Mechanical Coupling Elements and Joining Techniques

Posted on Nov 1, 2024 in Technology

Mechanical Coupling Elements and Joining Techniques

Gears

Pinion Gear

A normal 13-tooth pinion gear meshes with another gear whose radius is infinite. This system’s function is reversible.

  • If the pinion is fixed and the rack moves, the pinion rotates without moving the rack.
  • If the pinion rotates while the rack is fixed, the rack will move.

Screw-Nut

The screw-nut mechanism is used to turn the screw while preventing the nut from turning. The concept of a screw propeller is derived from a ramp wrapped around a cylinder.

Applications of Screws and Nuts

Screws and nuts have various applications, including moving loads, holding objects, and precise positioning.

Cams and Eccentrics

Cam

A cam is a metal or plastic part with a specific shape, mounted on an axis to move a rod or follower. There are linear and rotary cam types.

Eccentric

An eccentric is a disc or cylinder whose axis of rotation does not coincide with its geometric center. The distance (d) between the disc’s center and the axis is called eccentricity.

Crank-Piston Mechanism

This mechanism can transform circular motion into linear motion, and vice versa. Turning the crank moves the piston, and pushing or pulling the piston can rotate the crank. This linear-to-circular motion transformation is commonly used in internal combustion engines.

Ratchets

Ratchets prevent an axis from rotating in one direction while allowing rotation in the other. They consist of a sprocket and a pawl that engages the sprocket’s teeth, either through a spring or its own weight.

Ratchets can be:

  • Reversible: Allowing the blocking direction to be changed.
  • Non-reversible: Always blocking rotation in the same direction.

Freewheel

A freewheel, like in a motor starter, transmits motion from the starter to the engine. Once the engine is running and its speed exceeds the starter’s, the freewheel disengages to prevent damage to the starter motor at high revs.

Mechanical Coupling Elements: Removable and Fixed

Removable Joints

These parts can be easily separated without damage.

Fixed or Non-Removable Joints

These joints are not intended to be disassembled during the machine’s lifespan. Removing the parts would require breaking the joint.

Disassembly Joints: Threaded Elements

Types:

  • Pin or Set Screws: Screw threads into a nut.
  • Union Screw: No nut is used.
  • Prisoners: Screws threaded onto one piece and housed in a hole in the other.
  • Asparagus: Threaded rods at both ends with an unthreaded central part, often used in large, expensive metal parts.
  • Bolts: Used to join pieces.
  • Tapping Screws: Used for thin metal parts.
  • Wood Screws or Bolts: For joining wood pieces.

Other Joining Elements

  • Cylindrical Pins: Hold machine elements together and transmit large forces.
  • Keys: Prismatic steel parts placed between two pieces to join them and transmit effort. Can be longitudinal or transverse.
  • Tongue and Groove: Allows longitudinal displacement of a piece relative to the part holding the tongue.
  • Guides: Allow movement of one piece relative to another.
  • Button and Buttonhole, Velcro: Plastic strips that adhere to each other.
  • Brackets: Two metal pieces that, when pressed together, hold two items, like a chassis.

Fixed Joints

Rivets

Cylindrical rods with a head at one end, used to permanently join thin sheets or pieces.

Riveting Process:

  1. Cut the rivet to a length equal to the combined thickness of the parts to be joined.
  2. Make a hole in the parts with a diameter slightly larger than the rivet.
  3. Form a head on the opposite side, either manually or with a riveting tool.

Rivets (Large)

Rivets with a diameter greater than 10mm. The riveting process is done hot.

Pressure Fit

Pressure fit is used when the shaft is larger than the hole it will be placed in.

Bonding

Joining two surfaces by interposing a layer of a material with high adhesion power.

Adhesives can be:

  • Natural: Latex, gum arabic, dextrin.
  • Synthetic: Urea-formaldehyde, epoxy resins, vinyl resins, and polyisocyanates.

Fixed Joints by Welding

Cold Welding

Often composed of a metallic powder and plastic resin. Can join different metal parts and materials.

Hot Welding

Permanent joining of two metals using heat to melt the materials in the joint area until they fuse.

Types of Welding:

  • Soft: Heterogeneous welding at temperatures around 400°C.
  • Strong: Heterogeneous welding at temperatures around 1000°C.
  • Oxyacetylene: Heat is generated by a torch flame combining acetylene and oxygen. Butane can also be used, with a maximum temperature of 3000°C.
  • Electric: Used to join steel parts by creating an electric arc that melts a portion of the parts to be joined.

Electric Welding Methods:

  • Arc Welding: For large, thick parts.
  • Spot Welding: For joining thin sheets by applying pressure between two electrodes.

Weld Types:

  • Heterogeneous: Solder material is different from the parts being joined.
  • Homogeneous: Solder material is similar to the parts being joined.