Download - Gears I
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Gears I
Prepared by Dr Miloud S.
8/9/2011 EME 2026 : Engineering Design II, Tr.1, by M.S 1
EME 2026
Engineering Design II
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Content
8/9/2011 EME 2026 : Engineering Design II, Tr.1, by M.S 2
1. Springs2. Power Screws & Thread Fasteners
3. Flexible Elements/Derive Transmission
4. Gears
5. Sealing
6. Motors
7. Study Cases
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Gears
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S3
Types, Nomenclatures, Involute Properties
Spur gears
Helical gears
Bevel gears Worm gears
Gear trains
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Gears
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S4
Gear Definition
Gears are machine elements that transmit motion bymeans of successively engaging teeth. The gear teethact like small levers.
Gears are used to transmit power between shafts
rotating usually at different speeds.
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Gears Types
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S5
Spur Gear
A pair of spurgears formounting on parallel shafts. The10 teeth of the smaller pinionand the 20 teeth of the wheellieparallel to the shaft axes
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Gears Types
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S6
Rack and Pinion
A rackand pinion. Thestraight rack translatesrectilinearly and may be
regarded as part of a wheelof infinite diameter.
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Gears Types
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S7
Helical gears
Like spur gears helicalgears connect parallelshafts, however the teeth
are not parallel to the shaftaxes but lie along helicesabout the axes
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Gears Types
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S8
Bevel Gear
Straight bevelgears forshafts whose axes intersect
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Gears Types
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S9
Hypoid Gear
Hypoidgears - one of anumber of gear types foroffset shafts
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Gears Types
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S10
Worm Gear
A wormand wormwheelgives a large speed ratiobut with significant sliding
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Gears Types
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S11
Differentiation in terms of shafts
Connecting parallel shafts Spur, parallel helical, herringbone, rack & pinion
Connecting intersecting shafts
Straight bevel, spiral bevel
Connecting neither parallel nor intersecting shafts Crossed-helical, hypoid, worm & wormgear
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Gears Working Principle
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S12
A pair of meshing gears is a power transformer, a
coupler or interface which
marries speed and torquecharacteristics of a powersourceand a power sink(load).
Single pair may be inadequate for certain sourcesand loads
gear trainsnecessary
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Gears Working Principle
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S13
Example: Speed reducer
power source drives the device through highspeed low torque input shaft, power is fed fromthe device to the load through the low speedhigh torque output shaft
Speed reducers much more common than speed-up drives amplify torque
Gears used to accelerate a car from rest not to provide the initial low speeds (which could be
accomplished by easing up on the accelerator pedal) but increase the torque at the wheels which is necessary
to accelerate the vehicle.
Torque amplification is the reason for the gearbox'sincreasing sturdiness mentioned above.
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Gears Usage Examples
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S14
Small gears
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Gears Usage Examples
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S15
Gear box for a coal pulverizer
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Gears Usage Examples
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S16
Gear for cement industry
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Gears Nomenclature
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S17Module Circular pitch
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Gears Tooth Systems
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S18
Definition
A tooth system is a standard which specifies therelationships between addendum, dedendum, workingdepth, tooth thickness, and pressure angle
For spur: full depth or stub, pressure angle 20, 22.5 &
25
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Gears Conjugate action
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S19
Two surfaces tangent at
c Line ab= line of action
Circles drawn through P= pitch circles
P= pitch point Pmust be fixed to
transmit constantangular-velocity ratio
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Gears Conjugate action
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S20
Gears like a pair of cams.
Act through a small arc, then running off and
be replaced by another identical pair of cams.
Can run in either direction
Transmit constant angular-velocity ratio
With involute curves: gears are tolerant of changesin c2c distance with no variation in angular-velocityratio
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Gears Involute properties
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S21
Predominant form of teeth
Construction: Cord def
bis tracing point
Cord is unwrapped about cylinder
So bwill trace involute curve ac
Radius varies continuously (0 at a,max at c)
deis normal to involute at all pointsof intersection and
Always tangent to cylinder A
Circle on which involute is
generated is the base circle
G
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Gears Involute properties
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S22
Involute Animation
G I l i
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Gears Involute properties
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S23
Involute action
Base circle radii O1a&O2b
During rotation
gwill trace involute cdon
gear 1 & involute efongear 2
gis point of contact(moves along ab)
abis generating line (doesnot change postion,always tangent!)
Uniform motion
G M hi
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Gears Meshing
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S24
Tooth action
G M hi
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Gears Meshing
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S25
The animation shows clearly : contact point marching along the line of action path of contact bounded by the two addenda orthogonality between line of action and involute
tooth flanks at the contact point
how load is transferred from one pair of contactingteeth to the next as rotation proceeds relative sliding between the teeth - particularly
noticeable at the beginning and end of contact
guaranteed tooth tip clearance due to thededendum exceeding the addendum
a significant gap between the non-drive face of apinion tooth and the adjacent wheel tooth
G M hi
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Gears Meshing
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S26
Meshing = pitch circles rolling on one another
without slipping Pitch-line velocity
Circle tangent to pressure line/generatingline/line of action is base circle
G M hi
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Gears Meshing
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S27
Rack
Spur gear having infinite large pitch diameterInfinite number of teeth
Base circle is in infinite distance from pitch point
Base pitch
G M hi
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Gears Meshing
8/9/2011 EME 2026 : Engineering Design II, Tr.1, by M.S 28
Internal (annular) Gear and Pinion
Centers of rotation on the same side of the pitchpoint
Positions of addendum & dedendum circles arereversed in respect of pitch circle
G E l 1
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Gears Example 1
8/9/2011EME 2026 : Engineering Design II, Tr.1, by M.S29
A gearset of a 16-tooth pinion driving a 40-tooth driven gear. The module is 12 mm andthe addendum and dedendum are 1.0mand1.25mrespectively. The gears are cut usinga pressure angle of 20.
a) Estimate the circular pitch, the centerdistance, and the radii of the base circles
b) In mounting these gears, the center distancewas incorrectly made 6mm larger. Computethe new values of the pressure angle andthe pitch-circle diameters