Internal Geneva Mechanism

AIM

• To Simulate motion characteristics of Internal Geneva Mechanism using SOLIDWORKS MOTION with angular velocity of 10 rad/sec.
• Post-process the results to obtain forces acting on the driver and driven joints.

THEORY

Geneva Mechanism

• It’s a Gear mechanism used to produce an intermittent rotary motion from uniform input speed of the driver.
• When the driver member provides constant angular velocity, the driven member may generate simple/complex intermittent rotary motion.
• This mechanism comprises of a driver, driven wheel, pin and evenly spaced slots.
• The pin is attached to the rotating driving wheel which slides into the slots of the driven wheel that advances it by one step at a time.
• Depending upon the number of slots, “n” of the driven member, the driver must rotate “n” times for every one complete revolution of the driven link.
• There are three different standard type Geneva mechanisms known as ‘External’, ‘Internal’ and ‘Spherical’ geneva mechanisms depending upon the arrangement of slots in the driven wheel and the position of the driving wheel.

Internal Geneva Mechanism

• The driver member is contained in the same space occupied by the driven member.
• Dwell period is less than 180`°`.

Index and Dwell period

• The “indexing” period starts from the initial engagement of the driver pin with one of the slots of the driven member. This period ends with the complete disengagement of the pin from the slot.
• The “dwell” period starts at the end of the index period and ends exactly at the initial engagement of the driver pin with the next (ready to be indexed) slot.

DESIGN METHODOLOGY AND DESCRIPTION

Step1: Create the CAD model of the driving wheel by drawing a circle of diameter 420mm. Within the circle draw the pin of diameter 40mm which is 160mm from the origin. Extrude the Circle and pin separately with 20mm and 40mm respectively. Make sure the unit is in MMKS (Figure 1).

Figure 1. Geneva Driving wheel

Step2: Create the CAD model of the driven wheel by drawing circle of 1120mm diameter. Another circle of diameter 420mm is drawn and with slot size of 40mm. The slot along with the small circle is given a fillet of radius 50. A pattern is created with respect to the origin for the slots and the entire model is extruded. (Figure 2).

Figure 2. Geneva Driven wheel

Step3: Create the ASSEMBLY of the model by the following mates:

1. Mating the Front plane of the driving wheel and the Front plane of the assembly.
2. Mating the axis of the driving wheel with the Right plane of the assembly.
3. Mating the axis of the driving wheel with the Top plane of the assembly.
4. Mating the axis of the driven wheel with the Right plane of the assembly.
5. Mating the Front plane of the driven wheel and driving wheel with a distance of 20mm.
6. Mating the axis of the driven wheel with the axis of the driving wheel with a distance of 226.27mm (Figure 3).

Figure 3. Assembly

Step4: To create a SOLIDWORKS MOTION study, give the necessary initial conditions and the solid contacts for the required simulation of various conditions and obtain the plot.

RESULTS

1. Condition 1
   1. Rotor of constant speed 10 m/s.
   2. Without precise contact.
   3. With 60 Frames per second.
      
      Contact Force v/s time
      
      Figure 4: Contact force v/s time (10m/s)
      
      Angular Displacement v/s time
      
      Figure 5. Angular displacement v/s time (10m/s)
      
      From the above graph, its evident that the angular displacement and contact forces are high during the indexing period.
      
      Angular Velocity v/s time
      
      Figure 6: Angular Velocity v/s time (10m/s)
      
      The angular velocity remains constant with time.
      
2. Condition 2
   1. Rotor of constant speed 10 m/s.
   2. With precise contact.
   3. With 60 Frames per second.
      
      Contact Force v/s time
      
      Figure 7 : Contact force v/s time (precise contact)
      
      
      Figure 8: Angular Displacement v/s time (precise contact)
      
      Due to the precise contact, the variations in angular displacement is very small but the computations would be more accurate.
      
      
3. Condition 3
   1. Rotor of constant speed 20 m/s.
   2. Without precise contact.
   3. With 120 Frames per second.
      
      Contact force v/s time
      
      Figure 9: Contact force v/s time (20 m/s)
      
      Angular Displacement v/s time
      
      Figure 10: Angular displacement v/s time
      
      Reaction force increases with increase in speed and the angular displacement rate takes place at a faster rate.
      
      
      Figure 11: Angular Velocity v/s time
      
      
4. Condition 4
   1. Rotor of constant speed 20 m/s.
   2. With precise contact.
   3. With 120 Frames per second.
      
      Contact force v/s time
      
      Figure 12: Contact force v/s time
      
      Angular displacement v/s time
      
      Figure 13: Angular displacement v/s time
      
      Angular velocity increases linearly with time.

APPLICATIONS

• Geneva Mechanism is used in theatre movie projectors and movie cameras, where the film needs to be played frame by frame in intermittent motion.
• It can be used for tool changing in turret lathes.
• They are used in dial watches especially in the movement of three hands, which can give us accurate time.

CONCLUSION

• Analysed the kinematics and simulated internal geneva mechanism. The contact force increases at the time of contact and angular displaecement takes place more faster when there is increase speed.
  
  
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