Chap 10: Gear Drive

Question 1

gear drives vs belt and chain drives comparision

Answer 1

gear drives: more efficient and more compact
but costly and not easily maintainable

belt and chain drives: cheaper and easier to repair

Question 2

Straight spur gears description

Answer 2

noisy in ops.
used mainly for slow speeds to avoid excessive vibration (hand/powered winches)
speed ratio not to exceed 10:1

Question 3

helical spur gears description

Answer 3

teeth cut obliquely across gear width
angular contact creates side thrust (Fa) and is absorbed by bearings

quieter at high speeds and more durable
speed ratio not to exceed 10:1

Question 4

double helical gears description

Answer 4

teeth angles are opposite
thrust produced by each gear is counterbalanced
best for quiet, high speed, low thrust applications where heavy loads are applied
(large turbine, generators)

Question 5

rack and pinion description

Answer 5

converts straight-line motion to rotary motion and vice versa
for slow-speed reciprocating ops. (drilling machines)

Question 6

plain bevel gears description

Answer 6

turning of gears to an angle (turn corner)
for slow-speed applications without high impact force
speed ratio not to exceed 4:1

Question 7

spiral bevel gears description

Answer 7

teeth cut obliquely on the angular faces
for high speed & strength applications (drive axles / industrial machines)
durable

Question 8

worn and wheel description

Answer 8

worm is a screw
wheel has curved teeth for greater contact area
for high speed reduction in small space (steering mechanism, small power hand tools)
power supplied to the worm which drives the wheel
speed ratio not to exceed 70:1

Question 9

Hypoid gear description

Answer 9

like spiral bevel gear but pinion is offset from centre of the ring gear

Question 10

skew gears description

Answer 10

low load carrying capacity (due to small tooth contact area)
can be back driven
wide variety of speed ratios without change in centre distance of gear sizes

Question 11

pressure angles and their description

Answer 11

14. 5deg: quiet but weak in bending
    20deg: strong and quiet (in the middle of both)
    25deg: very strong in bending but noisy

Question 12

diametral pitch meaning

Answer 12

tooth size for imperial gear (inches)
number of teeth in the gear for each inch of pitch diameter
DP = no. of teeth / pitch dia

Question 13

module of gear meaning

Answer 13

tooth size fore metric gear (mm)
module = PCD/no. of teeth (mm)

module = 25.4/DP

Question 14

addendum (ha) meaning and formula

Answer 14

height of tooth above pitch circle

ha = module

Question 15

dedendum (hf) meaning and formula

Answer 15

height of tooth below pitch circle

hf = 1.25 * module

Question 16

clearance meaning and formula

Answer 16

difference between addendum and dedendum
prevent tooth from bottoming out during ops.
clearance = hf - ha

Question 17

working depth meaning and formula

Answer 17

max depth that the tooth extends into the space of mating gear

working depth = ha + hf - clearance = 2ha

Question 18

PCD formula

Answer 18

m*T

m - module of gear
T - number of teeth

Question 18

circular pitch formula

Answer 18

pi * (PCD/T)

T - number of teeth

Question 19

whole depth formula

Answer 19

hf + ha

Question 20

tooth thickness formula

Answer 20

CP / 2

CP - circular pitch

Question 21

outside diameter (OD) formula

Answer 21

PCD + 2ha

Question 22

root diameter formula

Answer 22

PCD - 2hf

Question 23

gear speed formula (using angular velocity)

Answer 23

v = w1 * r1 = w2 * r2

w1 / w2 = r2 / r1

v = speed of gears

w = angular velocity of gears

r = PCR of gears

Question 24

speed ratio of gears formula & speed and tooth relation

Answer 24

SR = input speed / output speed

Na / Nd = Td / Ta

Question 25

what is compound gear train and its advantages & applications

Answer 25

at least 1 intermediate shaft has 2 gears on the same axis

large speed ratio can be obtained

machine gear boxes, clock and watch mechanism

Question 26

angular speed of gears and teeth relation in compound gear train

Answer 26

For pair A & B: Na / Nb = Tb / Ta

For pair B & C: Nb / Nc = 1

For pair C & D: Nc / Nd = Td / Tc

Final formula: Na / Nd = (Tb * Td) / (Ta * Tc)

Na / Nd = (product of teeth on follower) / (product of teeth on driver)

Question 27

worm gear special property

Answer 27

one way direction

worm can turn wheel easily but not the other way round

Question 28

difference in right and left hand worm gear

Answer 28

right-hand wheel when turning worm CW: rotate CCW
when turning CCW: rotate CW

left-hand wheel when turning worm CW: rotate CW
when turning CCW: rotate CCW

Question 29

planetary gear train purpose

Answer 29

reduce speed of aircraft engine’s output shaft to a more acceptable speed for the propeller

Question 30

advantages of planetary gear train

Answer 30

output and input shafts are inline
drive occupies a smaller space (reducing aerodynamic drag)
can produce different speed ratios

Question 31

difference between involute and cycloid gears

Answer 31

involute: driving force by line contact (easier & cheaper to manufacture)
compound marking should be a line along the tooth

cycloid: total area contact (more power)
compound marking is full area contact

Question 32

what does the pattern of contact check of gears show

Answer 32

accuracy of gear teeth
alignment of shafts
end float

Question 33

how is gear backlash affected by

Answer 33

gear wear
thermal growth of gear
change in centre distance

Question 34

why is backlash necessary

Answer 34

prevent gear binding
space for lubricant
abnormal wear
thermal expansion & contraction

Question 35

what can cause excessive backlash

Answer 35

worn gear teeth
excessive play in gear shaft, bearings or bushes
worn keys and keyways
incorrect fitting

Question 36

end float/axial movement of gears causes

Answer 36

uneven wear on gear teeth

chafing of component casing

Question 37

what does concentric check do

Answer 37

check if flank of tooth at PCD is parallel to shaft axis

if not uneven wear will occur

Question 38

gear inspections issues to check

Answer 38

contact check
backlash & wear
end float
concentricity check
corrective & corrosion pitting (initial bedding causes pits & cause by corrosion)
discolouration (blueing in contact area due to overheating)
end loading (not align causing increased wear)
fretting (relative movement between gears, removes surface particles leading to corrosion)
frosting (breakdown of lubricant)
ridging & rippling (excessive loading/poor lubrication, plastic deformation formed)
scratching & scoring
spalling (fatigue failure results in surface of teeth breaking away)
cracks