Chap 10: Gear Drive Flashcards
gear drives vs belt and chain drives comparision
gear drives: more efficient and more compact
but costly and not easily maintainable
belt and chain drives: cheaper and easier to repair
Straight spur gears description
noisy in ops.
used mainly for slow speeds to avoid excessive vibration (hand/powered winches)
speed ratio not to exceed 10:1
helical spur gears description
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
double helical gears description
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)
rack and pinion description
converts straight-line motion to rotary motion and vice versa
for slow-speed reciprocating ops. (drilling machines)
plain bevel gears description
turning of gears to an angle (turn corner)
for slow-speed applications without high impact force
speed ratio not to exceed 4:1
spiral bevel gears description
teeth cut obliquely on the angular faces
for high speed & strength applications (drive axles / industrial machines)
durable
worn and wheel description
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
Hypoid gear description
like spiral bevel gear but pinion is offset from centre of the ring gear
skew gears description
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
pressure angles and their description
- 5deg: quiet but weak in bending
20deg: strong and quiet (in the middle of both)
25deg: very strong in bending but noisy
diametral pitch meaning
tooth size for imperial gear (inches)
number of teeth in the gear for each inch of pitch diameter
DP = no. of teeth / pitch dia
module of gear meaning
tooth size fore metric gear (mm)
module = PCD/no. of teeth (mm)
module = 25.4/DP
addendum (ha) meaning and formula
height of tooth above pitch circle
ha = module
dedendum (hf) meaning and formula
height of tooth below pitch circle
hf = 1.25 * module
clearance meaning and formula
difference between addendum and dedendum
prevent tooth from bottoming out during ops.
clearance = hf - ha
working depth meaning and formula
max depth that the tooth extends into the space of mating gear
working depth = ha + hf - clearance = 2ha
PCD formula
m*T
m - module of gear T - number of teeth
circular pitch formula
pi * (PCD/T)
T - number of teeth
whole depth formula
hf + ha
tooth thickness formula
CP / 2
CP - circular pitch
outside diameter (OD) formula
PCD + 2ha
root diameter formula
PCD - 2hf
gear speed formula (using angular velocity)
v = w1 * r1 = w2 * r2
w1 / w2 = r2 / r1
v = speed of gears
w = angular velocity of gears
r = PCR of gears
speed ratio of gears formula & speed and tooth relation
SR = input speed / output speed
Na / Nd = Td / Ta
what is compound gear train and its advantages & applications
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
angular speed of gears and teeth relation in compound gear train
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)
worm gear special property
one way direction
worm can turn wheel easily but not the other way round
difference in right and left hand worm gear
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
planetary gear train purpose
reduce speed of aircraft engine’s output shaft to a more acceptable speed for the propeller
advantages of planetary gear train
output and input shafts are inline
drive occupies a smaller space (reducing aerodynamic drag)
can produce different speed ratios
difference between involute and cycloid gears
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
what does the pattern of contact check of gears show
accuracy of gear teeth
alignment of shafts
end float
how is gear backlash affected by
gear wear
thermal growth of gear
change in centre distance
why is backlash necessary
prevent gear binding
space for lubricant
abnormal wear
thermal expansion & contraction
what can cause excessive backlash
worn gear teeth
excessive play in gear shaft, bearings or bushes
worn keys and keyways
incorrect fitting
end float/axial movement of gears causes
uneven wear on gear teeth
chafing of component casing
what does concentric check do
check if flank of tooth at PCD is parallel to shaft axis
if not uneven wear will occur
gear inspections issues to check
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
