Dry or Wet fasteners (re:thread404-173061) 2

[Drjones]

I am trying to resolve a dispute within our company in regards to wet vs. dry torque values. The dispute is over the condition of black-oxide cap screws when they come out of the box with a visible coating of "light oil". Our most experienced model builders beleive that these are still considered dry and I tend to agree with them. Our customer is convinced that these must be torqued to the corresponding "wet" values. Do we really need to wash all of our fasteners with acetone before assembly begins or can somebody on this forum provide a response that would convince our customer that we are right. (re:thread404-173061)

 

[Ron]

I'm not sure what size screws you are dealing with, but you can test the torque vs. elongation both ways to see which should be used for your application. Depending on how much tension you want to develop in the screw, you can set the elongation to that value (based on mill certs or test data for your screws), torque the screw until you achieve it with both the washed and unwashed condition to see.

I would agree with you that a light coating of oil for shipping would not likely contribute greatly to lubrication of the fastener for tightening purposes.

 

[desertfox]

Hi Drjones

Well I think I agree with your customer that the wet values should be used but is it possible you can do some tests and measure the tension in the the bolts using the dry torque values you have and compare practical results with theory.
My belief that wet values should be used is from an experience I had many years ago, I was called to investigate some tapped brass inserts that were failing at the recommended dry torque, it turned out the fitters contaminated the screws accidently with a grease residue they had on their hands as they picked the screws up, this got onto the threads, reduced the friction coefficient and the increased preload was sufficient to strip the brass threads. After I got them to clean up themselves and their spanners and stated not to go near any grease till after they had tightened all contacts onto the inserts we never had another failure.

regards

desertfox

 

[Drjones]

Great, now I have a forum dispute started : )
Hopefully after a couple days a consensus will form...

 

[desertfox]

hi Drjones

Have a look at table 2 in the link posted below:-

http://www.torqueleader.com/admin/pages/upload/docs/Mechanical Properties of Fasteners.pdf

regards

desertfox

 

[IceStationZebra]

--Any torque value you read off of a chart is only a guestimate needed to generate X clamp load!!--

If the clamp load is that critical the only way to truly determine the correct torque is to test it, as mentioned above.

I have investigated this for my current company and found two camps. One camp just uses the default SAE "K" factors. The second camp presumably has done internal testing and generated their own chart (and everyone's is different!). I used to have quite a bit of information on the subject but unfortunately my hard drive get erased by my IT department.

ISZ

 

[desertfox]

Hi Drjones

I was looking for some more sites which may throw up some more comparisons of wet and dry figures for torque or friction factors, you can see from table 2 in the first link that there is an overlap of the figures for both wet and dry.I found this other link that says that the torque factor for screws blackend and lightly oiled is 1 as the published data is for screws as supplied in this condition.
(see page 2 of the link)

http://www.albatros-ul.dk/?download=Bolts and Tightening.pdf

I have no idea what your torque figures are that you are looking at however I still think your best bet is some practical testing on the fasteners in question.

desertfox

 

[CoryPad]

It can be difficult to generalize friction coefficient behavior. Black oxide can be rough/hard/sharp, which can have high friction. Adding oil obviously lowers the friction. This lowered friction can be lower than bare steel or dry zinc electroplating, but higher than oil over zinc phosphate or new multilayer coatings. If I had to choose based on this limited information, I would go with "wet". You will find that the consensus is to conduct testing.

 

[MintJulep]

A few years ago I went through five well respected and authoritative fastener references and texts and plotted all of the listed "K factors" for the various combinations of material, plating and lubrication.

Tremendous variation.

Tremendous overlap.

Do not rely on handbook values.

If your use is sufficiently critical that you need to "know" that the preload is what it is supposed to be, and for some reason you are using torque control to achieve that preload, then you need to test your specific materials in your specific application.

 

[plasgears]

It would be prudent to test samples from the lot for ult strength. Also, take note of the torque to failure. In this age of bogus fasteners from questionable sources, it answers the question of whether they meet spec as advertised.

 

[racookpe1978]

I've never seen power plants accept torque values for high-value bolts UNLESS the threads were cleaned first, then all threads re-lubricated with a known thread agent. "Known" thread preparation agent is the key: old, dried out bottles or tubes were not accepted either.

They did not accept "out of the box" cleanliness of the threads - because they required the threads be lubricated; nor default grease/oil/nothing as a proper lubricant for torquing. If large bolts were re-used, they had to be thread chased, and have new nuts and washers. Preferably, new bolts were used on re-assembly.

 

[tbuelna]

You can eliminate the issue by using a torque-angle tightening procedure. It will produce a consistent fastener strain preload regardless of whether the threads are oily or dry. All it takes is a little bit of calculating and a reasonably priced tool.

Here's the tool to use:

http://buy1.snapon.com/catalog/item...=19918&supersede=&store=snapon-store&tool=all

Good luck.
Terry

 

[GuyFromDenver]

You best bet is to clean the fasteners then lube them with something that is compatible with your physical environment and then lube them with something that is consistent. Most moly-D's are predictable but won't work in strong oxidizing environments. In that case use Inlox (Google them They are in Southern Cali)It's pricey, but it is the most tenacious dry film lube that I have ever met, and it's friction is almost always dead on. By the way, angle of twist makes a lot of assumptions. The single best way to measure torque is the stretch of the fastener technique, but it only works with through bolts that you can mike over. Otherwise apply conservative tolerances when calculating torque that will satisfy your clamping requirements and still give you enough margin to accommodate variations. that will mean a little over design, but in my experience nothing pisses the customer off more then watching fasteners fly out of the works! It's worth it to play it safe while not going overboard!