TO DESIGN & FABRICATE A SIX SLOTGENEVA

WHEEL MECHANISM

OBJECTIVE:

The Geneva mechanism is a timing device. It isused in many counting instruments and in otherapplications where an intermittent rotary motion is required.Essentially, the Geneva mechanism consists of a rotatingdisk with a pin and another rotating disk with slots (usuallysix) into which the pin slides. The Geneva mechanism wasoriginally invented by a watch maker. The watch maker onlyput a limited number of slots in one of the rotating disks sothat the system could only go through so many rotations.This prevented the spring on the watch from being woundtoo tight, thus giving the mechanism its other name, theGeneva Stop. The Geneva Stop was incorporated intomany of the first film projectors used in theaters.

INTRODUCTION

The basic design criteria of a Geneva wheel is that the centerlines of theslot and crank are mutually perpendicular at engagement and atdisengagement. The crank, which usually rotates at a uniform angularvelocity, carries a roller to engage with the slots. During one revolutionof the crank the Geneva wheel rotates a fractional part of the revolution,the amount of which is dependent upon the number of slots. The circularsegment attached to the crank effectively locks the wheel againstrotation when the roller is not in engagement and also positions thewheel for correct engagement of the roller with the next slot.

DESIGN CRITERIA

DESIGN WORKThe designing work was carried out using Auto CAD 2010software. The required dimensions of the driver and thedriven wheels were taken as per the design equations.Auto CAD is the most widely used design software's whichhelps in designing 2 as well as 3 dimensional modelsusing simplified alphabetical and numerical commands.Both the driving and the driven wheels were drawn to therequired dimensions using the circle command. A slot wascut on the Geneva wheel using the trim tool. It was thenedited using polyline command and the remaining slotswere constructed using the array tool. The crank pin andthe driving wheel were drawn to the required dimensions

The Geneva Wheel Mechanism designed in thisproject has the following design specifications:Number of Slots = 6Radius of Crank = 60mmDistance between centers of Geneva Wheel and

Crank = 84.8mmOuter radius of Geneva Wheel = 70mmSlot width = 16mmLength of Slot = 53.2mmCrank pin diameter = 10mmShaft pin diameter = 10mmThickness of the base plate = 5mmDiameter of roller pin = 16mm

DESIGN SPECIFICATIONS

The Geneva Wheel Mechanism constructed by us wasmanufactured with 6 slots, having 6Parts overall.Model consist of one Geneva wheel piece, a circular & asemicircular lockingwheels, a Crank Pin and a Base plate.The Geneva wheel was manufactured by turning a 2mmthickMS Plate to the external dimensions.Then the profile was punch marked on the plate.The plate was then machined to required dimensionsincluding the cutting ofslots.

FABRICATION PROCEDURE

The locking wheel was also punch marked and machinedto the required dimensions.The crank pin was made by plain turning the requiredshape and the roller pin was fitted at the requireddistance 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 MSplate. The holes for carrying the shafts were then drilledby using a 10mm drill taking care of the distance betweenthe centers.

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 wheelrespectively]

γ = Л*(N+2)/N [were γ is the angle oflocking 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 ofthe driving crank, assumed to be constant]

GENEVA WHEEL ASSEMBLY

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 impactforces are minimized.

Ratio of the dwell time to motion time can be modified bystaggering the driver pins or combining the genevamechanism with chain drivers with chain drivers or gear trains

DISADVANTAGES

During the beginning and end of the indexing motion are marked bysharp acceleration and deceleration. However, as the ratio between thediameter of the follower and the diameter of the driver increases, thepeak acceleration and velocity of the indexing motion decrease. In otherwords, 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 foracceleration, velocity and displacement. The ratio of dwell time to motiontime is also set

CONCLUSION

The 6 slot Geneva Mechanism is successfully designed and fabricated basedon the selected design parameters.

BIBLIOGRAPHYDesign data book PSGWikipediaehow.comhowstuffworks.com