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Thermally Pre-Stressing Bolts

05/01/2012 2:11 PM

I need a few tips on how to calculate the amount of elongation of a bolt, I'm using a 1 1/2-6 B7 threaded rod with 9'' between the nuts and need to achieve 100 tons /200,000 pounds of compressive load, I'm doing this thermally be cause there is no way to get the proper torque with wrenches since I do not have enough room around the nuts

so I guess I'm looking for the formula for this kind of a problem so any tips on the proper way to calculate that would be appreciated

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#1

Re: thermally pre-stressing bolts

05/01/2012 2:17 PM

What are you post-tensioning?

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#2

Re: thermally pre-stressing bolts

05/01/2012 2:24 PM

Do you foresee washers under the nuts ? Surface quality of clamped surfaces ? Material of flanges? Do you have a seal ? Which kind ?

The result will be more precise than with torquing since the friction effect is avoided.

The 9" represent the distance between nuts with or without washers ?

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#3

Re: thermally pre-stressing bolts

05/01/2012 3:36 PM

You're trying to apply,"200,000 pounds of compressive load" with a single 1 1/2 inch threaded rod, over nine inches?

Over what temperature range will the rod operate?

What thread form, cut or rolled?

Lot's more information please.

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#4

Re: Thermally Pre-Stressing Bolts

05/01/2012 4:50 PM

well first off I guess I should have clarified in the first place that the rods are actually in a hydraulic press so in a sense they are more like press tie rods, they are not really going to be operating at any thing more then your normal ambient temperature

they are a cut thread, no washers just nuts

I went back and checked over the plans again and all we're going to need is 60 tons / 120,000 pounds of compressive load not 100 tons like I said in the beginning

there is 4 rods that like I said after pre stressing need to have a pre load of 60 tons compressive load each

we're planning on heating them up enough to where we can basically thread the nuts on by hand and let them cool

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#5
In reply to #4

Re: Thermally Pre-Stressing Bolts

05/01/2012 4:58 PM

Okay...

More curiosity: How will you disassemble, some time in the future?

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#7
In reply to #4

Re: Thermally Pre-Stressing Bolts

05/01/2012 5:37 PM

So, I, now, assume that you have plates (platens) in this press? We called these tie bars when I was working with presses and I see no reason for the large pre load. Nor a fully threaded rod, but that's up to you.

How much ram pressure can you generate? That's what's important, unless you will be trying to apply an even load over the entire surface of the platens, during heat cycles. Then you'll need a way to determine parallelism between the two platen surfaces. Lead bird shot works well for this. One in each corner and one in the middle and squeeze away.

Tell us much more about what you are really doing.

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#6

Re: Thermally Pre-Stressing Bolts

05/01/2012 5:05 PM

I assume that means six bolts, 1 1/2 inches diameter.

I think you cannot do it, the bolts will lose their preheat when they touch the adjacent metal, the nuts and the wrenches.

7.3 x 10-6 in/in is the rate of thermal expansion of steel.

29,000,000 lbsforce/inch2 is the Young's modulus (spring rate) of steel, =stress/strain.

Stress= say,19,000lbs/in2 then strain must be; 19,000/29,000,000=6.55x10-4

Extension =9x6.55x10-4 =0.0058inches

0.0058=9x7.3 x 10-6xt where t is the change of temperature. I make it about 90 degrees but you should check it, I did it on the back of an electronic envelope, and I'm not going back over it.

I still doubt it is a practical proposition.

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#8
In reply to #6

Re: Thermally Pre-Stressing Bolts

05/01/2012 7:16 PM

I've been retired too long, I missed an essential part.

The nine inch bolts are going to squeeze something, something elastic, something that will contract when squeezed and release some of the tension in the bolt.

I see you only have four bolts! but less total pre-load.

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#9
In reply to #8

Re: Thermally Pre-Stressing Bolts

05/01/2012 7:26 PM

Ok, I think I've got it. The 9" bolts are not going to squeeze something, he wants to lock the platens in place with the nuts.

He's got 9" tie rods. So, he has no more than 5 inches of daylight in his press, depending on platen thickness.

Or, I have no clue.

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#10
In reply to #9

Re: Thermally Pre-Stressing Bolts

05/01/2012 9:48 PM

I don't know anything much about presses so...

