Gear mechanism
Animation showing a six-position external Geneva drive in operation.
The
Geneva drive
or
Geneva mechanism
is a
gear
mechanism that translates a continuous
rotation
movement into intermittent rotary motion.
The
rotating drive
wheel
is usually equipped with a
pin
that reaches into a slot located in the other wheel (
driven wheel
)
that advances it by one step at a time. The drive wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps.
History
[
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]
An illustration that shows the four stages (motion stop at 90 degrees angle) of one full cycle of a Geneva drive.
The name, Geneva drive, is derived from the device's earliest application in
mechanical watches
, which were popularized in
Geneva
.
[1]
The mechanism is frequently used in mechanical
watches
, since it can be made on a small scale and is able to withstand substantial mechanical stress.
The
Geneva drive
is also called a "
Maltese cross
mechanism" due to the visual resemblance when the rotating wheel has four
spokes
.
In the most common arrangement of the Geneva drive, the client wheel has four slots and thus advances the drive by one step at a time (each step being 90
degrees
) for each full rotation of the master wheel. If the steered wheel has
n
slots, it advances by 360°/
n
per full rotation of the propeller wheel. The minimum number of slots in a practical Geneva drive is 3; it is rare to find a mechanism with more than 18 slots.
[2]
Because the mechanism needs to be well lubricated, it is often enclosed in an oil capsule.
[
citation needed
]
Uses and applications
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]
Movie projector with hand crank and Geneva drive
Geneva stop with five spokes
One application of the Geneva drive is in film
movie projectors
and movie cameras, where the film is pulled through an exposure gate with periodic starts and stops. The film advances frame by frame, each frame standing still in front of the lens for a portion of the frame cycle (typically at a rate of 24 cycles per second), and rapidly accelerating, advancing, and decelerating during the rest of the cycle. This intermittent motion is implemented by a Geneva drive, which in turn actuates a claw that engages sprocket holes in the film. The Geneva drive also provides a precisely repeatable stopped position, which is critical to minimizing jitter in the successive images. (Modern film projectors may also use an electronically controlled indexing mechanism or
stepper motor
, which allows for fast-forwarding the film.) The first uses of the Geneva drive in film projectors date to 1896, with the projectors of
Oskar Messter
and Max Gliewe and the
Teatrograph
of
Robert William Paul
. Previous projectors, including
Thomas Armat
's projector, marketed by
Edison
as the
Vitascope
, had used a "beater mechanism", invented by
Georges Demeny
in 1893, to achieve intermittent film transport.
[3]
: 138
Geneva wheels having the form of the driven wheel were also used in mechanical
watches
, but not in a drive, rather to limit the tension of the
spring
, such that it would operate only in the range where its
elastic force
is nearly linear. If one of the slots of the driven wheel is occluded, the number of rotations the drive wheel can make is limited. In watches, the "drive" wheel is the one that winds up the spring, and the Geneva wheel with four or five spokes and one closed slot prevents overwinding (and also complete unwinding) of the spring. This so-called
Geneva stop
or "Geneva stop work" was the invention of 17th or 18th century watchmakers.
Other applications of the Geneva drive include the pen change mechanism in
plotters
, automated sampling devices,
banknote
counting machines, and many forms of
indexable
equipment used in manufacturing (such as the tool changers in
CNC
machines; the turrets of
turret lathes
,
screw machines
, and turret drills; some kinds of
indexing heads
and
rotary tables
; and so on). The
Iron Ring Clock
uses a Geneva mechanism to provide intermittent motion to one of its rings.
A Geneva drive was used to change filters in the
Dawn mission
framing camera used to image the asteroid
4 Vesta
in 2011. It was selected to ensure that should the mechanism fail at least one filter would be usable.
[4]
[5]
Internal version
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]
An internal Geneva drive is a variant on the design. The axis of the drive wheel of the internal drive can have a bearing only on one side. The angle by which the drive wheel has to rotate to effect one step rotation of the driven wheel is always smaller than 180° in an external Geneva drive and always greater than 180° in an internal one, where the switch time is therefore greater than the time the driven wheel stands still.
The external form is the more common, as it can be built smaller and can withstand higher mechanical
stresses
.
[
citation needed
]
Spherical version
[
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]
Another variant is the spherical Geneva drive.
[3]
: 128
Kinematics
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]
Motion curves for one turn of the drive wheel, from top to bottom: angular position θ, angular velocity ω, angular acceleration α and angular jerk
j
a
.
The figure shows the motion curves for an external four-slot Geneva drive, in arbitrary units. A discontinuity appears in the acceleration when the drive pin enters and leaves the slot, occurring at the instant the rigid bearing surfaces make contact or separate. This generates an "infinite" peak of
jerk
(
Dirac peak
), and therefore vibrations.
[3]
: 127
See also
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References
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]
Further reading
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]
- Sclater, Neil (2011), "Cam, Geneva, and Ratchet Drives and Mechanisms",
Mechanisms and Mechanical Devices Sourcebook
(5th ed.), New York: McGraw Hill, pp. 180?210,
ISBN
978-0-07170442-7
. Drawings and designs of various drives.
External links
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]