Motion Analysis of Internal Geneva Mechanism

AIM - To create Internal geneva mechanism 3-D Model and do the Motion analysis Using solidworks MBD.

Theory - The Geneva drive or Maltese cross is a gear mechanism that translates a continuous rotation movement into intermittent rotary motion. the difference between Geneva Drive mechanisms and other gears is that Geneva Drive mechanisms have unusual teeth. Unlike in typical gears, the interaction between the part being driven and the part doing the driving is not continuous and the resultant motion is intermittent. Geneva Drives work by having the driving part, or wheel, interlocking with the driven part intermittently. The number of spokes that the driven part has and the speed with which the driving part rotates determines the frequency and distance of travel. There is an elevated segment on the driving wheel which keeps the driven part in position till the next revolution is complete. The external Geneva Drive is by far the most common type, although there are other variations possible in the design. The external drive can be built to withstand higher levels of stress and are generally more robust and sturdy than other variants.

The Internal Geneva Drive has the driving part below the driven wheel, with the driving part often being much smaller than the driven wheel. 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.

External Geneva Mechanism Internal Geneva Mechanism

Objectives -

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.

Procedures -

Driving Wheel -

We are going to design our first part which is Driving wheel based on 2D drawing. first select front plane and choose the option for sketching, draw a circle at Axis with diameter 420-millimetre diameter, similarly draw a small circle with diameter 40 millimetres and at a distance of 160 mm from the centre.

After completing 2D sketching we are going to use the Extrude option. to make the 2D drawing into 3D. Extrude Larger circle and smaller circle with 20 mm and 40 mm respectively, the final result of the first part is as shown below.

Driven Wheel -

our second part is driven part which is known as Driven wheel. again we are going to use front playing as a sketching plane. now Drawing a circle with diameter 1120. our driving wheel has slots for moving part of driver wheel. hence to draw a slot in the driven wheel we are going to draw x and y-axis as a reference.

Using offset with

after that, we are going to draw Internal circle with radius 120 millimetre

which has Fillet of the 50-millimetre radius.

Extruding with 40 millimetres

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.

Results -

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).

Contact forces with precise contact- With Precise Contact, Contact Force is calculated out to be nearly 1200 Newton.

Contact Forces without precise contact - The contact force between the Driver and Driven is calculated to be roughly 1200 newton.

Angular Velocity - 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.

Angular Displacement - 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.

20 RPM -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.

Contact force with precision- With Precise Contact, we get maximum Contact Force as 1200 N.

Contact force without precision - On the other hand, without Precise Contact, we get maximum Contact Force as 1200 N.

Angular Displacement -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 20 RPM of the Driver Wheel.

Angular Velocity - 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.

Observations -

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. 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 Links - Project UBAID AFZAL

Youtube animation Link - Internal geneva mechanism animation