linear rail/bearing wear and interchangability

The machine I'm working on has size 25 star/rexroth bearing blocks (1631-214-10) and rails. Are these an industry standard, or are bearing blocks and rails only interchangeable with ones made by the same manufacturer?

Also, is there a way to determine wear on the rails, and whether they are still within tolerances? I'm looking at whether the whole rail system will need replacing if I put new bearing blocks on the machine, and relatively new to this stuff.

Also, are there any sources for these particular bearing blocks that people favor?

I've done a tiny bit of searching on the compatibility issue, and my sense is that while many profiles are similar, I don't think they're intended to interchange. It might be that some of the newer Chinese suppliers have copied an existing system, but I couldn't tell you they'd fit the original.

On worn rail, you might be able to "map out" the rail using a known good bearing block, then using a test indicator to measure slop vertically and side to side. If you see variations of greater than a thou or so along the rail that might be enough (depending on application) to call it shot.

Bad wear should be something you can feel if you run your fingers along the cleaned rail. Watch for ridges, dings, etc.

fortunately the rails look very nice still. the existing bearing blocks seem like they've suffered from poor lube choices over their lifetime, along with complete disregard for maintenance by a previous caretaker. The guy running the shop when they bought the machine didn't seem to ever be happy that it was there. If I can get the bosch/rexroth people to connect me with a supplier tomorrow, hopefully I can get a block to test the rail. I'm trying to come to a decision about when we are starting to throw good money after bad.

No, the parts are not interchangeable between manufacturers and if they are in a cnc machine they are not interchangeable within manufacturers. Rails and blocks are like ball screws, the balls are preloaded. The only time you can just get a replacement block is if the assembly was not preloaded to begin with, and only good for a home built hobby project. After that you have to deal with stack height. For that size rail I would guess the standard stack height tolerance of a rail and block assembly is +/-.002" or .004", I can't remember if the .004" was total or +/-. Way more to it than this but these are the biggest deal killers to what you want to do.

I replaced the rails and blocks on one of my mills a few years ago so had to learn all this, and found out there are WAY more details to deal with than I ever imagined!

THK has an online store, where you can price and buy linear rails and trucks. Common sizes are stocked in Ohio.

Linear guideways are made to industry standard sizes — like ball and roller bearings are. But, each company utilizes unique patterns of the ball tracks/rail cross-sections, so you have to use trucks and rails from the same company.

You can research sizes and specs of the linear guides on THK’s website as well.

IF you get rails and blocks anywhere other than the MTB you better triple check that you got every one of the 20-30 or so CRITICAL details exactly right, or else you could be real disappointed! I got one minor detail wrong when I bought mine and lucked out, it only cost me an extra 3 days of work when installing them.

DavidScott said:

For that size rail I would guess the standard stack height tolerance of a rail and block assembly is +/-.002" or .004", I can't remember if the .004" was total or +/-. Way more to it than this but these are the biggest deal killers to what you want to do.

I replaced the rails and blocks on one of my mills a few years ago so had to learn all this, and found out there are WAY more details to deal with than I ever imagined!

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These are C1 preload according to bearing manufacturer website. That’s the lightest preload they offer, but higher preloads affect bearing life negatively with the increased tolerances.

What is stack height? This machine was once a $20-30k (maybe more) CNC Router. It doesn’t need to, or will it ever hold the tolerances of a mill, but it was designed to be a production/prototyping machine, and once I get the rest of the slop out of the drives, it should do its job satisfactorily for a while longer.

I learned today that when there is an odd sized bolt, you probably ought to leave it alone... the end caps of the bearings where the grease fitting is located are secured with two small bolts with 4.5mm heads. Had to order a special but driver, and then turn the o.d. of the nut driver down about .035” in order for it to fit into the recesses for the bolts. I popped the end cap off, because I wanted to see if the grease was making it to where it belonged.... oops. Amazingly I was able not to spill any of the balls onto the floor and lose them, and I was able to carefully wiggle it all back together. But I’m not going to make the mistake of dismantling one again.

The manufacturer of the router sells a very similar looking star bearing truck, so I imagine that they are field replaceable without consequence. When your machine isn’t designed to hold tenths, it is a little more forgiving to repair. Star/rexroth still makes this particular bearing truck, so, I guess unless I want to replace all of the rail, I will stick with them. Even if I had to replace all the rails and all of the reduction drives, rack, and pinion, I’m still the price of a new car cheaper than installing a new machine.

anchorman said:

What is stack height?

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Stack height is the combined heights of the linear rail, the bearing trucks, and their own individual tolerances that lead to the actual distance from bottom of the rail to top of the truck. These can vary a bit from production run to run due to all the elements (rail, rail ball tracks, ball sizes, truck height, truck ball path, etc.) combining together to change the actual stack dimension.

In many cases you you can shim the lower stacks to match the tallest, but if you're working on a precise machine you might specify greater control to avoid throwing other elements (ball screws and the like) off.

IMHO you need to down load the rail manufacturers data sheets - user guide for that series of rails. Most rails also have there parts numbers engraved on them. Im not Familiar with Rexworth's version.

The Hiwin stuff i use and am familiar with you very much can get replacement trucks in the lower three tolerance bands and at the lower 3 preloads if memory serves right. The rail is accurate enough from hiwin that you can use any of the lower 3 preloads with it. Its only there significantly higher accuracy rail were the trucks get factory matched to the rail. That said there more than just preload, there a wide range of different truck styles and lengths for a given rail then a range of lubrication and sealing arrangements.

Generally i buy this stuff online, local bearing places - dealers are about the most expensive source for this kinda stuff and over here they get it drop shipped to you any rate.

One thing i will add, i find regular lube essential on these, especially if even slightly exposed to dust and crap, its not so much just lubrication but it helps flush any crap out the bearing! If you have a machine that strokes about the same distance you will see what i mean with the build up of gunk at the end of travel. Equally little and oftern is better, seen more than a few customers that decided to try and empty a whole damn grease gun into one truck and just kinda hydraulically blew it apart. Had really good success and kinda recommend a synthetic oil based lithium grease for them too.

David is correct. Once you get into rails worthy of a machine tool and preloaded. They only come loaded on the rail.

Interchangeability is generally limited to the full assembly, not at the component level. So same bolt patterns and overall height, but not much else.

anchorman said:

These are C1 preload according to bearing manufacturer website. That’s the lightest preload ..

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I know with IKO, the interchangeable ones are only the no preload type. Preload does reduce lifetime. You also get free preload by mounting them on a less than ground flat surface. You also get free preload from stack variance on the interchangeable types when you bolt up more than one.

If you can't find a rep to find out, my guess would be no on C1 interchangeability. Also, the rails may look good, but if the cars are trash, the rails probably have some issues, maybe microscopic. Change both.

Most linear way machines I've worked on had shim plates between the block and mounting surface. When replacing the linear ways you almost always had to dust a bit off one or more plate to get correct geometry of the machine and accommodate differences in stack height.

Probably not the case with the OP's machine since I've never seen a cheap router built very well.