❏ A worm gear is a gear consisting of a shaft with a spiral thread that
engages with and drives a toothed wheel.
❏ It changes the rotational movement by 90 degrees, and the plane of
movement also changes due to the position of the worm on the worm
wheel (or simply "the wheel").
❏ They are typically comprised of a steel worm and a brass wheel.
❏ Worm gears are gears which are widely used for transmitting power
at high velocity ratios between non-intersecting shafts that are
generally, but not necessarily, at right angles.
❏ It can give velocity ratios as high as 300:1 or more.
❏ It is mostly used as a speed reducer, which consists of worm and
worm wheel or gear.
4. TYPES OF WORM GEAR
1. Cylindrical or straight worm.
2. Cone or double enveloping worm.
5. TYPES OF WORM GEAR
• The important three types of worm gears are given below.
1.Straight face worm gear.
2. Hobbed straight face worm gear.
3.Concave face worm gear.
8. TERM USED
❏ Axial pitch: It is also known as linear pitch of a worm.
❏ It is the distance measured axially from a point on one thread to the
corresponding point on the adjacent point on the worm.
9. TERM USED
❏ Lead: It is the linear distance through which a point on a thread
moves ahead in one revolution of the worm.
❏ Lead angle: It is the angle between the tangent to the thread helix on
the pitch cylinder and the plane normal to the axis of the worm.
❏ It is denoted by A.
10. TERM USED
❏ Tooth pressure angle: It is measured in a plane containing the axis of
the worm and is equal to one-half the thread profile angle.
❏ Normal pitch: It is a distance measured along the normal to the
threads between two corresponding points on two adjacent threads
of the worm.
❏ Helix angle: It is the angle between the tangent to the thread helix on
the pitch cylinder and the axis of the worm.
11. TERM USED
❏ Velocity ratio: It is the
ratio of speed of worm
in r.p.m to the speed of
worm gear in r.p.m.
Worm gear in guitar headstock
12. WORM GEAR WORKING
❏ The worm (screw) continuously rotates and drives the worm wheel
(meshed with it). Worm and worm gear form a lower pair as they have
sliding contact with each other.
❏ In a worm gear drive, power is always transmitted from worm to worm
wheel. Power cannot be transmitted from worm wheel to worm. This
phenomenon is called self-locking. It is highly useful in many
13. WORM GEAR WORKING
❏ Velocity ratio is determined by the
number of teeth on worm gear and the
number starts on worm. Power
transmission decreases with increase
in velocity ratio.
❏ Worm gear drives operate silently and smoothly.
❏ They are self-locking.
❏ They occupy less space.
❏ They have good meshing effectiveness.
❏ They can be used for reducing
speed and increasing torque.
❏ High velocity ratio of the order of
100 can be obtained in a single
❏ Worm gear materials are
❏ Worm drives have high power
losses and low transmission
❏ They produce a lot of heat.
A worm gear and a helical gear
❏ Gate control mechanisms
❏ Hoisting machines
❏ Automobile steering
mechanisms Lifts Conveyors
❏ Presses Speed reducer
❏ Tuners in Guitar headstocks
Worm gear in guitar headstock
❏ A Textbook of Machine Design : R.S. Khurmi & J.K. Gupta
❏ Slideshare : Presentation on Worm Gear : Leishangathem Ranjan,
Ranjan Rajkumar & Daihrii Kholi.