A recipe for A Slick Design

June 7, 2001
Low-friction coating technologies help designers improve the "touch" of mechanisms.

Janice M. Newton
Application Engineer
Whitford Worldwide
Westchester, Pa.

Edited by Jean M. Hoffman

Ergonomic investigations for vehicles in the design stage often begin with a programmable vehicle module. The PVM is a precision machine outfitted inside and out to look and feel like a real vehicle. Prefix Corp., Rochester Hills, Mich., builds vehicle mock ups for ergonomic evaluations. Dc stepper motors coupled to precision linear bearings let evaluators experience dimensional representations of several different automotive interiors, even the dashboards, within a matter of seconds.

Almost every object in a PVM can be moved, via discrete drives. Body panels attached to the PVM adjust with jacks to change the position of the panel relative to the roof. Seat height is also controlled for both bucket seats. Evaluators reposition the seats and the computer records the optimum setting selected. Subjects may also test how low-friction coatings improve operation of latches, cranks, slides, and other mechanical mechanisms.

The large spline shaft in an automotive drive was designed to let the drive shaft slip forward and aft slightly as the rear wheels undulate over bumps. Unfortunately, the spline exhibited stick-slip characteristics — accompanied by an irritating bump noise — whenever the road surface changed. Lowfriction coating eliminated the stick-slip problem and the irritating noise.

Low-friction coating on shear blades makes for easier cutting and eliminates sap buildup that causes blades to stick.

Ergonomics derived from two Greek words, "ergon" or work and "nomoi," meaning natural laws, has long focused on making products safer or more accessible. Increasingly, attention is being paid to aesthetic ergonomics or the process of making things more pleasant to operate.

But discovering which mechanical design feels best is more akin to art than science, because the process is mostly subjective. Special low-friction coatings can improve aesthetic ergonomics by reducing the effort required to operate a mechanism. These socalled matrix coatings contain low-friction materials in an engineering plastic binder such as polyimide (PI), polyamide-imide (PAI), and polyphenylene sulfide (PPS). The hard plastic binder encapsulates and protects the softer, friction-reducing materials or lubricants from wear.

Several types or combinations of low-friction lubricants can be dispersed in the coatings to influence mechanical properties. For example, PTFE gives the lowest friction coefficient at 0.02. While moly-disulfide (MoS2) based coatings have the highest loadcarrying capacity. And graphite filled versions can operate at temperatures beyond 500°F.

These coatings can be applied as 0.0003-in.thick films, making them suitable for tight-fitting components. They can improve the "feel" of mechanical actuation on a wide range of materials from wood to steel.

Ergonomic load ranges
Coatings help designers tailor the amount of force needed for mechanical operation. Forces range between discomfort, at the high end, and lack of feedback for the operator on the opposite. For example, between 2 and 8 oz is the comfortable operating load for a pushbutton, trigger, toggle, thumb screw, or other finger-operated device.

Small levers and other hand-operated devices such as throttle cables for lawn and garden equipment, overcenter latches, locks, and buckling devices tend to operate best when loads fall between 1 and 3 lb. And 1 to 9 lb is an acceptable operating force range for foot-operated mechanisms such as pump levers or automotive pedals. However, these values are both approximate and subjective. The best load range for each application should be determined experimentally.

Product designers may be inclined to take force values from standards such as MILSTD-1472D, Human Engineering Design Criteria for Military Systems, Equipment, and Facilities or the Human Factors Design Guide (HFDG, report DOT/FAA/CT-96/1) as benchmarks. But according to Paul Green of the University of Michigan's Transportation Research Institute (UMTRI), these tables only account for the loads themselves, not how human operators apply them or the strain caused by doing so or both.

For example, the forces needed to move a hand-operated jack may fall within "standard" limits. Still, some operators could strain muscles depending on their body position during the action. And people weighing more than 200 lb may easily operate devices that actuate with high loads, while those under 120 lb may have trouble.

Conversely, devices on the bottom of the load-scale range may not give operators adequate feedback. Drivers riding on especially rough roads may not be able to tell whether a switch or lever was actuated properly.

