Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/629709
• A worm drive can be designed with a gear ratio of more than 200-to-1, in comparison to that of a helical gear, which may be limited to 10-to-1 on a single reduction. The gear ratio for worm drives is the number of teeth on the worm wheel to the number of threads (or starts) on the worm. • The high gear ratio and configuration of the two gear elements allow for a compact design, making the worm drive a great option for space-limited areas. In addition, the number of moving parts is reduced along with the opportunities for failure. However, this may be partially offset by a loss in effi- ciency from large increases in torque. • Due to the extreme helical angle, switching the direction of power is nearly impossible. The worm wheel cannot easily be rotated independently to force movement on the worm. This self-locking ability eliminates the need for a backstop, which may be required in alternative gear systems. • With the precise movement of worm drives, particularly in double-envel- oping designs, backlash (play between gear teeth) can be greatly minimized. This is crucial in certain applications such as robotics. • Low noise and vibration results from minimal moving components in worm drives in comparison to alternative gear designs. Lubrication Challenges Worm drive designs have one major drawback: the relative motion between the mating teeth of the two elements is almost entirely sliding. This poses a significant challenge because the lubricant is continu- ally scraped aside. The sliding friction losses result in elevated temperatures and inadequate hydrodynamic pressure devel- opment. Consequently, wear debris generation can increase. In many cases, the higher temperatures will be the limiting factor on the worm drive before the loading limitations are reached. The load distribu- tion of enveloping gear designs can lessen this problem, but the challenge still persists. Non-throated (cylindrical) worm Throated (enveloped) worm wheel Three starts (threads) Side view Worm wheel Input www.machinerylubrication.com | January - February 2016 | 45 Figure 4. Single-throated (single-enveloping) worm drive Figure 5. A comparison of lubricant life and oil change intervals for mineral oil, PAOs and PAGs over a range of oil sump temperatures 160 150 140 130 120 110 100 90 80 70 300 500 1,000 5,000 10,000 30,000 polyglycol mineral oil Oil Change Interval (hours) Sump Temperature (°C)