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Worm gearing is most commonly used for high ratio reduction and increased precision positioning. Precision in position is gained by high ratio reduction. As the ratio increases, the output slows relative to the input, the reduction in backlash is similar to the ratio changes. Using worm and wheel designs, as an example, a simple drive with less than precise rotational position control will experience a significant increase in positional accuracy.
In terms of the working contact patch and service life, worm gearing does not share the contact parameters that parallel axis involute-based gearing does. Parallel axis gearing based on the involute tooth profile runs through a point during the engagement mesh where the two teeth exhibit no sliding, only rolling action. We define this contact point or rotational relationship as the pitch point. Worm gears have no point of pure rolling during their mesh engagement. Thus, all contact action is sliding. Interestingly enough, the predicted service life of a worm gear set is based largely on the wear rate as a function of the amount of material that can be sacrificed, the thickness of the tooth. Since predicted service life is based on wear rate and lubrication, and since the output wheel typically rotates slowly, traditional elastohydrodynamic lubricant development is difficult to develop or maintain. Thus, we usually rely on grease or a more adhesive lubricant and lubrication system to keep the gear teeth from touching. We also employ subtle geometric features of the tooth face, generally called a lubrication entry gap. The concept is to promote development of a wedge of lubricant to be drawn into the mesh. The geometric details of the entry gap are a function of the gear materials, lubrication, and interfacial speed.
Geometry also plays a role in the mesh efficiency. With the worm driving the wheel, the mesh efficiency is on the order of 60 percent to 80 percent. However, if the wheel drives the worm, the efficiency is generally less than 10 percent. From this, we tend to call worm and wheel gearing as non-backdrivable. It is not, but with these low efficiencies it is unlikely under typical circumstances that the wheel will effectively drive the worm. If, on the other hand, the gear mesh is exposed to an external vibration source, the wheel driving worm efficiency may climb as high as 40 percent, which invalidates the concept of non-back-drivability.
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发表于 2023-2-2 21:55
