Thread: Angular contact bearing source for ball screw supports?

I use double row bearings in one of my machines. Very happy with them.

Originally Posted by jhovel

you don;t have to worry about adjustment and preload and that's built in to match the bearing

There is no built in preload on double row angular contact bearings.

Yeah I will try to hunt some matched angular contact bearings (DF etc) from aliexpress and hopefully I get what I pay for. If not paypal resolution, but it won't be a huge amount lost anyway. Whether CA or B it doesn't matter hugely to me initially, as I can upgrade later.

I've done a bit more reading and seen some more people getting non matched pairs and making spacers, I just want to avoid this if it costs me an extra $50 / axis so be it. Nice to know its an option though.

I'm not confused about runout vs backlash, but I probably got them mixed up by typing. I've read arguments on using higher rated ABEC bearings for backlash elimination but I won't mention them here.

Overall I'm just looking to design the bearing block so I can order the screw to the right size and get things rolling. CAD work incoming...

Originally Posted by rcaffin

The trouble with Alibaba and AliExpress is that whatever is advertised is not what you end up with.
I was looking for a Crossed Roller Bearing, and Alibaba had several suppliers in the range of $1 to 420. OK, fine, hard to believe, but I asked for quotes. The quotes ranged from $95 - $230 per unit. There was some hand-waving about their advertised prices ('ah so sorry'), but by the end I did not believe ANY of them.
I didn't buy from them either.

Cheers
Roger

I order extensively from Aliexpress and have never had an issue. The quality is what you'd expect, and the platform far better then PayPal/ebay. As far as I'm aware the seller doesn't get paid until you provide a positive rating or 2 months have elapsed. They track the orders and want you to give feedback ASAP (of course) so there's a much higher incentive toi make sure they deliver what they say. Don't expect any Deckel parts to show up in your mailbox, but for the budget end of the market I've never had an item that doesn't meet expectations.

[QUOTE]I've read arguments on using higher rated ABEC bearings for backlash elimination but I won't mention them here.[/QUOTE]

People can argue about whatever they like. I can argue the sun won't rise tomorrow, and be passionate about it if I choose to. However based on established scientific principles, I'm going to bet it will. Based on the same scientific principles I'm going to say that a higher precision bearing won't help your backlash squat unless you increase the preload. This is not new stuff here, a ball screw support bearing is simply a spindle and there are well established spindle principles and designs. There have also been 100s millions of spindles produced over the years, some exceptionally cheaply, that use the same parts available now for peanuts.

Originally Posted by j3dprints

Yeah I will try to hunt some matched angular contact bearings (DF etc) from aliexpress

Matched bearings means the bearings dimensions all fall within 1/3rd of the total allowed tolerance of each other. It has nothing to do with anything else. You do not need "matched" bearings.

If you buy a set from china. How will you know they will even have preload ground into them? I would not trust a seller from China. Trust has to be earned, not given away.

The design should be for a home machine that you do not need such things as ground in preload bearings.

On the subject of over speccing I am selecting drives now too :P I posted this over at cnc zone but it seems you guys own machines yourselves, these look like my options:

I think I have arrived at 4 alternatives I need to choose between, I am liking the bang for buck for option 1 / 2. Any advice is appreciated. Servos look good but are pricey for me.

1) Leadshine open loop 3 phase steppers $255 AUD used with a motion controller and encoders on the axis
https://tinyurl.com/y9zhucyg

2) Leadshine closed loop steppers - $270 AUD
https://tinyurl.com/ybwxl3w4 . Can these be used in conjunction with an encoder on the load for positional feedback though? Sorry I do not know from looking at the diagram.
Also most other stores are $50 more, makes me pause.

3) Chinese servos - $422
albeit from a reputable Aliexpress operator BST Automation: https://tinyurl.com/yck8br4l
The more powerful servos don't go up by much more in price, and thus I would think you could overspec and re-use in another machine if upgrading.

4) DMM 400w servo - $441

Also 3/4 servos need belt drive setups which adds to the cost I guess.

Originally Posted by .RC.

There is no built in preload on double row angular contact bearings.

