Bolt Torque Coefficients

[DustinMechEng]

Hi All-

Does anyone know of a good resource for bolt torque coefficients? I need to determine the axial preload, given the installation torque, on a copper fitting being screwed into an acetal housing.

Thanks for the help!

 

[MintJulep]

What answer do you want?

For normal materials, it will be possible to find a reference with pretty much whatever value you want.

You really should test your materials and your application.

For uncommon materials you probably have no choice but to test it yourself.

 

[mk2000]

DustinMechEng

Have you tried machineries handbook or Matweb

 

[Cockroach]

There have been lots of threads on this subject as of late. Scroll back a few weeks or Goggle the topic under "bolt torque values".

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[IceStationZebra]

another way to check this is to strain gage the inside of the fitting, then install it and directly measure the loading.
ISZ

 

[CoryPad]

There are so many variables involved, it would be difficult to identify a single value. For example, what is the thread fit like? Rough machined copper threads going into a tight plastic fitting? Are you using any thread tape or dope? Using a simple equation like T = k d F (which is not necessarily a good idea), I estimate that k could vary from 0.1 to 0.4.

Lastly, fluid carrying joints rarely require preload. Rather, they need to be leak resistant, hence the use of thread tape, flares, gaskets, etc. Usually hand tight plus 90 degrees rotation is good. If the joint isn't sealed, advance another 90 degrees. Use some extra parts in the beginning to learn the amount of angle that begins deformation/fracture, and use a value below this (using a safe margin).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mickeymouse]

Book: "Fastener Standards"
Section N, Technical Data
Industrial Fasteners Institute
1505 East Ohio Building
Cleveland, Ohio 44114

From Fastener Standards as follows:
Equation:
T=KDW
Where:
T=torque (lb-in)
K=Torque Cofficient (not to be confused with coefficent of friction)
D=Nominal bolt size (in)
W=Bolt Tension (lb)

Arbitrary values for K are often assigned in non-critical applications as follows:
K=0.20 for nonplated steel fastners
K=0.15 for plated fasteners

In applications where additional lubricants, such as greases, oils, and waxes, are applied to the fasteners, K may be assumed to be as low as 0.12.

Generally, the above K values are conservative, and the actual preload induced by a given torque is slightly less than calculated.

A more exact value for K is expressed by the following equation....This is where I stop... you need to read the entire section if it is a critical fastener and you can't use the values listed above for K. The book has a table for torque cofficient K for bolt sizes 1/4-20 thru 1-14.

 

[Zapster]

Look at:

http://www.torqueleader.com/ams/userdownloads/Pages68&69.pdf

http://www.torqueleader.com/Support_and_Resources/Torque_Introduction/Torque_Values

 

[DustinMechEng]

Thanks for all the help you all, especially mickeymouse! That was exactly what I was looking for.

 

[CoryPad]

How is mickeymouse's post exactly what you are looking for? Copper external thread (NPT?) into acetal internal thread (both of unknown thread class, roughness, lubrication, etc.) cannot be considered equivalent to matched metric/UN threads on standard steel fasteners. You want a comprehensive listing of coefficients, so a better source would be Handbook of Bolts and Bolted Joints , available here:

http://www.crcpress.com/shopping_ca...?sku=DK5574&isbn=9780824799779&parent_id=&pc=

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MintJulep]

Cory,

Thanks for saying that!

Do you really think that the handbook you sort will list a value for this particular combination?

 

[Screwman]

The other thing to consider is that once you put those plastic threads under load you are going to have a serious creep issue and loose almost all your clamp load during the first 24 hours. long term clamp load retention for the vast majority of threaded joints in plastic is a nice dream that engineers hope exists, but that doesn't in the real world.
trying to calculate this is pretty much just an exercise because you will never retain that clamp in an acetal nut member.

 

[CoryPad]

MintJulep,

No the handbook doesn't have anything like this, but it does have the most permutations I have seen in a single publication.

Good comments from Screwman. I bet this is an NPT joint that doesn't need preload, so why bother. Also, if a minimum gasket force is necessary, it still would be better to use angle control.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mickeymouse]

Thanks screwmaster I missed the copper note.