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PROJECT REPORT
On
Project title: Geneva Cam Mechanism
5th Semester of
DIPLOMA ENGINEERING IN
MECHANICAL ENGINEERING
B.S. PATEL POLYTECHNIC, KHERVA
Submitted By Guide By Ankit Mevada (126440319162) Mr. H. S. Patel Harsh Patel (126440319161) Mechanical Engineering Department B.S.Patel Polytechnic, Kherva
Certificate
This is to certify that Mr. Mevada Ankit K. having Enrolment No:126440319162 have completed Part-I IDP Project work having title Geneva Cam Mechanism.
He has undergone the process of shodh yatra, literature survey and problem Definition. He is supposed to carry out the residue IDP Part-II work on same problem during Semester-VI for the final fulfilment of the IDP work which is Prerequisite to complete Diploma Engineering.
Institute Guide Head of Department
Mr. H.S. Patel Mr. K.P. Patel
Mechanical Engineering Mechanical Engineering Department Department B.S.Patel Polytechnic, Kherva B.S.Patel Polytechnic, Kherva
Certificate
This is to certify that Mr. Patel Harsh R. having Enrolment No.: 126440319161 has completed Part-I IDP Project work having title Geneva Cam Mechanism.
He has undergone the process of shodh yatra, literature survey and problem Definition. He is supposed to carry out the residue IDP Part-II work on same problem during Semester-VI for the final fulfilment of the IDP work which is Prerequisite to complete Diploma Engineering.
Institute Guide Head of Department
Mr. H.S. Patel Mr. K.P. Patel
Mechanical Engineering Mechanical Engineering Department Department B.S.Patel Polytechnic, Kherva B.S.Patel Polytechnic, Kherva
INDUSTRY DEFINED PROBLEM/PROJECT (IDP) STATEMENT FORM
STUDENT PARTICULARS-1
FIRST NAME ANKITKUMAR
LAST NAME KIRITBHAI MEVADA
MOBILE NO. +91 8735933694 +91 9825723235
EMAIL [email protected]
COLLEGE NAME B.S.PATEL POLYTECNIC, GANPAT UNIVERSITY.
ADDRESS
33/ Alok Tenament, Part-2, Maheshwari Nagar road,
Vastral, Ahmedabad.
BRANCH MECHANICAL ENGINEERING
SEMESTER 5th SEM. YEAR 2014-2015
TEAM NAME
SIGNATURE OF STUDENT
STUDENT PARTICULARS-2
FIRST NAME HARSHKUMAR
LAST NAME RAMESHBHAI PATEL
MOBILE NO. +91 7801862128 +91 9825575585
EMAIL [email protected]
COLLEGE NAME B.S.PATEL POLYTECNIC, GANPAT UNIVERSITY.
ADDRESS
A/67, Kashinath Parth, Mahadevnagar tekra,
Vastral, Ahmedabad.
BRANCH MECHANICAL ENGINEERING
SEMESTER 5th SEM. YEAR 2014-2015
TEAM NAME
SIGNATURE OF STUDENT
INDUSTRY PARTICULARS
---INDUSTRY COORDINATOR---
NAME KIRITBHAI B. MEVADA
CONTACT ADDRESS
33/ Alok Tenament, Part-2, Maheshwari Nagar road,
Vastral, Ahmedabad.
MOBILE NO. +919825723235
EMAIL -
-----INDUSTRY----
NAME LAXMI PHARMA MACHINES
ADDRESS
75, Bileshwar ind. Estate, Opp. GVMM, Nr. Ring Road,
Odhav-Kathwada Road, Odhav, Ahmedabad-382415.
CONTACT NO. +91 79 22900488 MOBILE +91982572325
NAME OF INDUSTRIAL ESTATE BILESHWAR INDUSTRIAL ESTATE
COMPANY LOGO
INDEX
SR. NO. TOPICS NAME
1. Introduction Of Project & background of the invention
2. Project Descriptions
3. Implementation Procedure, Study And Data Collection
4. Description Of Project
5. Manufacturing Aspects/Process Planning
6. Kinematics Of Jeneva
7. Advantages And Limitations
8. Cost Estimation
9. Conclusions
10. Expected Out Come/Future Modifications
11. References
Introduction
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.
