MOTION ANALYSIS OF INTERNAL GENEVA MECHANISM

AIM: To create 3D model of internal Geneva mechanism using 2D drawings and perform motion analysis.

GENEVA DRIVE: It 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 drive wheel also has an elevated circular blocking disc that locks the rotating driven wheel in position between steps. The name, Geneva drive, is derived from the device’s earliest application in mechanical watches, which were popularized in Geneva, being the classical origin of watchmaking industry. In the most common arrangement of the Geneva drive, the driven wheel has four slots and thus advances the drive by one step at a time (each step being 90 degrees) for each full rotation of the driver wheel. If the driven wheel has n slots, it advances by 360⁰/n per full rotation of the driver wheel. Geneva drive is used in movie projectors, tool changers in CNC machines, the turrets of turret lathes, screw machines, turret drills, some kind of indexing heads and rotary tables.

INTERNAL GENEVA DRIVE: An internal Geneva drive is a variant on the design. The angle by which the drive wheel has to rotate to effect one step rotation of the driven wheel is always smaller than the 180⁰ in an external Geneva drive and always greater than 180⁰ in an internal one, where the switch time is therefore greater than the time the driven wheel stands still.

FRAMES PER SECOND: Frame rate (expressed in frames per second or FPS) is frequency (rate) at which consecutive images called frames appear on display. Frame rate may also be called the frame frequency, and be expressed in hertz. The human visual system can process 10 to 12 images per second and perceive them individually, while higher rates are perceived as motion.

PROCEDURE:-

1) Using 2D drawings make sketches of driver and driven wheel and extrude them to required dimensions.

                                                                                                               DRIVER WHEEL

                                                                                                                DRIVEN WHEEL

2) Open the part files as assembly to assemble the both parts.

                                                                                                                  ASSEMBLY 

3) Right click on the part and select float if part is fixed.

4) Go to mate to constrain or limit the motion of the wheels. Make front plane of assembly and driver coincident.

5) Make axis of driver and right plane of assembly coincident. Similarly make axis of driver and top plane of assembly coincident.

6) Make axis of driven wheel and right plane of assembly coincident. Select front plane of assembly and front plane of driven wheel to make a distance constraint of 20 mm.

7) Select axis of both wheels to make a distance constraint of 224.85 mm.

8) Go to motion study tab. Select motion analysis from drop down list. Go to motor and enter axis of motor and value of 10 rpm.

9) Go to contact and select both the wheels. Select steel (dry) for both wheels.

10) Extend the time for analysis by dragging the line.

11) Click on calculate to calculate the motion study.

12) Go to results and plots to create graphs. Select Forces in category, contact forces in sub-category, and magnitude in result component.

13) Select face of slot and surface of pin. Click OK to show the graph of reaction force and time.

14) Similarly create graph for angular velocity and time.

15) Go to motion study properties to change the frames per second. Check use precise contact option.

OBSERVATIONS: The above plot of reaction force and time shows that as the driver starts rotating there is a sharp rise in the reaction force of 10,990.16 N at t=0 s due to the contact between the driver and driven wheel. After that the motion is stable and reaction force rises only when the driver enters the slot of the driven wheel.

OBSERVATIONS: The above plot of reaction force and time with precise contact of driver and driven wheel shows that at t=0 s there is a reaction force of 11110.80 N. As the driver starts rotating the reaction force decreases and increases with time sharply when the driver pin enters and leaves the slot of driven wheel due to the contact of surfaces of both wheels.

OBSERVATIONS: The above plot of angular displacement and time shows that at the start of motion at t=0 second the angular displacement of driver wheel is 38⁰ approximately. The driver pin leaves the slot of driven wheel at t=1.23 seconds when the angular displacement of driver is 90⁰. It enters the next slot at 2.7 seconds with an angular displacement of 180⁰, leaves at 7.4 seconds with an angular displacement of 15⁰. It again enters the next slot at an angular displacement of 90⁰ and t=8.68s and leaves at an angular displacement of 71⁰ with t=13.51 seconds. The slot moves 90⁰ when the driver comes again at 0⁰ at 14.65 seconds from 0⁰ at 7.23 seconds which includes time when there is no motion of driven wheel.Similar motion occurs in case of with precise contact motion study.

OBSERVATIONS: The above plots of reaction force and time shows that as the driver starts rotating there is a reaction force of 17910 N. As the motion progresses the reaction force decreases and increases sharply due to entry and exit of the driver wheel pin. After 13.33 seconds the driver wheel pin slides smoothly into the slots due to which there is not an abrupt increase in the reaction force till the end of motion study.

                                                                  INTERNAL GENEVA MECHANISM AT 20 RPM, 120 FPS WITH PRECISE CONTACT

OBSERVATIONS: The above plots of angular displacement and time show that when the driver rotates from 0⁰ at 3.6 seconds to again 0⁰ at 7.35 seconds i.e. 360⁰ there is angular movement of 90⁰of one slot of driven wheel.

OBSERVATIONS: The above plot of angular velocity and time shows that at time t=0 s the angular velocity of driver wheel is 120 deg/s. At t=0.01 s the angular velocity drops to 61.34 deg/s. It increases to a value of 73.48 deg/s at t=0.03 s and goes on increasing till it becomes constant at 122.66 deg/s at t=0.68 s because the pin is not in the slot of driver wheel. There is a sudden increase in the angular velocity when it enters the slot 137.87 deg/s at t=1.33 s as the pin strikes the surface of slot while entering the slot. There is no sudden increase in angular velocity of driver wheel after 16 s   as the pin of driver wheel starts sliding smoothly into the slot of driven wheel with angular velocity of 121.03 deg/s at t=16.10 s.

CONCLUSION: There is increase in reaction force at the start of motion because of contact between the surfaces of driver and driven wheel. The reaction force also increases when the pin of driver wheel enters and leaves the slot of driven wheel. With 360⁰ angular displacement of driver wheel there is one step or 90⁰ rotation of the slot of driven wheel.The plot of angular velocity shows increase, dwell and decrease of angular velocity with time kind of motion.

REFERENCES:-

WIKIPEDIA