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Shrink Fitting Threaded Fasteners

03/27/2012 6:05 PM

I have never heard of shrink fitting threaded fasteners. Can it be done and is it done?

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#1

Re: Shrink Fitting Threaded Fasteners

03/27/2012 6:51 PM

' shrink fitting'' is done for permanent fixing and is hard to remove, whereas threaded fasteners are for temporary fastening and can be removed later. Thread fasteners are not shrink fitted,,

ME 364 Manufacturing Technology Lecture Notes.inddme.emu.edu.tr/me364/ME364_combining_assembly.pdfFile Format: PDF/Adobe Acrobat - Quick View
processes for permanent (riveting, press or shrink fitting) or non-permanent (assembly with threaded fasteners) assembly; v joining processes, in which two (or ..




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#2
In reply to #1

Re: Shrink Fitting Threaded Fasteners

03/27/2012 7:07 PM

I'm not completely good with your reply. It is true as you say that fasteners can be removed, but they are also not just temporary. There are many applications where nuts and bolts are intended to be permanent, bridge and structural steel construction for example. There are also liquid locking compounds, (LOKTITE) that will make a threaded fastener joint permanent. Nuts are also tack welded to bolts to prevent removal. I'm thinking a shrink fitted threaded joint could be a legitimate application. I did a search and came up zero.

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#3
In reply to #2

Re: Shrink Fitting Threaded Fasteners

03/27/2012 7:12 PM

If you so desire make use of shrink fitted studs with threads on one end, fix it with a nut after jamming it in

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#4
In reply to #3

Re: Shrink Fitting Threaded Fasteners

03/27/2012 8:00 PM

Once again' you have ignored reality. And, apparently, the title of this thread, which is "Shrink Fitting Threaded Fasteners".

Cheers.

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#5

Re: Shrink Fitting Threaded Fasteners

03/27/2012 8:06 PM

The nut would shrink before you got it fully turned, you could end up with less than full engagement.

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#9
In reply to #5

Re: Shrink Fitting Threaded Fasteners

03/28/2012 10:47 AM

Do with the nut on.

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#6

Re: Shrink Fitting Threaded Fasteners

03/28/2012 2:57 AM

On reciprocating compressor cylinders, for making high pressure blocks compact, strength fastners are fitted by shrink fitting.

KOBE Japan had made LM6 model (Basemer Cooper Design) compressors for 2nd stage block they had used 6 number studs shrink fitted over the block's solid portion to improve the strength.

Unable to up-load the photo right now.

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#7

Re: Shrink Fitting Threaded Fasteners

03/28/2012 3:00 AM

Further, Steam turbine casing bolts are shrink fitted. Bolts are heated to a particular temperature and then tightened to particular torque.

Heating system is different for different manufacture'r design.

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#10
In reply to #7

Re: Shrink Fitting Threaded Fasteners

03/28/2012 2:11 PM

As far as I know the bolt heating has another goal. It allows to obtain a very well controlled pre-load when the bolt cooled back to normal temperature since by heating the bolt shaft elongation can be tightly controlled. It is a very time consuming procedure so that other ways have been developed as for instance assembly with pulling cylinders which allows to avoid, as in the heating process, torquing under load and the pre-load uncertainty due to friction which is as everybody knows a non constant and not fully predictable value.

As for the press fitting since the bolt is heated its diameter grows so that if it would have to be press fitted at normal temperature it should be "cooled" and not "heated".

This only as a remark.

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#12
In reply to #7

Re: Shrink Fitting Threaded Fasteners

03/29/2012 1:52 AM

This has nothing to do with "shrink fitting". These studs are heated in order to develop elongation. Once the necessary stretch has been acheived, the active nut is turned down to the spotface (only hand tight!). When the stud cools, the now-captured nut prevents axial contraction thus creating a preload without the use of torque!

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#15
In reply to #12

Re: Shrink Fitting Threaded Fasteners

03/29/2012 8:04 AM

That is a precise definition of shrink fitting. It is used also in large hydraulic assemblies, except your hand tight comment.

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#8

Re: Shrink Fitting Threaded Fasteners

03/28/2012 6:54 AM

In some aeronautical assemblies (for instance wings to body) a special sort of fastener is some times used. It has a tapered shaft with a low angle (a few degrees) and for assembly it is pulled in the conical calibrated hole with a nut on its threaded end. The result is equivalent to a shrink fit due to the high radial pre-load but it is easy to put in place - at normal temperatures and without external forces - due to the conical hole and fastener shaft. The assembly is quite complex since it requires a pre-drilling of all holes, a pre-assembly with special fixtures, a reaming (conical) of the two parts maintained in the expected position by fixtures and finally the introduction and tightening of the " threaded rivet". Assembly quality is controlled by the axial position of a reference surface on the "rivet". Tolerances are VERY tight. If the position is not correct then the hole is re-reamed and a pin with another nominal dimension is used. Same technology for repair.

