29-08-2011, 10:31 AM
Geneva drive or Maltese cross is a indexing mechanism that converts the continuous motion into intermittent motion. By means of this mechanism the rotary motion of the driver wheel is converting into intermittent rotary motion of sprocket. The film passed over the sprocket. Due to the intermittent motion, the film advanced frame by frame in front of lens for 1/24th second in frequency of 48 Hz.
Geneva mechanism is commonly used indexing mechanism where an intermittent motion is required.
The Inverse Geneva mechanism, which is a variation of the Geneva mechanism, is used where the wheel has to rotate in the same Direction as crank. It requires less radial space and the locking device can be a circular segment attached to the crank that locks by wiping against a built up rim on the periphery of the wheel.
The design and fabricating of a conventional Geneva mechanism is generally simple and inexpensive because there is no specially curved profile on any of the components except straight lines and circular arcs. However, due to the discontinuity of the acceleration at the beginning and ending positions, the shortcoming of using conventional Geneva mechanism is the large impact when the driving crank engages and disengages with the wheel slot.
1.1 GENEVA MECHANISM
Fig.1. GENEVA MECHANISM
In this mechanism, for every turn of the driver wheel A, the driven wheel B makes a quarter turn. The pin, attached to driver wheel A, moves in the slots causing the motion of wheel B. The contact between the lower parts of driver A with the corresponding hollow part of wheel B retains it in position when the pin is out of the slot. Wheel A is cut away near the pin as shown, in order to provide clearance for wheel B as it moves. If one of the slots is closed, A can make less than one revolution in either direction before the pin strikes the closed slot and, stopping the motion.
1.2 CLASSIFICATION OF GENEVA MECHANISM
1.2.1 EXTERNAL GENEVA MECHANISM: In this type of mechanism, the Geneva cross is connected with cam drive externally which is the most popular and which is represented by the device below fig.
FIG.2. EXTERNAL GENEVA MECHANISM
1.2.2 INTERNAL GENEVA MECHANISM: In this type of mechanism, the Geneva cross and cam drive are connected internally in the closed box, which is also common and is illustrated by below fig.
Fig.3. INTERNAL GENEVA MECHANISM
1.2.3 SPHERICAL GENEVA MECHANISM: In this type of mechanism the Geneva cross is in spherical shape and cam drive are connected in externally, which is extremely rare and is illustrated in below fig
Fig.4. SPHERICAL GENEVA MECHANISM
1.3 ADVANTAGES OF GENEVA MECHANISM
i. Geneva mechanism may be the simplest and least Expensive of all intermittent motion mechanisms.
ii. They come in a wide variety of sizes, ranging from those used in instruments, to those used in machine tools to index spindle carriers weighing several tons.
iii. They have good motion curves characteristics compared to ratchets, but exhibit more “jerk” or instantaneous change in acceleration, than better cam systems
iv. Geneva maintains good control of its load at all Times, since it is provided with locking ring surfaces.
1.4 DISADVANTAGES OF GENEVA MECHANISM
i. The Geneva is not a versatile mechanism.
ii. The ratio of dwell period to motion is also established Once the no of dwells per revolution has been selected.
iii. All Geneva acceleration curves start and end With finite ac-celeration & deceleration.
iv. This means they produce jerk.