Internal Geneva Mechanism using Solidworks MBD.

AIM : To create the 3D model of the internal Geneva mechanism using Solidworks.

OBJECTIVE :

1. Draw and Assemble internal geneva mechanism and do the motion analysis using Solidworks MBD when driven runs at 10 RPM and 60 fps.
2. Draw and Assemble internal geneva mechanism and do the Multi body dynamics using Solidworks MBD when driven runs at 20 RPM and 120 fps.
3. Obtain the following plots :

1. Angular Displacement.
2. Angular velocity.
3. without precise contact at 10 rpm and 20 rpm.
4. with precise contact at 10 rpm and 20 rpm.

INTRODUCTION :

Geneva mechanism

The Geneva drive or Maltese cross is a Gear mechanism that translates a continuous rotation movement into intermittent rotary motion.

The rotating drive wheel is usually equipped with a Pin that reaches into a slot located in the other wheel (driven wheel) that advances it by one step at a time. The drive wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps.

HISTORY

The name, Geneva drive, is derived from the device's earliest application in mechanical watches, which were popularized in Geneva, being the classical origin of watchmaking industry.

The Geneva drive is also called a "Maltese cross mechanism" due to the visual resemblance when the rotating wheel has four spokes, since they can be made small, and are able to withstand substantial mechanical stress. These mechanisms are frequently used in mechanical watches.

In the most common arrangement of the Geneva drive, the client wheel has four slots and thus advances the drive by one step at a time (each step being 90 degree) for each full rotation of the master wheel. If the steered wheel has n slots, it advances by 360°/n per full rotation of the propeller wheel.

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

uses and application :

One application of the Geneva drive is in film movie projectors and movie cameras, where the film is pulled through an exposure gate with periodic starts and stops. The film advances frame by frame, each frame standing still in front of the lens for a portion of the frame cycle (typically at a rate of 24 cycles per second), and rapidly accelerating, advancing, and decelerating during the rest of the cycle. This intermittent motion is implemented by a Geneva drive, which in turn actuates a claw that engages sprocket holes in the film. The Geneva drive also provides a precisely repeatable stopped position, which is critical to minimizing jitter in the successive images. (Modern film projectors may also use an electronically controlled indexing mechanism or Stepper motor, which allows for fast-forwarding the film.) The first uses of the Geneva drive in film projectors go back to 1896 to the projectors of Oskar messets and Max Gliewe and the Teatrograph of Robert william paul. Previous projectors, including Thomas Armat's projector, marketed by Edision as the Vitascope, had used a "beater mechanism", invented by Georges demeny in 1893, to achieve intermittent film transport.

Geneva wheels having the form of the driven wheel were also used in mechanical watches, but not in a drive, rather to limit the tension of the spring, such that it would operate only in the range where its elastic force is nearly linear. If one of the slots of the driven wheel is occluded, the number of rotations the drive wheel can make is limited. In watches, the "drive" wheel is the one that winds up the spring, and the Geneva wheel with four or five spokes and one closed slot prevents overwinding (and also complete unwinding) of the spring. This so-called Geneva stop or "Geneva stop work" was the invention of 17th or 18th century watchmakers.

Other applications of the Geneva drive include the pen change mechanism in plotters, automated sampling devices, banknote counting counting machines, and many forms of indexable equipment used in manufacturing (such as the tool changers in CNC machines; the turrets of turret lathe, screw machines, and turret drills; some kinds of indexing heads and rotary tables; and so on). The Iron ring clock uses a Geneva mechanism to provide intermittent motion to one of its rings.

A Geneva drive was used to change filters in the Draw 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 Mechanism :

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.

DRAWING FOR INTERNAL GENEVA MECHANISM :

STEPWISE PROCEDURE :

1. Create new file as part and Draw Driven part with given dimension.
2. Create new file as part and Draw driver part with given dimension with reference of Driven part to locate pin part
3. after sketching is over extrude both part with given dimension and save it.
4. Create new file as assembly and call the part using browse option.
5. now here you should now the motion of both driver and driven part and according to that we should position it.
6. first part call the driver part ,here after calling part is fix with all DOF , so should float the part and unlock DOF's.
7. after making Float, coincide the front plane of part and front plane of assembly.
8. then coincide the centre axis of part with top plane of assembly.
9. then coincide the centre axis of part with Right plane of assembly now part has 1 DOF which rotation.
10. call second part then coincide the front plane of part with mating surface of part 1.
11. second constraint will be the centre axis distance of both part measure it and apply coincide with measured distance.
12. now select front plane of assembly goto sketch and convert entities select second part bottom edge and click on ok and exit the sketch.
13. now coincide the outer face of second part with new sketch this will make second part to rotate in given axis.
14. lastly orient both the part as there is gap (no clash occur) and position both part for motion analysis.
15. now click of Solidworks MBD in menu and goto solidworks motion in ribbon bar.
16. firstly at bottom change "animation" into "motion analysis" .now according to given data apply 60 fps in setting of motion analysis.
17. after apply setting goto to apply rotary motor with 10 rpm speed to driver part.
18. then apply solid body contact with steel material.
19. then click on calculate to view animation of Internal geneva mechanism.
20. click on results and plots to get desired output.
21. apply same for 20 rpm and 120 fps.

RESULTS :

Animation of internal geneva mechanism :

1) Driver at 60 FPS and 10 rpm :

Link : https://youtu.be/tbanOlMXjyQ

2) Plots at 10 RPM and 60 FPS :

a) Angular displacement of driven (10 RPM) :

2) Contact of driver and driven part (without precise contact at 10 RPM):

3) with precise contact (10 RPM) :

3) animation of internal geneva mechanism with 20 RPM and 120 FPS:

link : https://youtu.be/40Cz8nASMzs

4) Plots at 20 RPM & 60 FPS :

a) Angular veocity of driver part (20 RPM) :

b) angular displacement of driven part (20 RPM) :

c) contact (without precise 20 RPM) of both the part :

d) with precise contact (20 RPM):

EXCEL FILE ATTACHED AS .ZIP FILE

CONCLUSION :

In this way , I learn how draw 2D sketch and convert it into 3D sketch and succesfully assembled the parts to create motion and transfer the motion using Solidworks and solidworks MBD.