Mounting accuracy key issue in gear system design

Some assembly is required. These dreaded words are frequently found on the packaging of many weekend projects. Whether it is assembling a desk for that new pandemic home office or building the new BBQ grill you received as a Father’s Day gift, we always hope that all the parts are included and that they fit together as expected. When it comes to gearing, there is a misnomer that to design the best gear system, you must use the most precise gears. However, even if the gear is produced with high accuracy and is not mounted properly, it is impossible to avoid the problems of bad tooth contact: noise, wear, and breakage. One of the errors that can occur in the assembly of gear pairs occurs with the center distance. Errors occurring with the center distance of two gears directly influence the backlash of the gear mesh. If the center distance value increases, the backlash value increases. As a result of this increased center distance, the gear teeth cannot mesh deeply enough into each other, and the contact ratio decreases. If the center distance value decreases, the backlash value also decreases. The gears’ rotation ability is inhibited if the backlash decreases too much. Table 1 shows the center distance tolerances for spur and helical gears.

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The tolerance values in this table are quoted from JGMA 110101 (2000) and are applicable for involute spur and helical gears made from steel. Another error that can occur in gear system assembly occurs with the pair's axial parallelism. The accuracy of two parallel axis gears comprises parallelism and shaft offset errors. These errors influence tooth contact in the direction of tooth trace. They may also result in bad tooth contact occurring at the tip of the tooth width. Any increase in these errors results in a decrease in the backlash and will cause both noise and tooth breakage. Tables 2 and 3 detail the shaft parallelism error and offset error tolerances for spur and helical gears, as detailed in JGMA 110201 (2000). Figure 1 details the location of these tolerances. Awareness of the different tolerances during assembly is necessary and will allow the gears to perform as designed. Setting the gears at the proper center distance and axial alignment will help to maintain the proper backlash and the desired tooth contact. With these two factors in check, the gear system will exhibit minimal noise and achieve the designed gear life.