BACKGROUND OF THE INVENTION
Geneva Mechanisms are widely used in motion picture film projectors to intermittently advance film through a film gate having a projection aperture. The film is moved or advanced by a Geneva Mechanism (also known as a “Maltese Cross”) until an image frame is in alignment with the projection aperture. The film is then held stationary for a discrete time period during which light is passed through the aperture, film frame, projection lens, and onto a screen. This intermittent frame-by-frame motion of the film is enabled by the Geneva Mechanism, which comprises one portion, the driver, which rotates continuously, and which causes intermittent rotation of a second portion, the star wheel. In a motion picture projector the star wheel shares its central shaft with a sprocket, the teeth of which are engaged with perforations in the film. Therefore, when the driver moves the star wheel, both the star wheel and the film experience a resulting intermittent motion. Other mechanisms, including servo motors and a Mitchell Movement, have been used to drive film through a gate in an intermittent manner, but the Geneva Mechanism has proven itself over the past century to be particularly well suited to accurately drive a load (the film) in an intermittent fashion when the time allowed for motion of the load is minimal. For example, motion picture film is typically projected at a rate of 24 frames per second, such that a new film frame is positioned in the projection aperture every {fraction
(1/24)} second, or approximately 42 ms. The typical projector Geneva Mechanism moves a film frame into the projection aperture with an indexing time of one-fourth of the frame period, or approximately 10.5ms.
During essentially all of this indexing time, a shutter must block the light incident to the film to prevent the appearance of “travel ghost” (smear of the image caused by film motion). The projectable frame time, which would appear to be approximately three-fourths of the total frame period, is further reduced to only approximately one-half of the total frame period because the typical motion picture projector employs a two-bladed shutter, which causes two blanking periods per frame of the film, in order to raise the apparent frame rate to 48 frames per second, and thereby greatly reduce the apparent flicker perceived by the human eye. Furthermore, it is necessary for these two shutter intervals to be nearly equal in duration in order to limit perceived flicker. Therefore, since one blanking period must be approximately one-fourth of the frame period in order to blank the projected image as the film moves, the other blanking period must be of essentially the same duration.
A basic Geneva Mechanism, as is commonly employed in motion picture projectors, is described in U.S. Pat. No. 1,774,789. In general, a Geneva Mechanism is used to convert uniform rotary motion to incremental rotary motion. Typically, such a mechanism includes a star wheel having a plurality of radially extending straight slots spaced equally around the periphery of the star. Interposed between these slots are concave cam guide surfaces, which, like the slots, are uniformly dimensioned and arranged. A driver component, comprised of a restraining cam, a drive arm extending from the base of the cam, and a drive pin near the far end of the drive arm, is employed for indexing the star wheel. The restraining cam has a side cam surface, which is convex and configured to interact with the concave cam guide surfaces of the star wheel. The close contact of this convex cam surface to the concave cam guide surfaces restrains the star wheel from experiencing rotary motion except during the periods in which the star wheel is driven by the drive pin. The star wheel is thus restrained intermittently, and in a manner such that the straight slots sequentially receive the drive pin.
The number of slots radially disposed around a Geneva Mechanism's star wheel is variable, and may be any whole number greater than 2. As the number of straight slots is changed, specific features of the mechanism such as component
sizes, the speed and duration of the intermittent motion, and the forces or loads applied to the drive pin and star wheel, and to the load (film) all vary as well. For example, the Geneva Mechanisms used in motion picture projectors (which vary some in detail depending on the projector) typically use a star wheel with four equally spaced straight-sided slots. Such star wheels with four straight slots are engaged with the driver pin for 90° of a 360° revolution of the driver, producing the intermittent motion. Thus, in the case of film projected at 24 frames per second, the star wheel and film experience movement during an indexing time of only approximately 10.5 ms of the approximately 42 ms available time per frame. By comparison, a star wheel with three straight slots will also experience intermittent motion once per revolution of the cam, but the engagement of the star wheel with the driver pin occurs over only 60° of the 360° revolution of the driver. If used in a motion picture projector at 24 frames per second, a star wheel with three slots would utilize an indexing time of approximately 7 ms per frame to move a film frame into the projecting aperture. While a three slot star wheel would thus decrease the time required to move the film and thereby increase the available projection time, the acceleration forces applied to the drive pin, slots, and the load (the film and film perforations) are greatly increased over those of a four-slot mechanism, making the three-slot mechanism undesirable for use in a projector. On the other hand, a star wheel with five straight slots will have any one slot engaged with the driver pin over 118° of the 360° revolution of the driver, for an indexing time of approximately 14 ms versus the approximately 42 ms available time per frame. This is not desirable in a projector, as the light efficiency to the screen would be significantly reduced when compared with those of a four-slot mechanism, and a more powerful lamp would be needed to obtain the same screen luminance. However, the acceleration forces on the drive pin, star wheel, and load (film perforations) would be reduced relative to a three or four straight slot star wheel.
Accordingly, it would be very advantageous to find a geometry for a Geneva Mechanism in `6 incurring a substantial increase in the acceleration forces applied to the drive pin, star wheel slots, and the load (the film and film perforations). U.S. Pat. No. 1,801,969 proposes to solve this problem by altering the slots of the star wheel to have curved surfaces. Thus, as the drive pin moves in engagement with these curved slots, the acceleration and velocity experienced by both the star wheel
and the load (film) are altered, and the indexing time is greatly reduced when compared with that of a star wheel with an identical number of straight slots operating at the same frame rate. However, the designs for the curved star wheel slots described in U.S. Pat. No. 1,801,969 fail to adequately provide for the forces applied both to the drive pin and to the star wheel slot surfaces with which the pin is in contact. As a result, a drive pin and/or slot surfaces manufactured per U.S. Pat. No. 1,801,969 would quickly experience overloading failure during operation. Furthermore, the curved star wheel slot geometry as described in U.S. Pat. No. 1,801,969 also fails to provide sufficient slot width at the mouth of the slot for the drive pin to enter and exit the slot without very heavy interference.
