- #1
Juanda
Gold Member
- 425
- 149
- TL;DR Summary
- I don't understand why the Bossard calculator gives greater differences for the resulting preload when the friction variance is more controlled.
Fastened joints are subject to scatter. It's impossible to get exactly the same value every time. This is especially relevant when using methods that rely on friction such as torque wrenches. The scatter is a combination of the possible differences in friction and the precision of the tool.
Bossard is a company that has released some very useful information regarding fastened joints mostly based on VDI 2230. To be precise:
However, I was very surprised when I compared the results of two different joints where the only difference was the “Friction coefficient Class”. According to F.049, the lower (alphabetically ordered) the class, the better because the range is smaller. As you can see in the following picture, the range for the friction coefficient is smaller in the upper classes.
A smaller friction range implies a smaller difference between the possible slopes in the following graph.
Now comes the surprise. I compared the output for two inputs with the only difference being the friction range. I made it so the friction range was equivalent to a “Friction coefficient Class B” vs “Friction coefficient Class D” which is two classes worse.
The necessary torque (maximum tightening torque) to reach the input utilization of yield (80 %) is greater in the D class since there is more friction to overcome. That makes sense. To find that number the online calculator uses the lowest possible friction to make sure you won't overtorque the bolt and brake it while preloading it in case the friction happens to be lower.
What breaks my mind is the fact that the "Difference" is also lower in the D class. That makes no sense to me. In the D class, we are more uncertain on what's the friction of the joint (0.08 vs 0.15 range). That's what makes it undesirable in the first place since that can produce a lot of variation in the preload. But we're seeing in the calculator that the class B is resulting in greater potential differences in preload (46 vs 43 %). As far as I know, these calculations should be linear so the greater certainty in the friction (a smaller friction range) should result in a smaller difference in resulting preloads.
Am I missing something or there is something wrong with the online calculator?
I probably could create an Excel sheet or small Python script to calculate everything but I'd rather not reinvent the wheel and use the tools already available out there. Also, I could commit mistakes I'd hope a big company such as Bossard has made sure to check for.
However, I need to be sure such tools are reliable.
Bossard is a company that has released some very useful information regarding fastened joints mostly based on VDI 2230. To be precise:
- F_044: Friction and friction coefficients. The document is sometimes referenced as F.049 which is the code of the page shown in the lower right corner.
- F_047: Preload and tightening torques.
However, I was very surprised when I compared the results of two different joints where the only difference was the “Friction coefficient Class”. According to F.049, the lower (alphabetically ordered) the class, the better because the range is smaller. As you can see in the following picture, the range for the friction coefficient is smaller in the upper classes.
A smaller friction range implies a smaller difference between the possible slopes in the following graph.
Now comes the surprise. I compared the output for two inputs with the only difference being the friction range. I made it so the friction range was equivalent to a “Friction coefficient Class B” vs “Friction coefficient Class D” which is two classes worse.
| Input for the class B (0.08 range in friction coefficient) | Input for class D (0.15 range in friction coefficient) |
| Input for the class B (46 % difference) | Input for the class D (43 % difference) |
The necessary torque (maximum tightening torque) to reach the input utilization of yield (80 %) is greater in the D class since there is more friction to overcome. That makes sense. To find that number the online calculator uses the lowest possible friction to make sure you won't overtorque the bolt and brake it while preloading it in case the friction happens to be lower.
What breaks my mind is the fact that the "Difference" is also lower in the D class. That makes no sense to me. In the D class, we are more uncertain on what's the friction of the joint (0.08 vs 0.15 range). That's what makes it undesirable in the first place since that can produce a lot of variation in the preload. But we're seeing in the calculator that the class B is resulting in greater potential differences in preload (46 vs 43 %). As far as I know, these calculations should be linear so the greater certainty in the friction (a smaller friction range) should result in a smaller difference in resulting preloads.
Am I missing something or there is something wrong with the online calculator?
I probably could create an Excel sheet or small Python script to calculate everything but I'd rather not reinvent the wheel and use the tools already available out there. Also, I could commit mistakes I'd hope a big company such as Bossard has made sure to check for.
However, I need to be sure such tools are reliable.
