Screws versus pins

[elisua]

What is stronger and better for continuous movement? Screws or pins for a mechanical component made of titanium Ti-6Al-4V

 

[berkeman]

Welcome to PF.

Do you mean bolts (or machine screws) versus pins? Can you be more specific in what the fastened joints look like?

 

[elisua]

This is a design concept for a fully titanium mechanical device that will be implanted in a patient. It needs to be determined in screws or pins could be placed where I highlighted in red. For purposes of being able to sustain patient daily motion, weight, torsion, I'd want to get some insight to see if overall it would be better to use screws instead of pins.

Still this is in the design concept phase, but just want to know do that I move forward in the best possible way.

 

[Bystander]

https://en.wikipedia.org/wiki/Joint_replacement ?

 

[Benjamin Calloway]

As far as weight and torsion capabilities, a cylindrical pin and the unthreaded shank length of a bolt have the same geometry, so both would be acceptable in that respect. The reentrant edge of a screw or bolt with a flanged head would cause a stress concentration, so if that's a concern, you might choose to use some unflanged pin. As far as sustaining cycles from motion, vibration, etc., that depends on how you'd plan to secure the pin or bolt. Pins are more readily able to be permanently secured from what I've seen.

 

[berkeman]

This is a design concept for a fully titanium mechanical device that will be implanted in a patient. It needs to be determined in screws or pins could be placed where I highlighted in red. For purposes of being able to sustain patient daily motion, weight, torsion, I'd want to get some insight to see if overall it would be better to use screws instead of pins.

Still this is in the design concept phase, but just want to know do that I move forward in the best possible way.

Thanks for the drawing. I'm pretty familiar with human A&P, but I'm having a hard time picturing where I would place that in a patient (Pt). Can you say more about the application? What are the expected stresses on those pinned joints? What is the range of motion for those joints? What is the mechanism of lubrication of those moving joints?

And what medical safety standards are you designing this to? 14971, 10993, 62304, and 13485?

https://www.in2being.com/iso-medical-device-standards/

 

[jack action]

If you use a shoulder bolt, there is no real difference compared to a pin. The shoulder bolt is just a pin with its own locking mechanism. The real question is therefore: How will the pin be secured and how does this locking and positioning system compare with a screw and nut?

 

[berkeman]

If you use a shoulder bolt, there is no real difference compared to a pin. The shoulder bolt is just a pin with its own locking mechanism.

Except a pin cannot loosen once it is flanged, right? A bolt can loosen unless it uses a lock washer or similar add-on. Still, I'm not sure how this all applies to implanting this inside a Pt and the medical safety standards...

 

[jack action]

Except a pin cannot loosen once it is flanged, right?

How are those flanges made or held in place though? A rivet? Interference fit? A circlip? A cotter pin? A set screw? A nut on a threaded end is just another way of locating the pin.

If I understood the drawing correctly some sort of clevis pin is needed (Should it be allowed to rotate?), not a bolt or a screw. The locating mechanism is really what needs to be determined.

There are two main types of clevis pins: threaded and unthreaded. Unthreaded clevis pins have a domed head at one end and a cross-hole at the other end. A cotter pin (US usage) or split pin is used to keep the clevis pin in place. Threaded clevis pins have a partially threaded shank on one end and a formed head on the other. The formed head has a lip, which acts as a stop when threading the pin into the shackle, and a flattened tab with a cross-hole. The flattened tab allows for easy installation of the pin and the cross-hole allows the pin to be moused.

A bolt can function as a clevis pin, but a bolt is not intended to take the lateral stress that a clevis pin must handle. Normal bolts are manufactured to handle tension loads, whereas clevis pins and bolts are designed to withstand shearing forces. The sheering strength of a threaded bolt is determined by its inner thread diameter.

Clevis pins should be closely fitted to the holes in the clevis to limit wear and reduce the failure rate of either the pin or the clevis.

 

[Flyboy]

… huh. Is it possible to TIG weld the pin ends in place before implantation? Or is that connection one that must be made during the procedure?