It should therefore readily be appreciated that there currently exists a need for an improved Geneva Mechanism, which includes a driver with a cam and a star wheel; in which a load can be advanced intermittently with a reduced indexing time required to experience the intermittent motion, while the load forces applied to the star wheel, drive pin, and the load are all controlled, without sacrificing any of the advantages normally attendant to the use of a Geneva Mechanism. It should also be apparent, that other improved configurations for Geneva Mechanisms, in which the indexing time required to experience the intermittent motion can be reduced while the forces on the load driven by the star wheel and on the drive pin are also reduced, would likewise be advantageous. Likewise, it should be apparent that yet other improved configurations for Geneva Mechanisms, in which the indexing time is effectively unchanged, but where the load forces applied to the star wheel, drive pin, and load are substantially reduced, would also have value. Finally, it should be readily apparent that such improved Geneva Mechanisms could be employed in devices generally, and in motion picture projectors particularly, with an advantageous affect.
SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide for a drive mechanism such as a Geneva Mechanism for intermittently moving a load, such as film, in a motion picture projector; where the time for intermittent motion is reduced, the load on the mechanism is reduced, the load on the object being moved is reduced, or some combination thereof.
The present invention relates to a Geneva Mechanism for intermittently driving a load. The mechanism comprises: a rotating driver having a drive pin; and a star wheel operationally associated with a load which is to be intermittently driven, with the star wheel comprising a plurality of radially extending slots, and each of the radially extending slots being curved along a substantial portion of its length. The drive pin of the rotating driver is engageable with one of the radial extending slots to drive the star wheel in an incremental angular manner, such that a drive pin load on the drive pin and forces applied to the load operationally associated with the star wheel remain in control, and the star wheel reaches a peak acceleration and deceleration in a continuous and controlled manner.
The present invention further relates to a Geneva Mechanism for intermittently driving a load which comprises: a rotating driver having a shaped drive pin; and a star wheel operationally associated with a load which is to be intermittently driven, with the star wheel comprising a plurality of radially extending slots which are located periodically around a circumference of the star wheel. The shaped drive pin has a controlled shaped surface and is engageable with one of the radially extending slots to drive the star wheel in an incremental angular manner while minimizing load forces on the drive pin, the star wheel and the load to be intermittently driven.
The present invention also relates to a drive mechanism for driving a load which comprises a first rotary member which provides an essentially uniform rotary motion, with the first rotary member comprising a drive pin; and a second rotary member operationally associated with a load which is to be driven. The second rotary member comprises a plurality of radially extending slots which are curved along a substantial portion of a length of the slots; with each of the slots being engageable with the drive pin of the first rotary member during a rotation of the
first rotary member, to provide an incremental angular rotation to the second rotary member while a drive pin load on the drive pin and forces applied to the load which is to be intermittently driven remain in control.
The present invention further relates to a projection system for motion picture film which comprises a film gate having a projection aperture, such that film frames of the film move past the projection aperture; a light source; illumination optics for directing light from the light source into the projection aperture of the film gate; a projection lens for projecting an image of each of the film frames onto a screen; a Geneva Mechanism for moving the film progressively past the projection aperture, with the Geneva Mechanism comprising a rotating driver having a drive pin, and a star wheel operationally associated with the film, with the star wheel comprising a plurality of radially extending slots which cooperate with the drive pin of the rotating driver to cause an intermittent rotation of the star wheel upon a rotation of the rotating driver and thereby cause and an intermittent motion of the film; and a shutter mechanism which blocks light incident onto the film frames of the film in synchronization with the intermittent motion of the film. One of the drive pin or the radially extending slots being shaped in a controlled manner to permit the star wheel to reach a peak acceleration and deceleration in a continuous manner.
The present invention further relates to a projection system for film which comprises a film gate having a projection aperture, such that film frames of the film move past the projection aperture; and a drive mechanism for moving the film progressively past the projection aperture. The drive mechanism comprises a first rotary member having a drive pin and a second rotary member operationally associated with film. The second rotary member comprises a plurality of slots which cooperate with the drive pin of the first rotary member to cause an intermittent rotation of the second rotary member and thereby cause an intermittent motion of the film. One of the drive pin or the slots being shaped in a controlled manner to permit the second rotary member to reach a peak acceleration and a peak deceleration in a continuous manner.