He does say the grip is nine inches so the rods are longer by at least two nut thickness' so does that tie in with your five inches (sorry, I should have thought of a better way to say that)

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#11

Re: Thermally Pre-Stressing Bolts

05/01/2012 10:53 PM

We used this method on some large bearing caps when I did a contract at a Nuclear facility. The bolts had a hole in the center and we inserted an electric heater for a pre determined time. Then we ran the nuts down hand tight and removed the heaters and when it cooled we achieved the desired stress or stretch. I had a similar task on an injection molding press where the tie bars were about 1 foot in dia and the mechanics could not undo the huge nuts. Again I used the heat and after soaking the bar for about 4 hours I was able to undo the nut. Great system just have to know the important facts in order to get the desired clamping.

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#12

Re: Thermally Pre-Stressing Bolts

05/02/2012 2:22 AM

"I'm doing this thermally be cause... I do not have enough room around the nuts".

  • First of all, "proper torque" isn't what you're trying to achieve (when will people get this right? )
  • Secondly, if you want to thermally elongate the tie bars you'll have to machine a hole down the centre in which to insert your heater rods be they gas, resistance or induction.

Although heating was at one time a common way of tightening tie bars, a modern and much easier way of reaching your goal is by the use of tensioning nuts. This can be done cheap'n'nasty with simple mechanical jacking-screw nuts or, it can be done much faster and easier with hydraulic nuts. In both systems the nuts are spun onto the tie bars until they meet the spotfaces. Then (with the hydraulic option), all 4 Nuts are quickly connected to a single pump and are pressurized. This stretches the 4 tie bars simultaneously, thus ensuring even and rapid preloading. Once the proper load has been achieved, the Hydraulic Nut locking collars are turned down by hand to retain the load before the pressure is released and the hoses are removed. Disassembly is the reverse procedure. And, since torque isn't involved, it's quite easy to calculate the necessary pump pressure for the required preload.

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#13

Re: Thermally Pre-Stressing Bolts

05/02/2012 7:24 AM

Instead of using your thermal trick (which will be difficult at best) try the following. Remove the 1.5-6 nuts and drill and tap a series of holes parallel to the main thread around the outer perimeter of the nuts. 1/4- 20 threads would be a good place to start. Insert 1/4-20 set or cap screws into these holes until they are almost flush with the far end of the nuts. Now screw the nuts on by hand until they bottom. When it becomes time to tension the assembly, tighten each of the outer screws in sequence. You will have the mechanical advantage of a finer thread and you will have enough smaller screws to achieve the strength you require. If you need more strength, add more screws. If there is not enough room on the nuts for the number of screws you require , use larger threaded collars instead of nuts. There are commercial versions of these tensioning devices available.

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#15
In reply to #13

Re: Thermally Pre-Stressing Bolts

05/02/2012 7:33 AM

Called "super bolt".

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#14

Re: Thermally Pre-Stressing Bolts

05/02/2012 7:31 AM

If you have the place I would suggest to use the solution Boltintegrity proposed. However the way you load the bolts could lead to fatigue and if an even small crack appears in only one bolt the structure is not anymore under uniform loading and it can locally be overloaded. This implies a well specified pre-loading in order to maintain the load VARIATION within acceptable limits. This was the reason for the different questions you did not answer. It is also very important to know how thick the plates are and the dimensions of the distance bushings or the diameter of the rods between the plates. All this enters the computation of the pre-load strech value.

If you use the nuts as indicated by BI then it will be better to measure the strech and not only the pressure.

There is a factor which is important for short tie rods, the crushing of contact surfaces. When you first load the piston applies the force on a surface different from the one where the load will be applied after the pressure is removed. This can lead to a pre-load loss. It is known phenomenon so that in general the operation is made at least 2 times.

The material used for the plates has to be of corresponding hardness if not at the surface a plastic deformation could occur which compromises the reliability of the function (loss of pre-load function of loading cycles).

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#24
In reply to #14

Re: Thermally Pre-Stressing Bolts

05/02/2012 8:51 PM

One factor that has not been mentioned so far is deformation of the threads inside the nut. That is, the rod length does not end at the face of the nut.

I have seen ads for the commercial version with jack bolts around it. It's been a while, but the name may have been "Super-Bolt."

