Machinery's Handbook, 31st Edition
2272 WORM GEARING Tooth Form of Worm and Wormgear: The shape of the worm thread in the normal plane is defined as that which is produced by a symmetrical double-conical cutter or grinding wheel having straight elements and an included angle of 40 degrees. Because worms and wormgears are closely related to their method of manufacture, it is impossible to specify clearly the tooth form of the wormgear without referring to the mat ing worm. For this reason, worm specifications should include the method of manufacture and the diameter of cutter or grinding wheel used. Similarly, for determining the shape of the generating tool, information about the method of producing the worm threads must be given to the manufacturer if the tools are to be designed correctly. The worm profile will be a curve that departs from a straight line by varying amounts, depending on the worm diameter, lead angle, and the cutter or grinding wheel diameter. A method for calculating this deviation is given in the Standard. The tooth form of the wormgear is understood to be made fully conjugate to the mating worm thread. Effect of Diameter of Cutting on Profile and Pressure Angle of Worms
Normal Pressure Angle of Thread Produced with 20-Inch Diameter Grinding Wheel
y 20
y 2 ∆ y
Cutter or Grinding Wheel Profile
Normal Profile of Thread Produced with 20-Inch Grinding Wheel
∆φ 20
Normal Profile of Thread Produced with 2-Inch Cutter
∆φ 2 Normal Pressure Angle of Thread Produced with 2-Inch Diameter
Cutter or Grinding Wheel Angle
Milling Cutter ∆ s
(A) Curvature Effect
(B) Pressure Angle Effect
Effect of Production Method on Worm Profile and Pressure Angle.— In worm gearing, tooth bearing is usually used as the means of judging tooth profile accuracy since direct profile measurements on fine-pitch worms or wormgears is not practical. According to AGMA 370.01, Design Manual for Fine-Pitch Gearing, a minimum of 50 percent ini- tial area of contact is suitable for most fine-pitch worm gearing, although in some cases, such as when the load fluctuates widely, a more restricted initial area of contact may be desirable. Except where single-pointed lathe tools, end mills, or cutters of special shape are used in the manufacture of worms, the pressure angle and profile produced by the cutter are different from those of the cutter itself. The amounts of these differences depend on several factors, namely, diameter and lead angle of the worm, thickness and depth of the worm thread, and diameter of the cutter or grinding wheel. The accompanying diagram shows the curvature and pressure angle effects produced in the worm by cutters and grinding wheels, and how the amount of variation in worm profile and pressure angle is influenced by the diameter of the cutting tool used. Materials for Worm Gearing.— Worm gearing, especially for power transmission, should have steel worms and phosphor bronze wormgears. This combination is used extensively. The worms should be hardened and ground to obtain accuracy and a smooth finish. The phosphor bronze wormgears should contain from 10 to 12 percent of tin. The SAE phosphor gear bronze (No. 65) contains 88–90% copper, 10–12% tin, 0.50% lead, 0.50%
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