16mm vs. 20mm supported linear rails...

[WilliamD]

I have always been set on 20mm supported rails for my future wood router. I want to have the least amount of fuss, and a bolt together setup is appealing.

I basically planned on getting the round recirculating ball type rails, found at VXB.

I have an opportunity to get the type of rail, but in the 16mm version, for half the costs, which is pretty significant for me.

My big question would be if there is a significant difference between the 16 and 20mm sets. I imagine flex wouldn't be an issue with either, since they are both supported. As far as loads, I only plan on using it with a standard router, and doing wood and foam.

I've searched the site, but mostly see a lot of debate over how thick unsupported rails should be. Any advice or input would be greatly appreciated. I'll spend the money for the 20mm if I have to, I don't want to be greatly disappointed after the purchase. Thanks!

[harryn]

Perhaps you can post a comparison table of mechanical specs on the 16 vs 20 vs 25 mm rail (load ratings, etc), and a little about your project size / construction method. This might make it easier to suggest something.

The answer would probably be different for a 12 x 12 inch router vs a 4 x 8 ft.

There are so many projects in the build logs that it might be possible to search on 16mm and find the answer.

Perhaps just as importantly, do you think you will want to build router # 2 in the future that is bigger + faster + use parts from #1 to do it ?

[WilliamD]

You make a good point, I should reveal more about my project.

Each axis will be 60", which is the length of the rail. The base will be made of 4x4" .25" wall steel tube for the legs, with 2x2" tube steel for the rest of the construction. The y axis will be a length of 3x6" wide 80/20, which the rails will bolt too. Much like the Joe 4x4 router.

I have no idea what the load ratings are, or even what kind of loads a 15 amp router can cause.

Hope this helps!

[WilliamD]

I found some load ratings, but can't make sense of them.

The 16 mm states the following

Basic Load Rating:
Dyn. CN - 774
Stat. CON - 1180

For the 20mm

Base Load Rating:
Dyn. CN - 882
Stat. CON - 1370

I have no idea how to interpret those load ratings, or even put them in an example I'd understand. Any help would be greatly appreciated!

[irving2008]

Originally Posted by WilliamD I found some load ratings, but can't make sense of them. The 16 mm states the following Basic Load Rating: Dyn. CN - 774 Stat. CON - 1180 For the 20mm Base Load Rating: Dyn. CN - 882 Stat. CON - 1370 I have no idea how to interpret those load ratings, or even put them in an example I'd understand. Any help would be greatly appreciated!

Basic Dynamic Load Rating (C)
This term means such load that, when a certain number of identical linear systems are individually run in the same conditions, 90% of them can run with the load (with a constant value in a constant direction) for a distance of 50 x 10^3 meters without damage caused by rolling fatigue.


Basic Static Load Rating (Co)
This term defines a static load such that, at the contacting position where the maximum stress is exercised, the sum of the permanent deformation of the rolling body and that of the rolling plane is 0.0001 time of the diameter of the rolling body.

The figures are similar to the calculations done for ball bearings... how long a life (in revs) at a given axial load... except for linear bearings its how long a life in metres of travel at a given load.... as long as you dont exceed the static loading of the smaller rail the only difference will be operational life under load....

the 16mm will support 1180N per bearing (about 118kg) while the 20mm 1370N (137kg) before the linear rail deforms .0001 x diameter of the ball (typically then that would be .0003mm)

For your needs I'd have thought the 16mm fully supported was more than adequate...