Torque values with bolt coatings.

I'm guessing here, but I would think that PTFE coated studs and nuts could be torqued just as they would be with other lubricants.

There is a "but": if the friction coefficient is too small then the bolt will NOT keep tight. For some threads ptfe has a too low friction value so that the bolts will get loose under own pre loading.

Friction is a pain in bolted joints creating the dispersion of pre-loads but is necessary to keep the parts together.

Very helpful information, thank you Kev_brown.

Good post.

However, this is potentially misleading:

Using a lubricant on a bolted joint is very important as it is easier to calculate the torque required for securing the joint. Without lubrication, you are subjecting the joint to much less torque to get the same bolt force.

Many torques specs are given by equipment manufacturers for dry fasteners. If you lubricated these, and failed to reduce torque accordingly, you would run the risk of stripping threads. Most mechanics are wise to use the manufacturer's specs on torquing fasteners.

In the second sentence, I think you meant to say something like: "With lubrication you use less torque to achieve the same bolt (clamping) force." This is the reason that torque specs for lubricated bolts are lower than for non-lubricated.

"Quote" Coatings such as galvanisation can also strip from the bolt/nut quite easily thus invalidating any friction co-efficient you were using resulting in you not obtaining the correct bolt load.

Kev, Thanks for your reply I completely agree with where you are coming from, So if I apply the lubricant (I will use Dow - Molycote 1000) will this change the force applied to the bolt - it will increase - (the tables to which I have to work, advise to stay away from coatings) Will this be a significant ammount?

So if I apply the lubricant (I will use Dow - Molycote 1000) will this change the force applied to the bolt - it will increase -

Ordinarily, you are trying to achieve a particular bolt tension and clamping force. Therefore, you would not apply a lubricant, torque the fastener to the same value and simply accept a greater tension and clamping force (and the possibility of stripped threads). Rather, you would apply a lubricant, and calculate the torque then required to achieve the correct bolt tension. Thus, bolt torquing charts show lower torques for lubricated bolts (such as PTFE coated ones).

In other words, torque is usually the dependent variable.

Rule of thumb is add 30% for dry torque or check Machinist handbook -Mcgraw/Hill

30% - that seems a lot, (hate to use the word "seems") considering most of the frictional surfaces are coated in PTFE -

I am leaning towards - Applying Dow Molycote 1000 to the underside of the nut / washer, while leaving the threads mostly dry.

The tables I'm using have been researched by leading torque / tension companys and a certain University.

Thanks for the answers / feed back from all.

mouse 32

I am bit surprised that there where there is a possibility to compute with quite a precision what occurs we discuss about qualitative values. There is a very well known and all over the world used equation for the computing of torques in any kind of thread. This equation takes into account the friction values on both sliding surfaces : under head/nut and thread.

To give an example dry friction can go as high as 0.16..0.18 and drops down to 0.07...0.08 if MoS2 based coatings are used. As I mentioned in previous comment if the friction coefficient is too low (which could happen with PTFE since the values are around 0.04) there is no self-locking which means that when the tightening torque is taken the pre-load could un tighten the bolt/screw. If we only consider a dry friction of 0.12 the usage of MoS2 can lead to even more than 30% increase for same torque or if one wants to maintain the pre-loading level constant a drastic reduction of the tightening torque.

The coatings can part from their support due to the high shear stresses during tightening. This depends on the kind of coating and the kind of bond between layer and base.

30% - that seems a lot, (hate to use the word "seems") considering most of the frictional surfaces are coated in PTFE -

I think Bwire was saying that lubrication reduces the torque requirement for a given bolt tension by 30% (as compared to a non-lubricated bolt). PTFE is a highly effective lubricant, so one would not ordinarily add additional lubricant -- you are already paying a premium for the PTFE coating.

This link gives some of the calculations used, and also provides a chart for lubed vs non-lubed fittings.

Why leave the threads mostly dry? Why not come up with a spec, and follow it?

You might also consider the use of direct tension indicating washers. This article describes their use, and touches on other subjects (such as the value of various coatings) that might be of interest.

I think Bwire was saying that lubrication reduces the torque requirement for a given bolt tension by 30%

Actually he wasn't saying precisely this (there is a difference between increasing by 30% and reducing by 30%) -- but the idea is that he was comparing lubed and non-lubed fasteners, I think (with PTFE coated clearly fitting into the lubed category).

Ok,

thanks for all comments,

Mouse.

Thanks,

To all Gents involved, you've helped me come to a informed decision. I will stick to my procedures and conduct a couple of tests - Based on your input and some of the tables I have visited over the last few days I don't expect much variation from my spec -

thanks again,

Mouse.

Sorry for arriving late; I had completely missed this one! Nevertheless, perhaps my two farthings worth might provide some further illumination:

If your aim in choosing the "right" lubrication factor is to help you get somewhat close to applying the 'proper torque', fine. If, however, you feel that this will ensure that the joint will be 'properly' tightened, you may want to reconsider. Please read on:

The point of tightening a fastener is so that the necessary clamp load can be applied to prevent joint failure. "Torquing" is an attempt to generate the required bolt stress. Between clamp load and torque, one must navigate a very mysterious, esoteric and ethereal course indeed.

Attempting to identify the friction factors and lubrication values which will determine how much torque will result in actual residual bolt stress is utter folly. It's like telling somebody how your hamburger is going to taste before even taking a bite. You can make an educated guess, you can throw the chicken bones, you can read the tea leaves and you can hope but, without checking after the fact, you'll have absolutely no idea.

Here's a link that expands on this. An interesting note is that it also points out that calibration of torque tools often serves no practical purpose if the goal is to accurately tighten fasteners...

http://www.heviitech.com/Hevii_TorqueDanger.html

Here's a question for you guys:

When reusing PTFE coated bolts, we add lubrication to them. How should I adjust calculated torque? Do I use the PTFE coeff of friction, or what I would use for a paste lubed bolt?

Thanks for any insight.