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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


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


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


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


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


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


Bevel Gear

Straight bevelgears forshafts whose axes intersect

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Gears Types


Hypoid Gear

Hypoidgears - one of anumber of gear types foroffset shafts

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Gears Types


Worm Gear

A wormand wormwheelgives a large speed ratiobut with significant sliding

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Gears Types


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


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


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


Small gears

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Gear box for a coal pulverizer

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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


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


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


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


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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Involute Animation

G I l i

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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


Tooth action

G M hi

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Gears Meshing


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


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


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


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


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

gears i

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