follw-up on pacific bearings was Re: shafting question

Rick,

Thank you for mentioning this. A friend of mine likes to remind me,
"WHY are they telling you this?" It often pays to remember that a
company is in business to sell their product.

While they DO have a greater contact area, being essentially a
"high-tech" PLANE bearing, one has to consider the material that is IN
contact... Frelon. Which is certainly? some variety of ptfe (teflon),
and also relatively thin due to its tendency to "cold-creep" (deform
under pressure). So what will eventually happen(with heavy loads) is a
thin frelon surface "backed-up" by whatever the frelon is housed
within. Not a bad design, and great for many apps, but definitely
having more "play" than our other choices.

And the physics of plane bearings means that the ratios of
cantilever-ed load to distance between bearings is fixed at some value
. A 2:1 ratio means the bearings must be(at least) twice as far apart
as the distance to the "overhanging" load). Going outside of this
value(say 1-1/2:1) produces poor results, and "sticking". Although the
use of high-tech material like frelon modifies the ratio, Ball-bearing
based linear bearings are more forgiving of poor ratio engineering,
IMO.

More pertinent than total load for us is the fact that a recirc-ball
type linear bearing can be set to a ZERO or even NEGATIVE(pre-load)
clearance; which the pacific bearing products expressly forbid. This
means that no matter how much load is capable of being carried, with
the Pacific bearings it will be carried with some "slop", due to the
necessary space between the bearing and shaft.

This is why I suggested the use of a bridge design over a gantry type
if using these bearings. With a bridge, there is no "stack-up" of
tolerance from the bearings supporting the table, and only a single
stack-up (the Y axis supports the Z) on the bridge.

One claimed advantage of the pacific bearings is that they DONT
(according TO pacific) increase their "clearance"(slop) as quickly as
some other type of plane bearings (such as sintered bronze; commonly
called "oilite"). This, in my experience with these bearings, appears
to be true.

Hope this helps.

Ballendo

P.S. Thomson, Igus and others offer a type of plane bearing similar
to, and in competition with, pacific.

P.P.S. It IS possible to use these bearings successfully in many types
of CNC machinery; just be sure to understand the trade-offs and limits
of the choice.

--- In CAD_CAM_EDM_DRO@y..., "Rick Miller" <apollo@a...> wrote:
<snip>
> This would seem to go exactly opposite of what they claim on their
website,
> that their bearings have a greater contact area and thus are capable
of
> higher loads than ball bearings. Either way, I keep thinking that as
they
> wear they will quickly introduce a lot of slop into the action, more
so than
> a ball bearing.
<snip>
> Rick