Background
The name derives from the device's earliest application in mechanical watches, Geneva, Switzerland being an important centre of watch making. The Geneva drive is also commonly called a Maltese cross mechanism due to the visual resemblance when the driven wheel has four spokes. Since they can be made small and are able to withstand substantial mechanical stress, these mechanisms are frequently used in watches.
In the most common arrangement, the driven wheel has four slots and thus advances by one step of 90 degrees for each rotation of the drive wheel. 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. The Geneva converts continuous rotary motion into intermittent rotary motion. When one of the slots in the driven wheel is obstructed, the device, then called a Geneva Stop, can be used to limit the number of turns of the input wheel. In this form, the ×Geneva was used to limit the wind-up in clockwork springs.
Abstract Converting constant rotary motion into intermittent rotary motion gives rise to a range of useful applications in silicon micromachining. This paper discusses the design and fabrication of one such mechanism called the Geneva Wheel mechanism. The standard SUMMiT process has been made use of in developing this. All the related mathematics of the Geneva wheel was developed and the system was analysed. The Geneva Stop Mechanism (MME), designed by EDIBON, is a mechanism that transforms continuous circular motion into intermittent motion. It is a positive drive in which the driven wheel is positively moved or locked. It is made of aluminum and consists of a rotating drive wheel and a driven wheel with six slots. The 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 half-moon locking disc that locks the driven wheel in position between steps.The drive wheel and the driven wheel are mounted in two graduated discs, where the angle of both wheels can be measured.
Design approach Many factors contribute to a successful Geneva mechanism design, such as materials used, surface finish, tolerances, loads, stress levels, lubricant, etc. Unsuccessful experimental applications of this mechanism usually result in two modes of failure: pin wear and wheel breakage. Of these two modes, wear is the hardest to control. The present design approach will be to reduce wear by altering the geometry of the Geneva wheel to reduce the contact stress while maintaining acceptable stress levels in other regions of the wheel. R. C. Johnson3 showed that an optimum wheel diameter exists for minimum wear stress. In this paper, consideration is given to two additional dimensions (pin diameter and tip thickness) on the wear stress and certain internal beam stresses. This paper will begin by defining the wheel geometry and then developing the relationships between this geometry and the wheel inertia, the maximum pin load, the contact stress, and the internal wheel stresses. These performance parameters will be normalized to the corresponding param- eters of a set of predefined “standard” Genevas for convenience in interpreting results. For the “standard” set chosen, curves will show the stress and load parameters as a function of inertia and speed. The normalized curves GENEVA WHEEL. Figure l(a) Geometry of the Geneva mechanism; (b) beam section geometry. will show the effect of geometrical differences between any Geneva wheel and the “standard” Geneva. Graphical curves for 4-, 5-, 6-, and 8-slot Genevas are shown although the concept can be extended to Genevas with any number of slots. The complexity and voluminous nature of 2 the calculations prohibit any complete closed form solution of the problem, and therefore it was necessary to use a digital computer (IBM
7094) for most of the results. For this reason, no detailed derivations will be given, and the emphasis will be on the results obtained and how they can be used in the analysis and synthesis of Geneva mechanism.
3
4
Geneva Wheel Mechanism
The basic structure of a four slot Geneva wheel is shown in Fig.1. The
system consists of a constantly rotating disk coupled with a slotted disk, which gives rise to the desired discrete motion. A rotation of 2p radians of the former causes 2p/N radians of rotation of the latter, where N is the number of slots available on the slotted disk. Thus, one complete rotation of the slotted wheel requires N complete rotations of the other disk, thereby also increasing he otal time period. The conversion mechanism of this disk system is as follows. Referring to Fig.1, pinwheel W rotates constantly about axis A and as shown below, has a pin ‘a’ attached to it. This pin ‘a’ engages into the slots ‘s’ of the Geneva Wheel G (a basic 4-slot Geneva mechanism is shown here) and rotates it as long as it is engaged with the slot. While the wheel W rotates continuously, the Geneva wheel G has a discrete rotation about axis ‘b’. Wheel G has a rotation time period of t when it is moving along with disk W and n idlingtime period, when the pin ‘a’ is not inside one of the slots ‘s’and is moving freely. The three quarter wheel ‘L’ is placed inorder to prevent any unintentional rotation of wheel G hile itis idling. For a four slot Geneva mechanism, the rotation timeperiod. By varying the number of slots on G, one can varythe time period and the angular displacement of the same. Ifthis system is now coupled with some optical system like amicromirror (through a rack and pinion kind of arrangement),then it can be used to deflect light rays in different directions(by discretely positioning the moving mirror by using thediscrete angular positions of the Geneva wheel) thereby givingrise to an optical switching technique.
In the following sections, four slot and six slotGeneva wheels have been analysed and a design layout hasbeen provided. Along the same lines, multiple slot wheels canbe designed. The basic criterion that has to be maintained indesigning any number of slotted Geneva wheel is that, the pinhas to enter and leave the slots radially. This will again bediscussed in detail in the following sections.
