12-03-2011, 11:02 AM
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TO DESIGN & FABRICATE A SIX SLOT GENEVA WHEEL MECHANISM
INTRODUCTION
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 was originally 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, the Geneva Stop. The Geneva Stop was incorporated into many of the first film projectors used in theaters.
DESIGN CRITERIA
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 WORK
The designing work was carried out using Auto CAD 2010 software. 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
DESIGN SPECIFICATIONS
The Geneva Wheel Mechanism designed in this project has the following design specifications:
Number of Slots = 6
Radius of Crank = 60mm
Distance between centers of Geneva Wheel and
Crank = 84.8mm
Outer radius of Geneva Wheel = 70mm
Slot width = 16mm
Length of Slot = 53.2mm
Crank pin diameter = 10mm
Shaft pin diameter = 10mm
Thickness of the base plate = 5mm
Diameter of roller pin = 16mm
FABRICATION PROCEDURE
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 2mm thick
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. The locking wheel was also punch marked and machined to 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 required dimensions.
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.
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