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Gear Surface Texture Is More Important Than Ever
The interactions of gear surfaces are becoming increasingly important in applications such as automotive electric motor drive systems and aircraft turbines.
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Source (all images): Mahr Inc.
The roughness of gears plays a major role in so many products developed over the past century. From the gears in dial indicators to machine tool spindle transmissions to automotive engines and transmissions, surface finish has always affected the interaction between gear teeth. In today’s world, the interactions of surfaces are becoming increasingly important in applications such as automotive electric motor drive systems and aircraft turbines.
There are three very important reasons for tighter control of gear surfaces. The first is increased efficiency. The correlation between the surface quality of transmission gears and energy efficiency has always been a concern for the automotive industry. An internal combustion engine might average an efficiency of 20%, (that is, around 80% of the energy it generates is lost as wasted heat). One possible approach to save fuel and reduce emissions is to minimize gear roughness in complex transmissions.
Alternatively, an electric car transmission is much less complex, as it may be a single-stage transmission. Although they are less involved, the surface quality of the gear teeth is critical because an electric motor can have up to 80% efficiency. Manufacturers’ most important goal and generally the most pressing challenge in electric car transmission is to prevent efficiency loss that would reduce vehicle range. To maintain or even increase the range, gear surfaces must be ground with high precision to minimize friction losses from the first mile traveled.
The second is torque and power throughput. A typical transmission for combustion engines is designed for comparatively low torque. Electric motor drive systems differ in that even typical stationary operation places extreme demands on the drivetrain; the load is comparable to that of a combustion engine under maximum load. This is because the electric drive reaches its highest torque and power throughput immediately after starting. The gears in electric motors must be able to cope with these extreme loads even in normal operation, which means that the demands on their surface quality are critical.
Finally, there is the noise produced from gear interaction. Every manufacturer strives to ensure its automobiles emit as little noise as possible. In combustion engines, noise primarily originates from the engine, rather than the transmission. In contrast, electric motor transmissions do cause undesirable noise. High roughness on gear surfaces increases this noise, whereas smoother surfaces minimize it. Ultra-fine surfaces on gear tooth flanks are therefore key to noise reduction.
Ultra-fine surfaces on the tooth flanks are the decisive quality feature for gears in electric motors and transmissions. Grinding machine manufacturers have already worked intensively with machining equipment and production processes to ensure the required ultra-fine gear surfaces, using continuous gear grinding and integrated polish grinding as an innovative process step. These advances reduce the roughness of a gear manufactured with continuous gear grinding, while not altering the gear flank topography or edge zone properties of the active tooth flanks. Thus, surfaces of unprecedented quality, currently with a roughness (Rz) of between 0.2-1 µm, are being produced; however, even smaller goals are expected in the future.
While the manufacturing of gears continues to improve, the high-precision metrology for testing gear roughness is also becoming increasingly demanding. Typically, the common practice is to determine the surface finish of gears using geometric gear measuring systems. However, these systems are designed for measuring coordinates, not roughness. With so many moving axes, system accuracy is often not appropriate for these tighter tolerances.
Also, these systems operate with a skidded probe system. The probes are too large to allow measurement down to the root of the tooth, even with small gears, where too much roughness can lead to hairline cracks and, as a result, to the complete rupture of a tooth. Additionally, they can only detect roughness and are unable to measure waviness.
A skidded probe is too large to be able to measure down to the root of the tooth.
The challenge is to utilize existing high-precision surface texture technology for surface measurement and to stage it in a way that reliably checks roughness on gears. This means employing a skid-less probing system capable of measuring high-quality surface finish roughness of Rz < 1 µm while also being able to sense fine waviness on the gear surface.
Naturally, gears come in various sizes and weights. While surface measuring system probes are capable of measuring even the smallest tooth root, the challenge is handling the various sizes of gears and positioning the skid-less probe in the correct position.
However, with a combination of a system best matched to the gear size, a system to easily load and orient the family of gears, and measurement automation, a high level of inspection can be performed to ensure today’s gears meet the new challenges they were designed for.