Clearance/movement between internal external threads

[Emo01]

Hi

I have a design where I have one dowel and one threaded hole.

I want to work out what the tolerance on the centre distance needs to be so I need to work out how much the screw can move in the hole, as the part is being wound up and down (adjusted) on the screw.

I know this is different for different classes of fit for the screw but I can't seem to find a clear answer. Essentially the more free play there is between the screw and threaded hole the less tight the tolerance on the centre distance needs to be. However I don't want specify a loose class of fit if I can avoid it.

This is the same as knowing the movement of a shaft in a clearance hole except for a screw in a threaded hole.

Cheers

 

[MikeHalloran]

Note also that the length of both male and female parts for both pin and screw enter into the picture.
I.e., it's basically a sophomore kinematics problem, like the monkey/clown that jumps and grabs a stick, with the further complication that the driving force is offset by the center distance.




Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

Act as if the screw has no clearance. Screw threads are self-centering under load.

 

[MikeHalloran]

Screw threads are self centering under axial load.
Not so much under cantilever/moment load.



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

If there is no axial load there's no point in using a screw thread.

 

[pylfrm]

Emo01,

You might want to provide a sketch of your assembly, or at least the relevant portion.

What type of screw thread are you using?


pylfrm

 

[greg2835]

Emo01,

You are on the right track that the class of fit will dictate the maximum movement of a screw shaft relative to a hole. It all has to do with pitch diameter and how much that is allowed to vary (which is defined by fit class i.e. 2A/2B).

For example, from ASME B1.1 a 4-40 2A screw has a pitch diameter .0950/.0925 and a 4-40 2B hole has a pitch diameter .0991/.0958. The most that screw can shift from hole center is (.0991 - .0925)/2 = .0033.

 

[3DDave]

If there is an accumulation of errors in pitch it might not shift at all.

Here's an article on the topic: Threads - Long Engagement Problems (pdf)

 

[Emo01]

Thanks for the replies