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#25
In reply to #24

Re: Thermally Pre-Stressing Bolts

05/03/2012 3:30 AM

It has not been mentioned because this is part of the bolt compliance computation. According to research made extensively in Germany this can be approximated with 0.5 D.

D- being the nominal thread diameter.

By the way you refer to # 14 and should refer to #13 and the super-bolt has been already mentioned. It is good to really read the previous threads.

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#16

Re: Thermally Pre-Stressing Bolts

05/02/2012 11:09 AM

ok so another correction from my original post I actually have 13" between the nuts with the rods being 18" over all

ok so the boss of the project does not want to go with the hydraulic nuts, and there is a way to heat them to disassemble the press ( the old fashioned way ) with a torch, there is a window like you would have in a hydraulic press

so basic plan is to set them up on a bench with a dial indicator and then hit them in the middle with the torch and watch the amount of expansion be it .010 or .020 or whatever then when we have reached the desired length drop them in and run the nuts on by hand and that should be it, old fashioned I know but its how the boss said to do it, I just have to figure out how much of elongation I need

thanks for all the replys

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#17
In reply to #16

Re: Thermally Pre-Stressing Bolts

05/02/2012 11:27 AM

If you do not consider the plates compliance and the other compliances in the game you can have failures very soon. The procedure is dangerous since you hit(heat) only a part which implies a higher temperature which in fact could modify the steel quality with all the consequences. Do get a written order or procedure specification to cover yourself in case of accident. Steel has a thermal dilatation coefficient in the order of 12*10^-6 mm/mm/°C. If all the dimensions are not known it is impossible to estimate the right elongation you need.

I am sorry to say that the boss takes big risks and could get a human accident on his shoulders.

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#21
In reply to #16

Re: Thermally Pre-Stressing Bolts

05/02/2012 1:16 PM

Matt, - Please tell your boss that at least one of us agrees with him. It can be done thermally. I think what we object to is the use of a torch to do the heating because the heating can not be done uniformly, simultaneously, and lacks temperature control that can lead to overheating. Your boss may have worked in a military arsenal where they commonly shrink tubes onto large gun barrels for reinforcement. But this is different. Please tell your boss that we can construct a resistance heating machine that will heat all rods simultaneously or a large induction heater that will do the same. About $500,000.00 should take care of it. When he sees this, perhaps jacking nuts (mechanical or hydraulic) won't look so terrifying.

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#23
In reply to #16

Re: Thermally Pre-Stressing Bolts

05/02/2012 7:07 PM

If you heat too much they can over stress and break we had this happen on one of the injection molding presses where the mechanics (that is what they called themselves) tightened the nuts too far during the heating and a 12 inch tie bar snapped about one year later.. Best part is I told them what to expect and it happened. No one would listen at that time.

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#18

Re: Thermally Pre-Stressing Bolts

05/02/2012 11:52 AM

This has all the markings of the blind leading the blind through the shifting sands of the unknown.

Good luck, I'm gone.

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#19

Re: Thermally Pre-Stressing Bolts

05/02/2012 12:36 PM

Matt, is your boss an accountant, the company gardener or an electrician? He most certainly can't be a mechanical engineer based on the directive that you've been given.

You'll be locally toasting the tie bars. Even when properly thermally elongating such fasteners, the heating process is relatively consistent and thus not as metallurgically risky as yours will be. Furthermore, I can guarantee that you'll be chasing your tail for days as you "hunt" for the proper elongation of each tie bar. For example, you'll torch the tie bars, run down the nuts, wait for things to cool, measure elongation, heat again, cool, measure, heat, cool, measure, heat, cool, measure, heat, cool, measure, ad infinitum...

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#20

Re: Thermally Pre-Stressing Bolts

05/02/2012 12:59 PM

Matt, here's a link to a spreadsheet which will help you with your calculations. We accept absolutely no responsibility whatsoever if you choose to use it for an unwise purpose (which appears to be the case) ...

http://www.heviitech.com/Downloads/HeviiBoltWorksheet264MetricImperialLocked.xls

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#22

Re: Thermally Pre-Stressing Bolts

05/02/2012 6:44 PM

well thanks to those who actually had a bit of help to offer, but for the rest first my boss is a mechanical engineer, and further they have been putting presses together this way for a hundred years or more commonly with a torch

so I guess not much more can be accomplished here

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#26
In reply to #22

Re: Thermally Pre-Stressing Bolts

05/03/2012 12:03 PM

Matt, you said "...and further they have been putting presses together this way for a hundred years or more commonly with a torch"

So... why this question at this forum? Never mind, I don't want to know. Good luck.

