Torque For Plated Hardware:More or Less? 4

[tc7]

An argument has erupted as to the correct installation torque for zinc and cad plated hardware as compared to plain clean steel hardware; our joint assy torque calculations are based on "K" factors suggested by Shigley which shows zinc or cad plating factors to be significantly lower than nonplated steel i.e., 0.2 and 0.16 (plated) versus 0.3 (nonplated). These factors result in lower torque for a desired preload when using plated vs. non-plated hardware [we use the simple equation(T=fKd)].

However, our lead engineer insists that zinc plated hardware requires 1.4x greater torque than for unplated hardware and will only provide an anecdotal reference which we cannot verify. Can anyone please resolve this with specific literature references?

Thankyou if you can.
Tom

 

[CoryPad]

Handbook of Bolts and Bolted Joints, edited by Bickford and Nassar, Chapter 2, Table 1:
[tt]
Coating Nut factor
------------- ----------
bare steel 0.2
Cd plate, dry 0.2
Zn plate, dry 0.295[/tt]


Regards,

Cory

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

 

[israelkk]

NASA RP-1228 Fastener design manual recommends not to use the K=0.2 or 0.3 blindly. It is more inclined to 0.15. It offers to best use the formula to calculate K based on the friction coefficient between the threads and between the bolthead or nut and the clamped surface coefficient of friction which are not always equal. It also give a table of static and sliding coefficient of friction for various combination of materials (greazed or dry).

I will be very carefull with those coefficients for critical use and I would recommend actual testing for every new batch of fasteners and clamped surfaces. It is best to use the largest fastener available that will give the desired clamping force even with the largest possible K while will be able to hold without failure if the actual K will be the lowest available.

 

[desertfox]

Hi tc7

Take a look at this site it as some excellent tables regarding friction factors and also supplies formula for torque and pre-load.

http://www.torqueleader.com/specify.htm

regards

desertfox

 

[tc7]

Israelkk-
I like the NASA Design Manual approach for calculating the K value for each particular application, but this requires data on helix angle and thread angle (same as lead angle???) which do not seem to be tabulated in H28 or ASME B1.1 or Machinery Hdbk. Do you know where to find this data?
Thankyou again,
T

 

[CoryPad]

The formula for K on page 19 of NASA RP 1228 has an error: [ψ] (helix angle) should be [λ] (lead angle). Mr. Barrett erred when transcribing the equation from Mechanical Engineering Design by Shigley and Mischke.

The thread angle [α] shown in NASA RP 1228 is 30 degrees. This is actually half the true thread angle [α] as defined in ISO 5408, so it is important to know from where your equations and data come.

You can calculate [λ] easily:

tan [λ] = l/[π] d_m

where

[π] = 3.141 592 654
d_m = mean diameter or pitch diameter

I recommend dispensing with K altogether and using friction coefficient equations. You can find them in VDI 2230, see faq725-600.

Regards,

Cory

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

 

[tc7]

Thanks Cory - your post is definitely a keeper!!!
T

 

[CoryPad]

I forgot to identify l in my previous post - it could be misunderstood to be a one...

l = lead or pitch

Regards,

Cory

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

 

[israelkk]

One more source is MIL-HDBK-60 "THreaded fastenters-tightning to proper tension" is available at

http://assist.daps.dla.mil/quicksearch/