Internal Geneva Mechanism

Aim:

Creating a 3D part of Driver and Driven and will be setting up the motion analysis for an Internal Geneva Mechanism

Introduction Multi body Dynamics:

(MBD) system is one that consists of solid bodies, or links, that are connected to each other by joints that restrict their relative motion. The study of MBD is the analysis of how mechanical systems move under the influence of forces.

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 main wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps.

Contact Force:

A contact force is any force that requires contact to occur.

Angular Displacement:

Angular displacement of a body is the angle in radians (degrees, revolutions) through which a point revolves around a centre or line has been rotated in a specified sense about a specified axis.

Geometry and Modelling :

1. Geometry of Driver wheel: Photo attached for reference.
2. Geometry of Driven wheel: Photo attached for reference.
3. Parts are assembled based on the dimensions determined using the drawing:

The center to center distance between driver and driven is equal to 226.15mm

(Photo attached for reference).

Procedure:

3D model is developed based on given drawings

1. The parts are assembled based on the dimensions determined using the drawings.
2. The motion of the wheels is to be constrained to rotation about their axes.
3. A motor is to be defined at the axis of driving wheel at required rpm.
4. Contact has to be established between the wheels and the simulation is ready to be run.
5. After running the simulation required plots are to be extracted.
6. Precision contact available in the settings can be used to determine the contact forces more accurately.

Image 1 : Plot _Contact force and time without precise

Image 2 : Plot_Angular displacement driven wheel

Image 3 : 60 frame per second_10 RPM_Precise

Image 4: Plot_contact force and time 10RPM_Precise

Image 5 : 120 frame per second_20 RPM_Precise

Image 6 : Plot_contact force and time 20RPM_Precise

Image 7 : Driver wheels angular velocity

Explanations:

1. The 'Precise Contact' gives a more accurate result as we can see that the curves in the plots with precise contact becomes less fluctuating and smooth.
2. When the driver tries to enter in the slot of the driven wheel it faces a jerk as the entry is not smooth therefore, we can see also from the plot that at the entry to the slot gives rise to a sudden increase in the contact force which decreases as the driver enters into the slot.
3. We can also see from the contact force plots of the 2 RPM's that as we increase the rotation of the driver the jerk force/contact force at the entry also increases and then it drastically decreases.
4. The angular velocity plot reveals that at first, there is a jerk felt by the driven wheel and we see peak in the angular velocity plot and then when it enters the slot there is acceleration in the driven wheel but as the motion progresses there is reduction in the velocity and hence retardation occurs. The maximum velocity is at the point where the pin of the driver reaches the bottom point of the driven wheel.
5. The angular velocity of the driven wheel also increases if we increase the rotation of the driver which is seen from the plot.
6. The angular displacement plot shows that the displacement occurs between -180 to 180 degree and it repeats the pattern in each revolution starting from 0.