by dirktheeng » Wed Sep 15, 2010 7:31 pm
I just looked again and called the engineers again. It looks like the clearance between the rail and wheel on the RSR25 is 0.1mm. They are not preloaded. 0.1mm is about 0.004", not so good when you want to go to 1000dpi. However, they did direct me to the RSR40, which has a clearance of zero mm. The engineer said that these are preloaded. In addition, there is a 40mm spacing between the rails... more ability to resist torque.
I also found that they sell a kit with rail and slider together. The price for the RSR25-480 is $38, the price for the RSR40-500 is $47.10. With first order discount this comes to $26.60 and $32.97.
Lets also consider the 2 different applications for the rails. The Y-axis is very different from the X-axis. 2 railse are used ont he Y-axis. The Y axis is rigidly locked in the direction of travel by the belts and cannot torque in the plane of the gantry because it is bolted to the gantry. Therefore, it can only roll about the axis of the gantry. Therefore, we only need to be concerned about controlling that roll. Since 2 slides are used, even if they aren't pre-loaded as in themselves, we can apply a preload by applying a small load in the direction of the gantry. In that case, we would want the side of the car with 2 wheels on the outside. And what we really care about is the roll with the common axis of the gantry. If they are preloaded by appying the small "pinching" force to the rails, the car wheels will not allow that motion. Basically, this would be akin to removing the inside wheels from your rails and applying a load to spread the outside rails. This would be necessary use the RSR25 series since there is clearance between the rail and the wheels.
This would not be necessary for the RSR40 series as the clearance is zero. With jurry rigging, the RSR25 could work for the Y axis, but definately not the X axis. It would seem that the RSR40 would be a better solution all around and could be confidently used for either.
The RSR40 could yeild a good Yaxis solution for about $66 total with the discount from misumi. I would suggest ordering an RSR40-500 after talking with Misumi. This is an all metal solution with apparently a zero load slop of zero. Givent That it is all steel (rails and cars are chrom plated, wheels are plain steel), the stiffness underl load is bound to be high. If you can spare the money, I think this would be a better thing to try first.