thomson linear?

[gcn2339]

I have a pair of spb10 bearings and I have 1 to 2 degrees of side to side motion... meaning I can rock the bar that is mounted on the rails... and you can see the bearing give....I talked to one company about replacing these bearings but they say this movement is normal?

Do linear bearings have this kind of play?

[Rhodan]

These aren't like the trucks that are almost crazy-glued to their rails. I find that the center area has lots of contact but the ends are sloppy. A single bearing I can rock end to end but if you install two on one rail and bolt a bar across them they are completely steady in every direction (except around the rails since there's nothing stopping that motion).

One on each bar as shown with something bolted across them would let you rock them back and forth in the directions of the shaft ends. You really need four of these on any axis - two on each rail.

[gth629e]

Hello,

I agree that that an ideal situation you would have 2 bearing blocks with bushings on each rod. However, I have here in my lap an spb10 block and bushing on an official thomson 10mm rod and there is NO slop end to end.

Of course I am only applying minimal force with my hand and feel no slop. I am not trying to discredit Rhodan and do not know the official tolerances allowed. You should be able to look them up on their website. Even so, in my experience if you have the correct rod for the correct bushing, and the correct bushing in the correct bushing block, you would not have any play.

How long have you had these installed, did the bearings leave raceways in the rod to allow slop?

Is the rod case hardened? Fully Hardened? Is it an officially linear motion 10mm bearing rod?

Is it possible that it is a standard rod instead of a metric one, sometimes people make substitutions on eBay lol. (I doubt thats the case).


Anyway, I hope I have offered some insight and not confused you further.

Feel free to check out my new website www.cncroutersource.com. Its growing fast and is updated daily, its only a few days old.

Brian

Brian@cncroutersource.com

[Rhodan]

Well, its quite possible they're supposed to be rock solid. I've only ever had used ones lol.

[the4thseal]

there is a kind of bushing block that has a "self aligning" tolerance to help with slight alignment errors....i do not know how those feel but it might be what you have....there are in the Thomson catalog.

[gcn2339]

it doesn't look like a case hardened rod.... my calipers measure dead on what is suppose to be though. I’m going to try replacing them and the rods just to be sure and I'll add another set of blocks.

[elwoodbeauchamp]

Originally Posted by gcn2339 it doesn't look like a case hardened rod.... my calipers measure dead on what is suppose to be though. I’m going to try replacing them and the rods just to be sure and I'll add another set of blocks.

Hi gcn2339

There are several factors that determine how tight the bearing and rod fit together.

1, are these bearings and rods "out of the box" new?

2, if so, do you know what class fit they are?

3, There are "got them at a great price" bearings and then there are "Precision" bearing fit. "self aligning" are just that. They allow some degree of slack in order for them to fit in a not-so precision application. You can call most all suppliers of linear bearings and they can give you a better idea how "precision" they need to be for your application.

4, 1 or more balls have been removed by accident when installing the rod and bearing(not that I have ever done that...) or by some-one adjusting the fit to be just a hair looser. If a new bearing and shaft are of the tighter fitting precision variety, they will seem to be "binding" even when perfectly matched. You cannot seem to make them glide smoothe.....by hand, anyway. Once they are mounted, aligned and bolted tight, you would swear they are not the ones you just had in your hand. I've seen this done way to many times...

5, Is your guide rod TGP (turned, ground and polished) , hardened and ground like Thompson rod, or drill rod or equivalent guide stock? If the rod is a stock size of 1018, or 4130 or drill rod, etc. most are already 0.02-0.07mm under size. These are pretty standard for press fit bushings to ride on as the bushing ID will usually squeeze down to the stock rod size.

There are still other factors that cause the rod to become "loose" in the bearing. These are "Linear Bearings", but, they are not for a single bearing on a rod application, however, an extra long bearing carriage available, that would, in your situation, reduce the slack by just a hair. Sometimes a hair is all you need to smooth the bump out of your application.

I'm sure you will find a solution soon enough.

[Rendisha]

If those blocks have super type bearings installed, then they do have self aligning feature. I have some of them and outer surface of races (one that touches bearing block) is convex so that it keeps contact with bearing block in the center of it's length - it acts like a rocker. For some reason, I never managed to download their datasheets, but if they are comparable with Rexroth super type bearings, then they should have some 0.5 degree of flex built into them. Yours seems to be a bit too much

[schbrownie]

Here's some additional info on bearings:

All of the bearing guys (Thomson, THK, NB, INA, Pacific Bearing, SKF, etc) have two types of bearings: a precision fit which has a flat exterior and a self-aligning fit which has a convex/football-shaped exterior. The self aligning bearings usually can rock about 1/2 degree in each direction, giving a total variance of 1 degree. In addition, bearings are setup with different preloads (each company calles it something different). Basically there's a standard ball bushing and a preloaded ball bushing. The bearings are basically the same, except the preloaded ball bushing uses balls that are a few ten-thousandths of an inch larger to make the bearing fit more tightly on the shaft.

Add to this that shafting comes with several different tolerance classes and the nominal dimension of these shafts range from 0.003" to 0.0002" under nomial (keeping in mind that 0.0005" will make a HUGE difference).

Add both of the two factors mentioned above together and you can easily generate more than 1 degree of misalignment. Use a micrometer and measure your shaft to make sure that you the correct sized shaft for your bearing. The outside of the pillowblock should have some kind of markings on it to tell you what the part number is and you can go to Thomson's website to find out the details about that bearing and what kind of shafting you should use with it.

Misalignment can severly reduce the useful life of a bearing and if you feel binding in your system you should do what you can to re-align your system.

By the way, Pacific Bearing and Thomson also make teflon based plane bearings and those bearings are more tolerant to using the wrong sized shaft and you won't lower the life of your bearings by using the wrong sized shafting (too rough of a shaft could lower the life, though). The bearings from Pacific have twice the load rating than those from Thomson (20,000 vs 10,000) and are less expensive. From those that I have talked to... Pacific is THE name for plane/teflon bearings and Thomson is THE name for ball bushings (although people don't speak as highly of Thomson as they used to now that they were bought by Danaher)

Hope this helps

~Jon