The AGMA quality standard governs both plastic and steel gears. Plastic gears provide a number of distinct benefits over their metal counterparts. These benefits make them more enticing in today’s world, as production speed and developing technology necessitate more plastic parts.
However, there are still questions remaining about the quality of plastic gear design compared to metal gears. Metal gears have been used for so long, it’s challenging to make the case that something so ubiquitous as plastic can so readily replace metal.
Challenges of Plastic Gear Design
Despite the plethora of improvements in, and changes to the plastic gear design guide, many engineers, tradesmen, and product designers still swear by metal gears. Plastic gear cannot be regarded as simply a different kind of metal gear, except that it’s cast in plastic. The dynamics of plastic material when heated demand that it be designed as a completely different kind of material. In reality, plastic gear design specifically necessitates consideration of hysteresis heat build-up, which metal gears are not vulnerable to.
The fundamental difference between metal and plastic gear design philosophy is that metal gear design is predicated on the strength of a single gear tooth, but plastic gear design considers load sharing across teeth. To put it another way, plastic teeth deflect more under strain and distribute the weight over a larger number of teeth. Load-sharing boosts the load-bearing capability of plastic gears in most applications. As a result, as tooth size decreases to a pitch of roughly 48, the allowed stress for a certain number of cycles of failure increases. Because of size impacts and other difficulties, there is little growth over a 48 pitch.
Why Plastic Gears Fail
The reasons that some plastic gears stop performing include wear and tear, material rutting, fracturing, pitting, and breakage. Notably, many of these vulnerabilities are the same as with metal gears. The root of the gear tooth and the contact point of a loaded gear made of plastic or metal are both susceptible to strain; specifically bending stress if it’s a greased gear. Conversely, non-lubricated gears may wear out before a tooth fails. As a result, contact stress is the most important consideration in gear design. At the tooth root, most plastic gears feature a complete filet radius. As a result, they are less susceptible to stress concentrations than metal gears.
Bending-stress data for engineering thermoplastics comes from fatigue testing performed at precise pitch-line speeds. When velocity exceeds the test speed, a velocity factor should be utilized in the pitch line. Continuous lubrication can raise the permissible stress by at least 1.5 times. Surface contact stress calculations, like bending stress calculations, require a variety of correction variables.
When the pitch-line velocity exceeds the test velocity, a velocity factor is employed. Changes in operating temperature, gear materials, and pressure angle are all taken into consideration using a factor. Another consideration in the construction of thermoplastic gears is stall torque. Gears are frequently subjected to stall torques that are significantly greater than the usual loading torque. When plastic gears are used at high speeds, they are susceptible to hysteresis heating, which can cause the gears to melt.
Benefits of Plastic Gear Design
Plastic gears are beginning to supplant metal gears in a growing number of applications as designers become more aware of their advantages over metal gears. Plastic gears made by injection molding are now used in everything from household appliances, windshield wiper motors, and a vast range of medical and industrial equipment.
Previously, plastic gear usage was restricted due to the ambiguity surrounding its reaction to moisture, temperature, and certain chemicals. Now, plastic gears are being used to their maximum capacity. Plastic gears are even outperforming metal gears in many applications, thanks in large part to advancements and new developments in polymers and plastic molding design, as well as material processing techniques and control.
The benefits of plastic gears include:
Less expensive to use – Plastic gears are often less costly to manufacture than metal gears. Plastic gears often save 50 percent to 90 percent compared to stamped or machined metal gears, since additional polishing is rarely required, according to Plastics Technology.
Higher accuracy – Plastic gears can attain high levels of accuracy – up to AGMA Quality 10 – with consistent material quality and precise molding process control.
Resistant to corrosion – Plastic gears, unlike metal gears, are corrosion-resistant. Because of their chemical inertness, they can be utilized in water meters, chemical plant controls, and other applications where metal gears would rust or degenerate.
Lightweight – Plastic gears are often lighter in weight than metal gears of a similar size. Steel has a specific gravity of 7.85, whereas nylon and acetal have specific gravities closer to 1.4.
Quieter operation – Plastics have greater noise-dampening qualities, resulting in quiet running gear. Plastics are now required for high-precision tooth designs as well as lubricious or flexible materials in the continued quest for quieter motors.
Flexible design – Plastic molded gears allow for more efficient gear geometries than metal molding. Molding is perfect for constructing designs like internal gears, cluster gears, and worm gears that would be prohibitively expensive to produce in metal.
Enhanced lubrication – Many plastics are naturally lubricious, making them perfect for computer printers, toys, and other low-load applications that require dry gears. Both grease and oil can be used to lubricate plastics.
The determination to utilize metal or plastic gears in design should ultimately come down to intended usage. There are many benefits to using plastic gears, but they aren’t suitable for every use. Although the list of benefits can make plastic gears look very attractive, the reality is that in some instances, plastic is the better solution, while in others, metal is the best option. Thanks to modern technology, there is sufficient data to show which option is the right choice depending on the final product needs. For more information about plastic gear design or for help deciding what to order, please contact us today.