Are you sure about that, RC? The last time I installed a set, my perceived 'free running abilty' or rolling resistance of the bearing noticably changed as I torqued the retaining nut to the prescribed value. I interpreted that as a tangible indication that the two inner halves were pulled together with a specific pre-load.....
The type I referred to is this one (with split inner rings as I said):
Double-Row-Angular-Contact-Ball-Bearings-FAG.jpg
I know there are single inner and outer ring angular contact bearings, but they have considerable freedom and even allow a small amount of misalignment - so they would not be suitable.
Then there are single row 4-point contact bearings which have similar angular contact angles - in a much narrower frame - but I have never used one. I wonder if they would suit this application?
FPCBB_M-cage.jpg

2) Leadshine closed loop steppers - $270 AUD
https://tinyurl.com/ybwxl3w4 . Can these be used in conjunction with an encoder on the load for positional feedback though?

A caution here. Very few CNC controllers are able to take encoder feedback when driving a stepper motor.

Cheers
Roger

Originally Posted by rcaffin

A caution here. Very few CNC controllers are able to take encoder feedback when driving a stepper motor.

Cheers
Roger

Hey Roger,

In this case the controllers are designed for closed loop with inputs from the encoders. Thus no lost position if a step is missed. Similar to a servo system in that regard, though very different speed/torque curves. Of course you do have to use the matching controller to get this feature.

Cheers

The Beryl Bloke

In this case the controllers are designed for closed loop with inputs from the encoders. Thus no lost position if a step is missed.

But what can the controller do when it loses a step on a stepper motor? No use issuing another step command: once the stepper motor loses it, it's finished. About the only thing such a controller can do then is eStop. Well, OK.

Cheers
Roger

Originally Posted by jhovel

Are you sure about that, RC?

Nope as I have never seen that style before other then the expensive special ones with a split outer ring specially made for precision applications.

But upon researching the split inner ring type I can not find any info regarding ground in preload. The tables all seem to show they all have internal clearance, like all the other general purpose bearings.

You could in theory I guess grind or lap the inner rings so they come together and reduce the internal clearance.

Maybe it was just pure luck. But the two bearings I fitted both exhibited the 'ground in' preload behaviour. I think I said that they were both automotive wheel bearings.
I'm about to complete a pipe centre with another of this type of bearing. Just the one bearing. When I assemble it, I'll take some photos of the parts and bearing and describe the bearing behaviour - one way or another. I'm hoping the bearing will provide both thrust and radial support, without any angular or axial movement possible. We'll see.

Interestingly, I have done more servo / stepper research.
Steppers are basically good enough, they are used on Tormach and I think most of us can agree we would be happy with a machine that performs like that. It does have proprietary software that probably goes some way to ensuring the machining you tell it to do will not cause lost steps (300ipm facemill 4mm depth of cut into stainless etc lol). Tormach now use 3 phase steppers, which have slightly better torque curves (but still not quite like servos). Their statement in their literature is that most hobbbyists give steppers a bad rep from overdriving them and losing steps.
Some people on US forums say after mapping their ballscrews, and accounting for backlash they can hold 0.001 inches with steppers, though they use feedback. 8Nm servos are commonly used on RF-45 machines for this purpose on x/y and leadshine provide a $300 solution per axis (minus power supply) you can access on aliexpress.

The only 'issue' is you can buy Chinese servos for the same price! Decisions, decisions. The vendors on aliexpress e.g. BST Automation who supply such servos have very good reps for other parts, e.g. genuine Hiwin rails and good ballscrews. The motors however are 'unproven' unlike Leadshine easy servo hybrid steppers which I think would be a sure bet.
I started a thread on cnczone hoping for an answer - Are Chinese servos a gamble? Servos vs Leadshine closed loop

Originally Posted by rcaffin

But what can the controller do when it loses a step on a stepper motor?

Sorry, I seem to have put that poorly. To quote the Leadshine blurb "The system includes a 2-phase stepper motor combined with a fully digital, high performance drive and an internal encoder which is used to close the position, velocity and current loops in real time"

So my understanding is that if a step command is issued to the motor and the motor's angular position does not change according to the encoder, the driver does what it can to correct that. That is probably not another step command as such, although I am guessing here. For all I know it may attempt to resend the original step command until it's executed in full. This process repeats a number of times before the system will stop and an error code produced. My understanding is likely simplistic.

In my simply way of looking at things the closed loop does the same function be it for steppers or servos.

Cheers

The Beryl Bloke

most hobbyists give steppers a bad rep from overdriving them and losing steps.

I will irritate a lot of very good people here and suggest that more than half of the comments in this area are made by people who have not actually got any practical experience. They are just channelling other peoples' comments.

Cheers
Roger