Rotating Ballnut - design ideas

[Andrew Wilding]

Jonathan,
You can ignore the stuff about critical frequecy/deflection of the screw with the spinning nut. I had a dim moment and forgot you were supporting the nut with bearings. Its late or at least that is my excuse!:cry:

Agreed with the 4th power bit but that would make the nut inertia even larger than the screw. How does moment of inerta or rotating bits compare with mass of linear bits when you look at a good old newton second law equation?

going tp bed now.

[Jonathan]

If I was to spin the screw the unsupported length would be 1800mm and overall length about 1900mm, so the calculations below are based on that.

J = m*pitch^2/(2*pi)^2

J = 45*0.01^2/4/pi^2 = 0.114 g-m^2

For the screw:

J = 0.5*m*r^2
Steel --> 7850Kg/m^3
Therefore:
J = 0.5*7.85*pi*r^4*l

So for mine:
J = 0.5*7.85*pi*0.0125^4*1.9 = 0.572 g-m^2

That makes the moment of inertia of the screw 5 times that of the gantry.

I will estimate J for the ballnut assembly.

[Jonathan]

Not much to add to the actual design here, just a 3D drawing which clarifies the bearing mounts:



Solid Edge got a bit carried away with the reflections here:

[Jonathan]

You can ignore the stuff about critical frequecy/deflection of the screw with the spinning nut. I had a dim moment and forgot you were supporting the nut with bearings. Its late or at least that is my excuse!:cry:

Interestingly I've since found that when you rotate the ballnut the critical frequency still has an applies - the end fixity is just better which, in my case, gives 1250rpm. So 12500mm/min. I think I'll be lucky to get that much out of my motors anyway so I'm not going to worry.

I just got prices for the screws:

2 of RM1610-2000mm with nut and end machined(both ends machined same as A type )
238 usd
1 of RM1610-900mm with nut and end machined(standard end machining)
72 usd
Sub-total
310 usd
Air express shipping
127 usd
Total
437 usd
-----------------------
Offer tow:

2 of RM2510-2000mm with nut and end machined(both ends machined same as A type )
269 usd
1 of RM1610-900mm with nut and end machined(standard end machinings)
72 usd
Sub-total
341 usd
Air express shipping
168 usd
Total
509usd

I'll go for the second option - the price difference is not as much as I expected. The smaller screw is for the Y-axis. I've decided to get both ends of the big screws machined to fit the BK type support ... that should enable me to tension the screw using the nut on either end.

I have also acquired a pair of 7206 FAG bearings cheaply, so I'm hoping I can use them instead of 7207. The obvious problem is that the bearing bore is only 30mm, and with the screw passing through it being 25mm the shaft will be very thin - about 1.5mm.

[Jonathan]

The ballscrews arrived today - I paid for them on 08/06 and I requested different end machining (longer portion for pulley and same machining on both ends of screw) so I think that's good service. He also charged me $447 in total even though I ordered the 25mm screws, so $72 less than originally quoted. The ballnuts backdrive easily and they seem nice and smooth.

I will start working on the rotating ballnut mount soon. I'm currently doing one last 3D drawing. I will use 5/8" aluminium for the plates that hold the bearings with some aluminium 'posts' to hold the plates together parallel at a fixed distance.

I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.

[Robin Hewitt]

I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.

Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:

[Jonathan]

Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:

That explains a lot!

I've just estimated the moment of inertia of the rotating bits of the assembly, so pulleys shaft, ballnut and bearings. Surprisingly it comes to slightly more than the inertia of the 2085mm, 25mm diameter screw! That's annoying since one of my main reasons for doing this was because I thought this value would be much lower. I've not included the inertia of the bearings I would require to rotate the screw, and I suppose I would use pulleys with it anyway. The inertia of the ballnut and shaft is still about two thirds of that of the screw.
This is going to make a big difference to the rapid speeds I can get since the torque required to move the 50kg gantry, due to the low coefficient of friction of the bearings, is very small.

Now I'm going to try modelling it with the stepper at different ratios - not 1:1, and see what happens.