The use of plastic gears is limited, largely because it takes special expertise to design them properly. Too, many engineers deal with gears so infrequently, that the seemingly complex discipline delays their consideration of plastic varieties. But plastic gears are often quieter, lighter, and more durable than metal gears. Too, plastic gears function quite well under many load types.

For example, intermittent loads generate elevated short-duration stress levels — acceptable when gear teeth have sufficient time to recover from bending. With continuous loads, allowable stress is defined by the gear design and material flexure rates, with the latter depending on how quickly teeth are loaded and unloaded. (Where root stress exceeds the allowable stress of the specified resin, the material must be reevaluated; where stress is extremely low, a redesign can reduce part-life cost.) One exception: Standing loads on plastic gearing cause creep and deformation, and should be avoided, unless a reinforced resin can be used.

Q & A

WHAT SPECIAL CONSIDERATIONS MUST BE MADE FOR ENVIRONMENT?

Whether they're inside a gearcase or exposed to open air, plastic teeth can be much hotter than their surroundings. Then they increase in size, effectively reducing the distance between them, thereby reducing backlash and other clearances. Conversely, when cool, gears tend to separate resulting in reduced tooth mesh contact. For this reason, operating temperatures must be factored into designs, especially when using dissimilar materials. When the gearcase or mounting plate is metal and the gears are plastic, the distance between gears changes due to the differing coefficients of linear thermal expansion, which may be much greater than an all-plastic system. Performance can be affected by these respective dimensional changes.

Moisture in the air surrounding the gears and the gearcase can also have an effect on the gear mesh, especially when the gearcase is metal and the gears plastic. Metal mounting plates don't absorb moisture but plastic gears do, so again, there's potential for size variation. In high humidity, expect gears to swell and reduce backlash. Where humidity is low, expect the opposite. Note these changes in size can often be addressed with a properly designed gear mesh.

One final note: Humidity can have an effect on material properties as well. For example, Nylon 6/6 is more brittle when dry than at elevated moisture levels. It is tougher and may perform better in at slightly elevated humidity.

DO PLASTIC GEARS REQUIRE LUBRICATION?

Lubrication provides a boundary layer between gear teeth in mesh, reducing friction and heat while guarding against premature wear. Lubricants like oil transfer heat from the gears to the exterior housing. External grease and oils may be applied to many materials. Alternatively, lubricant additives such as PTFE and silicone oil can be compounded into many plastics. Especially at high speeds, unlubricated plastic gears may not run well because of their high coefficient of friction. The gear teeth may heat rapidly, and even fail through deformation or excessive wear. Consider the use of a lubricant in plastic gearing, especially in systems where high loads, high speeds, or product life is an issue. Note however, that materials can behave differently when exposed to chemicals or lubricants and compatability of the two materials must be investigated.

WHAT ARE THE ACCURACY REQUIREMENTS OF PLASTIC GEARING?

Assembly tolerances must be carefully controlled in plastic gear trains — especially center-distance tolerances including bearing play and runout, which are often overlooked.

While mold manufacturers and injection molders may be capable of producing gears within demanding specifications, overly restrictive tolerances increase cost. As with all parts, don't overspecify beyond actual requirements. Remember: Even the inspection method required by tighter tolerances may add significant expense. Updated AGMA quality standards for elemental inspection now include the more commonly molded gears. Equipment for this elemental inspection can be quite costly and throughput may be slower than with common double flank roll testing (DFRT). That's why when tighter tolerances truly are needed, it might be wise to use elemental inspection for development and approval of initial samples, and base production on DFRT inspection.

This month's handy tips provided by Andrew Ulrich, senior engineer of Ufe, Inc., Stillwater, Minn. For more information, call(651)3514273.