Internal Geneva Wheel When the dwell period must be less than 180°, other intermittent drive mechanisms must be used. The internal Geneva wheel is one way of obtaining this form of motion. The main advantage of the internal Geneva wheel, other than its smooth operation, is its sharply defined dwell period. A disadvantage is the relatively large size of the driven member, which increases the inertial forces resisting acceleration/deceleration. For proper operation, the drive pin (crankpin) must enter and leave the slot tangentially. Structurally, the internal Geneva wheel differs from the external Geneva wheel in that the distance of the crank center from the wheel center is less than the wheel radius. However, this leads to significant differences in the mechanics of the system. The dwell period of all internal Geneva wheels is less than 180°, leaving more time for the star wheel to reach maximum velocity, lowering the acceleration. The highest value of acceleration occurs when the crankpin enters or leaves the slot, however the acceleration curve does not reach a peak within the range of motion of the driven wheel. The geometrical maximum would occur in the continuation of the curve, but this continuation has no significance since the driven member will have entered the dwell phase associated with the high angular displacement of the driving member. This geometrical maximum falls into the region representing the motion of the external Geneva wheel. The design of the internal Geneva mechanism is very similar to that of the external mechanism. The maximum angular velocity occurs when the crank angle is zero with respect to the centerline c. The maximum angular acceleration occurs when the crank enters the slot. Figure 3 shows a plot of the angular acceleration of the wheel with respect to the crank angle for 3, 4, 5, 6, 8, 10, and 12 slotted Geneva wheels with unit crank link length (r1=1). It can be seen that there exits a non-zero angular acceleration
component as the crankpin makes contact with the Geneva slot. In fact this is the maximum angular acceleration of the system during the non-dwell phase. Once again this leads to a singularity, hence an infinite jerk upon contact.
CNC Cut Geneva Drive
A Geneva Drive is defined by Wikipedia as “a gear mechanism that translates a continuous rotation into an intermittent rotary motion.” Although replaced by servo drives in many cases, these were once used in movie projectors and to power rotary tables in industrial assembly lines. This is where I first saw this type of drive, and after redoing practically the entirety of these machines, the venerable Geneva mechanism was still at the heart of it, working like a champ after 20+ years. Like most cam-type devices, they may not afford nearly the adjustability or the “easy” electrical programming of a servo of PLC controlled device, but they will run for literally decades on end with little maintenance.
So after that introduction, I recently machined one out of MDF on my router. I plan on making one that works a bit better, and is possibly motor-driven, but check out the video below for my prototype:
This is a crude model a this point, but I’ve included the G-code and DXFs for the mechanism at the end of the article. Everything was cut with a 5/16 inch bit that I’ve been using as a flycutter on my router, providing a relatively high pocketing speed. Unfortunately, it doesn’t provide for cutting small holes. This could be worked around by changing the tool, but it’s a prototype, so the middle hole was crudely marked with the cutter, then drilled with my manual mill. The cut inaccuracy is, I would assume, partly to blame for the reverse not working as it should in the video.
The design of this was done on Draftsight (see my review of this AutoCAD clone), with few problems. I’ve never designed one of these before, and there are some equations to use if you want to do it that way. Instead, I did it visually, drawing
the basic circles first, making everything fit, and rotating each piece around the central axis. This is probably easiest to do with a four-stroke device like I made, but I’m sure other numbers could be done this way.
After I cut the two rotating elements out, I placed them both on a spare piece of MDF, spaced out by the circular cavity, and drilled pilot holes for the axes. I then put a few washers under the driven element and bolted them both down lightly. It works OK, but I plan to redo this design so that the drive gear, the driven “cross”, and the base can be cut out at one time with a CNC router. I’m sure I’ll have to do some finishing work, especially if I end up putting a motor on it (maybe recycled from the “rotary engine” model project). Be sure to check back on this project, or just do yourself a favor and subscribe to the RSS feed!
The Geometry of the Geneva Mechanism
In the four slot Geneva stop, both wheels are the same size. In Geneva drives with different numbers of slots a little geometry soon reveals the relative sizes of the wheel.
For example, in the six-slot design, left, the slots are sixty degrees apart. The drive pin needs to to enter the top of the slot at an angle of ninety degrees. With the angles fixed, working out the dimensions is straight forward. In the six slot example, construct a right angled triangle with one the the angles being sixty degrees. The radii (radiuses?) of the two wheels are the lengths to the two shortest sides of the triangle and the wheels are separated by the length of the longest side.
The Geneva drive is named after the city of its invention where it was used in the construction of clocks. Originally the Geneva mechanism was used as a way of preventing springs from being wound too tight. One of the slots would be blanked off so the winder could only be turned a fixed number of turns. This mechanism is known as a Geneva stop or Geneva stop works.
In this case the spring would be connected to the smaller wheel, the slotted wheel is there to limit the number of turns. After five turns, the pin hits the blanked out slot, arrowed, and the spring is prevented from being over wound.
