Machine Design
How to get better motion with plastic parts

How to get better motion with plastic parts

Polymer bearing surfaces resist wear, don’t need maintenance, and can run surprisingly fast.

Authored by:
George Bartosh
Intech Power Core
Closter, N.J.

Edited by Stephen J. Mraz
[email protected]

Intech Power Core

Engineers might only glance at the dimensions and load capacities of roller and cam followers to ensure they will fit in their design before placing an order. But they should pay closer attention because these components can make or break the performance of motion subsystems critical to a host of industrial machines.

Choose the wrong roller or cam follower and your design can be destined for a shortened life plagued with maintenance headaches, contamination issues, and speed limitations. The wrong component can also drive up operating and manufacturing costs. The right components, by contrast, will contribute to a longer life of reliable motion that engineers and buyers expect.

But how do you pick the right roller or cam follower for the job at hand?

The trick is considering not just sizes and loads but also the materials that make up the roller’s bearing surface.

The polymer advantage
Many engineers still believe that heavily loaded applications need metal rollers and cam followers. They are unaware that polymer bearing surfaces withstand enormous forces if designed properly. At Intech Power Core, Closter, N. J., for example, we’ve delivered polymer rollers that withstand 8-ton loads. These rollers have been used in ski lifts and rigs that can raise an entire building. But the primary task of most rollers and cam followers is to transmit motion rather than support loads.

Regardless of their purpose, rollers and followers are commonly hybrid designs with polymer load-bearing surfaces over metal roller bearings or structural hubs. These hybrid components, often sized as drop-in replacements for standard-sized all-metal components, offer compelling technical advantages due to their use of polymers:

Self-lubricating. Engineering polymers best suited for rollers are self-lubricating, a property that last the life of the component. Bearing surfaces made from these polymers never require lubrication between the roller and rail. When combined with lubed-forl ife bearings , polymer surfaces eliminate maintenance costs associated with regular lubrication, even those scheduled after washdowns.

More importantly, self-lubrication puts an end to the loss of performance and catastrophic failures common when metal rollers and rails aren’t regularly lubricated.

Wear resistance. Metal-on-metal contact, even when both metal surfaces are lubricated, can result in excessive wear and galling. Metal-on-metal wear affects rollers and rails, which are far more expensive to replace. Plastic rollers eliminate this wear.

Clean running. Self-lubricated polymers eliminate two potential sources of contamination — stray lubricant and particulates from metal-onmetal contact. For this reason, polymer cam followers and rollers are a good choice for medical, semiconductor, foodprocessing, and cleanroom applications.

Smooth and quiet operation. Unlike metals, polymers can dampen shocks and vibrations. In cam followers, polymers absorb the damaging shocks and vibrations caused by reciprocating motion. This prolongs the life of machine bearings, especially in highspeed applications.

In rollers, damping contributes to quiet motion. Polymer rollers typically run about 10 dB quieter than their metal counterparts. They run smoothly too, thanks to their precise roundness. For example, total runout on a machined nylon 12 cam follower with a 1.5-in. OD would be less than ±0.001-in.

Speed and efficiency. Polymer rollers facilitate high-speed, energy-efficient motion in a three ways. First, polymer rollers weigh about 40% less than similarly sized metal rollers, reducing inertia. Second, polymer rollers typically use high-speed roller bearings, which have less rolling resistance than greased needle bearings found in metal rollers. Finally, polymers eliminate viscous drag caused by lubricants and worn metal surfaces.

Withstand harsh environments. Polymer rollers do not swell in moisture and resist chemicals and temperature fluctuations. They also do not corrode. These characteristics make them well suited to washdown applications, outdoor exposure and other harsh environments. (Specify cam followers with enclosed stainless-steel bearings and shafts whenever corrosion is likely.)

Lower costs. The true cost of cam followers and rollers has little to do with their purchase price and everything to do with their life-cycle costs. Polymers save over the long haul by eliminating the need for lubrication, extending maintenance intervals, and reducing energy costs. An even greater payback, however, comes from doing away with metal-on-metal wear. Polymer rollers, for example, pay for themselves many times over by extending the life of a motion system’s rails.

Polymer cam followers and rollers let designers use low-cost aluminum rails rather than expensive case-hardened and ground steel. Polymers also let machines be built without central lubrication subsystems, reducing costs even more.

Not just any plastic
Keep in mind that not all plastics make good cam followers and rollers. Even different grades of plastic within the same polymer family perform differently as bearing surfaces in followers, rollers, and other power-transmission components.

