Bolt & Cap Nut lock themselves together - need to modify design

[ThirdString]

I've taken over a project where basically two parts are mated using a 1 5/8 - 5 1/2 thread. One is essentially a bolt, the other essentially a cap nut. When the parts are screwed together, the nut bottoms out on the bolt head.

When it bottoms out, the two parts actually lock together (unintentionally). I have to tap the nut with a hammer while turning it to get them apart.

Any ideas as to why it locks together, and how the design can be changed to avoid this?

 

[rb1957]

i imagine that its working like a double nut situation ... when you put 2 nuts on a bolt, the inner one tightens up against the flange being bolted together, and the 2nd one is tightened up against the 1st.

it sounds like you need to add a washer (so that the bolt head doesn't bind up against the nut body).

 

[Timelord]

ThirdString,

Perhaps the thread is bottoming out first and locking the parts together. Add an unthreaded portion to the end of the bolt like a dog point that will bottom out inside the end of the nut before it runs out of thread.

Timelord

 

[rb1957]

re-reading the OP i'm having trouble with the terminology ... "bottoming" usually refers to a bolt screwing into a threaded hole (with a bottom) and usually means that the bolt stem contacts the bottom of the hole before the bolt head has tightened up the parts (like Timeloard's post)
but the OP also refers to "the nut bottoms out on the bolt head" which sounded to me like the bolt head is contacting the "nut" body (which i why i thought he needed a washer under the bolt head)

 

[ThirdString]

Thanks for the feedback, and sorry for the confusion. The nut body is contacting the bolt head. I was using "bottoming" incorrectly!

The shaft is undercut on the bolt between the threads & head, so the nut is only contacting thread-to-thread until the nut body touches the bolt head.

I would guess I washer would work, but for this application, that wouldn't work so great. I do think it is acting like a double nut.

And I could make it truly bottom out, where the end of the bolt contacts the cap part of the nut before the bolt head and nut body contact, but that's not ideal either.

One thing I noticed with 2 different sets (from different manufacturers) is that the one has some play in it as you're threading them together and that one doesn't lock. The one that locks (on all parts from this manufacturer) has a nice, tight-but-smooth fit for the threads. Does this play a role in it? It's my best guess so far, but I thought maybe there was some principle behind this.

 

[rb1957]

i've tried to think why a washer wouldn't work with your design but i'm having trouble seeing it ?

 

[ThirdString]

A washer would work with the design, but not the application. Having to put on a washer would make it less user-friendly. However, if that is our only option, that is what we will have to do.

 

[byrdj]

Wouldn't the locking be friction between the mating faces (threads and nut face) and the face loading force be caused by the very small bolt tensioning possible due to the rotational inertia during the assembling.

Thus lube and/or reduce the speed and driving torque of assembling

 

[byrdj]

For the friction of the tread faces, the amount of surface contact will determine the pressure when the parts are snugged up. If there is narrow line contact the pressure will be greater than if full face with the same snug up torque. This might explain why one manufactures is different