PTFE Coated Studs: Do They Work?
创建于03.20
PTFE Coated Studs: Do They Work?
PTFE Coated Studs: A Little Background
PTFE is short forPolytetrafluoroethylene, a chemical applied to common bolting materials (such as B7 stud bolts) to provide corrosion and chemical resistance.
Some other common coatings for PTFE coated fasteners are Teflon® and Xylan®.In this article, we’ll refer to all of them as PTFE.
PTFE coated studs have been used for many years within the bolting industry, especially in any application that requires corrosion resistance or in offshore applications (salt spray is hard on Grade B7 material). PTFE is also useful if you’ve had “galling” and need a lower breakout torque for safe removal.
PTFE should not be used in high-temperature applications. Fluoropolymer coatings manufacturers use temperatures in the 400F-500F as the maximum temperatures, so you typically do not see alloy steel bolts (such as B8) with PTFE coatings.
While PTFE coated studs (including Xylan® coated studs) have corrosion-resistant properties, which are functionally understood by assemblers, the technical aspects of fluoropolymer coatings are often misunderstood.
For example, as an assembler, I was told that the PTFE coaching acted as a lubricant, which meant I didn’t have to mess with lubricating them after I put them into a flange.
It sounds like the perfect solution. The only problem is: That advice was wrong.
Way wrong, in fact.
Yet many front-line assemblers today still think it’s correct.
Thankfully, in the decades since I first started putting together flanges, smart minds both in the lab and on the front lines have given us a much more accurate picture of how PTFE coated studs work.
In this article we’ll bust some myths, provide practical advice, and address some of the most common questions we hear from craft assemblers today, including:
- Are all PTFE coated studs the same?
- Do I lubricate PTFE coated studs?
- Can I re-use PTFE coated studs?
- Why do PTFE studs have low friction and lower break out torque than regular studs? (Grade B7 with 2H hex nuts for example)
- Does the PTFE coating help with corrosion resistance?
Click any of those questions above to jump directly to the answer you want. Or read on to get the complete picture of how PTFE coated studs affect assembly.
1. Are all PTFE coated studs the same?
Absolutely not.
While PTFE is the same as Teflon® on a chemical level, they are not applied the same way by each manufacturer. There also are several different types of base coats.
Therefore, the thickness of the coating on the fastener is not a standard — and quite frankly, it’s not controlled.
As a result, you must choose one manufacturer and test your k-factor for their product, and recognize that those values do not transfer to other PTFE coated stud manufacturers.
2. Do I lubricate PTFE coated Studs?
Yes!
At 30,000psi bolt load (not to be confused with 30% tensile strength), you start stripping your coating. Also, the coating will start binding. Therefore, you’ll have better accuracy and less bolt scatter (or differences in bolt loads on each bolt) by using lubricating them.
However, you will have to test your lubricant and manufacturer combination in order to correctly determine your K-Factor. Note: Don’t confuse K-Factor with low coefficient of friction, which is not used for this calculation.
3. Can I re-use PTFE coated studs?
You shouldn’t.
If you were to re-use PTFE coated studs, the corrosion-resistant coating on the threads will most likely be at least somewhat degraded or damaged, meaning your K-Factor will change again.
Physically the bolt might still hold up to corrosion. So visually, they would still look good, which would lead someone to think that the life expectancy of the stud would be longer. But appearances can be deceiving, and you shouldn’t reuse PTFE coated studs.
4. Why do PTFE studs have low friction and lower break out torque than regular studs?
What would you say if I told you that, in order to fit the nut on the stud with the PTFE coating, you must drill a bigger threaded hole (tap) through a 2H nut? Yes, it’s true. And there is are no technical specifications on this.
I couldn’t believe it either, but you will be effectively taking 30%-50% of the contact surface away. Therefore, it is not the PTFE that makes it easier to disassemble (and you have a low friction between stud and nut on assembly). This phenomenon is due to having considerably less contact area!
5. Does the PTFE coating help with corrosion resistance?
Yes…to an extent.
Let’s say you have a B7 stud (ASTM A193) and 2H nuts. The application process and proprietary materials of the PTFE coating is intended to help with corrosion resistance. However, since the coating is proprietary to the manufacturer, it is hard to say how well it helps with corrosion resistance.
One way to test this is with a salt spray test with ASTM B117, which sprays the stud bolts up to 3,000 hrs while not freezing the nuts.
Tags:Ningbo Hope Fasteners Co., Ltd.
By:Scott Hamilton
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