Challenge

Internal Geneva Mechanism

AAditya Kumar Jha29 Jun 2020

AIM: To perform Multi Body Dynamics using Solidworks on Internal Geneva Mechanism.

THEORY:

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.

Types of Geneva Mechanism

External Geneva Mechanism:- In this type of mechanism, the Geneva cross is connected with cam drive externally which is the more common, as it can be built smaller and can withstand higher mechanical stresses. The driver grooves lock the driven wheel pins during dwell. During movement, the driver pin mates with driven-wheel slot. 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.
Internal Geneva Mechanism:- An internal Geneva drive is a variant on the design. In this type of mechanism, the Geneva cross and cam drive are connected internally in the closed box. The driver and driven wheel rotate in same direction. 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 greater than 180° in an internal Geneva drive.
Spherical Geneva Mechanism:- In this type of mechanism, the Geneva cross is in spherical shape and cam drive are connected in externally, which is extremely rare. The driver and driven wheel are on perpendicular shafts. The duration of dwell is exactly 180 of driver rotation.

Working of Geneva Mechanism:

The drive wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps. In the most common arrangement of the Geneva drive, the client wheel has four slots and thus advances the drive by one step at a time (each step being 90 degrees) for each full rotation of the master wheel. If the steered wheel has n slots, it advances by 360°/n per full rotation of the propeller wheel. Because the mechanism needs to be well lubricated, it is often enclosed in an oil capsule.

Applications of Geneva Mechanism:-

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.
Geneva wheels having the form of the driven wheel were also used in mechanical watches.
Other applications of the Geneva drive include the pen change mechanism in plotters, automated sampling devices, banknote counting machines, and many forms of indexable equipment used in manufacturing (such as the tool changers in CNC machines; the turrets of turret lathes, screw machines, and turret drills; some kinds of indexing heads and rotary tables; and so on). The Iron Ring Clock uses a Geneva mechanism to provide intermittent motion to one of its rings.
A Geneva drive was used to change filters in the Dawn mission framing camera used to image the asteroid 4 Vesta in 2011. It was selected to ensure that should the mechanism fail at least one filter would be usable.

Advantages of Geneva Mechanism:

Geneva mechanism may be the simplest and least expensive of all intermittent motion mechanisms.
They come in a wide variety of sizes, ranging from those used in instruments, to those used in machine tools to index spindle carriers weighing several tones.
They have good motion curves characteristics compared to ratchets, but exhibit more jerk or instantaneous change in acceleration, than better cam systems

Disadvantages of Geneva Mechanism

The Geneva is not a versatile mechanism and produce jerk.
The ratio of dwell period to motion is also established once the no of dwells per revolution has been selected.
All Geneva acceleration curves start and end with finite acceleration & deceleration.

The 2-D drawing sheet required for making 3-D modal of Internal Geneva mechanism are as follows:-

For Driven Wheel

For Driver wheel

OBJECTIVES:

Creating 3D models for driver and driven wheels on SOLIDWORKS.
Performing motion analysis by rotating the driver wheel at different RPMs and FPS.
Obtaining the following plots

- Contact force (between driving and the driven wheel) as a function of time.
- Angular Displacement of the driven wheel.

Comparing the contact forces with and without "Precise contact"

Showing image for precise contact selection.
Exporting the results that we get to Excel and make the plots over there.

PROCEDURE:

We have done the various step to Perform the 3-D modelling and doing motion analysis of Internal Geneva Mechanism as shown below:-

For making Driver wheel:

First draw the above sketch by using tools like circle, perimeter circle, line, construction geometry, offset, trim, circular pattern, point, and fillet. Make sure that the whole sketch is fully defined.
And after that exit from the sketch and select the front plane and go to sketch and draw a circle of diameter 420mm from the point which is 160mm away from the intersection of first sketch construction circle, construction line and point and draw a circle of 40 mm diameter at that intersection point. The sketch of driver part is shown below.

