Ballscrew & Linear Rail Kit - Any Good

[JAZZCNC]

...for that price...? I mean, seriously, if the bearings are crap you buy some SKF quality bearings and replace them. It's a 5 minute job. Mine are so far perfectly OK. I just bought another kit. This one:

https://www.ebay.com/itm/393113635297

Will get it by next week and I expect it to be as good as the previous ones, which I bought on Aliexpress and not from the same seller.

Not a fair comment really because Clive S makes a valid point.? Many of these kits will and do come with standard deep grove bearings which are completely unsuitable for CNC or at best they use cheap nasty angular contact bearings which need constant adjustment and wear out within months.
Many many people have bought them only to find this out after the machine is built when they are chasing chatter or error from backlash etc.

SKF Angular contact bearings will cost £20 to £30 per pair and for the same amount or not much more you can buy much higher quality C5 BK bearings from Fred at BST. These are light years different in quality and will last years. So sometimes it's false economy to buy too cheap.

I agree these kits are light years better than threaded rod etc and are a great way to get started building a CNC machine provided you don't expect top quality and accept they will wear out and become sloppy quickly if not maintained and cleaned on a very regular basis. They won't tolerate harsh conditions very well either.

[WoodKnot]

Thanks for the advice on the bearings!

I was not sure if they even came with bearings?

I will take an image if there is any kind of stamp on them.

Presume I just take internal and external measurements and order the replacements

[A_Camera]

Thanks for the advice on the bearings!

I was not sure if they even came with bearings?

I will take an image if there is any kind of stamp on them.

Presume I just take internal and external measurements and order the replacements

They always come with bearings, one double bearing in the BK and one single in the BF part.

Once you remove the four screws holding the dust cover of the BK12 you will see two bearings. I think the BF12 has a similar (but only one) bearing, but that is the floating end, so it isn't really critical. They are simple 6001Z 28x12x8 bearings.





These are cheap, but never the less can be very good. 6001Z is one of the most common type, so they are cheap, even if you'd replace them with an SKF or similar. https://www.skf.com/sg/products/roll...oductid-6001-Z

Of course, if you replace them, you might as well replace them with the best ...or not. Replacing 8 BK bearings if you have four screws can cost between 250 - 400 USD (maybe even more), so you must take it as a fairly large cost for an improvement of a hobby machine. There is no need for rushing, it's easy to do even later on, once you have everything up and running.

Never the less, I would not recommend you to start with replacing them, since there is really no reason, unless you really feel some serious issue, or play. I know I can't feel any, so even if some people laugh at them, let them laugh. Replacing is easy, but you need a proper tool, because these are really tight sitting in their seats. Don't use a hammer to get them out because that might damage the seating. There are tools to remove bearings, get one if you not already have one. Anyway, I don't think you will need to replace them unless you really feel there is something wrong with them. When it comes to wear and tear, I don't think you'll ever wear out these cheap bearings. It's a difference if you'd build a professional placing machine with high speed servos running H24/7, but as a hobby machine with slow rotating ball screws you will never come near the performance limit. In fact, inlines and roller skate bearings get a lot more beating than these, after all, these always rotate on smooth surface and at comparably very low speed.

[Clive S]

They are simple 6001Z 28x12x8 bearings.

But they should be AC bearings back to back to get the preload. This is why I asked him to check !

[A_Camera]

But they should be AC bearings back to back to get the preload. This is why I asked him to check !

It should not be anything else than what they have specified. If you didn't ask and is not stated then that's it, be happy if the 6001Z is of good quality.

[JAZZCNC]

Never the less, I would not recommend you to start with replacing them, since there is really no reason, unless you really feel some serious issue, or play. I know I can't feel any, so even if some people laugh at them, let them laugh.

It's not about laughing at people it's about trying to let people know the difference between building the right vs the wrong way. Everything about these is Wrong and pointless for a CNC machine.
They are deliberately made and sold this way to keep the price low to make the kits look cheap with NO regard to whether fit for purpose or not.!

Those bearings in the BK mount are completely wrong for the application as they are not designed for lateral loads, they should have Angular contact bearings, not deep groove bearings like 6001ZZ.
They will wear excessively and they will introduce backlash, even if fit with SKF bearings because they are not designed to operate with lateral loads. Also, to get preload on the bearings and reduce backlash puts extra load on the bearings causing friction which robs power from the motors, again causing extra unnecessary stress on other components like drives, PSU, and lowering performance.

