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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
Ankit Mevada (126440319162)
Harsh Patel (126440319161)
Guide By
Mr. H. S. Patel
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
Department
B.S.Patel Polytechnic, Kherva
Mechanical Engineering
Department
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
Department
B.S.Patel Polytechnic, Kherva
Mechanical Engineering
Department
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
EMAIL
[email protected]
COLLEGE
NAME
B.S.PATEL POLYTECNIC, GANPAT UNIVERSITY.
8735933694
+91
33/ Alok Tenament, Part-2, Maheshwari Nagar road,
ADDRESS
Vastral, Ahmedabad.
BRANCH
MECHANICAL ENGINEERING
SEMESTER
5th SEM.
TEAM NAME
SIGNATURE
OF STUDENT
YEAR
2014-2015
9825723235
STUDENT PARTICULARS-2
FIRST NAME
HARSHKUMAR
LAST NAME
RAMESHBHAI PATEL
MOBILE NO.
+91
EMAIL
[email protected]
COLLEGE
NAME
B.S.PATEL POLYTECNIC, GANPAT UNIVERSITY.
7801862128
+91
A/67, Kashinath Parth, Mahadevnagar tekra,
ADDRESS
Vastral, Ahmedabad.
BRANCH
MECHANICAL ENGINEERING
SEMESTER
5th SEM.
TEAM NAME
SIGNATURE
OF STUDENT
YEAR
2014-2015
9825575585
INDUSTRY PARTICULARS
---INDUSTRY COORDINATOR--NAME
KIRITBHAI B. MEVADA
33/ Alok Tenament, Part-2, Maheshwari Nagar road,
Vastral, Ahmedabad.
CONTACT
ADDRESS
MOBILE NO.
+919825723235
EMAIL
-
-----INDUSTRY---NAME
LAXMI PHARMA MACHINES
75, Bileshwar ind. Estate, Opp. GVMM, Nr. Ring Road,
Odhav-Kathwada Road, Odhav, Ahmedabad-382415.
ADDRESS
CONTACT
NO.
+91 79
22900488
NAME OF INDUSTRIAL ESTATE
COMPANY
LOGO
MOBILE
+91982572325
BILESHWAR INDUSTRIAL ESTATE
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 onefourth 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 parameters 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
Mechanism
GENEVA
WHEEL
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 crosssection 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 j a .
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 hypercyclonical, 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
o
an example the scroll-shaped disc cam showed below indexes a wheel 180 when
o
the solenoid pulls the levers down and a further 180 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”.