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TO DESIGN & FABRICATE A SIX SLOT
GENEVAWHEEL MECHANISM
OBJECTIVE
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The Geneva mechanism is a timing device. It is
used in many counting instruments and in other
applications where an intermittent rotary motion is required.
Essentially, the Geneva mechanism consists of a rotating
disk with a pin and another rotating disk with slots (usually
six) into which the pin slides. The Geneva mechanism wasoriginally invented by a watch maker. The watch maker only
put a limited number of slots in one of the rotating disks so
that the system could only go through so many rotations.
This prevented the spring on the watch from being wound
too tight, thus giving the mechanism its other name, theGeneva Stop. The Geneva Stop was incorporated into
many of the first film projectors used in theaters.
INTRODUCTION
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The basic design criteria of a Geneva wheel is that the centerlines of the
slot and crank are mutually perpendicular at engagement and at
disengagement. The crank, which usually rotates at a uniform angular
velocity, carries a roller to engage with the slots. During one revolution
of the crank the Geneva wheel rotates a fractional part of the revolution,
the amount of which is dependent upon the number of slots. The circular segment attached to the crank effectively locks the wheel against
rotation when the roller is not in engagement and also positions the
wheel for correct engagement of the roller with the next slot.
DESIGN CRITERIA
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DESIGN WORK
The designing work was carried out using Auto CAD 2010software. The required dimensions of the driver and the
driven wheels were taken as per the design equations.
Auto CAD is the most widely used design software's which
helps in designing 2 as well as 3 dimensional models
using simplified alphabetical and numerical commands.
Both the driving and the driven wheels were drawn to the
required dimensions using the circle command. A slot was
cut on the Geneva wheel using the trim tool. It was then
edited using polyline command and the remaining slots
were constructed using the array tool. The crank pin and
the driving wheel were drawn to the required dimensions
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The Geneva Wheel Mechanism designed in thisproject has the following design specifications:
Number of Slots = 6
Radius of Crank = 60mm
Distance between centers of Geneva Wheel and
Crank = 84.8mmOuter radius of Geneva Wheel = 70mm
Slot width = 16mm
Length of Slot = 53.2mm
Crank pin diameter = 10mm
Shaft pin diameter = 10mmThickness of the base plate = 5mm
Diameter of roller pin = 16mm
DESIGN SPECIFICATIONS
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The Geneva Wheel Mechanism constructed by us was
manufactured with 6 slots, having 6
Parts overall.
Model consist of one Geneva wheel piece, a circular & a
semicircular locking
wheels, a Crank Pin and a Base plate.
The Geneva wheel was manufactured by turning a 2mmthick
MS Plate to the external dimensions.
Then the profile was punch marked on the plate.
The plate was then machined to required dimensions
including the cutting of slots.
FABRICATION PROCEDURE
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The locking wheel was also punch marked and machinedto the required dimensions.
The crank pin was made by plain turning the required
shape and the roller pin was fitted at the required
distance of 60mm from the crank center.
All the other components were turned to the requireddimensions.
The Base plate was then cut out of a 5mm thick MS
plate. The holes for carrying the shafts were then drilled
by using a 10mm drill taking care of the distance between
the centers.
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GENEVA WHEEL ANALYSIS
β = Л/2 – α [were α & β are the semi-indexing angle of the driven & α the driver wheelsrespectively]
For entry without shock: e = 1.414r [were r is the radius of the dist. btw the centre of the pinand centre of the crank & e the dist. Btw the centre of the crank and centre of the Geneva wheel
respectively]
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γ = Л*(N+2)/N [were γ is the angle of locking action]
γ = r/e = sinα = sin Л/N [were N is the no:
slots on the driven disc and ]
β = Л/2 – α = Л(N-2)/2N
tanΨ = λsinΦ/1- λcosΦ [were Φ is theangular position of the crank at ant time t &
Ψ the angular position of the driven disc]
ω = d Ψ/dt [were ω is the angular velocity of
the driving crank, assumed to be constant]
GENEVA WHEEL ASSEMBLY
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ADVANTAGES
Cheapest mode of producing intermittent motions .
Can be made out of simpler designs
If designed correctly , the pin enters and leaves the slot in a
direction tangential to the slot surfaces , so that impact
forces are minimized. Ratio of the dwell time to motion time can be modified by
staggering the driver pins or combining the geneva
mechanism with chain drivers with chain drivers or gear trains
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DISADVANTAGES
During the beginning and end of the indexing motion are marked by
sharp acceleration and deceleration. However, as the ratio between the
diameter of the follower and the diameter of the driver increases, the
peak acceleration and velocity of the indexing motion decrease. In other
words, the index motion gets gentler as the number of slots increases.
Another limitation of Geneva mechanisms is that they aren’t flexible.
Once the number of slots has been established, so too are the curves for
acceleration, velocity and displacement. The ratio of dwell time to motion
time is also set
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CONCLUSION
The 6 slot Geneva Mechanism is successfully designed and fabricated based
on the selected design parameters.
BIBLIOGRAPHY
Design data book PSG
Wikipedia
ehow.com
howstuffworks.com
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