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Bolted Connection Question 4

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rustyhooks

Mechanical
Mar 27, 2007
8
I am connecting two halves of a valve together. I have a 7/16-14 Grade 5 steel screw that is inserted into a clearance hole in a cast iron flange and then threaded into the gray cast iron flange on the other valve half. There are 8 of these in the bolt circle. The maximum load on each fastener when pressurized is 290 lbf. I know how to calculate the torque spec when the male thread and female thread are of equal strength (same material). However, I am not sure how to calculate the proper torque required when the female thread (cast iron) is weaker than the screw (steel). I would think the female thread would fail before the male thread in this case. Also, what is (if any) the difference between thread shear stress area and tensile stress area of a thread?
 
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Even if the internally threaded component has lower strength, it is desirable to have the externally threaded fastener fail in tension before stripping the internal threads. To do this, you must provide sufficient thread engagement length. Here is a basic calculation:

thread725-35222

The shear stress area of a thread is the area created if you strip the threads off (either external threads off a bolt's core cylinder, or internal threads out of a hole). The tensile stress are exists only for externally threaded components, and it is the area created when you fracture the component perpendicularly to the fastener's axis.

Regards,

Cory

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290 lbs is nothing for a 7/16 fastener to exert or withstand. At a mere 5 lb-ft torque it is exerting about 700 lbs of clamp force. If the bolt engagement is at least 1.5 diameters in cast iron then I think you'll find the fastener may be the weak link.

Is this a metal-to-metal joint or is gasket compression involved? Is this a new product? (too thin)Flange thickness has a powerful effect on sealing capabilities and flange stress.
 
Thanks to all for taking time to answer my question.

This is an existing valve that has been in service for 40+ years. Our own assembly procedures state that 20 ft-lbs of torque be applied to this fastener. This valve is now in a new application where vibration is a concern. I don't trust lockwashers. I believe a properly tightened screw is the way to go. Now I am questioning the 20 ft-lb spec. Yes, it is a gasketed joint. The gasket is 0.030" thick. The length of thread engagement is 0.470"

My Fastenal catalog suggested a torque spec of 50 ft-lbs for an unlubricated (.2 factor) 7/16-14 Grade 5 fastener. However, I think is based on the assumption that the female thread is of the same material grade. I became concerned when my cast iron female thread material was significantly weaker than the screw thread. I thought I might should back off that 50 ft-lbs. Maybe this is where the 20 ft-lbs came from years ago.

If I understand this correctly now, I can tighten this 7/16-14 fastener to 50 ft-lbs. This would be 75% of proof load for the fastener. In doing this, my cast iron female thread would still be OK from a stress point of view (due to the torque, not the 290 lbf) and I should not be worried about the female threads failing. Is this correct?
 
I think you are correct to be concerned about the cast iron female thread. Our standard for torquing most Grade 8 bolts or B7 studs is based on 60,000 psi stress in the fastener. However, if the bolt or stud is threaded into cast iron, we limit the stress to 30,000 psi regardless of the engagement length. This is common with many of our reciprocating compressors. The compressor manufacturer recommends that no fasteners threaded into cast iron are stressed above 30,000 psi.

With just over 1 bolt diameter engaged in cast iron, I would not torque to 75 percent of proof strength for the bolt.

Johnny Pellin
 
I would consider dropping to a gr. 2 bolt to get some stretch to prevent loosening without the worry of stripping the cast iron threads. You didn't mention how long the bolt is but hopefully it is longer than 1". You didn't mention the temperature range - but if it is extreme you should take heat expansion into account.

ISZ
 
If you are not sure about the torque and vibration is an issue I would go with some type of anti-rotation device. I completely agree with you that a lockwasher is a poor locking mechanism and I only use them on non-critical joints if at all.

For an anti-rotation device I have used a company called Stage-8 with great success. There are many out there that make many different kinds of very inexpensive devices or you can make your own. They are very straight forward and easy to use often times with no design modifications needed to older equipment.
 
Hi,
the calculations of thread stripping are based upon the assumption of how many threads do carry the load, which is not the same as considering the threaded length and, as JJPellin says, leads sometimes to limit the preload to a certain value regardless of the threaded length (because what you consider is that the first n threads are carrying the load, the remaining ones not). The problem is to estimate n. I'd suggest to do a verification "in reverse": given the max load on the threads, determine how many "active" threads are needed in order not to yield the female "weak" material, with some safety factor of course. The number n you will find will give you an idea if the load is bearable or absurd (or any graduation in between...)
Just an idea...

Regards
 
Hi rustyhooks

I used the formula's in the machinery's handbook relating to
thread engagement etc.
I calculate using a gray cast iron (grade 20) that with a thread engagement of 0.47" and a friction coeff of 0.2 that
the internal thread shear load will be 6857lbf for a 50ft/lb
torque setting.
The maximum the gray cast iron can withstand is just over
16000lbf to strip the threads so you have a safety factor of just over 2.
However I don't know whether your gasket material can take the extra load.

regards

desertfox
 
Go Desertfox,
I think you hit on the crux of this problem
in addressing the gasket concern. I assume
the tension in the screws is to ensure that
the gasket does its job in sealing the two
parts. Once that is ensured, I do not see
any advantage of greater force in the bolts.
If the gasket deteriorates because of the extra
force, the joint has failed. Thanks for that
reflection.
 
Hi dimjim

Thanks for your kind comment.
Yes your spot on about the pre-load to seal providing you have taken all loads into account ie vibration etc + a safety factor then there is no real need for extra force.

regards

desertfox
 
Thats why I was suggesting a softer bolt, you could get the proper pre-load with a lower clamp load. ISZ
 
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