When evaluating plastics for use in rollers, it’s important to look at stated load capacities as well as the polymer’s likely stability under expected operating conditions. All polymers have properties that change with variations in moisture, temperatures, loads, and chemical exposure. With power-transmission components, all these factors can trigger losses in physical properties and lead to premature failures.

The type of exposure engineers most often neglect is moisture. Many polymers lose much of their tensile strength — 50% is not uncommon — after long-term exposure to moisture. Hygroscopic polymers also swell substantially, sometimes by as much 3% or more, impeding the smooth motion needed for parts to work together.

Power-transmission components that are weakened and out of dimensional tolerance can doom an otherwise welldesigned motion subsystem. Yet, engineers often fail to account for the effects of moisture uptake. Or if they know of moisture’s harmful effects, they simply avoid polymers altogether.

The same goes for other types of environmental exposures. Engineers either fail to recognize that polymers change in response to operating conditions or simply steer clear of polymers.

At Intech, we’ve found we can design around problems with moisture and other environmental exposures by picking the right type of polymer. We use gravity cast and machined nylon 12 almost exclusively. Based on its balance of physical properties and cost, gravity cast nylon 12 is the gold standard when it comes to plastic power transmission bearing surfaces.

More exotic engineering polymers will also work in roller and cam follower applications too, but expect to pay more for them.


Nylon 12: The plastic of choice

From cheap plastics to the world’s most-expensive composites, many types of polymers have been machined, molded, or cast into rollers, cam followers, and other power-transmission components. Engineers at Intech focused their development efforts on a proprietary, gravity-cast nylon 12 called Power-Core PA12GC. It combines several advantageous material properties.

From a materials standpoint, Power-Core maintains its physical properties in a variety of operating conditions. This stability lets engineers accurately predict the material’s behavior and life even when it’s exposed to varying moisture, temperature, and chemical levels.

In particular, Power-Core’s stability despite high-humidity and even total immersion, makes it well suited to power-transmission applications. Most high-performance polymers absorb moisture, swell, and lose tensile strength. Power-transmission components made from moisture-absorbing (hygroscopic) polymers can end up too weak to carry the loads they were designed for and too swollen to work with mating components.

Power-Core also dampens vibrations, which lets it reduce noise and absorb shock loads that power-transmission components commonly see.

To make these components, Intech gravity casts nylon 12 over metal hubs or thermally installs it over rollers bearings to create the beginnings of a cam follower, roller, or gear. These blanks are then precision machine into „ nished components. Gravity casting contributes to Power-Core’s low internal stresses that gives the polymer a uniform crystalline structure. As a result, „ finished components have a consistent machining resistance that improves precision during manufacture and use. And under external load, the dense crystalline structure helps thwart stress-induced cracks and swelling that sometimes lead to premature failure.


Plastic components improve a rotary table

Power-Core rollers have found uses in applications ranging from industrial machines to transportation. Most of them are found in linear-motion subsystems of one kind or another, but there’s no reason nylon 12 rollers cannot be used in rotary subsystems such as an index table.

Designed for OEMs making servosystems, this index table makes extensive use of rollers and gearing made from Power-Core nylon 12. For example, the table’s drive uses a plastic, pinion-driven ring gear with helical-tooth profiles that eliminate wear and backlash. The Power- Core rollers mount on the unit’s base plate, supporting the rotating gear and mounting surface. They feature enclosed stainless-steel bearings, letting the unit survive washdowns and other corrosive environments.

Using nylon 12 for the drive and bearings helps this rotary table outperform all-metal designs in several ways:

• Less wear without lubrication. Power-Core contains a lubricant, so no external lubricants are needed. This design also avoids metal-on-metal contact that can shorten the working life of motion components.

• Backlash-free. Power-Core gears’ dimensional stability eliminates the need for backlash compensation associated with plastics that swell when they absorb water and other liquids. The tooth profiles are machined for backlash reduction, a feat that would be di‡fficult to achieve in metal.

• Fast and smooth. The gearing and rollers’ low-inertia lets the table operate at high speeds. By producing a tighter inertia ratio between the motor and driven load, the drive components also make it easier to tune controls for smooth, precise motion, particularly when moving light payloads.

• Resists washdowns and chemicals. Power-Core’s dimensional and thermal stability withstand washdowns, high humidity, and temperatures that would prohibit using less-capable polymers.

• Clean-room friendly. Power-Core 12 does not throw off‰ particulates associated with metal components. So clean-room use is a natural fit for this rotary table.

• Scalable. By manipulating the size of the gears and rollers, the table can be scaled to handle payloads ranging from ounces to tons.


© 2012 Penton Media, Inc.

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