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cwgf
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Join Date: Oct 2009
Posts: 3

Shoulder Screws - Pros/Cons

10/01/2009 7:06 PM

Over the years I have learned to avoid the use of shoulder screws for load bearing applications. I would like to compile a list of reasons why shoulder screws should not be used in design where any significant load can be developed, especially when the shoulder face seats against the thin face of a threaded insert. Thank you in advance for your thoughts on this subject.
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Pathfinder Tags: fastener machine design shoulder screw structural design
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TVP45
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#1

Re: Shoulder Screws - Pros/Cons

10/01/2009 7:40 PM

I guess I don't exactly understand your point. I've used shoulder screws a fair amount and found them to work pretty well, although they cost a pretty penny. What kind of threaded insert? I've put them in pressed inserts, but never threaded ones.

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cwgf
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Join Date: Oct 2009
Posts: 3
#2
In reply to #1

Re: Shoulder Screws - Pros/Cons

10/01/2009 7:50 PM

The issue I have found is that you cannot preload a shoulder screw enough to keep it from easily coming loose. The length between the threads and the shoulder is too short. The stress concentration in the shoulder is very high. It does not take much load on the screw to create very high stress under the shoulder. Finally, if someone wants to seat the shoulder screw onto an imprecisely controlled surface, like the face of a threaded insert, the screw shaft can be cocked at an angle.
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TVP45
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#4
In reply to #2

Re: Shoulder Screws - Pros/Cons

10/02/2009 7:36 AM

Ah, I see the difference. I've never used them way. I use them for location or for shear loads on the shoulder (where the threaded portion exists only to hold the thing in place and could probably be just an e-clip for all the good it does).

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nick name
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#3

Re: Shoulder Screws - Pros/Cons

10/02/2009 6:05 AM

"Shoulder screws are fasteners with an un-threaded shank. The un threaded shank is long when compared to its threaded portion, which is located under the head. Shoulder screws can be used to attach one machine part to another, or as a plain bearing in places where rotary or sliding motion is required."

Quote comes from a manufacturer's catalogue.

If transverse loads are present then the bending moment will be rapidly grow over the carrying capability of the thread, logically such screw should not be used where transverse loads do appear.

With respect to axial loads they support only what the thread accepts, not the shank.

As far as pre-loading is concerned the short thread leads to a high stiffness of the assembly and thus can loose the pre-load, it is thus recommended to use products to fix it in the female thread as "loctite" or similar or use locking inserts.

There are not pro/cons there is only the question if the part is used according to its properties or not!
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cwgf
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#5
In reply to #3

Re: Shoulder Screws - Pros/Cons

10/02/2009 8:24 AM

These are great comments all. Thank you for your insights.
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