Machine design Basics II - Screws, Fasteners, Springs, Belt

Explore lead screws, their design and specifications, and how rotation converts to translation, with square and Acme threads, backlash, and applications in lathe, 3D printer, and jack systems.

Explore lead screw specifications, including square and acme threads, pitch diameter, minor and major diameters, helix and thread angles, and pitch measurements for single and multi-start, right and left-handed designs.

Understand the self locking condition for power screws, where the friction coefficient exceeds the tangent of the lead angle, and how friction influences the efficiency of raising loads.

Analyze stresses in lead screws, including torque-induced shear and axial stresses at the root diameter, and bending stresses in the threads treated as a cantilever, using pitch and turns.

Explain how preload sustains voltage joint integrity and how loss mechanisms—improper preload calculation, vibrations, creep relaxation, and corrosion—lead to loosening, and outline design strategies to prevent it.

Explore bending stresses in torsion springs, where inner elements bear the highest stress, and apply Ki factor for spring index to compute sigma = Ki 32 F L /(pi d^3).

Describe the design equations and notations for a belt drive, including driving rpm N1, driven N2, diameters d and D, tensions F1 and F2, and angle of wrap.

Perform a radial and tangential force analysis for belting theory, using a small-arc approximation to split F and F+df into tangential and radial components, and derive equilibrium with friction mu.

Derives the belting model equations, solving df/dtheta - mu f = - mu m r^2 omega^2, yielding F1, F2 with hoop tension FC via boundary conditions at the wrap angle.

Describe the belt drive design procedure from Shigley's design book, including center distance, pulley sizes, power and speed, belt material selection, tension calculations, and friction checks to prevent slip.

Maintaining the belt's initial tension is essential for sustainable torque transmission; wear and elongation cause slip and sag, which idler pulleys or spring-loaded tensioners and adjusting center-to-center distance help prevent.

Explore the design procedure for v-belt drives, calculating pitch length and center distance, selecting cross sections A–E and pulley sizes from standard tables, and applying power and tension factors.

Select belt class and compute pitch length, inner length, center distance, belt velocity, and allowable power using k1 and k2 for a 5 kW drive from 1000 to 800 rpm.