The Geneva drive is used to provide intermittent motion, the drive wheel turns continuously, the pin on the drive wheel then turns the cross shaped piece quarter of a turn for each revolution of the drive wheel. The crescent shaped cut out in the drive wheel lets the points of the cross pass, the rest of the circle locks the slotted wheel into place while it is stationary. Drive motion can be changed by changing the number of slots in the slotted wheel. The Geneva drive mechanism is used commonly in film projectors to move the film on one frame at a time then hold it stationary as the bright projector light is shone through it.
Specialized Mechanisms GENEVA WHEEL Mechanism 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 when the driven wheel has four spokes. They are used in watches and for the main reason of being made small and can withstand mechanical stress. 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.
Analysis of the output motion of the cam
As discussed above, if the input member roller-crank completes one revolution, the roller must traverse the open cam problem twice. Consequently, the output motion of the cam while the
Roller retraces its path is governed by the problem that prescribes the motion of the cam in the rest part of the cycle. It is now clear that we can specify the motion of the cam in only a half the cycle and the remaining half is determined automatically, thus limiting control over the complete motion. However, by studying the motion in the second half we can design the cam problem to have better control over its motion.
Types Of Geneva Drive
As we know that GENEVA Mechanisms are very important in Automated machine tools, today we look towards 20 different types of Geneva mechanisms. Geneva mechanisms are nothing but the Indexing mechanisms like Dividing head. We can adjust the Geneva mechanism so as to rotate or transfer work around the working stations within some fixed Intervals
1) Locking-arm Geneva drive: The driving follower on the rotating input crank of this Geneva enters a slot and rapidly indexes the output. In this version, the roller of the locking-arm (shown leaving the slot) enters the slot to prevent the Geneva from shifting when it is not indexing.
2) Planetary gear Geneva drive: The output link remains stationary while the input gear drives the planet gear with single tooth on the locking disk. The disk is part of the planet gear, and it meshes with the ring-gear Geneva to index the output link one position. 7
3) Four bar Geneva drive: A four-bar Geneva produces a long-dwell motion from an oscillating output. The rotation of the input wheel causes a driving roller to reciprocate in and out of the slot of the output link. The two disk surfaces keep the output in the position shown during the dwell period.
4) Twin Geneva drive: The driven member of the first Geneva acts as the driver for the second Geneva. This produces a wide variety of output motions including very long dwells between rapid indexes.
8 5) Groove cam Geneva drive: When a Geneva is driven by a roller rotating at a constant speed, it tends to have very high acceleration and deceleration characteristics. In this modifi cation, the input link, which contains the driving roller, can move radially while being rotated by the groove cam. Thus, as the driving roller enters the Geneva slot, it moves radially inward. This action reduces the Geneva acceleration force.
6) Locking slide Geneva drive: One pin locks and unlocks the Geneva; the second pin rotates the Geneva during the unlocked phase. In the position shown, the drive pin is about to enter the slot to index the Geneva. Simultaneously, the locking pin is just clearing the slot.
7) Rapid transfer Geneva drive: The coupler point at the extension of the connecting link of the four-bar mechanism describes a curve with two approximately straight lines, 90° apart. This provides a favorable entry situation because there is no motion in the Geneva while the driving pin moves deeply into the slot. Then there is an extremely rapid index. A locking cam, which
prevents the Geneva from shifting when it is not indexing, is connected to the input shaftthrough gears.
8) Dual-track Geneva drive: The key consideration in the design of Geneva’s is to have the input roller enter and leave the Geneva slots tangentially (as the crank rapidly indexes the output). This is accomplished in the novel mechanism shown with two tracks. The roller enters one track, indexes the Geneva 90° (in a four-stage Geneva), and then automatically follows the exit slot to leave the Geneva. The associated linkage mechanism locks the Geneva when it is not indexing. In the positionshown, the locking roller is just about to exit from the Geneva.
9) Long-dwell Geneva drive: This Geneva arrangement has a chain with an extended pin in combination with a standard Geneva. This permits a long dwell between each 90° shift in the position of the Geneva. The spacing between the sprockets determines the length of dwell. Some of the links have special extensions to lock the Geneva in place between stations.
10) Modified motion Geneva drive: The input link of a normal Geneva drive rotates at constant velocity, which restricts flexibility in design. That is, for given dimensions and number of stations, the dwell period is determined by the speed of the input shaft. Elliptical gears produce a varying crank rotation that permits either extending or reducing the dwell period.
11) Internal groove Geneva drive: This arrangement permits the roller to exit and enter the driving slots tangentially. In the position shown, the driving roller has just completed indexing the Geneva, and it is about to coast for90° as it goes around the curve. (During this time, a separate locking device might be necessary to prevent an external torque from reversing theGeneva.)
12) Progressive oscillating drive: A crank attached to the planet gear can make point P describe the double loop curve illustrated. The slotted output crank oscillates briefly at the vertical positions.
