Eliminating backlash in worm gear pairs

Various techniques are available to minimize backlash in worm gear pairs.

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Gardening is a hobby enjoyed by many. The ability to transform a small piece of land into a beautiful landscape can be both cathartic and a source of pride. When designing the landscape, consideration will be given to various shapes, colors, and heights in order to develop a visually appealing effect. What lies beneath the vegetation is just as important. Rich fertile soil, proper drainage, and beneficial biodiversity will ensure that the landscape develops into the desired result. If you dig into the soil and find worms tunneling through your soil, you know that you have a good base to work from. In gearing, we have worms too; however, they do not represent good soil.

Worms and their corresponding worm wheels are a common gearing combination. They are frequently used as the reducing mechanism within a right-angle gearbox. They can efficiently reduce drive shaft speeds which increases the output torque. If the gearbox operates in one constant direction, a standard set of gears can be used. If the gearbox will be reversing directions and the output needs to have a precise location, such as in a repeating pick and place application, then an additional measure needs to be introduced to reduce the backlash.

The most common and poorly thought-out method of adjusting the backlash of a worm gear assembly is to modify the center distance. Once assembled, such adjustment requires a major rework of the gearbox housing. This method leads to premature wear of both the worm wheel and the worm as the clearances needed to properly run the gears is eliminated.

A better way to eliminate backlash in a worm gear pair is to use a worm wheel that is spring loaded. In this case, the worm is split along the face width and windows are cut into the body. In these windows are springs. As the gear halves are counter rotated, they pull the springs, thus creating a force which keeps the teeth positively always engaged with the worm.

A spring-loaded worm wheel.

Another method for eliminating the backlash is to use a spring-loaded split worm.

In this case the worm shaft consists of two pieces which are spring loaded and the teeth of the worm shaft is always engaged with the worm wheel.

Another method is to create a worm gear pair with a variable tooth pitch. This type of gear pair is known as a duplex worm gear pair. Whereas the spring-loaded worm gear pair’s capacity to minimize backlash and is limited by the spring rate, the duplex worm gear pair allows the backlash adjustment to be made by axially shifting the worm. This simplifies greatly the assembly and maintenance operations.

A worm gear pair with a variable tooth pitch.

These worm pairs consist of a dual-lead worm and a corresponding worm wheel. The dual-lead worm is generated such that there is a difference between the right tooth surface and left tooth surface, resulting in a unique tooth profile in which the tooth thickness varies continuously.

This allows the worm gear pair to be set at a nominal assembly distance, and when the worm is moved in the axial direction, the tooth thickness of the worm in mesh with the worm gear changes, making backlash adjustment possible.

Figure 1.

The amount of change in backlash (j mm) in relation to the axial movement of the duplex worm shaft (V mm) can be calculated from the formula:

where

When the worm is held with arrow mark pointing right, the tooth thickness is thinner on the right and thicker on the left. Therefore, moving the worm to the right causes the thicker teeth to come into actual engagement with the worm gear, thereby reducing the backlash (Figure 2).

Figure 2: Moving the worm in the direction of the arrow causes the backlash to decrease.

The KHK duplex worm is designed so that, for all modules, the backlash reduces by 0.02 millimeters for each 1 millimeter that the worm is shifted. As the worm wheels are produced with a backlash range of -0.045 to +0.045, a 2-millimeter axial adjustment can result in zero backlash pair. 

If you have a need to include a zero backlash worm gear pair in your application, these are the solutions available for your design. If you need to improve your garden, I recommend Red Wigglers. 

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is general manager of KHK USA Inc, a subsidiary of Kohara Gear Industry with a 24-year history of working in the industrial automation industry. He is skilled in assisting engineers with the selection of power-transmission components for use in industrial equipment and automation. Dengel is a member of PTDA and designated as an intern engineer by the state of New York. He is a graduate of Hofstra University with a Bachelor’s of Science in Structural Engineering.