Worm gearing . itch and wormdiameter for a given horse-power, the next thing to determineis the number of wheel teeth and worm threads to give the 16 WORM GEARS required ratio. Seeing that a comparatively small pitch hasbeen selected for all wheels, it is evident that a relatively greaternumber of worm threads is possible. DETERMINATION OF WORM WHEEL DIAMETER This will naturally be preferred as small as possible but itmust not be reduced at the expense of excessive tangentialpressure, and the number of teeth should not be less than 24 ormore than 40. The writer has found that almost any condit
Worm gearing . itch and wormdiameter for a given horse-power, the next thing to determineis the number of wheel teeth and worm threads to give the 16 WORM GEARS required ratio. Seeing that a comparatively small pitch hasbeen selected for all wheels, it is evident that a relatively greaternumber of worm threads is possible. DETERMINATION OF WORM WHEEL DIAMETER This will naturally be preferred as small as possible but itmust not be reduced at the expense of excessive tangentialpressure, and the number of teeth should not be less than 24 ormore than 40. The writer has found that almost any conditionscan be satisfied with between 30 and 40 teeth, bearing in mindthat a very heavy vehicle requires a larger wheel than a lightone, but the heaviest commercial vehicle need not have morethan 40, and questions of road clearances under the axle will toa great extent determine the diameter which must alwaysremain a matter of judgment on the part of the designer. The following proportions of teeth are recommended:. Fig. 2. Addendum,Dedendum, .3183 P It has been stated that the pitch of the worm can be measuredin three ways (Fig. 2): Axially =P Normally =P Circumf erentially = P. and these are inter-related as follows: PRELIMINARY PROPORTIONS 17 P = P cos aP = P cosec aTherefore, P = P cos a cosec a P = P cot a (2) It follows from this that the number of threads on a wormhas the following relation to its pitch line diameter. nd = nP cot a (3) Tzd .. n = ^—.— (4) P cot a ^ ^ and to find the number of threads, it is first necessary to knowthe value of the lead angle a which again depends upon thelead L, and this in its turn upon the gear ratio gear ratio G has the following value ttD ^-L (5) hence ,. ^D ^~ G (6) and the lead angle a is such that tan a— ,m (7) Substituting from equation (6) nD G tan « = —Tnd 7:0 D tan a- ^^ (8) from which a is at once found, and from this the value of n (equation 4). DETERMINATION OF LENGTH OF WORM AND NUMBER OF TEETH IN C
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishernewyo, bookyear1916