13) Sinusoidal reciprocator drive: This reciprocator transforms rotary motion into a reciprocating motion in which the oscillating output member is in the same plane as the input shaft. The output member has two arms with rollers which contact the surface of the truncated sphere. The rotation of the sphere causes the output to oscillate.
14) Controlled output escapement: The output in this simple mechanism is prevented from turning in either direction unless it is actuated by the
input motion. In operation, the drive lever indexes the output disk by bearing on the pin. The escapement is cammed out of the way during indexing because the slot in the input disk is positioned to permit the escapement tip to enter it. But as the lever leaves the pin, the input disk forces the escapement tip out of its slot and into the notch. That locks the output in both directions.
15) Parallel guidance drive: The input crank contains two planet gears. The center sun gear is fixed. By making the three gears equal indiameter and having gear 2 serve as an idler, any member fixed to gear 3 will remain parallel to its previous positions throughout the rotation of the input ring crank.
16) Rotating-cam reciprocator drive: The high-volume 2500-ton press is designed to shape such parts as connecting rods, tractor track links, and wheel hubs. A simple automatic-feed mechanism makes it possible to produce 2400 forgings per hour.
17) An external Geneva drive. The driver grooves lock the driven wheel pins during dwell. During movement, the driver pin mates with the driven-wheel slot.
18) An internal Geneva drive. The driver and driven wheel rotate in same direction. The duration of dwell is more than 180º of driver rotation.
19) A spherical Geneva drive. The driver and driven wheel are on perpendicular shafts. The duration of dwell is exactly 180° of driver rotation.
Design Design of the Geneva wheel has been done using the 4-level polysilicon surface micromachining technology by Sandia National Laboratories. All the four levels of polysilicon are required for designing this mechanism. The SUMMiT process [2,3] and the layout design have been discussed in detail in this section. 3.1 The Summits Process:- The Sandia Ultra-planar, Multi-level MEMS Technology (Summit) process[2,3,4] is a standard process developed by the Sandia National Laboratories. A cross-section of the main layers in the process is shown in Fig.2. In this process four layers of polysilicon alternated by sacrificial silicon dioxide layers are laid down. The first level is a silicon dioxide and nitride stack layer. The oxide layer in this
level is used as an insulating layer. he nitride layer acts as an etch stop and protects this oxide layer when the sacrificial oxide etch is carried out. he four polysilicon layers function as the structural layers used for developing various micromachined structures. The oxide layers alternating between the polysilicon layers are used to physically isolate the polysilicon layers. Once the whole structure has been developed, these oxide layers are etched away and polysilicon structures are released. The thickness of the various layers is given in Fig.2. Fig.2. Layers of the SUMMiT process Courtesy: Sandia National Laboratories. The two things that are unique to his process and which make a variety of designs possible at the micro level are the conformal SACOX2 layer and planarized SACOX3 layers. Wheel Design: Two types of slot designs for the Geneva wheel were considered. The designs were laid out using the Cadence software for MEMS layouts. Since the technology file available with the software allowed for only three levels f polysilicon, the structural poly0 level was not laid down. For any N slotted wheel, the angle by which the slotted wheel rotates for a given rotation of the constantly rotating wheel is 2p/N. The slots are thus placed at 2p/N radians intervals. An important requirement is that during every rotation, the pin should enter and leave the slots in such a way that the tangent to the constantly rotating wheel at the pin passes through the center of the slotted wheel. This means that if ‘r’ is the radius of the constantly moving disk,then the distance ‘D’ between the centers of the two disks has to be: D = r/sin (p/N) and the radius of rotation ‘R’ of the Geneva wheel is given by: R = r/tan(p/N) The minimum length of the slot through which the pin on disk W moves should be: S = D-[(D-R)+(D-r)]
= R+r-D Applying these relations to the wheel shown in Fig.1 we get: Aa = ab = Ab/ 2 Fig.3. Layout of a four slot Geneva wheel using Cadence The layout of a four slotted wheel in Cadence is shown in Fig. 3 with projections on the constantly rotating wheel to so that it can be moved with a probe. The constantly rotating disk can be rotated using a Sandia microengine[5,6,7,8] driven by comb-drives. The gears of the microengine can be made to mesh with the gears of the constantly rotating wheel that can be provided on it. To avoid unintentional rotation of the Geneva wheel, a truncated wheel (with a chopped arc angle of 4p/N radians) is placed on the constantly moving disk, which stops any rotation of the Geneva wheel when the pin is moving freely and is not engaged with any of the slots. The design should therefore, have the truncated disk and the Geneva wheel on the same polysilicon layer and the constantly moving disk in another layer, which would mesh with the microengine. The Geneva wheel and the chopped disk are made on the Poly2 layer and the constantly moving disk lies below in the Poly1 layer. The engaging pin on this disk is placed on Poly2 layer. The pins holding the
Geneva wheel and the other disks are then placed on Poly3 layer, which gets contacted to the Poly0 layer and allows rotation of the disks after release. A gear can be fabricated on poly1 layer concentric with the Geneva wheel, which can then be meshed with a rack to convert the intermittent rotation of the disk into discrete linear motion. This can then be applied to micromirrors and other systems requiring such motions.
