Pull-Out Force of Threaded Shaft in a Tube

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trying to mount an A-arm to a baja car with heim joints. i'll thread the heim into the A-arm and run a bolt through the heim to mount to the chassis. I'm concerned about when the vehicle lands from a jump, and the threads stripping out. The arms are made of 4130 steel and the heims are made of general steel.

In fact the maximal load is determined by the capability of system to absorm enegy in case of such dynamic situations.

I would consider 2 critical sections in the joint since it is the weakest element:

- the cross section of the first engaged thread, which is the most loaded one which due to notch factor at thread root is endangered

- the internal stripping of the joint thread at the internal diameter of the tube thread since the tube is a lot stronger than the joint.

If I know the dimensions and the materials I can give you an order of magnitude about the highest portant load.

The first mentioned point is less dangerous than on a normal bolt since the thread is in a tube and this kind of assembly reduces the notch factor.

There is of course a 3rd critical element: the bolt you use to fasten on the chassis. Its loading depends on the freedom of the movement.

I'm tryin to design it assuming the joint could absorb 1200lb of force, and trying to back out the dimensions of the needed hardware.

Hi Guest

This is a general observation assuming the failure will be a tensile one:

• Provided the threads are a good commercial fit, then an engagement of 4 - 5 threads will mean the bolt will break, not the thread. To calculate this, the area is taken at the base of the bolt thread and multiplied by a safe working stress for the material and this answer divided by 4 - 5 to allow for the pre-load when it is tightened and the shock loading.
• As the bolt is tightened and stretches then all the engaged threads have face to face contact.
• Good thread adhesive is applied to well degreased threads.
• I'm not a great fan of washers unless they are needed to spread the load.
• You should become a member!

Tony

Good morning Guest. I have always been aware of the fact that threads could be stripped out, but until I saw it happen, it just didn't register that it could happen so easily. I had a 1/2"-13 UNC bolt strip the threads out of its nut. The steel bracket holding the bolt/nut assembly was bent way out of shape; so the bolt/nut assembly put up quite a fight, but in the end failed. If its critical, why not take your assemlby to a test lab, where they could put it in a tensile machine. Then you'd know more precisely at what point it would fail. But then, you'd probably want to decrease that figure by a certain percentage, because your application includes a dynamic shock element, which won't be present in the tensile machine.

In the last few years, manufacturing of fasteners of all kinds has become 'global.' While this might have lowered costs of manufacturing, the 'savings' has not been evident at retail. What HAS happened is marked decrease in reliability and quality. At this point, I DO NOT rely on any non-certified fastener for any critical load. I will use USA manufactured Grade 8 or MIL-spec fasteners , and only with certificates for life critical applications. I have seen way too many 'big box' store hardware fail unexpectedly. Over-sizing is no guarantee of increased safety either. I have experienced bolt head twist off at very low torques on 1" hex head bolt! I have experienced steel chain failure at less than 25% of rated load capacity.

I now buy almost all threaded fasteners from Graingers or other industrial suppliers and Aircraft PMA approved sources. If you are building a strength critical assembly, be certain a 'high strength' component really IS.

Good Answer on being careful on where you buy your fasteners from. I have seen countless occasions where non-domestic, uncertified bolts fail prematurely.

As a rule, the threads on any bolt should engage to a depth of at least 1.5x the bolt diameter. This assures that the bolt will fail in tension before the threads fail in either member.

Good answers by previous posters. The percent of thread is an important factor in the analysis as it affects engagement. We have data that suggests that as long as you have at least 65% of full thread, accurately made, the joint will have ample strength when used with studs of the same type of material.

Also, a "formed" thread is somewhat stronger than a "cut thread" all else being equal.

I'm thinking that an acme or buttress thread form might also be worth considering. have you read : http://tinyurl.com/6fd7xq
milo