question concerning linear vs. roller bearings

[morphious69]

is there a sort of combination of these two where the linear block allows balls to circulate like ball bearing nuts do? Seems to be a really easy thing to build with a decent mill and some meager CNC skills.

[rowbare]

That is how profile rails work. See the diagram in this document. http://www.linearbearings.com.au/Por...ides%20HRW.pdf

I believe that the bearing for Thomson style round rails recirculate as well.

bob

[morphious69]

AH HA so now the pricing of these units make a lot more sense to me.

but I can definitely see making my own as opposed to the ball screws. Those look a bit indimidating for the nut. The threads look like I should be able to make my own tooling to make them though. wonder if making them in more than two parts would make it easier? and maybe more adjustable to get rid of backlash?

[RICHARD ZASTROW]

Are you referring to pre-loading the balls to reduce clearance? There are such linear ball and roller bearing blocks available with adjustable preload.

Dick Z

[morphious69]

what does preload mean exactly? like what doe sit mean mechanically? any diagrams of this ?

as for making them from more parts I was saying that more to make it easier to make them. though I am guessing if the part was cut using a custom tap and then cut in half it might make it relatively simple.

I am looking at making a ball screw thread style tap and a cutter that will work much like a gear hob to cut threads on rod.

[RICHARD ZASTROW]

The "threads" on/in ballscrews & nuts are ground to tight precision tolerances on thread grinding machines. Not DIY equipment.

Pre-load can be defined as the complete lack of clearance between the balls or the grooves they are riding in. Pre-load can also be increased to negative clearance similar to pre-loading ball or roller bearings.

The ballscrew/nut are hardened so that the balls (hard) do not "brinell" (dent) the grooves. Brinelling causes rapid wear and increased clearance. The pre-load then becomes backlash and destruction of the ballscrew/nut follows.

In normal situations, the ballscrew can be re-ground and/or re-balled with over sized balls to restore pre-load.

Hope that helps.

Dick Z

[BillTodd]

Morph,

I suggest you read this thread for DIY alternative to ball-screws:

http://www.cnczone.com/forums/showthread.php?t=13593

[morphious69]

Originally Posted by BillTodd Morph, I suggest you read this thread for DIY alternative to ball-screws: http://www.cnczone.com/forums/showthread.php?t=13593

yes I dreamed something like this up a long time ago only to find it in use in some sort of medical equipment I got an ad for through yet2 and nasa tech briefs.

my idea was to cut circular cuts into the surface of the outside of a roller bearing. problem I had was finding a roller bearing that was made with a heavier gauge of metal on the other casing so I could cut these rings into it. basically threads that do not spiral but just go back to where you started so they will not move while the threaded rod will. lest teh two sets of "threads" rotate around rather than slide past each other. It is a very good idea but it takes more space from what I had seen. The ones on there though seem to deal with this size issue pretty well. I had not thought of putting the bearings ont eh ends of a small rod with circular cuts into it. Funny thing is that would not even require thread cutting on a lathe, just a manual lathe to make those is all that is required really.

[morphious69]

oh and in case anyone is wondering I was rotating several pieces of all thread against each other that led me to dream up that idea. The things you come up with while just fiddling with various hardware while waiting in the check out lines. I have a few ideas I am still considering on this but I will take them over to that thread.