Technical > Design Guide > Wear

Initial contact between mating metal parts results in momentary welding of asperities (peaks) on each surface. As each part continues to move, the welded asperities are ripped off, leaving behind minute pits.

Every bearing and wear surface, no matter how smooth the finish, has asperities. The problem is common to impellers and housings, air cylinder pistons, machine slides, telescopic mechanisms, ball joints, plungers, gear teeth, hinges, journal bearings, valves, power screws.

Xylan coatings provide a thin layer of lubrication to prevent the asperities on mating surfaces from making physical contact with each other. The selection of the best dry lubricant — PTFE, moly, or graphite — for these applications depends primarily on the PV (Pressure x Velocity), atmosphere and temperature of the application.

In many cases, a dry film provides enough lubrication to eliminate objectionable wear. For example, a molded nylon detent for an automotive signal harness was failing prematurely due to heavy loads on the point of the detent. The sliding coefficient of friction between the detent and its mating cam was approximately 0.40, which resulted in severe abrasive wear.

When a 10 micron/0.0004 in. coating of a PTFE-bearing Xylan was applied to the detent, the coefficient of friction dropped to 0.12, and detent life was increased by over 200 percent.

Wear is often severe in bearing-type applications. Rods that slide through glands, rolling element bearings, slide assemblies, telescopic booms, ball reversers, rocker arms, ball joints, tracks, bushings and literally thousands of other applications are configured so that one part rolls or slides over another part.

In most cases, friction and wear of the parts are reduced when one or both are coated with a dry-film lubricant. Also, the coatings serve as a thin cushion, spreading high point loads in bearings and reducing element fatigue.

The energy that is transmitted and dissipated in a bearing is a function of the PV of the application. As the PV increases, so do heat and wear on the bearing surfaces.

Dry lubricants have a "limiting PV value" that they can withstand for a reasonable wear life. Typically, the highest limiting PV which a 25 micron/ 0.001 in. coating of Xylan can withstand is approximately 50,000 (PV). This limiting value varies from coating to coating.

There are two additional factors to consider. First: The ability of a coating to bear loads increases as thickness decreases. For instance, although a 25 micron/0.001 in. film may be able to bear PV of only 50,000, a 5 micron/0.0002 in. film (the practical lower limit using current application techniques) may be able to bear PV of 150,000. For this reason, the PV tolerance of a coating may be modified by the film thickness.

Second: The lubricants themselves. PV limits are not constant. They tend to increase with pressure and decrease with speed.

This is particularly true with moly coatings, which work better under high pressure and low speed, where galling is the principal reason for failure. Thus, with these coatings, a "speed factor" is necessary for final computation of the PV limit of the coating.

The wear rates of many of the Xylan 1000, 1400 and 1600 Series coatings are equal to that of bronze/steel bearings impregnated with PTFE-lead when applied in thin films (17.5 microns/0.0007 in.).

When specifying a coating for a bearing application, apply the coating to the larger "swept" area. This spreads the wear over a larger area and provides the greater amount of lubrication.

For break-in, frequent starts and marginal lubrication, remember: the period of greatest wear to a moving mechanism is when it is new. Even fluoropolymer coatings can benefit from secondary lubrication under these conditions.

Boundary lubrication failure
When equipment is started and stopped frequently, oil films tend to become too thin to function as effective lubricants and bring sliding metal surfaces almost into contact (a condition known as "boundary lubrication"). If metal-to-metal contact occurs, the condition becomes "boundary lubrication failure".

A thin coating of Xylan reduces the chance of failure and lengthens the life of such products as sprockets, seal plates for compressors, pump pistons, cams, ball joints, conveyor trolleys, gears, journal bearings — even though the anticipated PV limit of the application is far greater than the coating can endure.

These coatings solved wear problems under start/stop conditions in reciprocating plungers in electrical solenoids. Typical plungers are chrome plated (and extremely hard). But the starting and stopping at the end of each half-cycle put the plunger into boundary lubrication failure, causing the plating to wear rapidly. When a matrix coating replaced chrome plating, boundary lubrication failure was overcome and plunger life was extended by 90 percent.

A maker of chain saws uses Xylan 1010 as a fail-safe lubricant on the cage of the saw's main bearing. Clearance between the cage and connecting rod is only 100 to 150 microns/0.004 to 0.006 in. When the engine started, the bearing was in boundary lubrication. Without the coating, it tended to seize. As proof of the coating's ruggedness, these engines run eight hours per day and have a life expectancy of three years.

A PTFE-type Xylan coating is recommended for applications where initial wear is anticipated to be light to moderate; a moly-type coating is recommended for conditions of heavy wear, especially in high-load situations.

Fail-safe
In any circumstance in which a mechanism must function when needed, even if only once, Xylan coatings provide a good margin of security, even under the most critical circumstances. This includes such aircraft parts as bearings for turbine engines, solenoids, seat ejectors, actuators, door pins and firing mechanisms for ordnance.

Another category is equipment which would be damaged were a component to fail. For example, removing a frozen bolt from chemical processing equipment could cause damage costing thousands of times more than the bolt. In refinery equipment, the use of a wrench is infinitely safer than the use of a cutting torch.

A good rule of thumb: Apply coatings of approximately 25 microns/0.001 in. to the surfaces of these parts. This ensures that the component will function when required, and also provides good release and excellent corrosion protection.