screws are partially threaded, with the threads all engaged in the lower bone. Compression (Area and Force) will be measured using Pressure Senstive Fuji Film (a sheet containing liqhit in bubbles that burst at a certain force, the colour gradient codes for the force)

But my question is, whether a second screw, orthogonal to a first tightened one, can add additional force (compression) on the joint space.

thanks for your help!

 

[Ron]

Yes. Any additional screws will add compression to the joint space.

 

[rb1957]

you're trying to measure the axial load in the bones, right ?

your screws all appear to have a component in the axial direction, and therefore all of them will affect the axial load.

the "principle" you're thinking of (orthogonal vectors don't interact) is like this ... say you have a screw in the x-direction and another in the y-direction. the x screw has no y-component (and the y no x) so adding force in the y-direction doesn't affect the force in the x-direction. But if you want to measure the force in the y=x direction then both screws contribute to this result.

clear as mud ??

 

[Cockroach]

Very, very cool. But to answer your question:

1) Poisson's Ratio (multi-axial)
2) Use a rubber ball, like a Squash ball and place it in a thimble. Then use your screws to see how the rubber ball would deform.

Good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[bvanhiel]

baumisf,

The second screw will overconstrain the joint. You'll get some additional pressure in the joint space, but it will be impossible to predict how the screws are carrying the load. Furthermore, once the bone resorbs (assuming this in vivo) the two screws will serve to hold the joint space open.