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#21
In reply to #8

Re: Shrink Fitting Threaded Fasteners

03/29/2012 6:16 PM

I believe you are referring to "HUCK" fasteners, used in critical aerospace fastening applications.

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#23
In reply to #21

Re: Shrink Fitting Threaded Fasteners

03/29/2012 6:34 PM

No

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#24
In reply to #23

Re: Shrink Fitting Threaded Fasteners

03/29/2012 8:29 PM

What are you referring to then???

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#25
In reply to #24

Re: Shrink Fitting Threaded Fasteners

03/30/2012 2:23 AM

The generic name is Taper-Lock.The Hukk bolt has a pressed "nut". In fact it is not the traditional shrinking (Hukk) but a plastic radial deformation of the "nut".To allow a good assembly on surfaces which are normal to the hole the nut and washer have spherical surfaces.

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#28
In reply to #21

Re: Shrink Fitting Threaded Fasteners

03/30/2012 8:20 AM

because you mentioned the Huck (sorry for the tipping error in previous message) I show what they are so that it will be clear that there is not what I mentioned:

As you see the Huck is in fact a rivet and it is NOT shrink fitted it is only pre-loaded in axial direction as a bolt but without the torque which generates a pre-loading dispersion. the pre-load is given by the breaking force in pulling after the collar final position.

It is true that some fasteners of this type (Huck or others) have a real thread there where the "nut" is pressed on so that a dis-assembly for maintenance is made possible. Those bolts are NOT reused after dis-assembly.

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#11

Re: Shrink Fitting Threaded Fasteners

03/29/2012 12:58 AM

Need some clarification on this. Are you speaking of an interference fit of a shank with threads on the end (a stud) or the threads (nut and fastener) themselves being shrink fitted together?

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#13

Re: Shrink Fitting Threaded Fasteners

03/29/2012 2:01 AM

The issue with shrink fitting threaded fasteners is that as soon as they are tightened, their radial diameter decreases (since axial length increases). This is why most "fitted" bolts (frequently used on high torque transmission couplings) aren't really fitted. The trick is to shrink the fastener to such a relatively small diameter that the subsequent tightening (after the artifical "shrink" has disipated) still results in an interference fit.

Here's a description of such a fastener which should be familiar to those in the Marine industry:

"

The stretching force is applied through a hardened steel rod located down the centre of the bolt. Operating gear is screwed on to the head of the bolt. When pressurised, this exerts a force through the rod to stretch the bolt. As the bolt stretches in length it contracts in diameter according to Poisson's ratio.

Once pressurised, the bolt can be clearance installed in the hole. A nut is attached (if a through-hole application) and lightly tightened. The pressure is then released to bring about a reduction in the length of the bolt and a corresponding increase in its diameter. The reduction in length achieves an axial clamping load as the increase in diameter simultaneously achieves a fitted condition. The bolt is now ready for service. It can be installed remotely from the pump unit by using the stop valve fitted to the operating gear to lock in hydraulic pressure. Removal is the reversal of this procedure.

"

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#29
In reply to #13

Re: Shrink Fitting Threaded Fasteners

03/30/2012 8:34 AM

Is this one you think about?

Its working principle is the next picture:

The product is called Moorgrip an very often used especially where torques have to be transmitted with 100 % reliability thus not only based on friction. In this case they used with reamed holes and transmit via shear.

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#31
In reply to #29

Re: Shrink Fitting Threaded Fasteners

03/30/2012 3:29 PM

This is exactly the one I was writing about My friends at Pilgrim have been producing these for decades. Interesting concept, isn't it; This is truly a shrink-fitted bolt.

Whereas this style is un-naturally oversized and thus provides a true shrink fit, there is another version which is un-naturally undersized yet also provides a true shrink fit in a bore. It comprises a bolt with a tapered shank which fits into a sleeve with a matching taper in its ID but a parallel OD. The assembly is inserted into a bore with a clearance fit. The next stage is to draw the bolt through the sleeve using a hydraulic tensioner. This expands the sleeve to fill the bore completely. Bolt preload is acheived by another hydraulic tensioning step.

Disassembly is not quite the reverse of assembly; Once the preload has been released and one nut is backed off by 3 turns, hydraulic oil is injected between the sleeve and bolt shank. This "pops" the bolt out. Since sleeve expansion was well within elastic limits, it returns to it's original OD. The bolt is then withdrawn from the bore quite easily. Fascinating stuff, isn't it?

