BK Style Bearing Blocks for Ball Screws

[Machinery Jones]

I'm toying with the idea of designing my own 3-axis milling machine and to that end I was looking at some ball screw kits and and noticed a lot of them come with the 'BK' style bearing mounts. I understand that they're used to prevent axial forces being transmitted to the stepper motor during operation, but I can't get my head around how they achieve this with the way they're designed. Could someone please explain how they work?

[Muzzer]

This gives a good explanation.
https://www.thk.com/jp/en/products/ball_screw/support_unit/

I typed a reply but got a "bad gateway" when I tried to post. I usually copy the message before trying to post, just in case - but Sod's Law. Doh!

BK is a fixed brg that takes radial and axial thrust, whereas BF only provides radial support. If the ballscrew expands or contracts, no stress is imparted to the ballscrew or machine frame. The stepper is next to the BK so that there is no thrust on it.

[Machinery Jones]

Thanks for the link, some interesting info there. I've been having a couple of issues with the bad gateway error too (sorry for the late reply btw). I'm still a little bit confused about the need for two bearings on the fixed side, can you shed any light on this?

[Muzzer]

Each of those "angular contact" bearings is only designed to withstand thrust in one axial direction, so they are used in a back to back pair, ideally with some controlled preload. If they used only one such bearing, there would be some unhelpful backlash.

Yes, there's some problem with the page timing out due to this bad gateway issue. Must be driving Lee round the bend! Make sure you copy the message text before trying to post it!

[Machinery Jones]

Thanks, that's cleared it up for me. Hope Mr. Lee can get to the bottom of this issue!

[Lee Roberts]

Im on it tonight guys, ive had +343 emails about it today alone...

Sorry and thank you x