I don't understand why anyone would buy components that are not fit for the purpose, then spend more money on them to put or right and even worse spend money on them replacing bearings with the same inappropriate bearings. Especially when buying correctly the first time isn't that much more expensive and far less hassle.

[A_Camera]

I quoted the SKF page to show the specs, not that they should be replaced with SKF version of the 6001Z. Of course, if one replaces the bearings one should replace them with better quality AC bearings. To replace 6001Z with similar (but better quality) 6001Z is pointless, or not a good idea.

Anyway, to be honest, I don't think you'd wear out these bearings unless you are a heavy user of the machine. Most hobby users don't run their machines for hours every day, the speed is low (even with fast servos) and is very far from the performance top of those bearings so wearing out will take time. But sure, eventually they may wear out, and in that case they should be replaced, but as I said earlier, replacing is easy and can be done when needed, will result in a few minutes of work per screw, so no big deal.

[JAZZCNC]

Anyway, to be honest, I don't think you'd wear out these bearings unless you are a heavy user of the machine. Most hobby users don't run their machines for hours every day, the speed is low (even with fast servos) and is very far from the performance top of those bearings so wearing out will take time.

Sorry but your completely wrong on this assumption because there have been many examples here on this forum of people who have bought these BK bearings and even the cheaper BK which did use AC bearings and have had Big problems with backlash caused by excessive wear within the first few months of usage.

I myself had the same issues 12+yrs ago when ball-screws and BK bearings were not commonly available, and not at all on eBay. So buying from China thru aliexpress was the only choice. I myself and many others at that time nearly always had to shim and tweak the bearings and these were AC but low quality and poorly fit, often fit the wrong way around. And I'm sorry to say nothing has changed in this respect with these BK bearings and in fact, I have noticed over the years it's actually got worse.?

To satisfy the demand for cheap kits they started fitting them with deep groove bearings which like I've said above are completely unsuitable for CNC, Esp low spec Machines even more so because they often don't have the excess power to compensate for the excessive friction caused by preloading the bearings enough to eliminate backlash.
It's a catch 22 position.? If you DON'T preload you have backlash, if you DO preload the bearings can not handle the lateral loads and wear excessively. Then throw into the mix the constant checking adjustment to keep backlash out of the machine and it quickly becomes obvious why they are best avoided.

Anyone who thinks differently is just fooling themselves and others.


The 6001ZZ or any other deep grove bearings will easily handle the RPM's but that's not what kills them it's the lateral loads, they are not designed to take lateral loads and virtually all the load on a ball screw is lateral and it's this that kills them quickly.

[A_Camera]

I am not saying that there are no lemons, but that most are OK for hobby use and there is no reason to rush or assume that everything is bad. Of course if one is replacing the bearings one might as well do it right and get the proper ones. But you can't say that just because some people have issues all are having them. In my experience there was a huge quality improvement in these things the last few years, can't compare with how things were like ten years ago. Yes, they do use these simple bearings, but unless it is specified otherwise, there is nothing wrong with that, they didn't fool you. It's not for industrial use, if it was then I'd be more critical about them.

[Clive S]

I am not saying that there are no lemons, but that most are OK for hobby use and there is no reason to rush or assume that everything is bad. Of course if one is replacing the bearings one might as well do it right and get the proper ones. But you can't say that just because some people have issues all are having them. In my experience there was a huge quality improvement in these things the last few years, can't compare with how things were like ten years ago. Yes, they do use these simple bearings, but unless it is specified otherwise, there is nothing wrong with that, they didn't fool you. It's not for industrial use, if it was then I'd be more critical about them.

Yes, they do use these simple bearings, but unless it is specified otherwise, there is nothing wrong with that, they didn't fool you. It's not for industrial use, if it was then I'd be more critical about them.

That's the point they were not specified that's why I asked him to check. They are being sold in a kit with rails so should be AC type.

Quote Originally Posted by Clive S View Post
Have you checked what bearings are in the BK end bearings

Originally Posted by A_Camera
...for that price...? I mean, seriously, if the bearings are crap you buy some SKF quality bearings and replace them. It's a 5 minute job. Mine are so far perfectly OK. I just bought another kit. This one:

As the op is a beginner he would not know the difference between the bearings so why do you muddy the waters.