SawStop Sliding CrossCut Table review #1: Bearing Design review

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Blog entry by mummykicks posted 04-22-2016 03:56 AM 1513 reads 0 times favorited 0 comments Add to Favorites Watch
no previous part Part 1 of SawStop Sliding CrossCut Table review series Part 2: Table, fence, etc »

I’m a mechanical engineer with over 25 years of doing precision mechanical design. Everything from instruments used on the telescopes of Mauna Kea, to semiconductor capital equipment, to automation tooling and biomedical instruments.
I’ve designed more than a few motion systems in my time and have used linear guides of various types in the past and am very familiar with them. This of course is a significant handicap as a woodworker hobbyist, as I will like as not take something simple and complicate it.

Cross cut table bearing system review:

More than you probably want to know:
The most common linear guides used are recirculating bearing type. All types use ball bearings or roller bearings that roll but do not slide. The carriage on recirculating types does exactly what it sounds like. It picks the balls up from the back and moves them to the front, just like people using logs to move something heavy, they have to continually grab the ones that roll out the back and move them to the front.

These things are amazing, and the first time you size one for a given load, the bearing you get is impossibly small. Inevitably you grab the next size up (or two) just because it looks so out of scale relative to everything else.
As far as a cross cut table is concerned it isn’t the best choice, not from a load perspective, but from a footprint one.
You only get as much travel out of the stage as the length of rail you can fit, and you need at least two of the carriages separated by some amount. So if your table is 48” long, at best you’d get something like 40” of travel out of it.

Enter cross roller bearings, which are exactly the same as those logs. They are cylinders which ride in V shaped guides, and using them (or what the cross cut table uses) you get far more travel because the rollers roll along the bottom and cause the top to roll as as well. So for the cct a table of 47.5” can achieve 60” of travel.
The cct design is slightly different, and a bearing type I haven’t seen before (that doesn’t make it bad).
It uses sets of 3 ball bearings in V shaped guides rather than the traditional cross roller design which uses, well, rollers. I had to search quite a bit to figure out where they sourced them, and it appears to be from SKF, as none of the usual suspects (THK/NSK/Star linear) seemed to carry them.

IF the sawstop guys did the math (and I’m reasonably sure they did) there is no reason to doubt they are up to the task, but balls on flat surfaces are normally a bad idea because of contact stress, which is one of the reasons people use cross rollers because they provide line contact vs. point contact. Why didn’t they use the more traditional cross roller design? My guess is cost, but then there are much higher volumes and varieties of cross rollers out there so maybe it was so they didn’t have to seal them to keep dust out. The ball bearings can push dirt and debri out of the way easier than a roller which is likely to skid.

Contact stress can be very high, as it’s simply a force divided by the area available to support the load (google it if you are really nerdy or bored). The balls are not infinitely stiff, and neither are the rails, so they deform (flatten out slightly) until there is sufficient contact area to support the load. If the load is too high it flattens further until it deforms so much it exceeds the yield stress of the material. It inevitably leads to visible lines in the guides that now have been permanently deformed in the shape of the ball. This doesn’t necessarily mean it will fail, but it might have reduced life. Keep in mind that this things get rated in thousands of kilometers of travel under a given load, so a cct application is pretty benign.

The Youtube video sales pitch for this thing had the SS rep say something interesting about being able to adjust the play in the slide because the metal might move over time. Properly sized linear guides will NEVER wear out in an application like this unless they are run dry at many times their design load. Which makes me think they cut it close on this design for cost reasons. While I wouldn’t have used them, SKF has their act together, and if the sawstop guys sized it according to what SKF recommends it should be fine. If you are in a place that has one of these tables you can look at the V guide and see the line I’m talking about.

The guides are pre-loaded against each other to remove play and stiffen the bearing, and if there is excessive where one side can be adjusted to take out the play, however at that point the guides will likely soon fail due to spalling. Once you workharden the rail it embrittles and you can get chunks that come off over time. Again, if it’s sized right it shouldn’t be a problem, and it appears to be so at least from what I can tell. In any event I won’t be taking any rides on it like the felder guys demonstrate, but I won’t be worrying about my hobbyist activity wearing it out either.

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