Bruce Nesbitt
Dimension Bond Corp.
Chicago, Ill.
Edited by Jean M. Hoffman
Most engineers would jump at the
chance to replace bearing inserts
with dry lubricants. In many cases, of
course, this just hasn’t been practical.
PTFE and moly-filled coatings might
work on light loads that move slowly
with PV (pressure X velocity) levels
of 5,000 to 10,000 lb/in.-sec. But this
PV just isn’t high enough to replace
traditional bearings. Abrasion is another
problem and erosion can literally
wash the coatings away.
Four years ago, a coating technology
emerged that changed all this.
Formulations combine nanosized
additives with high-performance
engineering resins. The coatings
go on load-carrying surfaces that
see extreme pressure, high wear,
low friction, particle or ice buildup,
and static electricity. Many formulations
routinely exceed PVs of
50,000 lb/in.-sec.
The new coatings derive strength
from both their binder and additives.
Resins are blended with filaments/
whiskers or submicron particles of hard reinforcing
materials such as
boron carbide or
nanosized diamond
particles. Incompressible
in block or sphere forms,
fibers exhibit columnlike
behavior under load and
will shear or fracture, protecting
the soft binder from
erosion and scuffing.
This barrier under load lets the
new coating wear better by an order
of magnitude over PTFE and moly-filled
coatings. The bonded surfaces don’t wear
much even when used dry against rough
mating surfaces (~32 rms or greater). They
can also be formulated to lap rough counter
faces smooth while providing low friction.
Temperature stability of some resins is up
to 600°F. This is about 100°F higher than
PTFE.
The coatings are thin — from 0.0003 in.
(0.007 mm) for a single layer to 0.007 in.
(0.175 mm) for multiple layers. This makes
them strong in that strength is inversely proportional
to thickness. Thus the superthin
bonded materials take on the strength of the
substrate.
Wet applications
First to use the coatings were automotive
shock absorbers along with rotary and
plunger fuel pumps — where the surfaces
are wetted by oil or gasoline/diesel fuel.
More recently, bonded bearing surfaces have
replaced babbitt or lead overplate on journal
inserts and main bearings for racing engines.
Although both rod inserts and mains
are hydrodynamic (d/w > 4), surface-to-surface
contact has been a cause of scuffing at start-up as well as at high speeds
and elevated temperatures. Part of
the reason for this uneven wear is
lack of concentricity and parallelism
between the journal and insert
surface.
Now, a high-temperature-resistant
barrier coating over the babbitt
layer withstands erosion in the
hydrodynamic fluid as it supports
the shaft or rod journal. This permanently
bonded barrier protects lower layers from heat and erosion. It is
scuffproof (when oil is absent) at temperatures
over 500°F — though the coating cures
at temperatures below the softening point
of the babbitt. The bonded layer won’t melt
or smear, resulting in more uniform journal
clearance, better hydrodynamic properties,
and more even load distribution — and
elimination of some of the heavy metals historically
used in these bearings.
The first test of these bearings was in
NASCAR racing engines where output exceeds
800 hp and 10,000 rpm, sustained for
over 4 hr. Surface speeds of the bearings
reach 7,900 fpm, with PV levels over 10
times those seen in average daily driving.
Tighter tolerances
New application technology helps maintain
critical dimensions on the parent part.
The bonded material can be varied so the
clearance between the applied and mating
surfaces remains essentially constant, within
±0.0002 in., as parent dimensions vary
slightly. Thus, you can adjust the thickness
of the bonded material when the surface
needs to be thicker (or thinner) to improve
part fit.
In applications such as hydraulic or pneumatic
cylinders, the bonded material can
control fluid leaks between a piston or rod
and mating surface. In shock absorbers or
struts, replacing the split-bushing insert on
rod guides with permanently bonded bearing
surfaces keeps the calibrated leakage
uniform throughout the shock’s life. The strength of these lead-free (RoHS-compliant)
bonded surfaces increases component
life while lowering manufacturing costs.
The piston connected to the rod of the
shock absorber or strut presents a similar
problem. In conventional design, the piston
uses a PTFE band that is 0.020-in. (400-μm)
thick as a friction reducing outer band, surrounding
the powder-metal piston.
The problem again is wear, compounded
by distortion. The elastic memory of PTFE is
approximately 50%. When the PTFE is side
loaded — in cornering — the band compresses.
This lets hydraulic oil leak around
the outer diameter. The PTFE does not
return to the original shape and becomes
loose, letting hydraulic fluid seep around the
edges of and behind the band. And eliminating
inserts brings more bearing contact
along the entire vertical height of the piston’s
outer diameter.
There’s no rule of thumb but experience
has proven that bonded surfaces almost
always cost less than conventional inserts.
One reason is the elimination of production
steps. For example, tooling, machining,
and assembly of wear bands in a typical
piston represent a substantial portion
of the piston manufacturing cost. And with
bonded surfaces, parts are laser or gauge
inspected at the time of bonding so that no
out-of-spec parts are made.
Make Contact
Dimension Bond Corp. , (773) 282-9900
dimensionbond.com