Rotating Ballnut - design ideas

[mutzy]

Hello to all. I need some advice. I finally got back to tackling the rotating ballnut. After drawing on turbocad the htd gears on th shaft that fits into the bearing box/block(Shaft with gears machined on it with head piece that attaches to the ballnut) I realized that the 7207 2rs bearings will not fit over the machined gears or the large round headpiece that attaches to the ballnut. If I just find a larger ID like 47 or 50 angular contact bearing with 80 or 90 OD, can I substitute it? Of course I'll make the bearing box accomodate it. Thanks Mutzy

[Boyan Silyavski]

Hello to all. I need some advice. I finally got back to tackling the rotating ballnut. After drawing on turbocad the htd gears on th shaft that fits into the bearing box/block(Shaft with gears machined on it with head piece that attaches to the ballnut) I realized that the 7207 2rs bearings will not fit over the machined gears or the large round headpiece that attaches to the ballnut. If I just find a larger ID like 47 or 50 angular contact bearing with 80 or 90 OD, can I substitute it? Of course I'll make the bearing box accomodate it. Thanks Mutzy

Hows that will not fit? Over no, but there is a sequence of pressing the parts together. That's why the pulley is bored from a separate pulley and then pressed together. if you machine right on shaft, then you will have to machine a smaller pulley so the bearing passes on top of it. But then the ball screwe size pass trough hole, so you will have to check if that's at all possible. I believe it was possible but not sure now. Have to check

[mutzy]

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

[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