For these and similar reasons, UMTRI, recommends three distinct design phases when doing ergonomic investigations. First, the product and its application must be thoroughly characterized. Second, digitally model the application, incorporating all human factors standards needed to evaluate reach, body positions (whether seated or standing), and gross operating forces. Finally, conduct demographic tests. The approach fine-tunes the device for comfortable operation by most potential users.

But, warns Green, shortcuts such as linearly scaling forces from larger operators to smaller ones is generally not advisable. Just because a 200-lb operator can comfortably apply 10 lb of force doesn't necessarily scale to 5 lb for a 100-lb operator. UMTRI instead typically selects 50 operators of various sizes, ages, and weights to define a device's "comfort zone."

Reducing resistance
Small items such as car door latches, seat slides, cabinet latches, and clippers or shears have greater market appeal if they are easy to operate. However, there is no exact way to predict how much easier a mechanism will work after coating. But, relative coefficients of friction (COF), before and after coating, give a good indication of how smooth devices may operate. If the COF between two slides is 0.35 prior to coating and a conservative estimate of the COF is 0.1 after coating, then the force to operate the slide will be reduced by at least a factor of three.

COF depends on the materials and friction type, static or kinetic. The COF, ,, is defined as:

µ= F/N

where F is the force required to initiate or sustain relative motion and N is the normal force pressing the two surfaces together. For metal-to-metal contact, frictional force is generally independent of contact area, velocity, or temperatures below 200°C.

But, friction characteristics of engineering thermoplastic-based matrix coatings can be affected by pressure, velocity, surface finish, temperature, and fillers or lubricants. Here, it's probably better to follow what some call modern friction theory, where the frictional forces are said to result from the shearing of asperities or rough surface features that form between two surfaces in contact. Asperities make up the contact area between the two surfaces:

F = SAr

where F = friction force, S = shear strength of the contact area (the strength of the weaker material is used) and Ar = true area of contact. Ar also equals N/P, where P represents the penetration hardness of the material.

Combining the equations gives a relationship of COF to shear strength and penetration hardness:

µ= S/P

The equation helps illustrate why plastic binder coatings are dependent on operating conditions. It's well known that the mechanical properties of plastics change with temperature. Heat from increased contact pressure and sliding velocity tend to heat plastics which lowers shear strength, hardness, and COF. Consequently, it's important to work closely with a coating supplier to tailor the binder/filler combination to the application.

Devices that actuate smoother with less force may also favorably influence consumers. Metals and some plastic parts that slide against each other often do so with an unpleasant abruptness. This may be perceived by the consumer as poor design. In some cases, coated parts may also be safer to use. On tables saws with rip fences, for example, coatings may let the workpiece be pushed toward the saw blade with reduced effort, translating into better control of the workpiece and fewer accidents.

Reducing noise
Rattling, banging, and tinkling sounds of loose metal parts gives an impression of cheapness. Such noises come from two bare metal surfaces striking or scratching one another.

Coatings help dampen noise because sound strikes the coating surface, but doesn't propagate through it to the same degree as a bare-metal part. How much a coating reduces noise depends on many factors, including coating thickness, sound frequency, and air temperature. For example, coating supercharger rotors lowered emissions by 3 dB compared to uncoated models.

The "aesthetic" branch of ergonomics may have started 50 years ago at Rolls Royce when engineers spent thousands of hours testing pushbutton switches to find the one with the best feel.

Competitors of Rolls Royce, attempting to duplicate the switches, soon learned that "feel" is not as easy to replicate as are physical dimensions. Each design variation requires testing of potential users, who vote their preference based on three basic goals: Mechanical actuation must be smooth, easy, and generate as little noise as possible — preferably below 85 dB.

Sampling of the load/strength data (*) drawn from HFDG, report DOT/FAA/CT-96/1 and empirical data (+) gathered from application testing of Xylan fluoropolymer coatings on various mechanisms. HFDG values shown represent 80% of the maximum force that can be applied by a young male in the demographic 5th percentile.

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