Machinery Lubrication

Machinery Lubrication Jan Feb 2016

Machinery Lubrication magazine published by Noria Corporation

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Page 48 of 89

f all the different types of gear configurations, worm gear systems are consid- ered some of the most problematic because they present unique lubrication challenges due to their distinct design. To overcome these challenges, you must understand not only the complexities of worm gears but also which qualities to take into account when choosing a worm gear lubricant. Worm Gear Designs A worm gear is a non-parallel, non-inter- secting axis design consisting primarily of two gear elements: the worm, which is the driving gear in the shape of a spiral or screw, and the worm gear or worm wheel, which is the driven gear in the shape of a common spur gear. Technically, the entire worm gear system should be called a worm drive or worm gearset to avoid confusion. The worm always drives the worm wheel. This design character- istic is due to the extreme helical angle, which is nearly 90 degrees. The worm drive resem- bles the design of the crossed helical gear configuration, except the gear teeth on the worm of a worm drive will circle around the circumference of the worm at least once. Since the worm may have as little as one tooth that spirals radially around the helix, the number of teeth on the worm is more appro - priately identified by the number of starts or threads. There are three categories of worm drive designs that describe the degree to which the gears mesh together: non-throated (non-en - veloping), single-throated (single-enveloping) and double-throated (double-enveloping or globoidal). Non-throated or non-enveloping is the most basic design in which the worm and worm wheel are both cylindrical in shape. This allows for simplistic manufacturing, but the limited contact zone of a single point on one or two gear teeth can become problematic. In single-throated or single-enveloping designs, one of the gear elements (most commonly the worm wheel) has concave helical teeth for contour or envelopment of the gear teeth onto the worm. This enables the contacting zone to increase to a line. Double-throated (double-enveloping) or globoidal designs not only have concave helical teeth on the worm wheel, but the worm is also shaped like an hourglass so the two gear elements wrap around each other during motion. This results in nearly eight times more contact area (in the shape of a radial band) with three or more teeth in contact. As the contact surface area increases, the torque capacity, load-holding ability (shock load resistance) and durability are improved. Enveloping gear designs also have a lower anticipated wear rate as a result of the load distribution. Worm drive manufacturers attempt to optimize this contact relation - ship between the two gear elements for improved reliability. Other notable advantages of worm drives over potential gear system alterna- tives include: O G e a r L u b r i c a t i o n Benne t t Fi t ch | Nori a Corpor at ioN LESSONS IN LUBRICATION THE RIGHT WAY to Lubricate WORM GEARS 44 | January - February 2016 | Figure 1. Non-throated (non-enveloping) Figure 2. Single-throated (single-enveloping) Figure 3. Double-throated (double-enveloping)

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