The Bonded on Bearing

They are good enough for NASCAR engines. Bonded bearing surfaces are coatings made from high-performance polymers reinforced with hard nanosized particles.

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.

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