Assembling of self locking inserts in a through hole

[roberto1brazil]

Please, could someone help me with the following issue?

The shop floor of my company have to install inserts to obtain threads M5 x 0,8 in a 32 (quantity) threads holes (through holes) in a flange of 7,7 mm thick and approximately 260 mm external diameter. The metal base flange is already supplied with 32 threads holes (through holes) and with complete threads from top to the bottom (sketch #1). So, the threads of the metal base were manufactured to be assembled with inserts (self-locking) to get internal threads M5 x 0,8 with 4,5 mm length. The assembly of mating parts make use of bolts M 5 x 0,8 with silver plating coating (body length of 25,4 mm) See sketch #2 . Before installing the bolts, they are previously covered using AIR 4247 Graphite Grease and then they are screwed.
The problem is:
Impossibility of keeping and assuring that all the inserts will be in a stable position when screwing the bolts.

Important remarks –
-The tangs of the inserts are removed after their installation.
- Before assembling the parts the mechanic does a previous test verifying if when installing the bolts, the inserts do not slide inside its housing and go out from it. He screws the bolt (with grease) until its bottom go 2 threads beyond the lower surface. Sometimes the insert do not move but sometimes they do (sketch #3). When it occurs we change the inserts. Nowadays I think that it is like a lottery. The coefficient of friction between the insert and metal base threads has to be more than the one between bolt and the insert. The probability of having the first situation is higher than the inverse, but the inverse is possible to happen (as confirmed by previous test). Probably because some grease drips onto the surfaces between the insert and the screw.
-After assembling the mating parts with the bolts, it is not possible to check if the inserts moved down and came out from their housings. It is covered and there is no window. See sketch # 2.
-Question – What would be the effect of the self locking inset system in this case? Would it help to move (rotate) the insert when installing the bolts?

My opinion - There is a design problem. In order to be sure that the insert will never slides out, the metal base thread would have to be manufactured with an incomplete thread. Isn’t it? The threads holes should be manufactured until a depth thread of 6 mm. Acting in this way (in my mind) the problem would be eliminated. Besides all this explanation, I am not aware about the existence of any mechanism displayed by the manufacturer of the insert that could ensure to fix it in a position if the current assembly procedure is kept.

I am not a designer and because of that I must be sure about what I am going to discuss. Please, I would ask for your appreciation about and comments if possible.
My regards
Roberto

 

[MikeHalloran]

This is not a new problem.

One possible solution is, as you describe, incomplete threads beyond a particular depth, for which you will have to provide a tolerance. It's quicker and cheaper to make a full thread through, so expect a price increment.

One other possibility is to stake (deform) the threads of a through- threaded hole, often at both ends of the insert, to prevent it from sliding out in either direction. I think there are specialized tools for this operation.

A third possibility is to use Keenserts, which are slightly larger, and are locked in position by axial keys, driven in with a hammer or a press.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

From your description it appears you are using something like a Helicoil type insert. Helicoil type inserts always seem to have this type of problem, and the problem gets much worse with small thread sizes and inserts with internal locking features. Your situation also is compounded by the silver plate used on your bolt threads.

The basic reason the inserts are backing out of the threaded hole they were installed into is excessive interference between the bolt and internal thread locking feature of the insert. One way to prevent the insert from winding out the bottom of the hole is to use a dial-type torque wrench to monitor the prevailing torque when installing the bolts. If you can determine what level of prevailing torque will cause the insert to break loose, you can stop torquing a bolt if the prevailing torque limit is reached, remove it and try another bolt. You might also consider using bolts that don't have silver plating on the threads if it is not truly necessary.

One other thing that might help is to thread an unplated bolt into each insert once or twice after installation to "break in" the insert threads. It's better to find out that an insert has excessive prevailing torque when it can be easily replaced, rather than during installation of the component.

Lastly, I would recommend replacing helicoil type inserts with something like a keensert (key-locked insert) if at all possible. Key locked inserts are a bit more expensive, but I've never seen a self-locking keensert back out.

Hope that helps.
Terry

 

[roberto1brazil]

I thank you all for the comments and advices. Terry, you are right, the inserts are Helicoil. I can say you that your support was very important to me. Now I am feeling a little more confident about this matter.
Thanks Mike and Terry.

Roberto

 

[drawoh]

roberto1brazil,

I have been specifying helical thread inserts for a long time. Generally, I avoid the locking ones, but I have used them. I regard them as reliable. I would have worried about specifying them for your application.

It may be too late for this, but most of my problems with them have been with shops using old, dull taps.


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JHG