linear bearing 101 was Re: Linear Slide Components

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...