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Anonymous Poster

Tampering with the Bolt Head

10/30/2010 8:33 AM

How important is the height of a bolt head to the integrity of the bolted connection?

I would've thought the failure mode will be shear so a bolt head doesn't contribute to failure?

How much can you cut it back before it becomes a concern?

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#1

Re: Tampering with the Bolt Head

10/30/2010 8:58 AM

Notwithstanding the required reaction area if a bolt is to be torqued, the bolt head is in also in tension. This, and the nut on the other side (or the threads if it's in a blind hole) "clamps" a joint so that internal (or external) axial loads can't cause separation. Thus, a properly designed head is crucial to the integrity of a bolted connection.

Don't mess with it !

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#4
In reply to #1

Re: Tampering with the Bolt Head

10/30/2010 5:37 PM

I did but I knew my life didn't depend on it

Any coating disturbed will bite the OP.

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#2

Re: Tampering with the Bolt Head

10/30/2010 12:28 PM

What BoltIntegrity said plus, different bolts have different purposes, but if it is to be torqued the flats must be enough to grip and turn without rounding. Some bolts work in tension and the shank could pull out leaving a hole in the head if it is too thin.

Trust me, if it could have been made thinner it would have, long before now.

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#3

Re: Tampering with the Bolt Head

10/30/2010 1:40 PM

And, if you used an angle grinder to cut it back you could also significantly reduce the strength of the head and hasten failure all the more from the heat generated by cutting.

Unless the bolt is over sized for the job, don't do it.

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Anonymous Poster
#5
In reply to #3

Re: Tampering with the Bolt Head

10/30/2010 8:22 PM

Just reevaluated the scenario. The "tampering" I need to do is to cur through the bolt head to avoid interference.

I'd loose about 15% volume on a Gr8.8 M12 bolt head. It's only holding a handrail.

Is there a back of hand calculation that I can do to verify integrity?

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#8
In reply to #5

Re: Tampering with the Bolt Head

10/31/2010 4:36 AM

I you increase the number of bolts by at least 25%, you should be OK.

More would be better of course!!

That will offset the 15% loss of metal quite easily.....

If you go here you can read about the differences between the different qualities of bolts and nuts:-

http://en.wikipedia.org/wiki/Screw

Here is an example:-

The property classes most often used are 5.8, 8.8, and 10.9. The number before the point is the tensile ultimate strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.

IMPORTANT!!!! And the absence of marking/number on the head of bolts (usually the side of nuts) indicates a lower grade bolt/nut with low strength.

8.8 is the usual quality for road vehicles and are quite easily found everywhere, above this quality (10.9) they tend to be only available from specialist outlets.....

So just look for a better quality as well.....that will recover and lost strength fully. Machine or saw the excess off....

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#10
In reply to #5

Re: Tampering with the Bolt Head

10/31/2010 4:53 AM

Would replacing the existing bolt with one of these solve your problem?
Socket Bolt - Socket Cap Screws. Metric Low Head 12.9 Grade

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#6

Re: Tampering with the Bolt Head

10/30/2010 11:45 PM

Depends! Some bolts are designed to be loaded in shear, in which case the head shouldn't matter. But others are loaded in tension where the head is very important. You said it was on a hand rail, so I'll assume it is in tension. If it is fully loaded in tension, you shouldn't mess with the head.

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#7

Re: Tampering with the Bolt Head

10/31/2010 4:21 AM

this isn't very hard to see - if you cut all the bolt head off it doesn't work anymore - any amount that you cut off the bolt head reduces its ability to hold tension, without giving a little and then a little more. very little can be removed from a bolt head before the degrading of its rated capacity has to take place.

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#9

Re: Tampering with the Bolt Head

10/31/2010 4:52 AM

ONLY a handrail?

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Anonymous Poster
#11
In reply to #9

Re: Tampering with the Bolt Head

10/31/2010 8:33 AM

My thoughts, exactly.

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Anonymous Poster
#17
In reply to #11

Re: Tampering with the Bolt Head

11/01/2010 7:11 AM

I know, only a handrail and we used high tensile bolts.

Try telling that to the quality police!

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Anonymous Poster
#18
In reply to #9

Re: Tampering with the Bolt Head

11/01/2010 1:52 PM

JIMRAT:

The Instant I saw the comment:"ONLY A HANDRAIL" I thought of two different people using the Handrail. (One) a Prankster 5 yr old, testing the Rail Strength, by heaving against it. (Two) The Other being AN OVERWEIGHT GENTLEMAN, Tripping and GRABBING THE RAIL IN MID-FALLING POSITION TO AVOID A FULL STAIRWAY TUMBLE.

