Mechanical Elements: Motion Transformers and Bonding Systems
Mechanical Elements: Motion Transformers
A. Rack and Pinion
A rack and pinion system consists of a circular gear (pinion) meshing with a linear gear (rack). Depending on which component is the driver and whether they are fixed or mobile, we have several applications:
- Pinion rotates, rack fixed: The pinion translates along the rack. Example: Lathes.
- Rack moves, pinion fixed: The pinion rotates. Example: Former mechanical calculators.
- Pinion rotates and translates: The rack moves. Example: Automobile steering and automatic garage doors.
B. Screw and Nut
The most common application involves turning a screw while preventing the nut from rotating.
The helix concept involves wrapping an inclined plane (ramp) around a cylinder. This ramp-like structure forms the screw thread.
Applications of screws and nuts, from a motion transformation perspective:
- Moving loads and holding objects: Car jacks, vises.
- Precise positioning of objects.
C. Eccentric and Cam
These elements transform the circular motion of a shaft into reciprocating linear or circular motion.
Eccentric: A disc or cylinder whose axis of rotation does not coincide with its geometric center. The distance between the center of the disc and the shaft is called eccentricity.
Cam: A shaped piece of metal or plastic attached to a shaft. The cam’s movement displaces a rod or follower. There are two types of followers:
- Linear Cams: Limited applications.
- Rotating Cams: The maximum displacement of the follower is known as the cam’s lift. The follower’s return can be achieved through gravity or a spring. Cams are further classified by follower and cam shape:
Follower Shape:
- Peripheral: Causes reciprocating linear motion of the follower.
- Oscillating: Describes an alternating circular motion.
Cam Shape:
- Disc: Most common, used for opening and closing valves.
- Cylindrical: Rotation causes a determined axial movement of the rod.
- Face: Complex manufacturing.
D. Crank-Connecting Rod-Piston
This mechanism transforms circular motion into linear motion, and vice-versa. It consists of three main parts: the crank, connecting rod, and piston.
- Circular to Linear: The crank (connected to a motor axle) drives the piston. Example: pumps
- Linear to Circular: The piston drives the crank. Example: Internal combustion engines.
In internal combustion engines, the piston’s movement, caused by the combustion of fuel, rotates the crankshaft. The crankshaft, supported by bearings, manages the movement of multiple pistons. It operates under bending and torsional stress, and balance weights are used for static and dynamic balancing.
E. Ratchet
Ratchets prevent a shaft from rotating in one direction while allowing rotation in the other. They consist of a toothed wheel and a pawl that engages with the teeth. The pawl’s position, influenced by a spring or gravity, controls the direction of rotation.
Ratchets can be reversible (locking direction can be changed) or irreversible (always block rotation in the same direction).
F. Freewheel
A freewheel allows a driveshaft to drive a secondary shaft in one direction but not the other. The shafts decouple when the resistance on the secondary shaft exceeds the driving force.
Common uses:
- Rear-wheel bicycles
- Car starters (decouples the starter motor from the engine once the engine is running)
Bonding Mechanical Elements
Key bonding systems:
- Removable: Parts can be separated easily without damage.
- Fixed/Non-Removable: Disassembly is not intended during the machine’s lifespan, or is prevented for safety or design reasons.
A. Removable Unions
Threaded Fasteners:
- Bolt and Nut: A screw that threads into a nut.
- Screw Connection: A screw threads directly into a tapped hole in a part.
- Set Screws/Grub Screws: Used to secure a component to a shaft.
- Studs: Threaded rods with threads on both ends.
- Bolts: Used to join multiple parts.
- Self-Tapping Screws: Create their own threads in thin metal parts.
- Wood Screws: Used to join wood or wood-based materials.
Other Removable Fasteners:
- Pins: Cylindrical pieces used to hold parts together.
- Keys: Prismatic steel parts inserted between two parts to transmit torque.
- Keys/Splines: Allow longitudinal holding of a part.
- Guides: Facilitate controlled movement between parts.
- Buttons and Buttonholes: Common in textiles.
- Velcro: Plastic strips that adhere to each other.
- Hooks: Metal pieces that interlock.
B. Fixed Unions
- Rivets: Cylindrical rods with a head used to permanently join thin sheets or parts.
- Interference Fit (Press Fit): The shaft diameter is slightly larger than the hole, creating a tight fit.
- Adhesive Bonding: Uses a high-adhesion material to bond surfaces.
- Welding: Joining metals using heat.
Types of Welding:
- Cold Welding: Uses a metal powder and resin mixture. Achieves high strength.
- Hot Welding: Melts the base materials together. Can be homogeneous (same base material) or heterogeneous (different base materials).
- Soft Soldering: Heterogeneous welding using a tin-lead alloy, typically below 400°C. Used for electronics and electrical cables.
- Brazing: Heterogeneous welding using brass or silver-brass filler rods, up to 1000°C.
- Oxyacetylene Welding: Uses a flame produced by burning acetylene and oxygen.
- Electric Welding: Uses an electric arc to melt the base materials.