Lubricating miniature moving parts

Miniature components that slide, rotate, and reciprocate require lubrication for the same reasons that larger parts do — to reduce friction and allow consistent movement. However, because of their size, most can only be lubricated with dry films.

The main challenge with films is similar to that of grease or oil: Applying the lubricant where it's needed, and avoiding areas where it may cause functional problems.

In general, the smaller the item, the more precisely bonded films must be applied. For instance, medical guidewires of 0.010 to 0.038 in. are typical; they are lubricated with PTFE-based dry film to ease the passage of probes into the human body. Clearance between the core mandrel and the housing or tube ID is only 0.001 in. Similarly, for other items such as tiny valve parts, injector and metering slides, bearing retainers, pins, and detents, clearances can be on the order of 0.0005 in.

What's more, if small parts are completely coated, certain properties — those of low friction, release, and electrical insulation or conduction — can interfere with component functions.

For instance, some tiny ball-bearing retainers are coated on the ball side (cup) only, preventing them from spinning in their housing. In another case, surgical electro-stimulation needles are masked 0.025 in. from their electrically conductive tips to insulate their shanks, to prevent current from inadvertently passing into tissue.

Many conventional dry-film coatings don't work well in these specialized applications. At a micron level, some have the smoothness of a gravel road, while others tend to be too thick for dynamic applications. Holding these films to small tolerances that do not cause interference with their intended function is difficult, if not impossible. In some cases, the surface-to-surface motion between parts smooths these films, though not if parts are lightly loaded.

Polymers and particles

Certain dry films based on fluorochemicals address the roughness issues of conventional dry films. With extremely low friction coefficients, these materials cause little drag upon mating-surface contact — and they cause no stick-slip (chatter) or hotspots.

These dry fluorofilms incorporate a polymer binder (with a high glass transition temperature) and nano-size lubricating particles — for an overall coefficient of friction of less than 0.1. The films also follow contours such as shoulders and tabs without bumps, blisters, pinholes, or surface roughness.

Initially developed for hyper-speed bearings in dental drills, the material is FDA acceptable and PFOA free; its film provides primary lubrication, or failsafe lubrication where fluid lubricant cannot be used. Some formulations resist temperatures to 840° F. Compatible substrates include stainless steel, glass, Nitinol, and even plastics and rubber.

Tighter tolerances

Micron-thin dry films can be applied to tolerances that are more common to plating. For instance, bonded to the cup side of tiny bearing retainers to a thickness of 0.0003 to 0.0005 in., lubricant film is applied with tolerances of ±0.0002 in. Small gears, clips, valve slides, grippers, and probes can be similarly coated to prevent interference with their intended motion.

If necessary, each part can be inspected both microscopically and with a laser micrometer to ensure that no irregularities exist on the bonded surface.

Lubrication at high speed

Numerous aerospace instruments, specialty saws, small engines, medical and dental devices, power tools, and fluid pumps must operate at shaft speeds of five figures or more. Some even rotate as high as a half-million rpm. For instance, for gyros to produce enough gyroscopic force to resist torsion and maintain stability, and for surgical bone saws and dental drills to make smooth and exact cuts, rotating speed must exceed 10,000 rpm.

Though air bearings may seem like a useful solution in these applications, the size, weight, and immobility of these components make them impractical here.

Instead, designers of fast mechanisms often use highly refined versions of conventional ball bearings: Balls are ground and polished to exceptional uniformity and smoothness for minimal asperities. Typical roundness (deviation from spherical) is on the order of 1.0 × 10-5 in., and surfaces are smoothed to a maximum roughness of 1.0 µin. rms. To the naked eye, bearings have a mirror-like finish.

Even so, all the problems of conventional rolling-element bearings — scuffing, alignment, cooling, clearance, and overload — are evident on hyper-speed bearings. Loads on these bearings tend to be highly variable, too. PV values are typically less than 35,000 lb/in.-sec. At such speeds, lubrication is critical.

Bearing life depends on preventing contact between bearing retainers and rolling elements, with overloads and vibration being the most common causes of contact. At extreme speeds, any metal-to-metal contact is catastrophic.

To prevent this, some bearing suppliers use two-tiered lubrication that combines a conventional or synthetic fluid (that resists carbonizing into small, abrasive particles) with micropolymer surfacing applied to retainers — to eliminate all chance of abrasive contact. Here, abrasion-resistant particles (now common for many wear coatings) are unsuitable, because they abrade polished steel balls. Resilient materials are more suitable.

For more information, call (773) 427-2084 or visit surfacesolutionsgroup.com.