Thread Engagement
Thread Engagement
(OP)
I have a problem and need some guidance.
I have a bolt that is stretched hydraulically to 1700 kn. It's an M76x6 thread. I have always used 0.8 d to calculate the engagement on the nut/bolt.
Nut width = 0.8 x 76 = 60.80 = 61mm
Material Yield 680 N/mm2
Material US 850 N/mmm
I ran this through a thread strip calculator and it came out with a minimum engagement of 45 mm. This seems a little low. What I need is a nut width of 56mmy max.
Is this possible?
Thanks
I have a bolt that is stretched hydraulically to 1700 kn. It's an M76x6 thread. I have always used 0.8 d to calculate the engagement on the nut/bolt.
Nut width = 0.8 x 76 = 60.80 = 61mm
Material Yield 680 N/mm2
Material US 850 N/mmm
I ran this through a thread strip calculator and it came out with a minimum engagement of 45 mm. This seems a little low. What I need is a nut width of 56mmy max.
Is this possible?
Thanks





RE: Thread Engagement
can't know what your thread strip calculator does, somewhere inside
But, as you are on metric ground, I propose you base your considerations on german standard VDI 2230 edition 2014; chapter 5.5.5 Length of Engagement. This should cover yuor needs.
Regards
RE: Thread Engagement
If the nut and bolt are of the same material then 45mm might well be correct however you haven't told us what the bolt material is or what material the nuts clamping down onto.
If the material your clamping down on does have a lower yield than the nut material then the 1700kN force might not be achieved due to embedding.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
RE: Thread Engagement
The material of bolt and but are the same. The flange is of similar material.
RE: Thread Engagement
RE: Thread Engagement
But, most likely, they calcuate (approximate) stresses across the thread roots against tear-out forces caused by elongation. And, in actual fact, almost every nut is over-sized to resist tear-out (so the joint doesn't fail) but also to yield before the stud yields - because it is usually easier to replace a nut than a stud if failure occurs. And certainly much, much cheaper. SO, most nuts have enough thread to resist pull-out in the first 2/3 of their length.
So, the "real" nuts are thicker (taller) than the theoretical nuts due to manufacturer's choice of total yield (safety) margins, and tolerances in the standard nut sizes against weak material and manuf. tolerances.
RE: Thread Engagement
If you consider the safety related background of your application (which is hidden to us): Could you live with a thread strip accident?
R.
RE: Thread Engagement
The application is a rotating coupling running about 90 rpm.
The problem is that there is an obstruction stopping the nut going on the bolt. We cannot reduce the length of thread protruding from the bolt.
We use normally a ratio of 0.8 x thread diameter for engagement.
We have tried this formula in the past to give us guidance.
Fmax,nut = τ · Ashear,nut · C1 · C3
where
Fmax,nut is the maximum force the nut can withstand
τ is the nut material shear strength - 0.60 x 850 N/mm2 = 510 N/mm2
Ashear,nut is the nut shear area
Ashear,nut = (π d meff)/P · [P/2 + (d - D2)/√3]
π = 3.141 592 654
d is the thread major diameter - 76
P is the thread pitch -6
meff is the thread engagement length - 56
D2 is the nut pitch diameter - 72.478
C1 is a nut dilation factor - you can use 0.91
C3 is a nut thread bending factor - you can use 0.897
Max. Load Approx + 4666 Kn -> This seems rather excessive based on a thread length of 56mmm, personally I would of thought it would of been less.
RE: Thread Engagement
RE: Thread Engagement
If the situation is as per att. sketch, then:
- the bolted connection would not be securized against loosening (refer also CoryPads post)lest you prestress it to ~ 90% YS
- if it's the sectional area of the bolt you need for transmission of torque, then why not loose on the thread diameter and add a sufficiently strong washer?
- there should be several bolts on the circumference, any chance to increase their number & loose on the diameter?
RE: Thread Engagement
RE: Thread Engagement
However #1, is there perhaps a way out, if looking at the issue from a different angle? If this solution M76 + hydraulic stretching doesn't work out? At so low a tensioning torque, why not deviate to another tensioning principle? Or changing to a tapped hole, and so on.
However #2, what about securizing the connection? If this is a rigid coupling, there will be a bending moment i. e. an oscillation of the axial forces on the bolt. So loosening would imo present an issue.
Regards
R.