design of worm gear

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.

6. PROPORTIONS OF WORM GEARS

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)

14. Friction in Warm wheel

16. Since……

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:

25. THERMAL CONSIDERATIONS