linear bearing 101 was Re: Linear Slide Components

To highligh what ballendo is speaking of I dug out my IKO linear way
catalog looking at their smallest size,a 15mm tall X 15mm wide
single row rail, has a basic dynamic(moving)load rate of 950KG
this is for a single roller, 4 or more are used on a table or slide.
side to side moment rating of one bearing block is 11.9 kgf*m. front
to back moment is 10 kgf*m for one or 59kgf*m for a pair twisting
moment is the same.

--- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:
> John Craddock wrote:
> <snip>Can someone enlighten me as to the engineering principle
that
> >establishes that round ways and linear bearings or THK type rails
> >are superior to Bishop-Wisecarver V-Wheels and tracks given the
same
> >level of accuracy and tolerance in their manufacture.
>
> John,
>
> In one word, leverage. But there's more.
>
> B/W dual-vee wheels are at their heart, a dual race ball-bearing.
> This bearing is surrounded by a profile which gives the type its
> name; the dual-vee shape of the wheel periphery.
>
> There is quite a bit more distance from the point where the inner
> race of the dual-vee wheel meets its two lines of Ball BEARINGS TO
> the dual-vee shaped periphery than in other linear bearing systems.
> This sets up a lever with a long arm (The arm of which is acting
from
> the point where the 2 vees of the B/W wheel interface with the
single
> vee-shaped rail, TO the outer race of the wheels' bearings).
>
> Compare this to a linear THK style ground race bearing, which is
> actually more similar to the B/W system than a thomson type (which
> we'll get to it in a moment). In the THK style, the part
> corresponding to the dual-vee inner race is the ground rail. Then
> there is the line of recirculating ball bearings. IMMEDIATELY
outside
> of this line (one ball radius away!) is the other "moving" portion
of
> the system; the sliding/rolling block. An extremely short lever
arm!
> Additionally, the dual-vee wheel can have only 2 tracks of
bearings,
> whilst the THK (and thomson) can have many. These multiple lines
of
> bearings mean more contact area (for a given physical size) than
the
> B/W system. Thirdly, the contact AREA of the THK type can be
greater
> than the B/W, as each line of balls can be nearly completely
> encircled by its respective "inner" and "outer" races(the rail and
> the slider block). Due to the needs of mfg., the dual-vee wheels
can
> only have about 20-25% ball-to-inner, and ball-to-outer race
contact
> area. Add these three factors together and you can quickly see why
> the THK-type ratings are geometrically increased, compared to the
B/W
> ratings of the Dual-vee system.
>
> Please note in relation the above that by design, linear bearings
> (and most modern ball-bearings) have races which correspond to the
> shape the balls will take when loaded. There will always be an
> attempt to balance this increased contact area with the need
to "keep
> things rolling"- literally! No ball bearing designer wants the
balls
> to "skid" rather than roll, since this increases wear and runs the
> risk of lubrication film starvation. Many different shapes have
> been,and will continue to be tried; arcs, ellipses, gothic arches,
> etc. to find this needed balance of rolling contact and most
possible
> contact, This is why we see tapered roller bearings for heavy
loads;
> they can be assured of rolling, whilst having a large contact
area.
> And this needed geometry does not change as radically under load,
as
> does a ball (which becomes oval).
>
> We're gonna come back and touch on that sliding issue too, but
first
> let's talk about the thomson, or "round rail" style system. As
with
> the THK-type, the inner race of the dual-vee wheel corresponds to
the
> round rail of the thomson style. But this time, the dual-vee has
the
> advantage! It can easily be seen that a small ball rolling along a
> line on a cylinder will have a SINGLE point of contact. <pause for
> shameless sales pitch>
>
> (You know, this is gonna take more than just a message post to
answer
> fully. That's in my book one and book two. But I'll get the basics
> covered here)
>
> The OUTER race in the thomson system is a pressure plate which the
> actively loaded balls press against, and run in until they roll
into
> a relief area where they are recirculated. Again there is only one
> ball radius to this plate. And again, there are multiple ball
paths
> (circuits). And again, the pressure plates are ground with
> a "designed" shape to carry as much load as possible without
causing
> the balls to skid instead of the desired rolling... What we find
is
> that the thomson round rail style has a good compromise between
load,
> life and manufacturability. This is why for many years, the round
> rails "owned" the market. It was only with the availablility of
> relatively inexpensive precision grinding techniques and machines
> that the THK-type began its march to the top.
>
> Now let's look again at the B/W dual-vee system. By DESIGN, it has
> parts which "skid"! This is the so-called "wiping" action of the
> wheel against the rail. While this DOES do its intended function
of
> cleaning the rail, and allowing the system to be used in "dirty"
> environments; it comes at the price of: A need for increased
> clearance, due to expansion from frictional heat. Greater wear.
AND,
> that lever I first mentioned is NECESSARY to provide this wiping
> action! (a wheel of zero radius couldn't wipe. In fact,it would be
a
> ball of sorts, and we've come full circle <pun intended>)
>
> Hope this helps,
>
> Ballendo
>
> It is likely that one will need to look at some cross sectional
views
> for this post to make good sense. They are found on mfrs.
websites,
> and much of this is discussed in the engineering portions of the
> product catalogs and websites...