In part 1 of this series, we saw how springs and cams work by watching animated illustrations. (You can reread part 1 by clicking here.)
In this part of the series, we will watch animated illustrations to see how Geneva Drive mechanisms, universal joints, and constant velocity joints work.
GENEVA DRIVE MECHANISMS
The difference between Geneva Drive mechanisms (also called the Maltese Cross mechanisms) and other gears is that Geneva Drive mechanisms have unusual teeth. Unlike in typical gears, the interaction between the part being driven and the part doing the driving is not continuous and the resultant motion is intermittent. Geneva Drives work by having the driving part, or wheel, interlocking with the driven part intermittently. The number of spokes that the driven part has and the speed with which the driving part rotates determines the frequency and distance of travel. There is an elevated segment on the driving wheel which keeps the driven part in position till the next revolution is complete. The external Geneva Drive is by far the most common type, although there are other variations possible in the design. The external drive can be built to withstand higher levels of stress and are generally more robust and sturdy than other variants.
Figure 6. External Geneva Drive Mechanism
Variations of Geneva Drives include the Internal and the Spherical Geneva drives. The Internal Geneva Drive has the driving part below the driven wheel, with the driving part often being much smaller than the driven wheel.
Figure 7. Internal Geneva Drive Mechanism
The Spherical Geneva Drive has the driven wheel located at an angle (typically 90o) to the driving wheel. The continuous rotational movement of the driver is converted into an intermittent rotational movement that is at a different axis.
Spherical Geneva Drive Mechanism
The Geneva Drive derived its name from the town of Geneva, Switzerland. The Swiss are world-renowned watchmakers, so it’s no surprise that the oldest and most popular applications for Geneva Drives are in watches and clocks, and it is quite easy to see how they can be used to regulate the second hand on a timepiece. More robust uses are in film projectors where the mechanism is used to regulate the amount of time a frame on a film is exposed to the light from a projection lens and the image projected on the screen.
Universal joints are the joints between two members that allow for a transmission of rotation at an angle. A shaft typically transmits rotation, but a universal joint can be thought of as a mechanism in the middle of a shaft that allows the shaft to bend and still transmit the desired rotation. The basic design consists of a pair of rigid shafts with hinges that are oriented perpendicularly to each other and that are connected by a cross rod. This allows one shaft to be capable of movement in virtually any direction independent of the other shafts movements.
Figure 8. Typical Universal Joint
CONSTANT VELOCITY JOINTS
All constant velocity joints are universal joints but not all universal joints are constant velocity joints. Constant velocity joints remedy a major shortcoming of typical universal joints: the loss of angular velocity and/or torque as motion is being transmitted from one shaft to the other. This is because one shaft is not completely dependent on the other for motion and can move even if the other part is motionless. The constant velocity joint makes sure that the relationship between the shafts is constant.
Figure 9. Constant Velocity Joint.
Stay tuned for part 3, in which we’ll see how more mechanisms work.
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