4 Slot Internal Geneva Mechanism

Aim:-

To create a 4-slot Internal Geneva Mechanism and study Multi-body Dynamics associated with it.

Objective:-

Study variation of Contact force with and without Precise contact at the speed of 10 RPM and 20 RPM. Also, plot variation of Angular Displacement of the Driven Wheel over time.

Theory:-

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).

The Figure shown below represents an external geneva drive 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 overwinding of watch springs. For this reason, it is sometimes called a Geneva stop.

Internal Geneva Drive

Unlike External Geneva Drive, the slots on the driven wheel of Internal Geneva drive are closed at the outer edge and open at inner side. Also, a major difference is that while transferring one intermittent motion, driver of External Geneva has to rotate less than 180˚, but on the other hand, the driver of Internal Geneva rotates more than 180˚. This means that the dwell time of Internal Geneva is less than that of the External Geneva.

Spherical Geneva Drive

In Spherical Geneva Drive, the rotation axis of the driver and the driven part are at a right angle. It is usually the driven part, which is spherical in shape, more precisely hemi-spherical.

Applications of Geneva Drive:-

The most common application of Geneva Drive is Movie Projectors.
2) Bank Note Counting Machines
3) Automated sampling machines.
4) Plotters – Plotter is a computer printer for printing vector graphics.
5) Wrist Watches.

Procedure:-

To study Multi-Body Dynamics of internal geneva, its driver and driven components are modelled separately, assembled and then given motion to perform the study.

Modelling

• Driver – In Solidworks, select a plane and draw a 2-D sketch as shown below, two circles of diameter 420 mm and 40 mm, with smaller one off-centred by 160 mm Now convert this sketch into a 3-D model by extruding it in two phases, namely bigger circle by 20 mm and smaller one by 40 mm.
• Driven – Open a new part file. Draw the 2-D sketch as shown below with given dimensions. And then create a 3-D part from it by extruding it by 40mm.
• Assembly – A new Solidworks assembly file (sldasm) is opened and the two components, the driver and the driven are imported into it. Both the components are then restricted by using following mates.
  1. It is ensured that both the components are in float
  2. First, the driver is fixed by coinciding its front plane with the assembly’s front plane and its axis is fixed to remaining Top and Right Plane. This allows the driver to have rotational DOF.
  3. After this, the driven component is set-up by lifting its front plane by 20 mm up from assembly’s front plane. Its axis then coincides with one of two remaining planes at a calculated axial distance of from the driver.

Motion Analysis:

For motion analysis, the driver was connected to a motor. Both the driver and the driven wheel were then assigned steel (dry) material a contact was created between them.

1. At speed of 10 RPM – The driver is rotated at a speed of 10 RPM and all the results were captured at a rate of 60 fps (frames per second)
   1. Angular Displacement of the Driven Wheel - As we can see from the plot of Angular Displacement of the Driven Wheel with respect to Time, the wheel rotate by 90˚ in approximately 4 seconds and has a dwell period of approximately 2.5 sec for the speed of 10 RPM of the Driver Wheel.
   2. Contact Force (Without Precise Contact) - The contact force between the Driver and Driven is calculated to be roughly 2773 Newton.
   3. Contact Force (With Precise Contact) – With Precise Contact, Contact Force is calculated out to be nearly 1703 Newton.
   4. Angular Velocity of Driver Wheel - Though the driver wheel is set to rotate at a constant speed of 10 RPM = 60 deg/sec, but we do observe a slight fluctuation in the speed between 59.9999 deg/sec and 60.0002 deg/sec. This happens when the driver pin moves within the slot and experiences jerks due to frequent loosening of contact between the driver and the driven wheel.

1. At Speed of 20 RPM of the Driver Wheel – In this second case the drive wheel is rotated at twice the speed than earlier, that is 20 RPM and the same three results are again recorded. To keep the proportion same, these observations were captured at 120 fps.
   • Angular Displacement of the Driven Wheel - As we can see from the Angular Displacement vs. Time, the wheel rotate by 90˚ in approximately 2 seconds and has a dwell period of approximately 1 sec for the speed of 20 RPM of the Driver Wheel.
   • Contact Force (Without Precise Contact) – For the increased speed, the contact force between the Driver and Driven is calculated to be roughly 6387 Newton.
   • Contact Force (With Precise Contact) – On the other hand, with Precise Contact, we get maximum Contact Force as 6876 Newton.
   • Angular Velocity of Driver Wheel - Though the driver wheel is set to rotate at a constant speed of 20 RPM = 60 deg/sec, but we do observe a slight fluctuation in the speed. The maximum and minimum speed observed are 119 deg/sec and 120 deg/sec as shown in the fig.

Selection of Surfaces for Contact Surfaces:-

Contact Surfaces are selected such that the cylindrical surface of pin pushes the rectangular face of slot.

Observation:-

We observe that at the lower speed of 10 RPM Value of Contact Force without Precise Contact is greater (2773 N) than that with Precise Contact (1703 N). But this is not the case at higher speed of 20 RPM, where Contact Force without Precise Contact is lesser (6387N) than that with the Precise Contact (3782 N).

Also the Jerks, a cause for altering the driver’s speed is reduced at the higher speed.

Whenever the Driver Pin enters into the slot of the Driven wheel, a significant jerk is observed, which pushes the driven wheel back in the opposite direction. This is also reflected in the Angular Displacement vs. Time Plot, mentioned in red circles. It happens in both the cases, lower and higher speed of the driver wheel. This may be due to the fact that when driver pin leaves the slot, driven wheel due to its inertia and low friction between surfaces of two wheels continues to move forward of the calculated meeting position of pin and slot. We can use a low inertia driven wheel and increase friction to solve this issue.

Conclusion:-

For getting accurate results, one should make an exact calculation of the axial distance between the driver and the driven wheel, smooth movement of the pin in the slot, low inertia driven wheel and an appropriate amount of friction between the two components

Project Related Links:-

• Solidworks Files - Aishwary Mahajan Drive
• Youtube Link

References:-

• Wikipedia - Geneva drive
• 1700 ANIMATED MECHANICAL MECHANISMS
• Geneva Mechanisms
• A precise contact force measuring method for overhead catenary system