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 Vita scope, 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 over winding (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, banknote counting 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.
Internal Geneva drive
Internal Geneva drive. Animation showing an internal Geneva drives in operation.
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.
Assembly Drawings
Kinematics
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 ja.
The figure shows the motions curves for an external four-slot Geneva drive, in arbitrary units. There is a discontinuity in the acceleration when the drive pin enters and leaves the slot. This generates an "infinite" peak of jerk (Dirac peak), and therefore vibrations.
Geneva mechanism
Geneva mechanism, also called Geneva Stop, one of the most commonly used devices for producing intermittent rotary motion, characterized by alternate periods of motion and rest with no reversal in direction. It is also used for indexing (i.e., rotating a shaft through a prescribed angle).
In the Figure the driver A carries a pin or roller R that fits in the four radial slots in the follower B. Between the slots there are four concave surfaces that fit the surface S on the driver and serve to keep the follower from rotating when they are fully engaged. In the position shown, the pin is entering one of the slots, and, on further rotation of the driver, it will move into the slot and rotate the follower through 90°. After the pin leaves the slot, the driver will rotate through 270° while the follower dwells—i.e., stands still. The lowest practical number of slots in a Geneva mechanism is 3; more than 18 are seldom used. If one of the slot positions is uncut, the number of turns that the driver can make is limited. It is said that the Geneva mechanism was invented by a Swiss watchmaker to prevent the over winding of watch springs. For this reason it is sometimes called a Geneva stop.
Early motion-picture projectors used Geneva mechanisms to give the film a quick advance while the shutter was closed, followed by a dwell period with the shutter open.
APPLICATION OF GENEVA MECHANISMS IN INDUSTRY
Geneva mechanisms are generally used in induction unit. In industry we use this cam in pharma machines. This cam has great precision work. There are three basic types of Geneva motion uses in our industry 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 still 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 that 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 resiliency. 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 intermittent mechanisms are as suitable as the spherical Geneva, but this type is bulky and not practical for significant power levels. Molded or cast spherical Geneva’s are adequate for light duty applications. Typical Geneva with special characteristics are those driven by 4 bar linkages for improved acceleration characteristics, Geneva’s with variable dwells , Geneva’s used as planets in planetary chains and those combined with cycloid 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 centerline, 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.
Cycloid Gears
With cycloid intermittent gearing the input and output remains in constant
mesh. Cycloid gearing provides considerable latitude in selection of operating characteristics- decelerations, dwell periods, ratio of input to output motions etc. A basic cycloid 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-cyclonical, epicyclical & peri-cycloidal arrangements. These devices are very
versatile and can be used with Geneva’s 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 degrees of input motion, remaining at rest for the remaining 324 degrees of rotation. Accelerations 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 involutes 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 Geneva’s. Star wheel devices can rotate at different amounts at each index point. Accelerations and decelerations can be controlled more readily than in a mutilated gear pair." By careful shaping of the teeth. Internal pairs are also possible.
Cams
Various types of cams can be used to produce intermittent indexing rotations. As an example the scroll-shaped disc cam showed 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 cylindrical 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.
Advantages
• Available in a wide variety of sizes.
• Maintains good control of its load at all times.
• Have little wear leading to a very long life span.
Disadvantages
• Limited number of dwells from 3 to 18 per rotation.
• Has a greater instantaneous change of acceleration than a cam mechanism.
• Very difficult to change timing once design is chosen.
Conclusion − Taking into account the presented relationships it was created a computer program to determine the geometric parameters of Geneva mechanism elements. − The determined data were used for computer-aided design of the crank and wheel of Geneva mechanism using NX CAD module. − By using adequate relationships was made Geneva mechanism. − It was performed Computer aided manufacturing of virtual prototype of the Geneva wheel with NX CAM module − It was verified the CNC program obtained by post processing on Heidenhain iTNC 530 equipment.
Acknowledgement We wish to thank Prof. K.S.J. Pister, Department of Electrical Engineering, University of California, Berkeley for this invaluable guidance during the course of designing this system. We also thank Elliot Hui and Karen Cheng, University of California, Berkeley for giving us great technical assistance during the design of this system.
References
[1] I. I. Artobolevsky, Mechanisms in modern engineering design, Vol. III, MIR Publications Moscow, 1979. [2] Sniegowski J. F., “Chemical Mechanical Polishing: Inhancing the manufacturability of MEMS”. [3] Sniegowski J. F. and Rodgers M. S., “Manufacturing Microsystems-on-a-chip with a 5-Level micromachining technology”.
