Mechanical Joints and Fasteners: A Comprehensive Overview
Mechanical Joints and Fasteners
Rack and Pinion Mechanism
The rack and pinion mechanism is commonly used in car steering systems and subway doors. The pinion is a sprocket that rotates on its shaft, providing the driving force. The rack is a jagged piece that moves linearly in either direction according to the pinion’s rotation.
Handle with Screw and Nut
A crank describes a circle of radius r. The screw, also called a spindle, moves in unison with the crank and also describes a circular motion. The nut is a fixed part; the screw slides along the helical path of the threads inside it. For each complete revolution of the crank, the screw moves longitudinally a distance equal to the thread’s pitch.
Cams and Eccentrics
A cam, widely used in internal combustion engines, is a pear-shaped or irregularly shaped disc that relies on a mobile element called a rod or pushrod.
Cam Types
- Cam Corazon: Provides uniform motion to the rod until it reaches its upper limit.
- Cam Morin: The rod’s movement is uniformly accelerated in the first half of its stroke and uniformly decelerated in the second half.
Mechanism: Endless Screw-Crown (Worm Gear)
The worm is a cylindrical piece with one or more helical windings. The crown is a helical gear whose teeth’s angle of inclination coincides with the worm’s threads. The worm typically acts as the driving element, and the crown as the driven element.
Types of Joints
- Demountable Joints: Threads, pins, and tabs.
- Fixed Joints: Riveted (tubular rivet) and welded (heterogeneous and homogeneous).
Threads (Roscas)
A thread is any mechanical element with a continuous helical channel built over a cylinder. If the helix is outside the cylinder, it’s a screw; if it’s inside, it’s a nut.
Types of Threads
- According to the fillet shape: Triangular, trapezoidal, square, and round threads.
- According to the number of starts: Single-start or multi-start threads.
- According to the direction of rotation: Right-hand and left-hand threads.
Classes of Screws
- Bolts: Cylindrical screws with threads that allow attaching a nut or threading into a tapped hole.
- Studs: Headless bolts threaded along their entire length, attached to two screws (one at each end) or a nut and a tapped hole.
- Tirofondos (Lag Screws): Long screws, generally with a frustoconical body, used in woodworking without a nut.
Head Types
Hexagonal, round, cylindrical, or countersunk.
Dowel and Tongue (Key and Keyway)
A key is a prism used to connect two mechanical parts so they turn together. Axial dowel rods of rectangular section steel are used to connect a shaft to the elements embedded in it. Transverse dowels or wedges are used to attach elements that rotate on a shaft.
Dowel Types
Studded, thrust force, pin heel, flat, flat heel, concave, concave to heel.
A tongue (spline) is a special kind of keyway. The fundamental difference between pins and tabs is their shape and the faces used for adjustment. Pins are conical and adjust using their upper and lower faces, while tabs have a straight section and adjust using their lateral faces.
Pin
A pin is a rounded, elongated piece used as a support and to connect two pieces. It is usually subjected to shear stresses.
Pin Types
- Cylindrical pins: Used to position one piece over another, but not to fix it.
- Conical pins: Can be used as fasteners.
- Security pins: Used to immobilize parts where precise adjustment is unnecessary.
Riveted Joints
Rivets are cylindrical pieces fitted with a head and are deformed by compression.
Tubular Rivets
A tubular rivet is a variant of riveting, using a hollow rivet with similar characteristics to a solid rivet.
Welding
Welding is the stable joining of two parts obtained by applying heat.
Types of Welding
- Heterogeneous Welding: Soldering and brazing.
- Homogeneous Welding: Fusion welding (with liquid metal, torch, electric) and pressure welding (forge, electric points).
Heterogeneous Welding
Uses a filler material different from the base metal.
Homogeneous Welding
Does not use filler material, or if it does, its composition is identical to that of the base metal.
Heterogeneous Welding Examples
- Soft Soldering (S. Blanda): Joins metal parts using a metal or alloy with a low melting point (e.g., tin joints, galvanized steel parts, brass, lead pipes, and electrical connections).
- Hard Soldering (S. Fuerte): Joins metal parts using a metal with a high melting point (e.g., materials with high melting points, steel, cast iron, and bronzes).