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#14

Re: Shrink Fitting Threaded Fasteners

03/29/2012 5:09 AM

Shrink fit is usually done by temp difference. bushes into nitrogen etc

Screw threads would give a problem because by their nature the material gets thinner at the edge and would be prone to breakup whilst turning in. heating the outer material would not work either so basically IMHO NO

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#16

Re: Shrink Fitting Threaded Fasteners

03/29/2012 8:27 AM

When I was in Aviation Tech school back in the 70's, one of the demos the instructors performed was removing the valve seat from a cylinder. He heated the jug with a torch and when is was red hot he took a stick that had a wet sponge attached to it and stuck it into the hole in the middle of the seat and spun it. The result was that the seat cooled very quickly and unscrewed from the cylinder. This is not really an example of a fastener but a shrink fit fastening using threads.

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#17

Re: Shrink Fitting Threaded Fasteners

03/29/2012 9:23 AM

I have cut many threads in my life. Understanding the inter relation of function and Class of fit is one of those things a machinist needs to understand in ways many other people do not ever have cause to.

A Class 1 thread is really sloppy and should be employed in farm equipment and other situations where a nut or bolt is likely to see high levels of corrosion, and or muck, but needs to be successfully taken off and installed with a minimum of difficulty.

I won't run through all of the Classes in terms of description, but generally the larger the number of the Class the tighter the fit on standard threads.

Class 2 is what most commercially available fasteners are made to, Class 3 is much tighter fit in terms of the allowable tolerance.

The highest Class I know of is the famous (amongst we who cut them) Class 5 thread.

I have never cut one, but I know that is is employed in the US Space industry where they have fasteners which can not come off under any circumstances.

My understanding is that the Male and Female threads are cut to provide an interference fit on assembly, and to install them the practice is to freeze the bolt and heat the nut. Once assembled they are impossible to remove by any conventional wrenching method.

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#18
In reply to #17

Re: Shrink Fitting Threaded Fasteners

03/29/2012 12:34 PM

GA.

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#20
In reply to #17

Re: Shrink Fitting Threaded Fasteners

03/29/2012 4:45 PM

GA.

The threaded type you described is used beyond just the aerospace industry. I've seen it used standard for certain "nuclear" rated valves. Even in a conventional power plant some high pressure applications use this method. It was common for a steam valve body with a threaded hole to be heated, the stud cooled and then driven into the body making them as you said, nearly inseparable.

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#22
In reply to #17

Re: Shrink Fitting Threaded Fasteners

03/29/2012 6:34 PM

In very high quality Bolts used in Aviation, the threads are "roll formed", a method which utilizes material displacement rather than material removal (cutting). The area to be threaded is precision ground to a specified tolerance to allow the displacement of material that will provide an exact major diameter thread tolerances after rolling. These have a radiused root diameter, while standard threads have flatted roots. The flat equals (Pitch divided by 8)

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#26
In reply to #22

Re: Shrink Fitting Threaded Fasteners

03/30/2012 2:39 AM

Not only, all series threaded fasteners have threads rolled.

This has several reasons as well economical as functional. Economical because it requires less time thus costs less. Functional because it drastically increases fatigue resistance of bolts in use. The highest stress occurs at the last active thread.

Rolling introduces a compression strain which has as direct effect the reduction of stresses under load and thus a better life expectancy..

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#19

Re: Shrink Fitting Threaded Fasteners

03/29/2012 12:50 PM

Why would you ask this question? Do you have an application in mind that might require such an esoteric operation?

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#27

Re: Shrink Fitting Threaded Fasteners

03/30/2012 2:41 AM

I would like to make a remark, it is possible that due to my limited English I did not understand OP question was it centered on shrink fitted threaded fasteners or on fasteners with shrink fitted threads ?

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#30

Re: Shrink Fitting Threaded Fasteners

03/30/2012 9:09 AM

Okay, here is one idea ...

Cold shrink the bolt with threaded end and when shrunk tighten the threaded end immediately in the pre-formed threaded hole corresponding to the size of the shrunken threaded end and force in the remaining unthreaded bolt into the socket which should be pre-formed to be a bit larger than the unthreaded part of the bolt. When the bolt will get warmer and reach the room temperature both the thread and unthreaded sections of the bolt will expand and grip the part. Same way you can use this method to shrink fit bolts with external nuts.

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#32
In reply to #30

Re: Shrink Fitting Threaded Fasteners

03/30/2012 3:40 PM

There is a fundamental flaw with this concept . You neglect to consider that as the bolt expands radially when it returns to ambient temperature, so does it expand axially. When this happens, any preload that had been applied to the bolt while it was cold is now reduced. Hence the bolt loosens Although such a concept may have merit in a slip joint, it is truly ineffective if the joint requires clamp load.

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#33
In reply to #32

Re: Shrink Fitting Threaded Fasteners

03/30/2012 9:57 PM

This is a valid point at this juncture.

If the joint requires clamp load, then complete joint shall be frozen to the required low temperature and then hot nut shall be used.

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