2. INTRODUCTION
• 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.
• The worm wheel or gear is similar to a helical gear with a face curved
to conform to the shape of the worm.
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3. TYPES OF WORMS
• There are two types of worms. They are as follows:
1. Cylindrical or straight worm.
2. Cone or double enveloping worm.
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4. TYPES OF WORM GEARS
• 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.
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6. Terms 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.
• 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 λ.
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8. • 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.
• Velocity ratio: It is the ratio of speed of worm in r.p.m to the speed of
worm gear in r.p.m.
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9. How worm gear works
• 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 applications.
• 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.
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15. Advantages of Worm Drives:
1. Worm gear drives operate silently and smoothly.
2. They are self-locking.
3. They occupy less space.
4. They have good meshing effectiveness.
5. They can be used for reducing speed and increasing torque.
6. High velocity ratio of the order of 100 can be obtained in a single step
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16. Disadvantages of Worm Drives:
1. Worm gear materials are expensive.
2. Worm drives have high power losses and low transmission
efficiency.
3. They produce a lot of heat.
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18. Areas of further Research and Development
specific lubricants
Special design for effective lubrication and cooling
New material to accommodate less losses of material in worm wheel
Development of specific pair of material for higher transmission
capacity and efficiency
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