linear rods versus linear rails

there seems to be a bit of confusion on this hell I am to, I hear one place say rods are best for this or that then I hear the same thing about rails?? I am wanting to figure out which is best on what standing point's accuracy, load, which one keeps it's accuracy longer, life span, etc?? there are three type's used most and easiest to find is as fallows

standard rod's using round linear bearing's

rod's connected to a support rail system split round bearing

rail system using bearing cartridges (flat) such as a ssr15 thk rail

I am sure a lot of people have experience with one or two of these systems
and some I am sure with all of them at a time or another simple asking you to share your experience with them

and may be help out some new builders make a better informed decision when selecting rails

Hi,
Round type (thompson etc) dont have any curved grooves machined into them for the ball bearings to ride in like LM guides (flat type linear rail), hence they need more bearing columns to distribute side load as the bearings only make contact on their outer edge, vs a more supported LM guide which has half of the bearing in contact with the rail.

LM guide (flat rail) type generally have a higher load capacity and last longer, but depends on size, you would need to check manufacturer's specs to compare LM and round, then assess your budget and which would be easier to mount.

As for round rail, supported and unsupported, as a general rule, supported should be used for higher load / smaller diameter rail / long span applications etc, and unsupported for short length / large diameter rail or light load use etc.
Check specs, or do a flex test with scale etc.


Hope this helps you at least a little. :)

I've worked on a machine for a friend.... early '80 cincinatti... round rods, split bearings - the bearings were shot and totally worn out. they would have lasted longer if there was a way to lube them. Rod diameter 2 1/2 inches, weight of just the machine table on them 7000 Lb. Machine was used for mold making = lots of back and forth movement!!!!

S

PS: I'm parting out the machine! (and it has almost new rods and bearings in the 'Y' axis!)

thank you have a new question I have seen people with listings for rail's lm sr20 ssr25 and such having the ends machined to line them together to get extra length I would thingk this might effect accuracy at least a little bit what do you people think?

Quote:

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Originally Posted by haylspa

thank you have a new question I have seen people with listings for rail's lm sr20 ssr25 and such having the ends machined to line them together to get extra length I would thingk this might effect accuracy at least a little bit what do you people think?

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Get hold of the hiwin documentation, it has the answers to most of your questions, and relates the tolerances given to their rails. Other manufacturers will provide the spec data, but aren't as clear as some of the hiwin illustrations. They show how one can line up two sections of rail, but you can get pretty long sections already. As long as the carriages and rails are of the interchangeable type, there should not be any issue.
They will also give load specs per various rail sizes, which is good info to have.
In order to set up any precision set of rails, you need a good dial test indicator, generally one that reads in .0001".