The Geneva drive or Maltese cross is a gear mechanism that translates a continuous rotation into an intermittent rotary motion. The rotating drive wheel has a pin that reaches into a slot of the driven wheel advancing it by one step. The drive wheel also has a raised circular blocking disc that locks the driven wheel in position between steps.

The name derives from the device's earliest application in mechanical

watches, Switzerland and Geneva being an important center of watchmaking.

The geneva drive is also commonly called a Maltese cross mechanism due to the visual

resemblance.

In the most common arrangement, the driven wheel has four slots and thus advances

for each rotation of the drive wheel by one step of 90°. If the driven wheel has n slots, it

advances by 360°/n per full rotation of the drive wheel.

Because the mechanism needs to be well lubricated, it is often enclosed in an oil

capsule.

Uses and applications

One application of the Geneva drive is in movie projectors: the film does not run

continuously through the projector. Instead, the film is advanced frame by frame, each

frame standing still in front of the lens for 1/24 of a second (and being exposed twice in

that time, resulting in a frequency of 48 Hz). This intermittent motion is achieved using a

Geneva drive. (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 go back to 1896 to 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 Demenÿ in 1893, to achieve

intermittent film transport.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, indexing tables in assembly lines, tool changers

for CNC machines, and so on. The Iron Ring Clock uses a Geneva mechanism to

provide intermittent motion to one of its rings.

Internal Geneva drive

Internal Geneva drive.

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.

Indexing Mechanisms

Geneva Mechanisms

There are three basic types of Geneva motion as shown in the figures below.

External the most common type.

Internal

Spherical ..This is rarely used..

Because the driven wheel in a Geneva motion is always under full control of the driver there is no problem with overrunning. Impact is till a problem unless the slots of the driven wheel are accurately made and the driving pin enters these slots at the proper angle. For best results the pin should be shaped so tht the pin picks up the driven member as slowly as possible. Impact can also be reduced by leaving the top and bottom of the slot open. The fingers that form the slot will then have some . However strength is of primary importance and the slot must be bridged by a web .

External Geneva and Internal Geneva have been used for both light and heavy duties. They are frequently used as inputs to high speed devices e.g high speed mechanical counters use a Geneva between the first and second wheels. Mutilated pinions, which connect succeeding stages, could not absorb the shocks transmitted from the first to the second wheel.

When input and output shafts must be perpendicular few intermittant mechanisms are as suitable as the spherical Geneva , but this type is bulky and not practical for significant power levels. Moulded or cast spherical Genevas are adequate for light duty applications.

Typical Geneva with special characteristics are those driven by 4 bar linkages for improved acceleration characteristics, Genevas with variable dwells , Genevas used as planets in planetary chains and those combined with cycloidal cranks.

Mutilated Gears

Gears can be used in several ways to produce intermittent motion. A typical unit is the "mutilated gear" shown in the figure below. In this case some of the teeth have been removed from the driver and a partial holding surface has been added to each gear to prevent slight rotation of the drum gear during the dwell period.

Mutilated gears can be run without holding rings but it is not desirable no matter how slow the motion, the teeth of the driver will sooner or later top the teeth of the output gear. Since the teeth will meet near a centreline, even small input torque can produce large toggle forces that can damage the teeth.

Mutilated gears of the type shown in the figure below are subject to large impact loads and accelerations if the driving speed is high. The shape of the first teeth that will mesh is sometimes modified to reduce impact but only a slight advantage is gained. Attempts have been made to slack mount the first teeth but only a slight advantage is gained. Geneva or star wheels are usually preferred for high speeds and for high power applications.

A desirable feature with mutilated gears is their indexing accuracy and in addition to the inherent accuracy of the gears, the output is always under control on the input. Mutilated gears as shown below are used in almost all counters, they are inexpensive reasonably precise and efficient. They stand up well under the type of loading found in instruments. The mutilated pinions as shown below are virtually identical to the gear above except that the locking ring on the output gear has been eliminated. Every other tooth on the input end of the pinion has been cut away so that the remaining teeth can hold the pinion during dwell periods . In counters the driver has only two teeth, but it can have any even number of teeth.

Cycloidal Gears

With cycloidal intermittent gearing the input and output remains in constant mesh. Cycloidal gearing provides considerable latitude in selection of operating characteristics- decelarations, dwell periods, ratio of input to output motions etc. A basic cycloidal mechanism is shown in the figure below. In this type the drive pin or roller must on the pitch circle of the planet gear if the output crank is to stop, Otherwise, the output will either slow and not stop or actually reverse the motion.

There are many other variations of this type of mechanism including hyper-cycloidal , epicycloidal & peri-cycloidal arrangements. These devices are very versatile and can be used with Genevas for additional output variations.

The arrangement as shown below can be classed with the hyper-cycloidal gear arrangements since the driver moves around the inside of a ring gear. In this case however the driver is constrained from rotating by a fixed pin. The input shaft turns the eccentric, which is mounted with a sliding fit within the internal gear and is concentric with it. The amount the ring gear is indexed by the internal gear is determined by relative size diameter of the eccentric and location of the pin. In this case the ring rotates 36 degrees for every 360 degees of input motion, remaining at rest for the remaining 324 degrees of rotation. Accelarations are low and the two gears are always in mesh. Since the inner gear is really only link of a four bar mechanism, the sliding pin can be replaced by a

link loosely pinned to the gear and to the frame. This arrangement is reliable, inexpensive, quiet and compact. A company "Ikongear" manufactures a gear reduction design similar , in principle, to this mechanism. below).

Star Wheels

A different type of intermittent motion mechanism is the star wheel. In the arrangement shown in the figure below pins are used as teeth on the driver, but involute teeth can be used instead. This is another versatile mechanism. It provides considerable freedom in choosing operating parameters. The output wheel for example can be made to rotate more than one revolution. This is not possible with pure simple genevas. Star wheel devices can rotate at different amounts at each index point. Accelerations and decelerations can be controlled more readily than in a mulitalted gear pair." By careful shaping of the teeth. Internal pairs are also possible.

Cams

Various type of cams can be used to produce intermittent indexing rotations. As an example the scroll-shaped disc cam shown below indexes a wheel 180o when the solenoid pulls the levers down and a further 180o when the solenoid is releases the levers.

A face cam as shown below is also often used for indexing. The reciprocating drive arm moves a pin or roller back and forth in the zig -zag groove in the face of the wheel. This simple arrangement is used in moderate speed counters. As with many inexpensive cam drives efficiency is not high. Impact is light particularly if the drive arm is itself driven by a properly shaped cam. In this form of drive there is little danger of over travel.

A cylinderical cam as shown below can serve as the driver in another type of indexing drive. a typical commercial unit can handle moderate to high loads at speeds of 1000 steps /minute.

The cam system shown below is used often for Sequenced grabs. Every time the centre shaft is lowered down the ring in which internal slots are machined is caused to rotate a fixed angle. This is used for sequenced grabs in which each alternate ring position conforms to a grab open position and the other positions conform to a grab locked closed position.

The mechanism shown below is a diagrammatic representation of a ball point retracting mechanism. In practice this mechanism is a cylindrical mechanism arranged such that at each press of the end projection the pen is sequentially extended and retracted. The diagram below show the cylinder flattened out to illustrate the action.

One significant advantage of cam drives over most other intermittent motion drives is that the cams can be shaped to control such dynamic factors as impact, acceleration and dwell periods. However since small changes in cam contour can result in significant changes in performance each design must be tailored to the particular application.