Bolted Connection Question
Bolted Connection Question
(OP)
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?





RE: Bolted Connection Question
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
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Bolted Connection Question
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.
RE: Bolted Connection 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?
RE: Bolted Connection Question
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
RE: Bolted Connection Question
ISZ
RE: Bolted Connection Question
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.
RE: Bolted Connection Question
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
RE: Bolted Connection Question
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
RE: Bolted Connection Question
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.
RE: Bolted Connection Question
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
RE: Bolted Connection Question