Bolt Torque using PCBolts 1

[bandrsntch]

I have a problem involving determination of proper torque using 3/8-24UNF K-monel 100ksi YS bolts into a Monel 25K YS plate. It is generally desired that the externally threaded part will break before the external or internal threads slip. FED-STD-H28/2B accounts for this by ensuring adequate Length Engagement. In my case I have .44 inch which is more than the .391 required by H28/2B. When using PCBolts software program, if you let it calculate everything it comes up with a required torque of 14ft-lbs based on limiting the YS in the set end to 2/3 of YS(25ksi). If you plug in a required preload 4640lbs(.60 YS of the bolt) then it gives a torque of 22ft-lbs. I believe to latter is correct. Any thoughts?

 

[kstaylor]

bandrsntch,

What is PCBolts software? Who is the vendor/publisher of this software? Is this software commercially available or is it internally proprietary to your organization? I tried to do an internet search on PCBolts and got no results. Could ou share some information on this program?

 

[bandrsntch]

Excuse me for not checking this first. PCBolts is a Dos based program developed for Naval Sea Systems Command(NAVSEA PMS390T) and is dated 1995. It may be available on Navy computer systems, but is probably not well known commercially. In view of this, I will revise my question to ask what is the generally accepted value of preload for fasteners to ensure adequate clamping force. I have seen values given from 60% to 90% of bolt Yield stress. I assume the minimum length of thread engagement formulas per FED-STD-H28/2B will ensure the internal set end threads will not be stipped and allow full YS to be developed in the bolt before failure.

 

[diamondjim]

I am missing some information or do not fully
understand the problem. You are using 100ksi
bolts in 25ksi threaded material? How are you
tightening the bolts? I doubt you could ever
torgue the bolts to their yield level without
destroying the internal threads if the first
statement is true. I would back off and use
lower yield strenth bolts and take them to a
higher yield. Does the guideline give any
ratio that should not be exceeded between the
bolt and the internal threaded part?

 

[bandrsntch]

Bolts is K-monel, QQ-N-286E, (Cold Drawn and age hardened)UTS/YS=140/100 ksi
Set End is Monel, QQ-N-281D Amd 2, Class A Annealed) UTS/YS= 70/25 ksi
Length of Engagement is .44 inch
Per FED-STD-H28/2B:
"If failure of a fastening system using standard threaded fasteners should occur it is generally more economical that the externally threaded part will break rather than that either the external or internal thread will strip. In other words, the length of thread engagement should be sufficient to develop the full strength of the screw. Thus, the length of internal thread and the dimensions of this thread, particularly its minor diameter, should be such that, taking into account a possible difference in strength of material of the internal and external threads, the threaded portion of the externally threaded part will break before either the external or internal threads strip. Due to this situation, lengths of engagement formulas are derived from shear formulas with tensile stress area AS replaced by 2AS because the required area in shear is twice the tensile stress area in order to develop the full strength of the externally threaded part. This relationship is based upon experiments made by the National Bureau of Standards in 1929, in which it was found that for hot-rolled and cold-rolled steel, and brass screws and nuts, this factor varied from 1.7 to 2.0. The effect of combined stress is not taken into account in calculation of LE because the added shear load affects both tensile and shear stresses in approximately the same proportion."

 

[boo1]

Applied dynamic force
Many details of the applied forces are misssing:
Are they "dynamic in nature", what are the lower limits?
What is the Axial force? Shear?

SURFACE PRESSURE ANALYSIS
Limiting Surface Pressure for the Material = 25000.00 Lb/in²
Surface Pressure under the Nut Face Excessive preload loss
is anticipated to occur due to creep under the nut face/bolt
head causing excessive embedding. The use of flanged fasteners or hardened washers should be considered.

 

[boo1]

Applied dynamic force
Many details of the applied forces are misssing:
Are they "dynamic in nature", what are the lower limits?
What is the Axial force? Shear?

SURFACE PRESSURE ANALYSIS (I agree with KSTAYLOR above)
Limiting Surface Pressure for the plate material = 25,000 Lb/in² with a 100,000 Lb/in² fastener?
Surface Pressure under the nut face causes excessive preload loss is anticipated to occur due to creep under the nut face/bolt head causing excessive embedding. The use of flanged fasteners or hardened washers should be considered. Dia?

 

[bcd]

The correct torque depends heavily upon the thread lubrication. Your torques definately seem too low. PCBolts must allow you to enter a number for the torque coefficient. Important note: The torque coefficient is very different from the coefficient of friction.

Hydraulic shock procedures recommend bolts and studs be stressed to 2/3 their yield to improve their capacity to withstand both tension and shear. For your fastener, the axial load for 2/3 yield is 5853 lbs.

Assuming you will use molybedenum disulfide as the lubricant, I suggest a torque coefficient of 0.15. You should be able to find this is any machine design textbook. Fine threads reduce the torque coefficient by 6 to 10%, and it tends to increase with smaller fasteners. So just use the general value.

Based on this, I get a torque of 27 ft-lbs.

Your length of engagement is enough to develop the full strength of the fastener. Don't worry about the surface pressure. It's in compression.

 

[bandrsntch]

No nuts involved, this is a flange that is bolted to a threaded adapter plate. Torque coefficient provided by PCbolts is .148. If I understand this correct, one should torque bolts to 2/3 of their yield strength and provide sufficient thread length to ensure set end is not stripped out. The bolts are self-locking. This whole problem started because the customer wanted to use lock-tite instead of anti-sieze lubricant to reduce the stresses in the set end because of the effect of the torque coefficient. Length of engagement should do this, NOT using lock-tite(Torque Coeff .231).