Hello all,

I am in need of a means to calculate thread strength.

In the assembly of hydraulic actuators, a thread will be machined into the bore of the tube, and onto the mating component. Although we have not suffered any failures, our thread sizes and lengths are always worked, arbitrarily, from the fact that they have worked before. It would be far better to have a definite figure of what the breaking strength of the thread will be.

In looking it up, it appears hard to find a definitive answer. Likewise, to me, it is unclear whether the failure mode of the thread would be under tensile/compressive load, or shear. Either way, I am unsure of quite how the area would be determined.

I would be most grateful if anyone could help me find an answer.

Parents
• The first thing to realise is that it depends on the material, the thread form, and possibly the tolerance of the thread cut.
For example, cheap mild steel may be taken as having an ultimate tensile strength of 25 tons per square inch, but High tensile bolts are from an alloy that manages about 3 times this.
In small fasteners a lot of the stress in the core comes from just torquing against the thread - in fact any metric thread (i.,e 60 degree iso triangle form)under about half an inch / 12mm diameter can be tightened to tensile failure - larger sizes can be tightened until the thread strips out but the core remains instead....

So do not over-tighten very small bolts, as that eats into the tensile strength available for load bearing. Really these are only ever fully torqued  for clamping parts together where the load is directed to shear the joint, and the purpose of the nut and bolt is only to force the mating surfaces together really firmly, and the bulk of the joint rigidity comes from the friction that this clamping action causes.

So either the force is set by the area of the core of the bolt and the UTS in the normal snap a rod way force - UTS * area, or by the tearing off the thrianges of thread - in which case the area is the base of the thread area unwound - bit that is in shear, not tensile failure, so about 4-5 times higher .

A loose fitting thread (one where the peaks of the nut and bolt threads do not meet at the 'middles of the mountains' as it were, rather only the tips overlap. have a correspondingly reduced thread shear area, and will shear strip the threads  early.)

For anything remotely critical, I would recommend a test on bolt makers samples,

Also note that in things that really matter it is important to allow for the fact that the load may not share equally among all fixings . To be avoided at all cost is a situation where one fails and then another is overloaded and fails - that can lead to catastrophic chains of failure. Such systems have very specific  tightening settings and a sequence that must be followed.

I may not have the right mental picture of what you are doing, so not all of this may apply.

M,

• The first thing to realise is that it depends on the material, the thread form, and possibly the tolerance of the thread cut.
For example, cheap mild steel may be taken as having an ultimate tensile strength of 25 tons per square inch, but High tensile bolts are from an alloy that manages about 3 times this.
In small fasteners a lot of the stress in the core comes from just torquing against the thread - in fact any metric thread (i.,e 60 degree iso triangle form)under about half an inch / 12mm diameter can be tightened to tensile failure - larger sizes can be tightened until the thread strips out but the core remains instead....

So do not over-tighten very small bolts, as that eats into the tensile strength available for load bearing. Really these are only ever fully torqued  for clamping parts together where the load is directed to shear the joint, and the purpose of the nut and bolt is only to force the mating surfaces together really firmly, and the bulk of the joint rigidity comes from the friction that this clamping action causes.

So either the force is set by the area of the core of the bolt and the UTS in the normal snap a rod way force - UTS * area, or by the tearing off the thrianges of thread - in which case the area is the base of the thread area unwound - bit that is in shear, not tensile failure, so about 4-5 times higher .

A loose fitting thread (one where the peaks of the nut and bolt threads do not meet at the 'middles of the mountains' as it were, rather only the tips overlap. have a correspondingly reduced thread shear area, and will shear strip the threads  early.)

For anything remotely critical, I would recommend a test on bolt makers samples,

Also note that in things that really matter it is important to allow for the fact that the load may not share equally among all fixings . To be avoided at all cost is a situation where one fails and then another is overloaded and fails - that can lead to catastrophic chains of failure. Such systems have very specific  tightening settings and a sequence that must be followed.

I may not have the right mental picture of what you are doing, so not all of this may apply.

M,

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