MBD of Internal Geneva Mechanism using Solidworks
Objective:
- Build the driver and driven wheel and assemble them partially.
- Analyze the assembly under different rotational speeds and forces.
- Analyze the difference in the contact forces with and without 'Precise contact'.
- Observe the variation of angular velocity with time.
Introduction
The Geneva drive or Maltese cross 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 main wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps.
Theory
The Geneva wheel is also commonly called as the Maltese cross mechanism due to the visual resemblance. In the most common arrangement, the driven wheel has 4 slots and thus advances for each rotation of the driver wheel by 1 step of 90 degrees. If the driven wheel has 'n' slots, it advances by 360/n degrees per full rotation of the driven wheel. Since the mechanism needs to be well lubricated, it is often enclosed in an oil capsule.
Nomenclature
Geneva Mechanism
The Geneva drive or Maltese cross 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 main wheel also has an elevated circular blocking disc that "locks" the rotating driven wheel in position between steps.
Animation showing a six-position external Geneva drive in operation.
(i) Contact Force
A contact force is any force that requires contact to occur.[1] Contact forces are ubiquitous and are responsible for most visible interactions between macroscopic collections of matter. Pushing a car up a hill or kicking a ball or pushing a desk across a room are some of the everyday examples where contact forces are at work.
(ii) Angular Displacement
Angular displacement of a body is the angle in radians (degrees, revolutions) through which a point revolves around a centre or line has been rotated in a specified sense about a specified axis. When a body rotates about its axis, the motion cannot simply be analyzed as a particle, as in circular motion it undergoes a changing velocity and acceleration at any time (t). When dealing with the rotation of a body, it becomes simpler to consider the body itself rigid. A body is generally considered rigid when the separations between all the particles remain constant throughout the body's motion, so for example parts of its mass are not flying off. In a realistic sense, all things can be deformable, however, this impact is minimal and negligible. Thus the rotation of a rigid body over a fixed axis is referred to as rotational motion.
The Present study is to analyze the motion study of an internal geneva mechanism and plot various graphs and compare the plots and explain the results.
Model Building Process
1. After HyperMesh is launched, meshing is done on the mechanism. At first trial, Auto-Mesh Surfaces was used which has an Average element size of 5 and Minimum Element size of 4. However, it was found that the meshing done was coarse especially the meshes on the pin weren't fine enough. Since the pin is what will go inside the Geneva Wheel Slots, a finer mesh had to be done.
2. The model is re-meshed with an Average Element Size of 2 and a Minimum Element Size of 1
3. Finally two outputs which would measure the torque and angular velocity at the driver and geneva wheel are added.
Results and Discussion:
Angular Velocity of wheel(Driven)
Angular Velocity of wheel (Driver)
Geneva Wheel Torque(Driven) :
Geneva Wheel Torque(Driver):
Animation of Internal-Geneva Mechanism
The animation above in youtube vedio reveals several weak points in the Geneva mechanism. For instance, for each rotation of the Geneva (slotted) gear the drive shaft must make one complete rotation. Thus for very high speeds, the drive shaft may start to vibrate. Here, it can be seen that the Geneva Wheel is vibrating while the drive wheel makes it rotation. Another problem is wear, which is centralized at the drive pin. Finally, the designer has no control over the acceleration the Geneva mechanism will produce. Also, the Geneva mechanism will always go through a small backlash, which stops the slotted gear. This backlash prevents controlled exact motion.