Tech Briefs Magazine - June 2021 - MD-14

Other roller-type forms include Vgroove
rollers and rails capable of negotiating
curved tracks, recirculating
roller-chain bearings used for heavy
loads on round rails, and bearings that
employ ceramic elements as a means of
reducing system weight. There also exist
linear bearings that use air as the lubricating
medium.
Selection
can clear the rod rails. Shafts supported
on their ends can use closed bushings.
Linear bushings are tolerant of dirt as a
result of having no moving elements.
Rolling-Type Bearings
Open ball cages - sometimes called
guidepost or leader-pins - are used with
die posts for guided, precise motion of
the upper and lower dies as they close.
These bushings are close-toleranced to
provide accurate location for mating dies.
For linear motion control, both recirculating
and non-recirculating ball and
roller bearings are available. They both
provide low starting friction as compared
with slide-type bushings. Nonrecirculating
types sandwich the balls or
rollers along the complete lengths of
paired rails or ways. The rails move relative
to each other as the rolling elements
rotate between them. These designs can
provide very accurate linear motion and
some designs use crossed rollers for
even higher accuracies. The stroke
length is fixed by the number of elements,
however, making long strokes
somewhat impractical.
Recirculating ball and roller bearings
have no theoretical limits on travel distance
because the rolling elements travel
the length of the bearing as it moves
along the rod but then double back
through the carrier, creating an endless
loop of load-bearing elements. These
designs operate on round rails or profiled
tracks, depending on the design of
the bearing.
14
Cov
ToC
Recirculating ball bushings are perhaps
the most familiar as they are direct
descendants of sliding bushings. As with
the sliding varieties, recirculating ball
bushings can be open or closed based on
the support of the linear rails; they can
be mounted in pillow blocks, flanged
units, etc. and they are often sized to
directly correspond to the dimensions of
sliding bearings.
Care must be taken in dirty environments
to protect the many moving balls
within the bearing. In some instances,
the entire rod is covered. Manufacturers
will often supply seals, wipers, etc. as standard
or optional with these bearings.
Ball bushings are stiffer than linear
bushings and consequently less compliant
with regards to shaft misalignment.
Ball bushings can run on hardened
round rails. Recirculating roller bushings
usually run along profiled tracks
that have flat ways. Ball bushings are
sometimes available with different levels
of precision with the looser versions able
to compensate for misalignment of the
shaft. Self-aligning ball bushings also are
available. Ball bushings have some ability
to rotate on the shafts, so if used with
parallel shafts, one shaft can sit lower
than the other without noticeably affecting
operation.
Ball bushing rails can be end-mounted
successfully and can span some distance
between supports. Mounting of profile
rails usually requires support over the
full length of the rail, demanding careful
preparation of the mounting surface.
Most linear bearing suppliers offer
selection guides that step the user
through the design problem. Considerations
include the precision of the application,
from lowbrow operations such as
drawer slides to semi-precision operations
such as saw guides, to exacting
applications such as machine tools.
Bronze bushings might be appropriate
where loads are high and smooth mo -
tion is required but neither speed nor
accuracy is important. On the other
hand, where loads are high and accuracy
is necessary, a recirculating roller bearing
and profile rail might be the preferred
choice.
Ball bushings are easily contaminated
by particulates - the sealing problem
being somewhat more difficult than that
of radial bearings because of the long
lengths of exposed shafting involved. As
was suggested, seals, wipers, and way covers
all offer means of minimizing contamination
but in very dirty environments,
sometimes a sliding bushing
makes a better fit. The friction of these
can be quite high when compared with
rolling-element linear bearings.
In addition to offering the components
to piece together linear-bearing
systems, many manufacturers sell assembled
systems under various names such
as linear slides, linear tables, etc. Many
manufacturers can supply systems with a
variety of actuating options.
In sizing a system, engineers will need
to have some information at hand
including the maximum expected speed
and acceleration, the maximum force
the system will see, and the length of
stroke. Repeatability is a concern. Mo -
ment loads acting on the system along
the x, y, and z axes also must be considered.
Maintenance plays a part in system
selection, as do vibration and noise constraints.
This
article was contributed by Thomasnet,
New York, NY. For more information, visit
http://info.hotims.com/79414-282.
Motion Design, June 2021
DimiSotirov/Shutterstock.com
http://info.hotims.com/79414-282 http://www.abpi.net/ntbpdfclicks/l.php?202106TBNAV
Tech Briefs Magazine - June 2021

Table of Contents for the Digital Edition of Tech Briefs Magazine - June 2021
