Bolt Bending

Why not drill and tap your hole at the angle you need but be sure to check the side shear strenght of the bolts you intend to use.

I agree that the bolts should be examined for yield strength, in order to determine whether you have the correct ones for the application. Without seeing the parts, I must say that I prefer the design scheme you first described; where the surface is machined flat and the threaded holes are drilled and tapped so as to be perpendicular to the flat plane. You do not mention the type of loading that happens to this assembly; is it purely a side shear, or a combination of a side shear with a vertical (regarding the bolt axis) tensioning that bends the bolts? I suspect the latter. Is there a marked deformation of the mounting plates when the bolts fail?

Is the M12 fastener large enough for the application, and if not, do you have enough material to support a larger size such as the M14? What is the thickness of the mating members, and what is the span between the holes? All of this has much to do with the selection and positioning of the proper fasteners.

I would hesitate to use a through-bolted design that incorporates additional hardware, as where you had "play" in one (1) pair of holes, you will now have "play" in two (2) pairs of holes. And, add the angularity that may be present in nuts of commercial supply, and you might be worse off to use a through-hole plan here. Without knowing more-I really cannot say. Can you provide a sketch with some basic dimensions?

Best Regards,

Ing. Robert Forbus

VG suggestion.

Hello "somarp",

Please consider that the through hole now extends the engagement position by the plate thickness, and seems contradictory as to what you are trying to achieve. By removing the thread and adding a through hole you are also adding a clearance, and a chance for the bolt to "slip" causing more movement allowance compared to one clearance hole. Changing to a through hole may make sense in your thread engagement in the plate was less than 1.5 X the bolt diameter, assuming a correctly drilled tap drill size hole, a good tap drill chart will give you the percentage, or you can calculate it.

Generally when two components are bolted and an angled surface is encountered a "spot face" is used. This spot face essentially is a counter bore (circular machined surface) on the angled surface to assure that the bolt head or nut is parallel to the contact faces. Adding any type of deflection stress to the bolt will decrease its strength.

As for a redesign, likely it is best to oversize your bolts or go with a higher grade bolt, use parallel clamping surfaces and bolt contact points, and don`t forget to look at the root cause of the failure, hence if you strengthen this assembly, and there is a problem causing your bolts to break which originates somewhere else then the next weakest link will let go or give. This may be a more expensive component of the assembly then your bolts!

Mirco.

I was coming here to say this. GA.

If the angle is so critical then go with PC's suggestion, you can compensate for any surface angle.

Hi somarp:

Regardless of the type of hole you use (through or tapped) ensure that the bolts are torqued the correct amount.

If the bolts are tightened too much you will exceed the bolt material yield strength and the bolts will fail.

As odd as this sounds a properly engineered joint will withstand a greater load than the bolts alone, therefore if the bolts are not tight enough the load could exceed the bolt material yield strength and the bolts will fail.

This is due in part to the additional friction imposed by a properly preloaded bolted joint on the mating surfaces. The friction resists the slip in the joint.

I would recommend that you ensure that the bolts are tightened to the original specifications before redesigning the equipment.

Any redesign you do will shift the culpability from the original designer to you.

mechtech

If you go with through bolting, both surfaces of the joint MUST be parallel. If a tapped hole, the bolt head should be on the flat surface. Check bolt strength, but be aware that a stronger bolt could hold so well, that the welded joint could fail. This latter can be caused by high shock loads which causes rapid acceleration.

I am not smart enough to give a technical explanation of why it works but over 70% of the bolt failures I've experienced over the last 40 years were prevented from reoccurring by substituting hardened steel washers for the plain ones we were using.

This fix was verified by a materials engineer who went on to tell me that it's a recognized cure and that such a fix can often double the amount of torque that can be applied.

Hope this helps.

L. J.

Without pictures it is difficult to know what is causing the failure, but, I'm guessing shear load. Plate B slips and shears off the bolts.

Adding a single solid pin that is either a taper fit or press fit is a very cost effective method of preventing radial slippage.

I also liked the hardened washer suggestion very much, VERY cost effective. The improved washer will allow better torquing of the bolts and better distribute the clamping load over a larger area.

Improving the bolts is also a possibility. If you are already using a good grade then check for proper torquing during assembly. Under torquing causes slippage and shear.

If vibrational loading is high in you machine then thread locking either through Loc-tite or lock washer is indicated.

Sincerely,

Mr. Gee