static supporting shafts, y?

[max_imum2000]

ok here is a question
when building a cnc, lets say the y axis, the ballscrew are on bearing and there are 2 shafts to hold the z assemble, the z-assemble is connected to a linear bearing upon the y axis stainless steel shafts, ok

now , why dont we install this shafts at its 2 ends by bearings also ?, so it can rotate radially as well , i think it will reduce the friction as well in case there are something wrong in any part of these shafts

thanks all

[NC Cams]

The ball screw and true ball screw bearings are designed to absorb/apply axial thrust - they are NOT designed or intended to support radial loads other than the weight of the ball screw. This pertains to the nut as well as the preloaded bearings.

Linear bearings are designed to support radial loads and concurrently support them while you are moving the table (whatever) via some external axial force applying member.

As a colleague once said, "the key to doing something right is to assign the right job to the right guy'. Machine elements react the same way....

[Vaughan Sage]

I read an article in a YANK magazine ages ago where they were reducing stick-slip of FRICTION type slide bearing by using round rods in a seat & rotating them. When trying to move very slowly friction slides tend to stick tight until "limiting friction" is overcome then all the springiness in the thrusting elements goes boing & your carriage jumps, hence the name stick slip. If you have roller bearings this almost eliminates it, as doe air or fluid floating bearings. You would have to be chasing micro precision & super rigidity to want to go for this method. The principle is simple... by overcoming friction in Y, the friction disappears in X! Very heavy round guide rods with tight iron or bronze oiled bearings would give no stick-slip if kept slowly rotating, & bearings like this should absorb/damp milling vibrations very well if the oily bearing area is big, quite cheap to build too, but bar beds will always deflect. Ability to rotate the rods allows you to use a dial indicator check for straightness too. Nuff of my blurb!

[NC Cams]

Vaughan's post is true but you need to know the following to understand how/why FRICTION type bearings/bushings work.

Whenever lubrication (IE Grease or oil, jello or whatever viscous fluid is used) is involved) you have will have "sticktion" until you get enough of a horizontal vector for the development of an EHL (elasto hydrodynamic lube film) between the metal parts by the lubricant. EHL films are speed, area, load and finish sensitive but it is a proven concept.

Translation: the metal parts start to hydroplane/water ski on the film (oil, water, air, grease, jello, whatever). Yes the film is only microns thick but it is an EHL film and the parts do hydroplant.

THe benefit of steel against bronze in an unlubed state or the like involves the "natural lubricity" that occurs between dissimilar metals. This is also called "surface action".

Be careful. Although hard/soft metal mating usually offers the best "natural lubricity", you can't run steel agains pure copper or pure aluminum.

Why? copper and or aluminum wans to sieze to steel.

Why doesn't bronze when it is a copper alloy? Bronze is an alloy of copper and tin or lead. The Pb or Sn provide the "surface action" needed to enhance natural lubricity so as to prevent siezure.

Why don't aluminum bearings (ala car main bearings) sieze? The aluminum is alloyed with tin, lead, bismuth and/or silicon and iron and the alloys provide "surface action".

Not only did I do ball/roller bearing engineering but also did sleeve bearing and bushing engineering work. That's how I learned about EHL (which also applies to ball/roller bearings but we'll save that explanation for another post).