Bottoming out threads of blind hole

[BenjC]

I'm trying to find some guidance of the behaviours of load and stress distributions on threads when a bolt is bottomed out against the base of a blind threaded hole with pre-load applied. Are there design guides that elaborate on this topic? Do bolts that bottom out the threads of the hole hold torque? If not then why?

 

[GregSY]

It's pretty much 'all bets are off' when you bottom out. Of course, your question doesn't give nearly enough specifics, and there are many variables. Bottomed out threads definitely hold torque, but how much becomes the question. There is a guideline that applies to this topic - don't do it.

 

[technocrat]

Bottomed out bolts hold the torque reaction from the bottom end surface area friction of the fastener. Normal (non-bottomed) holding torque is provided by the thread face area friction which is far larger than the end area. At some number of engaged threads, the addition of extra engaged threads becomes insignificant. Design guidelines are do not use the bottom of the hole as a torque reaction surface, it is invariably too small compared to the engaged thread surface area available.

 

[michiganbuck]

I used to do a lot of assemblies when building broach bar set-up. It can be good to count the turns of a machine screw. 7 to 11 turns is often the number of turns.

 

[DDoug]

I'm trying to find some guidance of the behaviours of load and stress distributions on threads when a bolt is bottomed out against the base of a blind threaded hole with pre-load applied. Are there design guides that elaborate on this topic? Do bolts that bottom out the threads of the hole hold torque? If not then why?

I think your looking at bolt retention thru bolt stretch/preload, and trying to
relate that to bottoming preloading.

While the female threads are being pre-loaded, the fastener has not been stretched,
in the "normal" way (unthreaded neck outside of the threaded portion)

 

[moonlight machine]

I think your looking at bolt retention thru bolt stretch/preload, and trying to
relate that to bottoming preloading.

While the female threads are being pre-loaded, the fastener has not been stretched,
in the "normal" way (unthreaded neck outside of the threaded portion)

I have never run into a case of a bolt being intentionally bottomed in a threaded hole. However I have seen studs bottomed out thousands of times. All the studs on an IndyCar have the end turned to thread minor for about .2 or so depending on dia. and thread depth and have the end turned to the drill point angle. The studs are bottomed and tightened, this loads all the threads evenly and allows the stud to stretch when it is tightened. Another way to do the same thing is to put a ball bearing in the hole and tighten the stud against it, this is what BSA did on the cylinder studs on some of their 500cc single cylinder engines.

 

[dian]

???. so bolt has infinite modulus in compression? the load on inside thread has to be deducted from max. load, no? or you mean the bottom threads get loaded from torquing the stud and top threads can be loaded as usual?

 

[jscpm]

Trying to force a threaded bolt to turn in the bottom of a blind hole can only weaken the threads.

 

[Illinoyance]

Once he bolt bottoms out how can you create preload?

A bottomed bolt will have buggered threads at the end. Removal of the bolt will likely mess up the threads in the hole.

As others said: Don't do it.

just to be a technical bastard: A fastener in blind bole will be a screw or a stud. If it goes in a through hole with a nut on the end it is a bolt.

 

[Labrat]

I think it's important to differentiate between 1) a bolt that runs its nose into the bottom of the hole and 2) a bolt that runs out of thread and the nose does not touch the bottom of the hole.

Unless you do something specific to avoid it, you're probably going to end up with scenario 2. This isn't going to work well at all.

In the cases like moonlight described, yes that will work. Your torque may be slightly different. Think set screw.

 

[jim rozen]

I'm trying to find some guidance of the behaviours of load and stress distributions on threads when a bolt is bottomed out against the base of a blind threaded hole with pre-load applied. Are there design guides that elaborate on this topic? Do bolts that bottom out the threads of the hole hold torque? If not then why?

I'm not going to provide any fancy-pants FEA analysis, but I can say one thing right off the bat: if the threads are bottomed in the tapped hole, the *head* of that bolt isn't going to be holding anything tight.

 

[Strostkovy]

In my experience a bottomed out bolt means I have a lot of work ahead of me.

A coworker once decided to use an inch and a half bolt where there was only room for an inch, and torqued the ever loving shit out of it.

Result being I got to pull the bumper off of the customer's vehicle, repair the resulting dent, and repowder coat it the night before he picked up.

It was 3/16" steel plate. I don't know what was going through his head, if anything.

 

[Superbowl]

I'm trying to find some guidance of the behaviours of load and stress distributions on threads when a bolt is bottomed out against the base of a blind threaded hole with pre-load applied. Are there design guides that elaborate on this topic? Do bolts that bottom out the threads of the hole hold torque? If not then why?

Without at least knowing the configuration of the end of the bolt, flat,dished, dog point, etc. your question is addressable,

 

[tdmidget]

Oh by all means do it. Do it really well. It provides work for field machinists who machine them out because the stud galls to the bottom.

 

[plastikdreams]

Where do people come up with this shit...there must be some forum that they convene on to come up with it.

 

[Strostkovy]

Actually, what about studs used in engines and differentials and the likes? Are those bottomed out or does the taper at the end of the thread bind up?

In any case they loosen only when you don't want them to.

 

[EPAIII]

I think this is THE point. Generally speaking, the purpose of specifying and using a torque value is to have the proper preload on the part being fastened. An engine's head is perhaps the classic example of this. The combustion in the cylinders creates a lot of pressure that tends to LIFT the head off the block. It is the preload that prevents it from actually lifting and therefore for preventing the combustion gasses from escaping.

If bolts were used to hold the head on and they were bottomed out, then they would either not even be in contact with the head or they would have an incorrect and LOW value of preload.

So yes, you probably can install a bolt that bottoms out to the correct torque value, but it simply will not do it's job. This is a very good reason for using the style of bolt that has an internal rod to indicate the correct torque because it is measuring the preload instead of the torque and that, after all, is the whole purpose of torquing bolts.

Once he bolt bottoms out how can you create preload?

.....<snip>.....

 

[JS]

He asked the question here as well....

[: Bottoming out threads of blind hole - Mechanical engineering general discussion - Eng-Tips :]

Maybe he's found an odd application...

 

[plastikdreams]

Bottoming out studs, sure...cap screws or bolts, negative ghost rider. As said previously said, a bottomed thread will not be able to establish proper compression...unless everything is designed and machined to exacting tolerances and assembled perfectly...even then, I wouldn't trust it.