# Theory of Machines Questions and Answers – Gear Terminology

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This set of Theory of Machines Multiple Choice Questions & Answers (MCQs) focuses on “Gear Terminology”.

1. The smaller and generally the driving gear of a pair of mated gears is called _________
a) pitch
b) module
c) rack
d) pinion

Explanation: Pinion is the smaller and the driving gear of a pair of mated gears. It is used in gear train drives and is usually the smaller gear. The pinion engages the larger gear or rack that helps in the turning of the wheels of the vehicle.

2. The distance measured along the circumference of the pitch circle from a point on one tooth to the same point on the adjacent tooth is called ________________
a) pitch diameter
b) circular pitch
c) pitch point
d) line of centres

Explanation: Circular pitch is the distance measured along the circumference of the pitch circle from a point on one tooth to the same point on the adjacent tooth. So basically, it is the distance between corresponding points of consecutive gear teeth.
p = d π/ N; where, p is the circular pitch, d is the standard pitch diameter and N is the number of teeth.

3. The number of teeth per unit length of the pitch circle diameter is called ________________
a) diametral pitch
b) module
c) clearance
d) gear ratio

Explanation: Diametral pitch is the number of teeth per unit length of the pitch circle diameter.
It is used to classify different types and sizes of gears. It is a function of the pitch circle.
P = T/d.
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4. The ratio of the number of teeth on the gear to the number of teeth on the pinion is called ______
a) velocity ratio
b) module
c) gear ratio
d) pitch point

Explanation: Gear ratio is given by T/t and is defined as the ratio of the number of teeth on the gear to the number of teeth on the pinion. Velocity ratio is the inverse of gear ratio.
Gear ratio is inversely proportional to the radius of the pitch circle and also the number of teeth on the driving gear.

5. The ratio of pitch diameter to the number of teeth is called _________
a) module
c) dedendum
d) backlash

Explanation: Module is the ratio of pitch diameter to the number of teeth.
Module indicates the size of the gear and classifies whether the gear is big or small as module is a unit of size.
m = d/T or m = p/π.

6. The circle passing through the upper tips of the teeth is called _________________
a) pitch circle
b) circular pitch
c) dedendum circle

Explanation: Addendum circle is the circle passing through the upper tips of the teeth. The addendum circle lies on the outside cylinder for external gears whereas the addendum circle lies on the internal cylinder for internal gears. The diameter for the addendum circle of an internal gear is called as inside diameter.

6. The radial height of a tooth above the pitch circle is called ____________
b) dedendum
c) rack
d) line of centres

Explanation: Addendum is the radial height of a tooth above the pitch circle. Basically, it is the radial distance between pitch diameter and diameter of the outside circle. Addendum angle is the angle between the face cone and the pitch cone.

7. The circle passing through the roots of the teeth is called ________________
c) dedendum circle
d) rack

Explanation: Dedendum circle is the circle passing through the roots of the teeth. It touches the bottom of the spaces between the teeth of a gear.

8. The radial depth of a tooth below the pitch circle is called _______________
a) dedendum
c) rack
d) line of centres

Explanation: Dedendum is the radial depth of a tooth below the pitch circle. It is the radial distance between the pitch circle and the root circle. Dedendum angle is the angle between the root cone and the pitch cone.

9. The radial difference between the addendum and the dedendum of the tooth is called _________
a) clearance
b) rack
c) line of action
d) line of centres

Explanation: Clearance is the radial difference between addendum and dedendum of the tooth. Clearance circle is a tangent to the addendum circle on the gear. It has a clearance which equal to the distance between the dedendum of the gear and the addendum of the mating gear.

10. The locus of the point of contact on two mating teeth from beginning of engagement to end of engagement is known as the _______________
a) angle of action
b) arc of contact
c) path of contact
d) line of action

Explanation: Path of contact is the locus of the point of contact on two mating teeth from beginning of engagement to end of engagement. It is also called as contact length. It is the tangent to both the base circles and passes through the pitch point.

11. The portion of path of contact from the beginning of contact to the pitch point is called _______
a) arc of recess
b) arc of approach
c) path of recess
d) path of approach

Explanation: Path of approach is the portion of path of contact from the beginning of contact to the pitch point. It is given by the formula (Ra2 – R2cos2 φ)0.5 – Rsinφ; where Ra is the radius of addendum of the larger gear, R is the radius of the larger gear and φ is the pressure angle between the two gears.

12. The portion of the path of contact from the pitch point to the end of contact is called _________
a) arc of recess
b) arc of approach
c) path of recess
d) path of approach

Explanation: Path of recess is the portion of the path of contact from the pitch point to the end of contact. It is given by the formula (ra2 – r2cos2 φ)0.5 – rsinφ; where ra is the radius of addendum of the smaller gear, r is the radius of the smaller gear and φ is the pressure angle between the two gears. Path of approach + Path of recess = Path of contact.

13. Arc of contact = ___________________ Complete the equation.
a) Path of contact / cos θ
b) Path of contact / sin θ
c) Path of contact x cos θ
d) Path of contact x sin θ

Explanation: Arc of contact = Path of contact / cos θ; θ = the pressure angle between the two gears.

14. The locus of a point on the pitch circle from the beginning to end of engagement of two mating gears is called as ___________
a) Arc of contact
b) Path of contact
c) Path of approach
d) Arc of approach

Explanation: Arc of contact is defined as the locus of a point on the pitch circle from beginning to end of engagement of two mating gears. It is subdivided into two sections namely arc of approach and arc of recess. Arc of approach + arc of recess = arc of contact.

15. The ratio of angle of action to the pitch angle is called ___________
a) space width
b) angle of recess
c) angle of approach
d) contact ratio

Explanation: Contact ratio is the ratio of angle of action to the pitch angle. It is also defined as the number of pairs of teeth in contact. Contact ratio = arc of contact / circular pitch.

16. The angle turned by the gear from the beginning to the end of engagement of a pair of teeth is called ______________
a) angle of approach
b) angle of recess
c) angle of action
d) angle of contact

Explanation: Angle of action is the angle turned by the gear from the beginning to the end of engagement of a pair of teeth. Angle of action = Arc of contact / r; where r is the radius of the pinion.

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