Basic of Gears | Machine Design – Interview Questions and answers

In this post we will see interview questions related to basic of gears , types of gears , gear manufacturing . This questions and answers are helpful for Mechanical students who preparing for viva, Oral , Interview .

1. Mention some applications of gear drives.
Gear drives are employed in many fields such as from smaller instruments to the heaviest
and most powerful machines,crushers etc. Some of the common applications of gears
are in hoisting machinery, rolling mill, machine tools such as lat machines, etc.

2.How are gears classified?
Gears are classified based on
(a) Axes of gear shafts as
i) Parallel – Eg. spur, helical, herring-bone gears.

ii) Intersecting – Eg. Bevel gears.

iii) Non-parallel and non-intersecting – Eg.- worm, gears, Skew gears.
(b) Profile of gear tooth – i) Involute gears. ii) Cycloidal gears.
(c) Position of teeth on wheel rim.
(D) Tooth parallel to axis of gear – Eg. Spur gears. ii) Tooth inclined to axis of gear – Eg. Helical gears.
(d) Pressure angle – i) Gears with 201 pressure angle. ii) Gears with 14 1/20 pressure angle.

3.Illustrate the materials for making gears .
1. Ferrous metals such as carbon steels, alloy steels of nickel, chromium and vanadium.
2. Cast-iron of different grades.
3. Non-ferrous metals such as brass, bronze, etc.
4. Non-metals like phemolic resins nylon, bakelite etc.
Among them steel with proper heat treatment is extensively, employed in many of ‘the engineering applications.

4 .Specify the types of gears-failures.
a) Tooth breakage. b) Pitting of tooth surface. c) Abrasive- wears. d) Seizing of teeth etc.

5. At what occasions non-metallic gears are employed.
Non-metallic gears are employed ‘where we require silent operation and low power transmission.

For example, in instruments like pressure gauge and so on.

6. How are the following terms defined?
a) Pressure angle (a) is the angle making by the line of action common- tangent to the pitch circles of mating pars.
b) Module m is the ratio of pitch circle diameter to the number d of gear teeth, and is usually represented in millimetres.

7. Define the following terms.

a) Back lash is the difference between tooth thickness and the space into ‘which it meshes, measured along the pitch circle. If we assume the tooth thickness as t, and space width as t2 then backlash = t2 – tl

b) Gear ratio

(i) is the ratio of number of teeth of larger gear to that of smaller gear. At is also defined as the ratio of high speed to the low speed in a gear drive. Usually, the gear ratio should always be greater than one.

8. What preliminary design considerations should be, adopted, When selecting gear drive?
All kinds of gears can not be useful for all kinds of work. Hence following factors should be considered for selecting a specific type of gear drive.
i) The amount of.power to be transmitted.
ii) Space availability.
iii) Amount of gear ratio for single step.
iv) Causes for gear failures and their preventing methods.
v) Proper material
vi) Life of gears required, usually 10,000 hours.

9. What is interference in gears? How can you overcome it?
Gear profile usually starts, from base circle and ends with tip teeth are made in such a way that their contact is along the pro the top surface of teeth is made, flat, the tip of the teeth of one gear dig I into the bottom flank of mating gear. This action is called interference.

10. On what basis gear cutters are selected?

Gear cutters are selected based on the following conditions.
1) Properties of materials for work piece and tools.
2) Cost of production.
3) Structure of gears such as spur gear, helical gears and so on.
4) Module of the gear.

11. . How do gears fail?
a)Gears may fail due to tooth breakage by overload and misalignment of shafts.
b) corrosion of teeth by improper lubricants.
c) tooth wear because of insufficient lubrication.
d) interference because of no under-cut.

12. What stresses are induced in gear tooth?
1. Surface compressive stress.
2. Bending stress.

13. What is meant by a corrected gear?
In normal gear, there may be an undercut between base circle and root circle which weakens the teeth. This undercutting can be avoided by making some modifications on the gear profile. This modification is addendum modification or profile correction or profile shift. The gear, which has this correction, is called as corrected gear.

14. What are the effects of little backlash and. excessive backlash on gears
Too little backlash may lead to over loading, overheating and ultimately seizure resulting eventual failure of the system. On the excessive backlash may cause, non uniform – transmission of motion. backlash may also cause noise and impact loads.

15. Define form factor?
Form factor is a constant, employed in the design of gear which, design the shape and the
number of teeth.

16. What is helix angle? How this angle differentiate helical gear from
Helix angle -is the angle between the axis of the gear and the through tooth face.For
helical gear, teeth are cut at an inclined axis, specified as’helix angle and its value ranges
from 8 degree to 25′ the case of spur gear, tooth-are cut parallel to the axis, the spur gear is zero.

17. What is a herringbone gears?
A herring bone gear is made of two single helical gears attached other hence called as double helical gear in which the teeth of be set in the opposite direction to the teeth of another gear arrangement the axial thrust produced in one gear will be null’, thrust produced in another gear, and the resultant thrust is improves the life of the gear.

18. What is a 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 and uniformly truncated frustum of a cone.

19. Based on the included angle between the shaft axes, called as shaft angle
a) External gears < 900)
b) Internal gears > 90*)
c) Crown gears 90

20. In which gear-drive, self-locking is available?
Self locking is available in worm-gear drive.

21. Write some applications of worm gear drive.
Worm gear, drive find wide applications like milling machine indexing head, table fan,
steering rod of automobile and so on.

22. What are the merits and demerits of worm gear drive;
Merits
1) Used 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 from the drive.
3) Low power transmission.