[endgrainguy]

I bought a set of these (SK 10?) used on online auction site, and was surprised when I received them how tiny and lightweight they are. They seem good and tight, but I'm still wondering if they're not meant for something much more lightweight than my intended purpose -a horizontal y axis for a woodwoking router (fixed gantry,moving table) with 15" travel on 26" long rails. They did come with 4 trucks on each rail. Will they work for me? Should I use more than 2 trucks/rail? (I was hoping I might be able to sell the extra trucks). The bolts that will fit their tapped holes are really small! Thanks!

[ger21]

It depends a little on which model you have (2 balls or 4 balls), but generally, 15mm rails can handle several hundred pounds of force. They should be fine.

[kiwibum]

It depends a little on which model you have (2 balls or 4 balls), but generally, 15mm rails can handle several hundred pounds of force. They should be fine.

The RSR15 are 2 ball units, then come under THK miniature range, specs can be found here https://tech.thk.com/ put "SRS 15" in the product search and you will find out more about them.

Basic Dynamic load rating down on to the rail is 4.41 kN where pulling up or side loading is 3.44 kN, depending on which version you have, this is for RSR15VM, the RSR15M is 6.6 kN in all directions. Compare this to 4 ball unit HSR15 which has dynamic load rating of 8.33 kN in all directions. The miniature series are used for "MGN/MGW series can be used in many fields, such as semiconductor equipment, pCB assembly equipment, medical equipment, robotics, measuring equipment, office automation equipment, and other miniature sliding mechinery." Quote is from HIWIN catalog but specs are similar between manufactures from what I can tell so far. One nice thing about the ones I have is they rails should be stainless steel.

How do I know all this? I've been struggling with the same question, I did the same as you except mine are IKO LWL15:-). These are equivalent to the RSR15 you have. Although I wont see mine until May when I get to the US to collect them from girl friend, I suspect I've got the wrong thing. I've been doing a lot of reading about the different types, sizes etc and starting to question they will be suitable for what I want to do.

Whether they will be suitable for you will depend on how much use they will get and material you are cutting. I think they will be fine for regular hobby use on balsa, light wood and the odd hard wood (with reduced feeds) but I get the feeling they wouldn't last being used commercially every day. If you want another set of 26.5" equivalent rails with 4 trucks let me know I might be reselling mine.

[LeeWay]

Just a little extra info that may not have been covered. I use RSR 12's on the Z axis of my router. 4 trucks on two rails that are about 12" long. They seem plenty strong after several years of use. I keep them oiled up pretty good.

I don't think the balls in these will stay in place once removed from the rails. I also have a set of these that have worn balls, and these do fall out when the trucks are removed. Not sure if it's cause they are worn or not.
You won't be able to recoup much money selling 4 little trucks like this. You may get a little more for plain rails that you can for a set of trucks, but separately, these just don't fetch what complete units can. Strictly talking Ebay here.

At one time I bout 10 brand new in box THK HSR25 trucks with no rails. I think I gave about $200 shipped for those. I have since seen similar pricing on the same size blocks. It's more difficult to spot just rails. I wound up getting the rail from THK Online. I would avoid that in the future and go with Hiwin for new or keep watching Ebay. I am satisfied with these, but the rail came at a premium price, which meant the blocks were all that cheap.

I think the rails you have will work fine for you. They do last quite a long time. Since you have 4 trucks on each rail, it should increase strength. I would consider using at least 3 on each and 4 if you have the room.
These are stronger than you might think.

You might consider placing these top and bottom or top and front on the gantry. The first would require more precision in mounting. Such configs would help to ease the loads on the rails.

[ger21]

Basic Dynamic load rating down on to the rail is 4.41 kN where pulling up or side loading is 3.44 kN, depending on which version you have, this is for RSR15VM, the RSR15M is 6.6 kN in all directions. Compare this to 4 ball unit HSR15 which has dynamic load rating of 8.33 kN in all directions.

3.44Kn is 773 lbs of force. Will you be putting anywhere near that load on them? A router will probably only have 30-50 lbs of force on it when cutting hardwoods, unless you have a 5HP spindle cutting full depth in 1 pass, then you might have a bit more, but your homebuilt machine will probably tear itself apart before the linear rails fail.

[kiwibum]

3.44Kn is 773 lbs of force. Will you be putting anywhere near that load on them?

Actually this isn't the problem as you state but I think people are missing the point about this loading as it depends on use.

"The basic dynamic load rating is defined as the constant load in both direction and magnitude under which a group of identical Linear Ways are individually operated and 90% of those in the group can travel 50 x 10^3 meters (50Km) free from material damage due to rolling contact fatigue." (HIWIN catalog)

Doing life time calculations and safe loads for environments with vibration and shocks divide by 6-8 times, this brings the loading force down closer to 100lbs. To move 50Km at 50IPM would take ~750 hours, for commercial use this could be 12-24 months with the 10% chance of developing "rolling contact fatigue". Since these units were purchased used from a commercial environment, although they would have been used for their intended purpose of light loads, they will possibly have done a lot of miles. They will now go into a high load, high vibration environment of a CNC router. From the hours calculation above they will be fine for hobby use but not really for commercial precision, they also only have light preload.

Then as another thread mentions it's the "moment loads" that are important http://www.cnczone.com/forums/showpo...24&postcount=9
With a shorter Z axis these will be fine from what I've worked out.

I'll probably be keeping/using my LWL15 since I shouldn't be seeing anywhere near that number of hours of use in my machines life. I hadn't done that calculation until now and it gives a little confidence that they will last the distance.

Regarding the balls staying in place once removed from the rail, it all depends on whether the balls are caged or not, I don't think it's related to wear. This is all based on opinion not practical experience at all. There appears to be dozens of different types of trucks/rails when you look deeper into it, caged, not caged balls, 4 or 2 balls, different preloads for precision (low, high), axial loading, radial loading, low profile, stainless or high carbon steel, truck length, type of wipers etc......... Then maybe I'm looking at it all a little too deeply.