Rotating Ballnut - design ideas

[Boyan Silyavski]

Can't I just get a larger ID bearing for near the machined on gears to compensate for this?

I could explain deeper why not, but isn't it easier to bore 4$ pulley than machine a complex part and buy twice as expensive bearing for which you will need bigger aluminum housing which is also more money?????


I would say generally No. The short answer is: do as i suggest, its best for many reasons or before jumping to buy bigger bearing, just sit down and make a detailed inertia calculation, what snappiness of the machine you need, what will be your maximum speeds and so on. And if that will change the motor you need to bigger!

Why? lets have as an example my machine 8x4 / in reality 1300x2600mm in metric/. If you invest in long ballscrews you generally make the machine right. Which means the gantry will be heavy around 180 to 300kg. If you gantry on that span is less than 160kg, then go with Rack and Pinion as that means your machine is not made to cut aluminum, so save yourself the trouble.


Generally if one is there already, one will use servo motors to move the gantry. But next is even more valid if steppers are used.To obtain the right speeds of the machine, the screw will be xx10 , best is the 2510. No need for bigger screw.

So most probably a gearing has to be used or bigger servo motor. Which means much more money if motor is not Chinese.

Also i have found that with normal servo motor the most desired ration is like on mine machine 20t:30t. fast enough and geared enough. It gives me a machine that has 307.2 pulse por mm ---- 1/307.2=0.003 mm resolution with maximum speed 20m/min . In reality that speed is 30-40 as my servos can spin to 6k rpm instead of the normal servos 3k rpm

Anyway, if you feel like, of course there are bigger bearings. But it seems easier for me to preload correctly 2x the same bearings than different sized ones.

[mutzy]

So by attaching the Larger bearing to the smaller shaft/gear/bearing combo, it raises the whole inertia dilemma? If there is a possibility of lowering the height of the machined gears to let the 7207 pass over it might work. But do you still believe it's cheaper, easier to bore out a $4.00 pulley?

Mutzy

thanks for the help

[Boyan Silyavski]

So by attaching the Larger bearing to the smaller shaft/gear/bearing combo, it raises the whole inertia dilemma?

No. Changing the gear ratio raises the inertia dilemma.

If there is a possibility of lowering the height of the machined gears to let the 7207 pass over it might work

have to check that on some specifications but i think this was not possible. Willlook later at night

But do you still believe it's cheaper, easier to bore out a $4.00 pulley?

Or you will have to redraw the rotating part and the aluminum block part/ all the parts/ from scratch. I dont know what to believe, depends how you value your time.

[mutzy]

Boyan, were you able to put some brain cells around the idea of keeping the same number of teeth, but lowering the overall height of the machined gear?

Mutzy

[Boyan Silyavski]

Boyan, were you able to put some brain cells around the idea of keeping the same number of teeth, but lowering the overall height of the machined gear?

Mutzy

Yes, you made me do it

From table below we see that without changing anything, these 3 sized pulley will be possible to incorporate ion the rotating part without further change of components



So from these 3, it seems the best will be the P22-5M-15F . 22t is not so bad, outside diameter of the teeth is 35.01 so basically same as the 35mm of the OD of the part and the ID of the bearing. That will work.




Not to have in mind that 400w servos and bigger have shafts starting from 12 if not from 14mm in diameter. So smallest pulley there could be even the 12 teeth, also 14, 16, 18, 20. But in reality could be also 13, 15, 17, 19, as i can draw the pulley with as many teeth i would like to and it suits me.


from my Favourite belt calculator https://www.bbman.com/belt-length-calculator/ having in mind minimum distance from motor shaft to center of pulley is 90mm, and if bigger than 400W servo even 100mm, we could see that:

12t puley is No good, less than 6t engagement

But all puley more than 13t including are ok. Now it should be considered that the bending radius of 13t puley is too small, this for the durability of the belts. And believe me it will generate heat on the belt and pulley. But as belts is 3-4 $ its not of a great concern.


So yes, it could be done. 13t pulley and bigger.

now when you divide 22/13 , 22/14 etc. its not a whole number which i dont like. the first one that divides well is the 22/16=1.375 so nothing has to be rounded, 16t pulley i see as a minimum for a good bending radius. So that would be the one i would have used.


Obviously 2x 750W servos at both sides will do the job and if something extreme is desired even bigger could be fit. having in mind gantry around 180-250 kg and max machine speed at 30m/min with acceleration around 3000m/s2 which is my machine tested and it really rocks. You should do calculations again for the specific scenario but more or less thats it.
Also all time i am speaking of Samsung servos and brand servos in general. I have never tried the chinese servos but i think they will do well also. Plus really they are almost same price bigger or smaller/ servos must be correct size, not bigger or smaller /.

It should be noted that when i am engraving small stuff i lower the acceleration at 1000, talking about aluminum stamps and so. But this is very delicate work which normally is done on a mill.

I think that answers your question.

Boyan

[mutzy]

Thanks for your extremely detailed researxh. Now i'll think about what i need to do, i'keep you posted thanks. Mutzy

[Black Forest]

But all puley more than 13t including are ok. Now it should be considered that the bending radius of 13t puley is too small, this for the durability of the belts. And believe me it will generate heat on the belt and pulley. But as belts is 3-4 $ its not of a great concern.


Boyan

Remember that changing out a belt is no small project with the rotating nut design. So durability of a belt should be a big concern because of the downtime and hassle of disassembling everything in order to change out the belt. The whole ballscrew must be uninstalled to change a belt. No small task regardless of the size of the table.

[Boyan Silyavski]

Remember that changing out a belt is no small project with the rotating nut design. So durability of a belt should be a big concern because of the downtime and hassle of disassembling everything in order to change out the belt. The whole ballscrew must be uninstalled to change a belt. No small task regardless of the size of the table.

Exactly, that's why i said 16t as minimum. Everyday 8h machining belts would withstand at least say 2 years. Using it as a hobby machine at least 6-8 years.