Principle: A machine’s practical mechanical advantage is defined as the ratio of the force expended by the machine to overcome a resistance to the force applied. The screw’s mechanical advantage is likely to be greater than that in any other form of machine. The worm gear is a form of screw that makes use of this inherent mechanical advantage. A screw and a worm gear have a cylindrical body with helical grooves cut into the surface. The screw’s mechanical advantage is the ratio of the distance traveled in one revolution to the force applied to its pitch.
A worm gear uses the mechanical advantage of the ratio of plane length to height. When a screw is turned through one revolution, the screw advances the distance between successive threads. This distance is called “the pitch of the screw.” The inclined plane is another ancient and familiar machine and was used by the Egyptians to move stone blocks into position for the pyramids. The worm gear is also the mechanical application of the inclined plane: a plane that is rolled up in a helical form. If a gearset were assembled so the worm wheel could not rotate and the worm left free to move along its axis, depending on the direction of rotation the revolving worm would move in a forward or backward horizontal motion. The thread of the worm advances in an axial direction. When the worm is held between thrust bearings and the worm wheel is free to rotate, the revolving worm turns the worm wheel. When the worm is considered to be in a stationary fixed position, the resulting conjugate action would be identical to that of a rack and pinion. The worm wheel’s pitch circle is a cylinder concentric with the axis. The worm pitch surface, on the other hand, is a plane parallel to the axis of the worm.
There are two distinct types of action that take place when the worm and wheel are rotating. These actions are termed approach and recess. The approach action is a sliding action that takes place when the gear tooth slides down the side of the worm flank toward the worm’s central axis. This is a wearing away action that is harmful to the wheel surface. The recess action takes place when the gear tooth slides out of mesh climbing up the worm tooth away from the central axis. The friction forces are then much lower and in a positive direction. This action improves the contact, and thereby the load capacity. Whenever the worm is the driving member, the recess action is the preferred operating mode. In a full-recess action the worm pitch line coincides with the outside diameter of the gear. It is sometimes necessary to add two or three teeth to the worm gear or increase the worm’s lead. The approach ends and the recess action begins at the pitch line. The reversal in the direction of sliding will have the tendency to break down the oil film that can only be eliminated with all recess action.
There are many advantages to the worm gear. From the earliest of times the worm gear has been widely used in many different applications because of several inherent advantages, even though it is a complicated gear to design and manufacture. Worm gears provide within one pair of gears the largest available ratio and the most compact design for gears with ratios > 10:1. Shaft axes can be at a right or acute angle. Due to the high contact ratio and material combination, worm gears have the lowest sound levels of all gear types. According to the German VDI standard 2159, the anticipated sound (noise) level for a worm gear enclosed drive is approximately 7dBa lower than a bevel-helical drive at the same input power and speed. A worm gear has more internal vibration damping than any other gear form. Due to the large area of contact and the material’s properties, worm gears have the highest load bearing capacity. They are capable of sustaining high peak torques. Design ratings allow for momentary 300 percent peak loads in comparison with 200 percent for other gear forms. When it is an advantage to the application, self-locking characteristics can be utilized, within limitations. The transfer of motion is smoother than with other gear systems due to the gradual tooth engagement, and precise backlash control with provisions for minimum backlash is obtainable. Lastly, high precision worm gears can be produced with a total angular velocity deviation of less than two arc minutes.
Worm gearing is considered by many to be more complex in the understanding of its design, operation, application, manufacturing, and assembly than most other types of gearing. Even so, when properly applied the worm gear is still one of the strongest, most reliable, and long-lived gears that can be selected. When the performance is below what is anticipated, the reason is usually found to have been misapplication or a misunderstanding of either efficiency, lubrication, or the thermal rating. In the application of the worm gear three areas are of special significance—threads, effective length, and tooth thickness—which we will consider in the next installment of this column.
