Worm gearing . se asa spur wheel; the exceptions exist in very heavily loaded com-mercial vehicles where one of the brakes is applied to some partof the transmission system, whereby, should the wheels belocked by this brake, and the car be caused to skid, the tractive 36 WORM GEARS resistance will rise to a high figure tending to shear off or atleast to bend the wheel teeth, while at the same time, consideredpurely as a worm gear, the conditions are not by any meansdestructive for the high tooth presstire occurs with zero rubbingvelocity and consequently there are no heating effects. CHAPTER V
Worm gearing . se asa spur wheel; the exceptions exist in very heavily loaded com-mercial vehicles where one of the brakes is applied to some partof the transmission system, whereby, should the wheels belocked by this brake, and the car be caused to skid, the tractive 36 WORM GEARS resistance will rise to a high figure tending to shear off or atleast to bend the wheel teeth, while at the same time, consideredpurely as a worm gear, the conditions are not by any meansdestructive for the high tooth presstire occurs with zero rubbingvelocity and consequently there are no heating effects. CHAPTER VII STRESSES IN WORM GEARING In a subsequent chapter, the stresses in the gears as affectedby the width of the wheel and the number of teeth in contactwill be investigated, but before considering, these, it will beadvisable to analyze all the stresses in the gears, as we can thendetermine the specific tooth pressure between the workingsurfaces and this value will be needed in the final proportioningof the Fig. 13. In the example already considered, the contact between theworm and the wheel is made at five different points simultane-ously, but owing to the fact that there are five threads to theworm, any diagrammatic representation of the same is muchcomplicated; let us, therefore, consider the same worm divestedof four of its threads as in Fig. 13; then some part of the remain-ing thread is brought in contact with every part of the surfaceof the wheel tooth against which it presses, and the successivelines of pressure commence at a and end at /. The sm-face ofthe worm thread forms a continuous spiral around the axis, and,therefore, each one of these lines of pressure makes successivelythe same angle with the axis of the worm. In Fig. 13, these 37 38 WORM GEARS lines of pressiire are shown as a skeleton diagram, and it is seenthat they form a portion of the sm-face of a conical spiralencircling the worm. Taking Fig. 13, the portion of this surfacewill be seen to be approx
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