Re: [CAD_CAM_EDM_DRO] follw-up on pacific bearings was Re: shafting question

Thanks for bringing up some of these points I hadn't considered; I just
received the Pacific Bearing catalog in the mail today and plan on reviewing
it further. At this point, I am starting to lean away from their use though,
mainly due to the clearance problem...
They do have some nice-looking slide assemblies in there though,
complete with ballscrews. Probably far too expensive for my tastes though.

Rick

----- Original Message -----
From: <ballendo@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, November 13, 2001 6:24 PM
Subject: [CAD_CAM_EDM_DRO] 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
>
>
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