# Unit3 Warm and warm gear.pdf

30 May 2023
1 sur 26

### Unit3 Warm and warm gear.pdf

• 1. Unit 2 Worm gears Worm and worm gear terminology and proportions of worm and worm gears, Force analysis of worm gear drives, Friction in Worm gears, efficiency of worm gears, Worm and worm gear material, Strength and wear ratings of worm gears (Bending stress factor, speed factor, surface stress factor, zone factor) IS 1443-1974, Thermal consideration in worm gear drive, Types of failures in worm gear drives, Methods of lubrication
• 2. The worm is a threaded screw, while the worm wheel is a toothed gear.
• 3. Terminologies A pair of worm gears is specified and designated by four quantities in the following manner: z1/z2/q/m z1 = number of starts on the worm z2 = number of teeth on the worm wheel q = diametral quotient m = module (mm) The diametral quotient is given by, q=d1 /m d1 is the pitch circle diameter of the worm d1 and d2 are pitch circle diameters of the worm and the worm wheel respectively
• 4. (i) Axial Pitch The axial pitch (px) of the worm is defined as the distance measured from a point on one thread to the corresponding point on the adjacent thread, measured along the axis of the worm. (ii) Lead The lead (l) of the worm is defined as the distance that a point on the helical profile will move when the worm is rotated through one revolution. The pitch circle diameter of the worm wheel is given by , d2 = mz2 As seen in the figure, the axial pitch of the worm should be equal to the circular pitch of the worm wheel. Therefore,
• 5. (iii) Lead Angle The lead angle (γ) is defined as the angle between a tangent to the thread at the pitch diameter and a plane normal to the worm axis. From the triangle, (iv) Helix Angle The helix angle (ψ) is defined as the angle between a tangent to the thread at the pitch diameter and the axis of the worm. The worm helix angle is the complement of the worm lead angle. The helix angle should be limited to 6° per thread. For example, if ψ = 30° then the worm should have at least five threads. (v)Pressure Angle The tooth pressure angle (α) is measured in a plane containing the axis of the worm and it is equal to one-half of the thread angle.
• 10. FORCE ANALYSIS Assumptions: (i) The worm is the driving element, while the worm wheel is the driven element. (ii) The worm has right-handed threads. (iii) The worm rotates in anti-clockwise directions (P1)t = tangential component on the worm (N) (P1)a = axial component on the worm (N) (P1)r = radial component on the worm (N)
• 17. Self preparation points 1.Self locking and reversible worm gear drive
• 19. SELECTION OF MATERIALS The selection of materials for the worm and the worm wheel is more limited than it is for other types of gears. The threads of the worm are subjected to fluctuating stresses and the number of stress cycles is fairly large. Therefore, the surface endurance strength is an important criterion in the selection of the worm material. The core of the worm should be kept ductile and tough to ensure maximum energy absorption. The worms are, therefore, made of case hardened steel with a surface hardness of 60 HRC and a case depth of 0.75 to 4.5 mm. The following varieties of steel are used for the worm: Normalized carbon steels—40C8, 55C8 Case-hardened carbon steels—10C4, 14C6 Case-hardened alloy steels–16Ni80Cr60,20Ni2Mo25 Nickel–chromium steels—13Ni3Cr80, 15Ni4Cr1
• 20. STRENGTH RATING OF WORM GEARS The maximum permissible torque that the worm wheel can withstand without bending failure is given by the lower of the following two values
• 23. WEAR RATING OF WORM GEARS The maximum permissible torque that the worm wheel can withstand without pitting failure, is given by the lower of the following two values: