load on end of bolt 3

[trebor]

I would like to find a formula to figure the amount of lifting force in lbs. when you exert a given amount of torque on a bolt. My most recent project involves (6) 5/8"-11 bolts
on a 8" bolt circle, I would like to develop a total load of about 60.000 lbs. Thanks Bob

 

[GregLocock]

This is not an easy calculation to get right. If you think of a thread as an inclined plane then it will give you some idea of the relationship between torque and bolt tension, but the reality is that much of the torque is 'wasted' in friction.

Shigley discusses this in reasonable depth from a practical point of view. Cheers

Greg Locock

 

[diamondjim]

My chart shows that one bolt at 75 percent
of the proof load is 23500 pounds, so I would
assume that 6 times this would be the result.
Am I missing something? This is for grade 8 bolts.

 

[butelja]

The standard way to calculate this is covered in nearly every machine design handbook. The formula that relates torque and axial force is:

T = K*D*F

Where:
T = Applied torque
K = Dimensionless factor
D = Nominal bolt diameter
F = Axial force

The tricky part is that you are probably lucky to come within +/- 50% when uncertainties in applied torque and K value are considered. Check Machinery's Handbook for some of the various K values. K is a function of the thread type (UNC or UNF), bolt surface coating, and lubrication. Lubricated bolts will be more consistent than dry installations, because dry friction coefficients are more variable.

 

[diamondjim]

If you want to determine the results of your
calculations, you might just want to take the
bolts to yield and record the results of at least
6 bolts. Butelja is right in that there is great
variance but not of the values that he is stating.
If the value was plus or minus 50 percent, the bolt
industry would soon be out of business. The shorter
the bolt the more likely the values are close.
The longer the bolt, the easier it is to simply have
much of the torque value going toward twisting of
the bolt itself. Almost all bolts should be lubed
as the results are more consistant. It also depends
on the type of joint. I think tightening the nuts
gives better results than torquing the heads of
bolts. Would you trust the lugs on your car wheels
if the variance was plus or minus 50 percent? Like
the internet, much of what is published in text books
are often only opinions or best guess theories. Hooray
for the guys in the shop who will test these theories.
If your calulations were off by 50 percent, imagine the
extra waste caused by overdesigning. Test all theories
to failure. Bolts vary from manufacturer to manufacturer
as well as having pseudo grade bolts being sold. 5/8
bolts are cheap and testing 20 to failure should not
be cost prohibitive and give you greater certainty in
your calculations and theory.

 

[butelja]

OK, maybe +/- 50% was a little excessive. I'll back off to +/- 35% based upon accurate torque control and experimental testing to determine the proper K factor. There really are variations that wide if torque control is the method used to tension the bolt.

For critical joints where a (roughly) +/- 35% cannot be tolerated, methods other than torque control are used. Note that many of the methods listed below are relevant to large bolts such as would be encountered in the power generation or chemical industries. Other more accurate methods include:

1.) Measuring the actual bolt elongation via ultrasonic methods to determine preload.
2.) Hydraulic bolt tensioners that use a hydraulic cylinder to apply the preload.
3.) Large bolts are sometimes heated in a controlled manner, then the nuts are run down snug, allowing the thermal expansion to determine the bolt preload.

 

[diamondjim]

I do agree with Butelja. A simple answer
is not appropriate. The fact that you will
be using only 6 bolts, I would assume that
you are using a sophisticated way of measuring
the tightening these bolts. I love the way
technology is being used to reduce the costs
and force us to be more realistic about research
and actually use the results in a practical way.

 

[kelv]

can of worms by the sound of it,

maybe this will help?

http://www.boltscience.com

just as an interesting point, automotive engineers never use lubricant on bolts. they are used straight out of the box (either zinc plate and passivate or phosphated).
Why?
ask the other guys - they seem very keen....

more later...

 

[gbent]

If both sides of the bolts can be made accessable, measure the elongation. Prep the bolts by grinding the ends of the bolts parallel. If all are make the same length, you don't have to record which bolt is where. Use the joint length as the stressed length. Stress is Young's modulus times strain, and load is stress divided by area. If you want to investigate friction, you can try various lubricants, and record the torque to achieve the required stretch. If you do, please post the results back here.

 

[CoryPad]

kelv,

You are incorrect regarding automotive fasteners - a majority of these are lubricated. The lubricant is integrated within the fastener coating, which allows them to be used straight from the box. The latest coatings used are either electroplated zinc + topcoat or multilayer phosphate/zinc/organic coatings, both of which have a lubricant (e.g. PTFE, polytetrafluoroethylene) added to the final layer. Having integrated lubricant allows for low lot-to-lot variation of friction coefficient, plus it eliminates any associated assembly line time/cost/errors. The benefit of fastener lubrication is developing maximum preload from a given input torque.

 

[olympic]

For information regarding stresses created in bolts see the text"Machine Design" by Robert L. Norton. There are items listed that may taken for granted. ie. A bolts cross sectional area when sliced thru a thread is different than assuming a mean diameter of the pitch circle.
Also information is given regarding the k of material that is surrounding the fastener w.r.t. the fastener's k-value.

Good reference

 

[diamondjim]

Kelv, my comment to the auto guys is obviously they
do not have to take them off. I think a little
lube would help. Lets hear it from the lube guys.
Have you ever tried to unloosen a bolt that the tire
guys have used air guns to tighten? It is an experience.
I guess we are starting another topic.

 

[butelja]

Diamondjim,

In our litigous society, it is probably a lot safer from a liability standpoint of the manufacturer to favor joints that will not vibrate loose rather than make it easy for mechanics to work on some time down the road. It seems that servicibility is way down on their list compared to manufacturability and reduced cost/weight. It's probably only a matter of time until they weld the hood shut and call it a disposable car.