View Full Version : Ball screws?
10-16-2001, 09:24 PM
What exactly is a ball screw? and How does it work? I understand that it eliminates backlash but where do the balls go do they recirculate? My 14th edition Machinery's Handbook doesn't mention ball screws. Is there a pattern or thread class? Are they more precise and why?
10-17-2001, 03:53 AM
Ball screw come in various classes of precision. You can have ball screws that are roll formed and fairly accurate. High Grade precision ball screws are normally milled, hardened, and ground on CNC machines.
The Ball nut recirculates the ball bearings with ball bearing guides that channel bearings from one end of the nut to the other end. (an endless loop)
Accuracy depends on proper helical pitch more than anything else. The support bearings on the end of the ball screw are critical for consistant positional accuracy.
When assembled the ball nut has zero backlash (in theory, at least) with the ball screw.
In applications such as the Deep Ultraviolet Lithography Mask Positioning Tables used to expose Silcon wafers for chip production, fuzzy logic along with negative feedback systems provide accuracy to about .01 microns (1 micron = 1 millionth of a metre) or .000000393700787402".
Hope this helps a little.
Another explanation, maybe, of the general idea:
The screw part has a semicircular groove going along it that is the "thread." The nut has a similar internal semicircular groove. Without balls, the screw would slide right through the nut. But, what they do is put ball bearings in between the screw and nut, 1/2 of each ball in the groove in the screw and 1/2 in the groove in the nut, thereby locking the two together. The nut portion has a return path so the balls can go from one end of the nut to the other, then loop back and start in again.
10-17-2001, 01:53 PM
SGW is right, I forgot to mention the grooves for the balls - I was too excited from the mere mention of "ball screws" to think of "that". ;-)
You could always go to a machinery dealer and look at one installed on a machine. A picture is worth a "thousand words". The machinery trade shows always have Bearing OEM's there showing off all those Chrome steel goodies. I have never been to one yet that did not have ball screws displayed as well.
If you get a chance you should go to one. Often there are deals to be had and lots of free information. The biggest is in Chicago - I always seem to miss that one - bad timing.
I'll echo Thrud's suggestion to go to a machine tool trade show if you get a chance. It's an experience. I've gone to the EASTEC show in Springfield, MA, a few times. 7 big buildings full of more stuff than you would believe. Mostly it's totally ridiculous for a home shop -- Cincinnati Milicron is always there demonstrating a machining center that's about 10' square and 15' high -- but it's fun to look, anyway. Wear comfortable shoes....
You can often get free passes to these shows if you register ahead of time, typically on a web site. Admission at the door is expen$ive, so try to find a free way to get in.
10-23-2001, 11:43 PM
Thanks I will keep and eye out. My wifes uncle works for a big shop as an engineer and he gives me some copies of Manufacturing Engineering. I will see IF I can tag along to one of the shows.