Internal Geneva Mechanism
AIM:
To use the following 2D CAD drawings to create the 3D model of the internal Geneva mechanism and get the results for the above cases.
THEORY:
Geneva mechanism, also known as Geneva Stop, is one of the most commonly used devices for producing intermittent rotary motion, charectized 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).
In the figure 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.
Early motion-picture projectors used Geneva mechanisms to give the film a quick advance while the shutter was closed, followed by a dwell period with the shutter open.
In this challenge, it is required to assemble an internal Geneva mechanism based on the following dimensions:
GEOMETRICAL SETUP:
1)Open SolidWorks. Open a new part model. Make the dimensions to MMGS. Create a new sketch in the front plane.
2) Draw 3 circles of diameter 240mm, 1040mm and 1120mm. Right click on the second circle and make it into a construction geometry.
3) Draw a horizontal line from the 240mm circle to the 1040mm circle.
4) Click on The offset entities option. Make offest distance as 20mm. Also check Select Chain, Bi-directional, Cap-ends(Lines). Slot is created. Fillet the ends with fillet radius 4mm.
5) On the other end, draw a line intersecting one side of the slot and the 240mm circle. Fillet the line with fillet radius 50mm. Mirror it to the other side. Trim off the excess.
6)Using the circular pattern, make three more such slots, each slot having an angular spacing of 90 deg from the next. Trim off the excess.
7) Exit Sketch. Extrude sketch by 40mm. Driven component is ready.
8) Create another sketch in front plane. Draw a line, bisecting the angle between the axis of two consecutive slots(90/2= 45 deg).
9) Select the perimeter circle option. The two points being the axis of the two slots. The centre of this circle should lie on the 45deg line. Draw two lines, perpendicular the each other, from the axes of the slots to the centre of the circle. The two line should intersect one side of the respective slots where they originate from, where the fillet starts. This circle is completely constrained. Right click this circle and make it into a contruction geometry.
10) From the centre of the circle, draw another circle of diameter 420mm. Place a point on the 45 deg line such that it is 160mm away from the center of the circle. Draw a circle of diameter 40mm with center as that point.
11) This is how the assembly of the driver will be with the driven component. Measure the distance between the centers of the driver and driven circle. It comes out to be 234.09mm.
12) In another model sketch the driver (420mm diameter circle, 40mm circle away from center). Exit sketch and extrude the larger circle by 20mm and the smaller circle by 40mm.
ASSEMBLY:
1) Open a new assembly model. Open The driven and driver model. Right click the driven part and click Float, so it can be moved around.
2) Next click on the mate option. Under entities to mate, select the Front Plane of the driver and the Front Plane of the assembly. Click on coincide.
3) Go to the View options, and select Hidden Axis. The hidden axes are now visible.
4)Next mate the central axis of driver and the Top Plane of the assembly. Click on coincide.
5) Then mate the central axis of driver and the Right Plane of the assembly. Click on coincide.
6)Similarly, mate the central axis of driven component and the Right Plane of the assembly. Click coincide.
7) Then mate the Front Plane of the driven component and the Front Plane of the assembly. Keep the distance as 40mm.
8) Finally, mate the central axes of the driver and driven component. Keep the distance as 234.09mm.
9) Assembly is now complete.
MOTION STUDY:
1)Select the SolidWorks Add-Ins option. Under that Select SolidWorks Motion. Open Motion Study 1.
2) Change the type of motion study from animation to Motion Analysis.
3) Click on the Motor Option. Under reference, select the central axis of the driver. Keep Motor speed as 10rpm (In second case 20rpm)
4) Click on the Contact option. Under Type of component, select Solid. Under Surfaces to be selected, select the driver and driven component. Under Nature of material, make both driver and driven component as Steel(Dry).
5) Click on the Settings option. Make the frame speed as 60fps(120 fps for 20rpm speed). There is the Precise components which has to be selected when plotting contact force with precise contact vs time.
6) Make the duration of the analysis as 20seconds. Click on the Calculate Results options. Then Click on the Plot Results option and obtain variation of contact force, angular displacement and angular velocity with respect to time. Repeat the steps for speed equal to 20rpm.
RESULTS:
When speed= 10rpm
Contact Force vs Time (No Precise Contact)
Contact Force vs Time (With Precise Contact)
Angular Displacement of Driven Component vs Time:
When Speed= 20rpm
Contact Force vs Time (No Precise Contact):
Contact Force vs Time (Precise Contact)
Angular Displacement of Driven Component vs Time
Angular Velocity of Driving Component vs Time:
INFERENCE:
1) At 10 rpm, without precision contact, the maximum contact force is 800999 N while with precision contact the maximum contact force is 822163 N. The angular dispacement of driven component changes in a sinusoidal manner from -180 deg to +180 deg.
2) At 20 rpm, without precision contact, the maximum contact force is 814789 N while with precision contact the maximum contact force is 876054 N. The angular dispacement of driven component changes in a sinusoidal manner from -180 deg to +180 deg. The angular velocity of the driver decreases from 600 deg/sec to 0 in 10 secs and increases from 0 to 90 degs/sec in the next 10 seconds.
Video Link: https://www.youtube.com/watch?v=c09ujhUS0pY&feature=youtu.be
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