Then exit from the sketch go to feature and select extruded boss/base and select the outer circle and extrude it to 20mm. and again go to extruded boss/base and select smaller circle , choose option surface/face/plane and select top surface of first extruded portion and extrude upto 40mm. Make sure that the merge result option is on. Hide the first sketch. And the extruded driver part is as shown below.

For making driven wheel:

First draw the above sketch by using tools like circle, line, offset, circular pattern, and fillet. Make sure that the whole sketch is fully defined.
Then exit from the sketch go to feature and select extruded boss/base and select the sketch and extrude it to 40mm. And the extruded driver part is as shown below.

For assembly:

Open SOLIDWORKS select assembly select the driver and driven wheel first driven part open click on it then driver part open click on it. Go to the driven part make it float. Then by using mate tool prepare the assembly various mate used in assembly are as follows:-
Coincident - between front plane of driver front plane of assembly.
Coincident- between axis of driver top plane of assembly
Coincident- between axis of driver right plane of assembly
Coincident- between axis of driven right plane of assembly
Distance- between front plane of driven and front plane of assembly and distance is 20mm.
Distance- between axis of driver and driven.
The assembly obtained is as shown below:

Motion analysis:

Open the assembly and go to solidworks add-ins and then click on solid works motion
Now go to motion study select the type of study as motion analysis.
Go to motor and select the axis of driver and set the rpm.
And then go to contact select the driver and driven component. And then select the material.
And click on calculate to perform motion analysis.
And at last go to the result and plot option we can see the various plots like Angular velocity vs time and contact force and time.

Animation of internal Geneva Mechanism

Plots:-

Contact force (between driving and the driven wheel) as a function of time at 10 rpm.

Contact force (between driving and the driven wheel) as a function of time at 10 rpm 60 FPS without precise contact, the becomes smoother from previous one .

Contact force (between driving and the driven wheel) as a function of time at 10 rpm 60 FPS with precise contact, now the graph become much more smoother than than without precise contact.

Contact force (between driving and the driven wheel) as a function of time at 20 rpm 120 FPS without precise contact.

Contact force (between driving and the driven wheel) as a function of time at 20 rpm 120 FPS without precise contact, now the graph become much more smoother than than without precise contact.

Angular displacement vs time for driven at 10 rpm

Angular displacement vs time for driven at 10 rpm 60 FPS without precise contact

Angular displacement vs time for driven at 10 rpm 60 FPS with precise contact, there is no change seen in the graph it similiar as previous.

Angular displacement vs time for driven at 20 rpm 120 FPS without precise contact when we have changed the rpm the displacement rise time decreases and the at 20 sec the angular displacement value also decreases.

Angular displacement vs time for driven at 20 rpm 120 FPS with precise contact, the graph is similiar to previous graph.

Angular displacement vs time for driver at 10 rpm 60 FPS without precise contact, thre is no any change inthe graph

Angular displacement vs time for driver at 10 rpm 60 FPS with precise contact

Angular displacement vs time for driver at 20 rpm 120 FPS without precise contact

Angular displacement vs time for driver at 20 rpm 120 FPS with precise contact

Angular velocity of Driver at 10 rpm 60 FPS with and without precise contact appears to be same.

Angular velocity of Driver at 20 rpm 120 FPS without precise contact

Angular velocity of Driver at 20 rpm 120 FPS with precise contact ,this much smoother than previous graph.

Angular velocity of Driven at 10 rpm 60 FPS without precise contact

Angular velocity of Driven at 10 rpm 60 FPS with precise contact, the graph becomes much smoother than previous one.

Angular velocity of Driven at 20 rpm 60 FPS without precise contact, the graph becomes much good as previous one.

Links for Solid parts, assembly part and excel file obtainfrom graph:-

https://drive.google.com/drive/u/0/folders/10BjV6JCfnfaBJiBv-0kkqpjQnVo7FczP

CONCLUSION:

The motion study helpful in simulating the performance of the internal geneva mechanism and also gives outline of the model for better output.

REFERENCES:

Internal Geneva Mechanism

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Ship work like thisin your own course.

Talk to a career counsellor

Ship work like this
in your own course.