Re: [CAD_CAM_EDM_DRO] Bearing Lock Nuts on 5/8 in ballscrews

Hi Mike:
There are a couple of things you need to pay attention to:

1) Make sure the thread you cut on the ballscrew is PERFECTLY concentric to
the screw.
If it wobbles, you'll only load one side of the bearing and it will be
trashed in no time.
This precludes trying to die-cut the thread, or even chasing the thread with
a die...the thread really should be ground, but you can get away with a
single point turned thread so long as it's cut properly...no burrs, no
tearout, no pitch errors no pitch diameter errors etc etc etc.

2) Ditto for the nut...running a tap down a drilled hole won't do here...you
need to turn the face, and then single point bore and thread all in ONE
setup...no rechucking, and no chasing with a tap.
The mating face must be FLAT...running a file over it to clean up the burs
is no good.
Make the nut first... it's way easier to use the finished nut as a gauge on
the shaft when fitting the thread, than the other way around.

3) The best way to be kind to the bearing, is to have a section of smooth
shaft between the bearing and the nut, that carries a parallel ground
sleeve.(kind of like a very thick precision ground washer)
If the sleeve is made to a very close sliding fit on the shaft and the ends
are perfectly square and parallel, and the length of the sleeve bore exceeds
the diameter of the shaft by 50% or so, then the sleeve will act to even out
the load that the bearing experiences when it's preloaded, and will
compensate to some extent for MINOR errors in flatness of the nut mating
face.

4) Belleville washers are OK as a preload element, but they allow compliance
in the system, so you have to think a bit about why they're there and what
they will do.
They're good to compensate for thermal growth, for example, but they will
allow lateral movement of the screw if the force on the table exceeds their
ability to resist it, so you need to calculate the loads and find out how
hefty to make them.
Too floppy is useless...the table will hop around every time you change
direction or make a cut.
Too stiff, and you might as well not bother with them at all, and just go
with a plain preload nut.
For a leadscrew application in a hobby class machine, I wouldn't bother with
Bellevilles.

5) With regard to bearing locknuts, all of the above applies about accuracy.
Nyloc nuts and othe such self-locking nuts are good only if the design of
the spindle end can mitigate the errors inherent in their locking mechanism.
I've had best success with a pair of thin nuts that jam together, or a
lockwasher that's got a tab fitting into a keyway on the shaft and tabs that
can be bent into castellations on the nut.
These won't push the nut sideways like Nylocs or setscrews will.
I just make my own nuts...that way I know the mating faces are flat.

6) Oh yeah...don't forget that you have to set up angular contact bearings
in opposed PAIRS... your post implies that you had hoped to preload a single
bearing, which, of course, can't be done.
Keep the bearings reasonably close together...a single bearing at each end
of a long skinny screw is the worst possible solution.
Ideal is a pair of opposed angular contacts at one end and a needle roller
bearing at the other to allow the screw to expand and contract with changes
in temperature.

Hope this helps

Cheers

Marcus

----- Original Message -----
From: "mmeyers1111" <mmeyers1111@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, May 31, 2005 5:34 AM
Subject: [CAD_CAM_EDM_DRO] Bearing Lock Nuts on 5/8 in ballscrews


Hi folks...

I'm going to turn the ends of some Rockford 5/8 inch ballscrews and
I was wondering if anybody has experience with bearing lock nuts and
who could comment on a few questions. The lock nut will be used to
preload an angular contact bearing with a 12mm = 0.4724 inch inside
diameter that is mounted inside a bearing block. I've never used
ballscrews or bearing lock nuts so this is very new to me. I'm doing
this to the ends of 4 ballscrews. I'll be turning them as R-30 Type
C ends (http://www.rockfordballscrew.com/semchart.htm).

1) I noticed that McMaster-Carr (http://www.mcmaster.com) has
bearing lock nuts in 2 different sizes that would be reasonable for
a 5/8 in screw (.391"-32 and .469"-32 ). The bearing portion (non-
threaded portion) of the ballscrew shaft will be turned to 0.4724
inch to accept a 12mm ID angular contact bearing. So I guess I'm
thinking that the .469 - 32 TPI threads would be the best choice and
the bearing should slip over the threaded portion without trouble.
Does the .469 -32 lock nut sound like the most reasonable choice?
Yes/no?

2) I noticed that the McMastrer-Carr bearing lock nut indicates that
it is `self retaining' and I don't see any bearing lock nut washers
available for it. Other nuts I've seen seem to require special
washers. Is it true that that all I need is the lock nut itself and
that a washer would play no role in making sure it stays `locked'
Yes/no?

3) Do bearing lock nuts typically have special threads that require
a special die to cut them, or can I use my lathe to cut 32 TPI
threads with the compound rest set at 29.5 degrees? I'm assuming
this isn't a simple yes/no. Sorry.

4) Does anybody use a simpler less expensive approach to preloading
angular contact bearings on a shaft so that I don't have to purchase
special bearing lock nuts?

5) Can anybody suggest other/better lock nuts from other suppliers?

Sorry for the length of this folks.

Mike