acme screw streses??

[mrbaseball]

I just received some 1/4 inch flange bearings that were samples that I planned on using to support the ends of the acme screw for the x, y and z motion. they are really small and I'm concerned that they may not be heavyduty enough. There should not be any stress on the acme screw ends right?? they are not supporting anything other than the acme screws weight? The bearing are just there to support the free movement of the acme screws.

thanks

[fyffe555]

Ah, no.. the bearings support the screws rotation yes but if they are the only bearings on the screw they also locate the screw so there's a significant load stress along the axis that the bearings pass to the machine frame. That is, the screw is turned and the force applied to the nut with an equivalent force applied back to the frame of the machine via the screw bearings. So, for a screw with, say 0.1" lead and a motor of 200oz-in torque the bearings have to carry up to 785lbs...

[mrbaseball]

How do folks get away by using NO bearings on their machines?? i.e.- Jgro style machines??

[mrbaseball]

Here u what I received an was planing on using. I was goign to place two on each axis to support and enable the acme screw rods.

http://www.igus.com/igubal/sizepresent.asp?p=EFOI

EFOI-04

load : http://igus.bdol.com/igubal/kbrm2x.asp

So I guess these won't cut it??

I'm using 1/2 inch x 10 acme screw with the ends turned down to 1/4 inch.

[fyffe555]

OK, good questions. the JGRO machines do use bearings, or at least the original drawings had them.. Some people use no bearing at the motor and this is a bad idea as the motor and its bearings where not designed to support and take the thrust from the screw. If the loads exceed the motor bearings ratings ( some are less than 20lbs) then you damage the motor.

Going back to the loads on the screw bearings. You have to support the screw so it will rotate. You also have to locate the screw and stop it moving along its axis. If the screw is not fixed in position it will move, as will the gantry and so you get backlash. You're producing a force with the screw to move the axis, that force is also trying to move the screw along its axis, the bearings support along the axis must be equal to that needed to move the gantry while cutting at least. If you've got 200oz in motors and 1/2-10 acme then that force available can be up to ~700lbs. If the gantry is not able to move for some reason the entire 700lbs would be the force that's applied to the screw bearings along the axis. If the typical force needed to move the gantry and cut is 20lbs then that's what would be applied to the bearings, if th eforce is 100lbs then thats what would be applied to the bearing. If you use your igus bearings then they can support a maximum of ~120lbs static and ~28lbs dynamic which isn't a lot but might be enough for what you're cutting. If you hit the stops on your gantry or cut something that produces more than 28lbs force you'll damage something.

The igus bearings are not thrust bearings. You could use these for rotational support and add thrust bearings to one end of the screw to positively hold the end of the screw in place.

The JGRO design uses a regular roller bearing as both support and thrust bearing. The screw is clamped to the bearings inner race by a nut on either side of the inner race. While these bearings are not really thrust bearings they are still strong enough to carry the thrust loads that a machine of the JGRO size would produce. Worst case the bearing would wear prematurely and you replace them.

Andrew

[mrbaseball]

OK, good questions. the JGRO machines do use bearings, or at least the original drawings had them.. Some people use no bearing at the motor and this is a bad idea as the motor and its bearings where not designed to support and take the thrust from the screw. If the loads exceed the motor bearings ratings ( some are less than 20lbs) then you damage the motor.

Going back to the loads on the screw bearings. You have to support the screw so it will rotate. You also have to locate the screw and stop it moving along its axis. If the screw is not fixed in position it will move, as will the gantry and so you get backlash. You're producing a force with the screw to move the axis, that force is also trying to move the screw along its axis, the bearings support along the axis must be equal to that needed to move the gantry while cutting at least. If you've got 200oz in motors and 1/2-10 acme then that force available can be up to ~700lbs. If the gantry is not able to move for some reason the entire 700lbs would be the force that's applied to the screw bearings along the axis. If the typical force needed to move the gantry and cut is 20lbs then that's what would be applied to the bearings, if th eforce is 100lbs then thats what would be applied to the bearing. If you use your igus bearings then they can support a maximum of ~120lbs static and ~28lbs dynamic which isn't a lot but might be enough for what you're cutting. If you hit the stops on your gantry or cut something that produces more than 28lbs force you'll damage something.

The igus bearings are not thrust bearings. You could use these for rotational support and add thrust bearings to one end of the screw to positively hold the end of the screw in place.

The JGRO design uses a regular roller bearing as both support and thrust bearing. The screw is clamped to the bearings inner race by a nut on either side of the inner race. While these bearings are not really thrust bearings they are still strong enough to carry the thrust loads that a machine of the JGRO size would produce. Worst case the bearing would wear prematurely and you replace them.

Andrew

OK, thanks, where are some good sources for thrust bearings?? Do I need two thrust bearings per acme rod?? X? Y? Z? or just one??? Do they make flange mount thrust bearings??

Will these work?? http://www.mcmaster.com Part # 6655K13

OR

http://www.qbcbearings.com/RFQ/B610HTML/B610P035.html 1/4 inch

thanks again for the reply

[jeffs555]

You would need two thrust bearings per screw. The thrust bearings you linked would work if you could come up with some way to mount them. Most people use angular contact bearings, which handle both radial and axial loads. Angular contact bearings are directional, so you use two per screw, and mount them back to back. You can also get dual row angular contact bearings which are like two single row bearings mounted back to back. Like Andrew said, on many home machines, people just use radial bearings, and accept the fact that the axial loads will wear them faster than normal. For light home use, they can last a very long time.

Jeff