Molded Plastic Gears

This month we are going to discuss several critically important factors designers need to know when developing an application with molded plastic gears.

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A designer never wants to mold a standard “generated” gear design. Why? Because the molding process gives us several opportunities to optimize the gear geometry in ways that are either not possible with classical cutting methods or practical-based on cost and difficulty. One mold makes many gears, so invest your resources in the design and the mold. Consider the gear in Figure 1 that meshes against a steel driver:
• Optimize the Specific Sliding Ratio: This is very important because the tooth tip of the driver will slide into contact with the plastic gear at the start of the active profile until it gets to the operating pitch diameter, where theoretically, sliding ceases and the involutes roll. As the driving tooth passes the pitch point, sliding again occurs and reaches a maximum at the end of the active profile. Specific sliding is directly proportional to the rate of frictional heat that is generated at the surface layer of the tooth. We generally want to limit the sliding ratio to less than 3.0 so that the gear does not heat up, make changes in the crystalline structure, distort or in the worst case—melt. Specific sliding is strictly a function of geometry. Unlike metals, wide swings in operational temperature have a dramatic impact on plastic mechanical properties, i.e. strength, fatigue, and creep. Generally, plastic gears either have internal lubrication in the material or are grease lubricated. However, if the application is in an oil bath that can dissipate the flank temperatures, sliding and wear concerns are greatly reduced. Either way, the designer needs to keep his eye on this parameter as the optimization unfolds, reduce specific sliding with addendum modifications, increasing pressure angle or tooth count, and move contact away from the base circle.
• Profile Contact Ratio: Maximize the total line of contact first to get the highest profile contact ratio without tip to root interference or excessive sliding.
• Helical Overlap Ratio: Spur gears only have a profile-contact ratio. However, helical gears offer greater contact as a function of the helix angle and face width. Higher contact ratio is desirable for increasing strength due to load sharing. Generally, the more teeth in contact the less noise is produced due to the averaging out of errors. In applications where minimum mass is required,  gear face width can be reduced by increasing the helix angle. The total contact ratio is a function of the profile plus the overlap ratio.
• Equalize Plastic Gear Tooth Strength Against the Mating Gear: Many times in power dense applications, a plastic gear will mesh with a much smaller metal pinion. This opportunity will allow optimization of the plastic gear to yield thicker, stronger teeth. Gear designers know that thinning and reducing strength of one gear allows an opportunity to transfer that strength to the mating gear. The other major benefit comes from compliance of the plastic teeth. Since the steel pinion is generally much stiffer than the plastic mate, plastic gear deflection tends to comply with the steel flanks – especially in unfilled materials. Therefore, if a higher accuracy is given to the steel member, the plastic gear will conform to it under a certain load level. As a result, many times, the index and lead errors are not nearly as critical for noise concerns and load distribution in the compliant material.
• Balance Gear Body Strength (min mass/volume): In Figure 1, the body of the gear is optimized for stiffness, rim thickness and strength for the application.

Figure 1

In molding, maximize tooth-ending strength through custom root geometry to obtain the lowest possible stress concentration in the high stress fillet area. This is much easier and more effective to do in a molded verses generated gear. Optimize the plastic net shape structure to give minimum mass, and always provide uniform wall thicknesses critical to good a molding process. The gear in Figure 1 was optimized with high-level gear software, finite element analysis and mold-flow technology.
• Balancing Tip, Root, and Backlash Clearances for all Environmental Conditions:
In general commercial and industrial metal gears may not require much in the way of an evaluation of thermal energy. However, plastic gears generally do. Most plastic gear failures I have encountered revolve around this very issue. Plastic gears should be designed to “Tight Mesh Condition” (Figure 2).

Figure 2

This means the geometry is designed at the MMC gear parameter (i.e. tooth thickness, OD, tight centers), with the highest temperature and greatest humidity environmental application in mind. Under this condition, the minimum acceptable backlash is specified. Note: This requirement is for both members of the mesh.

The idea is to avoid any binding during an extreme application condition. It is a tragedy to see a gear drive over-heat where you get even a very small interference condition. We have seen the compression and release mechanism of gear teeth with each revolution cause greater and greater thermal material expansion to the point where the gear teeth begin to melt and extrude. In fact, in one test case a gear drive stopped running overnight. Disassembly revealed what seemed impossible—the plastic gear had no teeth. What happened was that continuing flank and tooth to root interference resulted in a hysteresis that caused the temperature to rise exponentially and continuously expand the material until it could contact no more. When the drive cooled down, there were no teeth left and quite a gap existed between the two gear bodies. Optimized gear geometry will also account for differences in effective center distance resulting from thermal expansion between the gears and the housing. Always remember to evaluate the gear design at the opposite thermal boundary. Make sure the worst case “LMC Cold & Dry” condition of profile contact ratio remains greater than 1.0.
• Design of a Customized Gear Cavity Generating Tool: In addition to the noise, vibration dampening, compliance, and mass benefits of plastic gears, there is a huge financial incentive to molding net shape components. A plastic net shape versus the least expensive steel (smooth blank) gear is can be as little as 1/10 to 1/5th the cost even with amortization of the tool. However, molding tools are very expensive relative to hobs and shaper cutters.

Therefore, understanding and accounting for the critical factors in plastic gear design will take the risk out of the process and help you be successful. Plastic gear molding is a very specific and highly technological expertise.  It is not commodity molding. The gear designer should look for and work very closely with a molder who specializes strongly or specifically in precision plastic gearing.

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is a senior technical specialist in the development of gearing, drive motors, and power closure devices in the automotive industry. He currently serves on the AGMA Plastic and PM Gearing Committees. Eberle is a master six-sigma black belt and has authored many papers on gearing, measurement system analysis and process statistics. He can be reached at Fred_Eberle@hci.Hi-Lex.com.