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View Full Version : How do Thomson linear rods work?



samualt
09-01-2004, 12:27 AM
I've seen some neat machines posted here on CNCZone but little details on some aspects of them. One being how to use Thomson linear rods. I see ads on Ebay (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=11810&item=3836870154&rd=1) where people are selling Thomson rails with bearings, but what good are they? I mean, you just get a cylinder type bearing that slides on a rod-like rail with seemingly no way to bolt to the bearing! Ad after ad shows the same thing. So how are you supposed to connect your flat metal plate to a round bearing that has no bolt holes?

:confused:

ynneb
09-01-2004, 12:43 AM
Yes a good question, I have wondered the same thing.
I have seen blocks that slide over the round bearings and then the blocks are attached to the machine. I dont know how the blocks are fastened to the bearings though. The other thing that strikes me odd with the rods is that they have to be fastened and supported from each end. This means that these rods are supporting the weight of the machine. I know you can get midway supports, but they look like trouble getting them in accurately.

I guess this is why you see a lot of people using THK rails. It solves all of the above problems.

ESjaavik
09-01-2004, 04:18 AM
It is just a bushing, but with the benefits of very low friction. You can make a slit in the bearing block and thus adjust the preload. You can also get them with a segment missing (open on one side) so it can be used on a fully supported rail.

The simplest for of bearing block is just a block with a hole. Or the hole can be part of the component it will carry.

samualt
09-01-2004, 05:13 AM
ESjaavik:
I still don't get it. Does a pillow block fit over the bushing/bearing? Or, are you saying the plate or device itself must have a cylindrical hole that the bushing/bearing fits into and clamps onto in some way. Sounds very complicated for such a simple linear bearing.

I've been pricing these things and it really looks much more expensive than THK rails. I mean, once you buy two rods, four end mounts for the rods, four bushings, and then figure on buying four pillow blocks or have four special bushing-holders machined, your going to end up paying 3 times or more what THK rails cost. I must be missing something...

ger21
09-01-2004, 08:52 AM
I just drilled some holes, slipped them in, and used snap rngs to hold them in place.

http://www.cnczone.com/forums/attachment.php?attachmentid=647

When I bought these, I thought it would be a lot cheaper than THK's. I spent about $70 for the 4 bearings and 2- 18" SS shafts. Since building this, I managed to find some 18" THK's for about $50. But it can take months to get good deals on Ebay.

If buying these new vs THK's, I'd get supported shaft, so no end supports needed, And integrate the bearings into the axis to eliminate the pillow blocks (They fit in standard size holes, and you can get adjustable ones to account for misalignment). This would probably still be a bit less than THK's, if purchased new. But with Ebay, if I were starting over, I'd go THK every time.