d shaped thread body

[616]

I plan to machine a flat on the threaded portion of a shoulder screw. The thread is 5/16"-18 and is .5" longand the shoulder is 3/8" dia. The purpose of this is to stop the rotation of a nut when a member rotates about the shoulder. Between the shoulder and the nut will be a washer with the same d profile as the threaded portion. The flat stops the washer from turning.How big of a flat can be machined with out weakening the attachment?

 

[CoryPad]

The simple answer is you cannot machine a flat on the threaded portion without weakening the attachment. Perhaps you can adjust the flat size and the axial thread engagement to provide acceptable performance. You should test this since there aren't standards/equations covering this.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[CorBlimeyLimey]

The strength of the screw is based on the Root dia of the thread, not the crest, so you should be able to machine a flat down to the root without significant reduction in strength. Whatever you end up doing should be tested.

 

[616]

I am currently researching this thru a company named bolt science. I will post there response.Thanks for the replies so far

 

[CoryPad]

616,

Does the threaded region of this item develop preload? If so, then a machined flat will weaken the part. If there is no preload in the threaded region, then perhaps you don't have a problem.

CorBlimeyLimey,

Externally threaded fasteners fail in tension at a diameter greater than the root diameter, so your statement is false. Also, the full thread form is hugely important to resist shear forces for both the externally threaded part as well as internally threaded part. The proper overlap of the external and internal threads results in a fastener being tension limited rather than failing by shear through the threads.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[griffengm]

It may be that the strength of your resulting fastener will be akin to that of trilobe self tappers. I don't know this, it just seems logical. The trilobes would not fully engage the resultant thread.
Maybe some others can comment?

Griffy

 

[616]

This is the response I recieved and I tend to agree

"You are effectively loading the fastener off axis and hence inducing a
bending moment into the threads that will reduce its fatigue life.

To compute the likely magnitude of the effect detailed calculations
would be required. If the loading is small and static then such a
reduction may work - but there is - to the best of my knowledge no easy
way to establish that."

Thanks again

 

[Screwman]

Griff,
I think that you would see more of a degredation with the single D than from the trilobe parts. The tensile stress area of a trilobe part is about 95 - 97% of a full round body. This really isn't apples to apples with the single D because he is concerned with a thread height engagement issue. The out-of-roundness (K ratio) of a tri-lobe part is less than the thread height, but occcurs in three locations, so I don't know how you would correlate.

I agree fully with Corey that this will have an effect on the tensile load resistance of the part in question. How much, I don't know and I can't think of a good way to calculate it. I think that if you were to use a high hex series nut, that the added engagement length would be enough to overcome the reduction in thread stripping area created by the D configuration. But you would have to do testing to be sure.
One problem with threaded fasteners is that despite that fact that they appear to be very basic elelments, they have a configuration that makes them very difficult to characterize outside of the normal envelope. We end up doing a lot of testing whenever there is something outsdide of the normal situation and we continue to be surprised from time to time with the results that come out of the tests.

 

[CoryPad]

they have a configuration that makes them very difficult to characterize outside of the normal envelope

Well said.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[arto]

how about opposing flats - like on a valve stem?

 

[griffengm]

616,
I just reread your original post and am wondering why not just "D" or key the shoulder. The assembly you describe sounds similar to that used on the steering fork of a bicycle to retain it to the frame. If so, there are more standard washers to use than redesigning one.
A similar setup is used to key a washer to a shaft after which tabs on the washer are bent around the flats on the nut to fix it from working loose.

Griffy

 

[616]

griffenqm,

I have seen this before in flat monitor hinge assemblies. Usually I look for a convention when designing. I try to be careful and consider the application and the forces at work. I believe your idea is a viable alternative. I have already decided to "go another way" and skirt the issue.The pursuit of this idea and the input on fasteners has been invaluable. Thanks again