(Did this get anyones Attention.)

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#19
In reply to #18

Re: Tampering with the Bolt Head

11/01/2010 5:56 PM

Yep - but the fact the OP is 'modifying' obviously means he/she has not thought through to the court case.

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#12

Re: Tampering with the Bolt Head

10/31/2010 9:22 AM

Usually you can replace a bolt with a machine screw, which would give you more options. You really need a bolt if torque is essential, most construction applications are not quite that critical. Don't mess with a bolt head though. As one of the previous posters pointed out, if they could have got away with less metal, they would have got away with less metal.

Would there be a way to countersink the workpiece so that a countersunk machine screw would fit flush? (I know, not a really good option, but maybe worth looking at if the brackets are nice and solid, and the chance of injury is high if you don't lower the height of that screw. I am imagining some skateboarder rocketing down a rail like the Silver Surfer and suddenly discovering a row of bolt heads under his board!) Or you can install a countersunk washer to turn a projecting screw head into a nice smooth surface that won't bash the hands. This is a good option for sheet goods like airplane skins or auto bodies, no reason it can't work here for this application.

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#13

Re: Tampering with the Bolt Head

10/31/2010 11:28 AM

Carriage bolt (coach bolt) as discribed in wiki, has a domed or countersunk head, and the shank is topped by a short square section under the head. By replacing to this type could give more clearance and probably you could avoid interference. But they have a square section grips into the part being fixed. How could you accomudate this?

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#14
In reply to #13

Re: Tampering with the Bolt Head

10/31/2010 1:11 PM

They have to be installed so that they cannot turn and ONLY used with a nut that can be!!!!

This cuts them out for most applications....

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#15
In reply to #13

Re: Tampering with the Bolt Head

10/31/2010 2:04 PM

Select a bolt whose square is slightly over size to the handrail flange or drill a hole slightly undersized to the the square shoulder. The slight interference will hold the bolt enough to tighten it from the nut side.

Bob

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#16

Re: Tampering with the Bolt Head

10/31/2010 4:45 PM

Weld it.

Hypothetically, even if a future failure had absolutely nothing to do with the modified bolt, you'd better believe that an angry mob, deaf to any attempts at reason, would be on its way marching up the hill wielding recently-sharpened pitch forks and waving blazing torches, looking to string-up the guy who butchered the head .

Save yourself the hassle and lay a bead onto whatever you're trying to secure. If necessary, you can always grind it off.

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#20

Re: Tampering with the Bolt Head

11/02/2010 11:25 AM

I looked at the answers and I dare ask a question:

have you thought how the hexagonal head height was determined ?

Have you ever seen a force flow path through the head ?

Why for instance, as a comment very well suggested, are there bolts with shorter heads ?

The comments lead to the assumption this was not the case.

Now to make a long story short:

the height was determined in order to transmit the torque from the spanner without a plastic deformation of the head edge.

If the torque is limited then the head can be shorter.

The integrity of the assembly - if the lower preload will not compromise the function - is not reduced for different reasons, 1st the shear strain due to the preload path between shaft and support face is not uniform, it has its highest value near to the shoulder and decreases toward the top. There where the OP wants to act the strain is low and the transmitted froce only a small part of the preload.

Stress distribution for a standard head. Load for similitude was defined as 100N/mm² of shaft area.

Head reduced with about 25% in height all other dimensions and loads same as above.

It is clear that the maximal stress which determine the assembly behaviour is at the fillet. It is increased by ≈ 10% for a 25% height reduction!

If one has a look at other head forms as torx for instance due to the higher torque transfer capacity the head can be shorter.

I think it is always better to look at details as well not only accept a "rule" without understanding the base of choice.

Sorry to participate so late but I was out of home and had no access to the wwweb.

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#21

Re: Tampering with the Bolt Head

11/02/2010 12:15 PM

I thought that an even better image of material use is given by the von Mises stress display. here are the 2 for normal and reduced head:

Pictures show how material is stressed in the bolt head.

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#22
In reply to #21

Re: Tampering with the Bolt Head

11/04/2010 7:25 AM

Thanks for this information, it shows me i'm not dead as i am still learning something.

Jim

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