Under the hood maintenance update.
Its been 2.5 years since the first iteration of the machine. The ballscrew bearings have started to sound strange. I am not talking about the ballnut bearings. Those have been recently lubricated and run super smooth. There’s a bearing block on each side of a ballscrew. A fixed block on the side of the motor and a free block on the far end. The fixed side is the most critical. This is not something that can be dealt with lubrication, they need to be replaced.
A 1605 ballscrew uses the FK12 or BK12 type of bearing blocks on the fixed side. The blocks themselves have a very simple geometry (i.e. are easy to manufacture). But then why some of them can be found for $10 or less and some for $50 or more? Cause maybe the bearing blocks are identical, but the bearings inside them are not. The former category carry deep groove ball bearings (aka skate bearings), the latter carry angular contact ball bearings.
Guess which kind mine were…
So I started researching about angular contact ball bearings. In essence angular contact ball bearings are designed to withstand both radial and axial loads (as opposed to deep groove ball bearings’ radial only ability). They are available in a variety of configurations, but for this particular application I will just focus on the duplex face-to-face (DF) and duplex back-to-back (DB) arrangements. Here’s how I tried to visually summarize them.
In DF the load lines converge towards the bearing axis, in DB they diverge from the bearing axis. This makes DB more rigid, but DF more forgiving. Both arrangements need to be preloaded. In DF the inner races touch and the outer races need to be pushed together. In DB the outer races touch and the inner races need to be pushed together. The best way to get this right and not have to fiddle with spacers and guesswork is to get them in matched pairs. In that case the preload (i.e. gap between them) is specific and already taken care of during manufacturing.
The contact angle is significant as the higher the angle the higher the axial load that can be handled (image below taken from SKF).
As angular contact bearings are not a commodity (compared to skate bearings), there are not so many retail options out there. These are the ones I ended up with as a sweet spot between angle, precision, preload, price: 7001 for size, AC for 25deg contact angle (as opposed to C for 15deg), P5 for precision (DIN P5 = ISO Cass 5 = ANSI ABEC 5), DF for, well, DF and A for light preload. They come as a matched pair.
(Since I was going to swap the fixed sides, I got some new ones for the free sides too. There’s a single deep groove bearing there that can float in the bearing block.)
Are they really P5? All I can tell is that they wring together. I’ve only seen that with a pair of 1-2-3 blocks before, so some precision grinding is definitely there.
The total preload is between 0.10mm and 0.15mm.
Making sure that this
minus this
is more than the preload. No shim needed, bolting the top plate on the main body is enough to apply the preload.
Swapping was a breeze. It was understanding what I was doing and why that took most of the time.