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Design and Application of the Worm Gear
William P. Crosher
William P. Crosher
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ASME Press
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Because of the difference in the meshing action, the lubrication of worm gearing is a more complex problem than for other types of gearing (Fig. 10.1). Due to the side-slide an effective lubricant film is not formed as with helical, bevel, or spur gearing. The direction of the slide and roll is a separate action from the direction of movement of the line of contact. The line of contact movement takes place from the tips to the roots of the driven gear teeth. This action forces the lubricant into a smaller area and then a wiping action and squeezing of the oil takes place. The formation of a thick film is hindered by the slow speed of the worm wheel. A high sliding velocity is produced by the rotating worm's faster speed. As the velocity increases the oil film is better able to support the load with a corresponding decrease in friction. There is only a brief period of contact at any part of the tooth, a combination of rolling and sliding action, the direction is parallel to the line of contact.

When the direction of slide is at right angles to the line of contact, as with the globoidal worm, an oil wedge is more readily formed. The selection of lubrication properties for globoidal worm gears are, however, complicated by the existence of an oil pocket between the tooth faces bounded by the contact lines. Cylindrical worm gearing produced with an oil entry gap and assembled with contact on the leaving side assists entry of the lubricant (Fig. 10.1). Contact in globoidal worms tends to the entry side which complicates the ingress of lubricant. The contact pattern shows the clearances gradually diminishing from entry to the contact area. The entry gap is achieved by machining on design centers with a modified hob profile, whose diameter is that of the worm plus two clearances. The more the oversized hob profile, the larger the clearance and the smaller the contact pattern. Flycutters work in much the same manner, if the cutter has a radially adjustable blade it is moved outwards to the desired location.

When there is sliding between two components the surfaces that rub against each other cause destructive damage unless separated by an oil film. This film will be influenced by the worm sliding speed and the viscosity. Lubrication between any two surfaces that have relative motion consists of either a thick film (hydrodynamic) or thin film (boundary). To provide conditions for a thick film, gears must be provided with a localized bearing pattern by selection of the geometry with associated data. Thick film lubrication is impossible to maintain when the surfaces are between a worm and wheel. A worm gear's performance is effected by the oil film between the rubbing surfaces.

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