The smooth motion with which worm gears should continuously operate and an acceptable and reliable service life depend not only on the design and quality of manufacture, but on the selection of two entirely different materials that are paired together. The torque that can be transmitted by a worm gear set is always limited by the surface pressure that the materials, from which they were made, can accept without deteriorating.

There are three principal factors that will effect the performance of a set of worm gears in relationship to the materials that were chosen:

(1) WEAR: The material must have the ability to withstand the drive conditions without showing appreciable wear throughout the life of the drive. That is an amount of wear that effects the performance of the drive.

(2) HEAT: The amount of heat generated is of great importance for several reasons. It has a major effect on the ratings. The temperature of the lubricant must be within acceptable limits; more heat indicates higher efficiency losses. The steady running temperature of a continuously rated gear is approximately proportional to the coefficient of friction with other conditions being equal. There may only be a slight difference between a good combination of materials and the ideal selection but a gear at 90 percent efficiency will operate at twice the temperature of a gear set that is 95 percent efficient.

When gear sets only run for relatively short periods of time the effects are not so pronounced; the gears have time to cool before they are once more in operation. Large center distance units can take six to eight hours of continuous operation before they reach their maximum temperature (Fig. 5.1).

(3) STRENGTH: The physical strength of the materials must be such as to avoid the risk of tooth breakage. Worm gears have an inherent advantage in that they have a much greater resistance to tooth breakage than other forms of gearing. Most designs are expected to result in more than adequate safety factors. The conditions of heat and wear are expected to restrict the rating long before the strength limit is reached.