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#27
In reply to #22

Re: Thermally Pre-Stressing Bolts

05/03/2012 4:41 PM

The question is totally valid: when a plant has a 100 years old experience one has to use it without asking known details since 100 years to poor people who do not have such an experience being too young for it !

Why does your mechanical engineer (the boss) not give you the means to compute what you need ?

I would be very glad to learn from him.

A a subsidiary question how many presses and for which forces have they build in the century ? Just for curiosity.

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#28
In reply to #22

Re: Thermally Pre-Stressing Bolts

05/06/2012 11:23 PM

"...not much more can be accomplished here". Not entirely true. Matt, it takes 10 min to calculate or verify the stretch/load relation you're looking for using FEA (finite element analysis). I can get you the elongation required to produce the desired load. I'll use the material and dimensions of the bolt you provided. Keep in mind this will be informative data only and in no way implies validation of what you're trying to do or your process. Use the values to compare with your hand calcs to see where you stand. I'll get you something by 9:00am.

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#29
In reply to #28

Re: Thermally Pre-Stressing Bolts

05/07/2012 3:46 AM

Usage of FEA is verygood when you have ALL informations. A streching can be determined when the compliance of fastened parts is known. As far as you do not have a full drawing your results are not the right values. If you look at the bolt/rod which is only part of the problem your results are meaningless.

If I am for you not clear enough I can elaborate on request.

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#30
In reply to #29

Re: Thermally Pre-Stressing Bolts

05/07/2012 8:38 AM

Your being very clear Nick. Any analysis is meaningless without ALL the information. Not just FEA. I did not imply to do a complete and full analysis of his system or process in 10min. It would be absurd to assume such a thing. I'm merely providing an alternative answer to Matt's original question which is simple in nature taken at face value based on the information he provided. It's purely theoretical, academic, and in no way implies relavance or accuracy with respect to his entire system.

In case I'm not understanding your concern, please fell free to elaborate while I settle in at my desk for another day of drinking coffee, eating donuts, and getting fat!

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#32
In reply to #30

Re: Thermally Pre-Stressing Bolts

05/07/2012 10:49 AM

You have to write if you understand or not, not me.

I understand what I mean but due to your suggestion I am not 100% sure you fully understand the problem. Thus my proposal to elaborate in case that you feel the need for it.

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#33
In reply to #32

Re: Thermally Pre-Stressing Bolts

05/07/2012 11:05 AM

Well I'm not much for cryptic conversations using bold typeface so it's entirely possible I don't understand what you mean. I'm not sure I "need" an explanation it but I admit you have me curious so please elaborate.

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#34
In reply to #33

Re: Thermally Pre-Stressing Bolts

05/08/2012 5:21 AM

I appreciate the acid comment but since you are not sure to "need" the explanation I shall not spend my time and take yours to give it. When you will truly be interested it will be a pleasure to do it, let me know.

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#35
In reply to #34

Re: Thermally Pre-Stressing Bolts

05/08/2012 10:29 AM

It wasn't an acid comment. It was sincere exactly as I wrote it. You are hinting at something which I clearly don't catch onto so to me that is being cryptic. That's not the way I roll. If you have something to say then say it. I was sincere when I said I was curious of what you had to say. Do I "need" to know what's inside your head? No.

It seems we do not understand each other. That's fine. No need to continue this subject. Let's move on.

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#31
In reply to #22

Re: Thermally Pre-Stressing Bolts

05/07/2012 9:18 AM

Matt, your looking at about .024in of stretch in a 1.5in x 9in tie rod (1.281 grip length for the nuts on each end) to achieve 60tons of tension. The stretch is linear at 5000 lb per .001in of stretch. Whether or not you should apply 60tons is something you need to verify with your engineer (boss). As I mentioned earlier, this is purely informative and you should correlate this .024in to hand calcs by a PE. In no way do I imply this is accurate information related to your entire system. It's theoretical based on a simple cylinder with uniform cross section and nothing more. Just for shits and giggles! See below.

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