Chapter 34 : Transmission of Power by Gear Trains 697
34.2. FRICTION WHEELS
Consider two plain circular wheels A and B having sufficient
rough surfaces and pressed against each other as shown in Fig. 34.1.
Let the wheel A be keyed to a rotating shaft, and the wheel B to the
shaft, to be rotated. A little consideration will show, that when the wheel A
is rotated by a rotating shaft, it will rotate the wheel B in the opposite
direction as shown in Fig. 34.1.
The wheel B will be rotated (by the wheel A) so long as the tangen-
tial force exerted by the wheel A does not exceed the maximum frictional
resistance between the two wheels. But when the tangential force exceeds
the frictional resistance, slipping will take place between the two wheels. In order to avoid the
slipping, a number of projections (called teeth) are provided on the periphery of the wheel A, which
will fit into the corresponding recesses on the periphery of the wheel B.
34.3. ADVANTAGES AND DISADVANTAGES OF A GEAR DRIVE
A gear drive has the following advantages and disadvantages :
(a) Advantages- It transmits exact velocity ratio.
- It has a high efficiency.
- It has a compact lay out.
- It can transmit a large power.
- It has a reliable service.
(b) Disadvantages - The manufacture of toothed wheels requires a special equipment and tools.
- Any error in teeth machinery causes vibrations and noise during operation.
- Any defect in one wheel damages the whole set up.
34.4. IMPORTANT TERMS
Fig. 34.2. Terms of gears.
The following terms, which will be mostly used in this chapter, should be clearly understood
at this stage :
- Pitch circle. An imaginary circle, which would transmit the same motion as the actual
gear, by pure rolling action, is called pitch circle as shown in Fig. 34.2. The diameter of
the pitch circle is known as pitch circle diameter. - Addendum circle. The part of a gear outside the pitch circle is called addendum, and the
circle (concentric with the pitch circle) drawn through the top of the teeth is known as
addendum circle as shown in Fig. 34.2. The diameter of the addendum circle is known as
addendum circle diameter.
Fig. 34.1. Friction
wheels.