MBD Simulation of an Internal Geneva Mechanism

AIM

MBD Simulation of an Internal Geneva Mechanism

OBJECTIVES

• To create 3D models of driver and driven wheels and assemble them to create the Internal Geneva Mechanism using Solid Works.
• To perform motion analysis, by rotating the driving wheel by 10 rpm(using 60 Frames per second) and 20 rpm(using 120 Frames per second) respectively.
• To plot the contact force between driving and driven wheel as a function of time.
• To plot the angular displacement(of driven wheel) and angular velocity(of driver wheel).
• To enable the “precision contact” option and compare the contact forces with and without “precise contact”.

INTRODUCTION

The Geneva Mechanism is a gear mechanism that translates a continuous rotation movement into intermittent rotary motion characterized by alternate periods of motion and rest with no reversal in direction. The rotating driver wheel is usually equipped with a pin that reaches into a slot located in the driven wheel that advances it by one step at a time.

Geneva mechanism is used indexing equipment. The tool changers in C.N.C machines, the turrets of the turret lathes, screw machines, turret drills, indexing heads and rotary tables are some of the examples used in the manufacturing industry.

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.

Here, motion analysis of an internal Geneva mechanism is performed to determine the contact force and its effect on the angular velocity of the driver is studied.

THEORY

If the driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel.In the simplest form, the driven wheel has four slots and hence for each rotation of the drive wheel it advances by one step of 90°.

From the internal Geneva Mechanism, it is clear that the wheel in this kind of Geneva mechanism rotates in the same direction as the driver, the center distance is less than the wheel radius and the mechanism requires less radial space.

From the given figure, using the law of sine’s:

A contact force is any force that requires contact to occur. Contact forces are responsible for most visible interactions between macroscopic collections of matter.

The angular displacement is defined as the angle through which an object moves on a circular path. It is the angle, in radians, between the initial and final positions

MOTION STUDY SETUP

The 3D model of the parts are made as per the dimensions provided.

Driven wheel(Isometric view)

Driver wheel(Isometric view)

The Driver and Driven wheels were assembled using the following mates

• A coincident mate between the Front plane and front face of Driven wheel
• A coincident mate between the front face of Driver wheel and rear face of Driven wheel
• A concentric mate between a circle of diameter 1120mm(on the front plane) and the curved outer surface of Driven wheel.
• A concentric mate between the circle with diameter 420mm(on the front plane) and the curved outer surface of Driver wheel

The centre to centre distance is calculated and defined as discussed earlier.

Assembly of Internal Geneva Mechanism (Front view and Isometric view)

For the motion study the following steps were followed

• A Rotary motor was used selecting the axis of the Driver wheel and specifying a constant speed of 10 RPM.
• Then the contact was selected as Solid Body Contact between the Driver wheel and Driven wheel using material as Steel(Dry) for both the parts.
• Under the ‘Motion Study Properties’ the Frames per second was changed to 60 and the ‘Use Precise Contact’ box was checked.
• The time marker was set to 20 seconds.
• Then the ‘Calculate’ option is used to conduct the Motion Study.
• The same Motion Study was also done disabling Precise Contact.
• The required plots were obtained using the ‘Results and Plots’ option.
• The same Motion study was repeated for Motor speed 20 RPM at 120 Frames per second.

Enabling Precise contact

The Google rive links for the parts and assembly are given below-

Internal Geneva Mechanism Driven wheel-

https://drive.google.com/file/d/1UBf7SfwHgpMDzWZTOVuh2IPqTbie5eDU/view?usp=sharing

Internal Geneva Mechanism Driver wheel-

https://drive.google.com/file/d/1_pBSd6OfVw1f9qfqhkJkSaQ8VZrhz7lm/view?usp=sharing

Internal Geneva Mechanism Assembly-

https://drive.google.com/file/d/1yQ9iRNWHFtMQD4Zk1aKwM-ug02UVQ7gM/view?usp=sharing

RESULTS

For 10 RPM and 60 Frames per second

Contact Force (between the driving and driven wheel) as a function of Time

i. with precise contact

ii.without precise contact

Comparing Contact Forces with and without precise contact

Angular Displacement of Driven wheel

Angular Velocity of Driver wheel

For 20 RPM and 120 Frames per second

Contact Force (between the driving and driven wheel) as a function of Time

i. with precise contact

ii. without precise contact

Comparing Contact Forces with and without precise contact

Angular Displacement of Driven wheel

Angular Velocity of Driver wheel

EXPLANATION

• In the plot of Contact Force, we get a peaks at different points because when the pin of driver moves inside the driven member slot there is contact/clash between the surface of driver and driven wheel.
• Contact force between driver and driven wheel is highest at the initial stage, when it enters the slot due to the movement of the driven wheel.Also the contact force increases with increase in angular velocity from 10 RPM to 20 RPM.
• Also when precision contact is used, solid elements are used in meshing the body. Thus the solution is accurate compared to the solution obtained without precision contact, where planar elements are used.
• The values of Angular displacement changes with increase in angular velocity.
• As we have selected constant speed rotary motor the Angular velocity of the Driver wheel remains constant.

CONCLUSION

So, the Motion Analysis of Internal Geneva Mechanism was successfully performed.

REFERENCE

https://drive.google.com/file/d/1qH2QpECmT2SfwvN51lSs70QHvmlfd9t0/view?usp=sharing