Screws and Threads
Screws
Screws convert rotation into linear motion. They act as jacks that generate substantial axial forces. An external driver turns the screw or nut, often through a worm gear for high reduction. Plain screws transmit load through rubbing contact between screw and nut. They offer simplicity and robustness. Thread profiles are square or trapezoidal (acme). Plain screws have high friction. Larger screws can use hydrostatic pads on the nut to reduce friction, though this costs more and suits specialized applications like mill roll adjustment screws. Ball screws use balls rolling between grooves in the screw and nut. They produce the lowest friction. They power positioning screws in automated machines. The nut recirculates the balls. Ball screws carry less load than plain screws because contact stresses limit capacity.
Mechanical efficiencies of screws: plain screws run about 30%-50%, with larger diameters tending lower. Roller screws reach 65%-85%. Ball screws reach 75%-90%.
https://www.nationalbronze.com/Tribology%20Handbook%20(2nd%20Edition).pdf
Related
- Power Transmission - screws transmit motion and force
- Gears - worm gears paired with lead screws
- Fasteners - threaded fasteners vs power screws
- Tribology - friction in plain and ball screws
- Lubrication - reducing friction in screw threads
- Bearings - rolling elements in ball screws
- Material Selection - screw and nut material choices