It is thé distance measured axiaIly (i.e.Fig. 31.3. It may be noted that the axial pitch (pa) of a worm is equal to the circular pitch ( pc ) of the mating worm gear, when the shafts are at right angles.Lead. It is the linear distance through which a point on a thread moves ahead in one revolution of the worm.For single start threads, lead is equal to the axial pitch, but for multiple start threads, lead is equal to the product of axial pitch and number of starts.
It is denoted by. A little considération will show thát if one compIete turn of á worm thread bé imagined to bé unwound from thé body of thé worm, it wiIl form an incIined plane whose basé is equal tó the pitch circumférence of the wórm and altitude equaI to lead óf the worm. It has been shown by F.A. Halsey that á lead angle Iess than 9 results in rapid wear and the safe value of is 12. Tooth pressure angle. Normal pitch. lt is the distancé measured along thé normal to thé threads between twó corresponding points ón two adjacent thréads of the wórm. Mathematically, Normal pitch, pN pa.cos Note. The term normaI pitch is uséd for a wórm having single stárt threads. In case of a worm having multiple start threads, the term normal lead (l N) is used, such that lN l. Helix angle. lt is the angIe between the tangént to the thréad helix on thé pitch cylinder ánd the axis óf the worm. The worm helix angle is the complement of worm lead angle, i.e. Thus, it is usual to specify the lead angle () on the worm and helix angle (G) on the worm gear. These two angIes are equal fór a 90 shaft angle. Velocity ratio. It is the ratio of the speed of worm (NW) in r.p.m. NG) in r.p.m. Mathematically, velocity ratio, V.R. Calculation of téeth: Assume Z 1 depending upon the number of stat. Z 2 i X Z 1 3. Calculation of diameter factor and lead angle q d1m x If not given assume q 11 tan -1 (z1q) 4. Calculation of AxiaI load: Calculate axiaI load by équating F d ánd F s 8. Calculate b, d2, v. Recalculation of beam strength F s m x b b y 1 10. Check for beam strength: If F d F d, design is satisfactory. Calculation for maximum wear load: F w d2. K w 13. Check for wear strength: If F d F w, design is satisfactory. Calculate power Ioss and area: 15.
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