Thread Pitch Error.

[richardandtracyParticipant @richardandtracy]

I have also seen really thin walled nuts with a double hex external shape. These were slightly ovalled to become self locking & the thin wall did the same job as Duncan's slot. The ones I saw were titanium for aerospace use.

Regards,

Richard

[duncan webster 1Participant @duncanwebster1]

Posted by Brian Wood on 09/03/2017 08:22:33:

Hello Duncan,

​Thank you for the sketch, it explains what I couldn't imagine with crystal clarity.

​I'm going 'off air' for a day or two so I'll let the design mull away in the background, it is certainly one I have never come across. In the meantime, what application were these used on?

Regards
Brian

It's back in the mists of time, but I think I came across it when involved in the design of a large hydraulic press. Very big nuts, sort of 12" BSW if there is such a thing. Not tightened with a spanner, the male thread had a hole down the middle down which you inserted an electric heater, run down the nut to contact, heat up the male, run down the nut by a calculated amount, then let it all cool down.

[Alan VosParticipant @alanvos39612]

Posted by duncan webster on 08/03/2017 19:05:42:

It's a rubbish sketch, the thread is mean to be parallel. You can see that the threaded part of the nut is in tension, and why they would be so expensive

It looks like a means to spread the load across more turns of the thread than would be the case if solid. Less material. Less weight. There are industries which are prepared to pay silly money to save weight.

[Brian WoodParticipant @brianwood45127]

Hello Duncan and Alan Vos,

​Over 30 years ago I had some peripheral contact with the use of bolt heaters to set preload on big studs in critical naval pressure vessels and remember well the carefully choreographed and very prolonged period of time it took to set the ring of closing bolts on those vessels. A full team of metrology experts and measuring tackle was needed with extension bars down inside the bolts to give the necessary reference from the bottoms of the bolts.

​Allowing for the time it took to allow cooling and then stable measuring conditions with at least 4 bolt heaters working round in a sequence like cylinder head closure, only to have to do it over when there was too little or too much preload, was hugely expensive in keeping the team on site to see it done and finished to specification.

​Another set of operators of similar plant elsewhere pulled their bolts up hydraulically and got much faster results; I guess these annular nuts, despite high initial cost, become cost effective in the long term if they can be relied upon to be 'fit and forget' for that particular operation.

​The design rather cleverly positions all the deformation on top of the nut where it can be much more readily assessed with simpler measuring tools, and hence give preload information, while at the same time allowing hydraulic tensioning of the stud to be done and relaxed for measuring as each bolt is set. As Alan observed, the thread itself is in a position of close contact on the stud without stretch to influence the picture, all the stress is concentrated in the material at the top of the annular groove.

​I don't think though that these rather exotic cases will seriously challenge the values I gave Richard in his quest for a rule of thumb guide which started this thread, interesting as they are.

​Regards Brian

 

Edited By Brian Wood on 11/03/2017 10:25:37

[Tim StevensParticipant @timstevens64731]

When you tighten a nut onto a bolt the thread in the nut is compressed slightly, while the bolt thread is extended slightly. It can help to remember this when choosing a pitch which is not quite right. Better to make a nut slightly greater in pitch, or a bolt which is slightly smaller pitch, than it should be, if bang-on is not an option.

Not that it really makes a difference, I suppose.

Cheers, Tim

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