bevel and worm gears

Bevel Gears 1.

What is bevel gear? Bevel gear is the type of gear for which the teeth are cut on conical surface in contrast with spur and helical gears for which the teeth are cut on cylindrical surfaces. The structure of bevel gear is similar to a uniformly serrated frustum of a cone.

2.

When do we use bevel gears? When the power is to be transmitted in angular direction, i.e., between the shafts whose axes intersecting at an angle, bevel gears are employed.

3.

How are bevel gears are classified/ Bevel gears are classified in two ways 1.

2.

4.

Based on the shape of teeth. a)

Straight bevel gears.

b)

Spiral bevel gears.

Based on the included angle between the shaft axes, called as shaft angle . a)

External gears

b)

Internal gears

c)

Crown Gears

What is a crown gear? A crown gear is a type of bevel gear whose shaft angle is equal to the pitch angle of gear. Let

= Shaft angle

= pitch angle of pinion = pitch angle of gear Then 5.

=

What is the specific feature of meter gear?

and the pitch angle of pinion is not

Meter gear is special type of crown gear in which the shat angle is and gear are equal and each angle is equal is . That is

6.

=

and pitch angle of pinion

and

Fill in the blanks following: a)

Bevel gears having shaft angle of

are known as ………….

b)

When the spiral angle of a bevel gear is zero, it is called as ………

ANSWERS:

7.

a)

Crown gears.

b)

Zero bevel gears.

Specify material used for the production of bevel gears. 1)

Cast iron of different grades.

2)

Carbon or alloy steels.

3)

Plastic like nylon, mica.

4)

Brass, bronze and so on.

Among them cast iron and steel gears are mostly employed. 8.

Defined the following terms. a) Pitch angle. b) Shaft angle. a)

Pitch angle is the half of the cone angle subtended by a bevel gear at its apex.

b)

Shaft angle is the sum of the pitch angles of pinion and gear of a bevel gear-pair. For example,

Let

= Pitch angle of pinion. = Pitch angle of gear

= Shaft angle Then 9.

=

Discuss about the following parameters. c) Tip angle.

d) Root angle.

a) Addendum angle.

b) Dedendum angle.

a)

Addendum angle is the angle between the line of cone distance and the line passing Through the top surface of tooth.

10.

b)

Dedendum angle is the angle between the line of cone distance and line passing through the root surface of tooth.

c)

Tip angle is the angle between axis of gear and line passing through the top surface of tooth.

d)

Root angle is the angle between the axis of gear and the line passing through the root surface of tooth.

Define the following terms. a) Cone distance or Pitch cone radius, b) Face angle. a)

Cone distance or pitch cone radius is the slant length of pitch cone, i.e, distance between the apex and the extreme point of tooth of bevel gear.

b)

Face angle is the angle subtended by the face of the teeth at the cone centre. It is equal to the pitch angle plus addendum angle. It is also called as tip angle.

Worm Gears SHORT QUESTION AND ANSWERS: 1. What is a worm-gear drive? Worm gear drive is a type of gear drive in which pinion is made as lead screw shape and the wheel is made as helical gear structure and it is employed for transmitting power between the shafts whose axes are at right angle to each other. Also this gear drive is used for high velocity ratio of about 100, sometimes upto 500.

2. What kind of contact occurred between worm and wheel how does this differ form other gears? In worm gear drive sliding contact is occurred between the worm and wheel whereas other gears such as spur helical and bevel gears transmit power by rolling contact. 3. In which gear drive self locking is available? Self locking is available in worm gear drive. 4. When do we use worm gears? When we require to transmit power between nonparallel and non intersecting shafts and very high velocity ratio of about 100, worm gears can be employed. Also worm gears provided self locking facility. 5. Write some applications of worm gear drive. Worm gear drive find wide applications like hoisting equipments milling machine indexing head table fan steering rod of automobile and so on. 6. Suggest one material of worm gear drive. (a) Worm (b) Worm-wheel (c) Bevel gears a) Worm - Steel to transmit high torque b) Worm wheel - Phosphore bronze to get low coefficient of friction when the wheel is operated with worm c) Bevel gears - Hardened steel to sustain heavy load and to transmit high power 7. Define the following terms. a) Pitch b) lead a) Pitch of worm is the distance, measured axially, form a point on one tooth to the corresponding point on an adjacent tooth. b) Lead of worm is defined as the distance measured axially between the corresponding points of adjacent teeth for the same helix. Based on the number of stars lwad can be varied form pitch. Usually. Lead = Number of starts × pitch 8. State True of False. a) The contact between worm and wheel is the sliding contact. b) The materials used for worm gears should have high coefficient of friction. Answers: a) True b) False 9. What are the merits and demerits of worm-gear drive Merits: 1) Compact layout for very high velocity ratio of about 100. 2) Smooth and noiseless operation 3) Self-locking facility is available. Demerits:

1) Low efficiency 2) More heat will be produced and hence this drive can be operated inside an oil reservoir or extra cooling fan is required in order to dissipate the heat form the drive 3) Low power transmission.