Helical Screw 3D model

Helical gears are one type of cylindrical gears with slanted tooth traces. Compared to spur gears, they have a larger contact ratio and excel in quietness and less vibration, and able to transmit large force. A pair of helical gears have the same helix angle but the helix hand is the opposite. A screw and a bolt (see Differentiation between bolt and screw below) are similar types of fasteners typically made of metal and characterized by a helical ridge, known as a male thread (external thread). Screws and bolts are used to fasten materials by the engagement of the screw thread with a similar female thread (internal thread) in the matching part.

Screws are often self-threading (also known as self-tapping) where the thread cuts into the material when the screw is turned, creating an internal thread that helps pull fastened materials together and prevent pull-out. There are many screws for a variety of materials; those commonly fastened by screws include wood, sheet metal, and plastic.