Screw is usually a cylindrical object or an object with a round hole with uneven spiral linear stripe on the surface. The cylinder object is called "screw", while the object with a round hole "nut" and the spiral linear stripe “thread”. The thread of a screw is called "external thread", and a screw consists of external thread and “axle” sections. The thread of a nut is called "internal thread". The nut and screw with thread matching each other together form a “screw pair”.
With the screw mechanism, the rotary motion can be turned into linear motion and moment can be turned into linear force. By this force acting force transmission mechanism, the acting force can be enlarged. A small rotary force (moment) applied on the axle can be converted into a large axial force. Thread pitch is the axial distance between two adjacent threads. The smaller the thread pitch is, the larger the mechanical benefit is, i.e. the larger the output force to input force ratio is.
This machinery demonstrating the screw operation mechanism comprises a screw, a nut and a crank, of which the nut is fixed on the hander. When you turn the crank on the right side of the machine, the screw will perform rotary motion along the thread and go through the nut in a linear manner along the axle.
Screw was invented at the latest among the six kinds of simple machinery and made its first appearance in ancient Greece period. Historians think its inventor might be Archimedes or Archytas of Taranto (428-347BC).
With the self-locking property of screw (detailed explanation will be provided later), a threaded fastener can be used to fasten several objects together via screw, e.g. wood screw, sheet metal screw, bolt and nut etc.
Self-locking property is a key property of screw in many application aspects, e.g. screw cap, vice, C-clamp and screw jack etc.
Screw can be used as linkage for power transmission in machinery, e.g. turbine, guide screw and ball screw etc. Due to its low efficiency, screw linkage is hardly used for transmission of high power. It is usually used for low power and intermittent transmission, e.g. position actuator. It can also be used for moving materials by the rotary motion of spiral surface around the rotary axle, e.g. the Archimedes screw, twist drill and screw conveyer.
With precise calibration screw, a micrometer can accurately measure the length.
As there is a large area of sliding contact surface between threads, most screw machinery has "self-locking property"-apply a moment on the axle and the axle will rotate. However, if an axial load force is applied on the axle, the axle will not rotate reversely.
This property is apparently different from the other simple machinery, e.g. lever is a machinery that can move bi-directionally; if the load force applied on the resistance point is too large, the lever will move toward the reverse direction and work on the applied force (applied force will work negatively). Most kinds of screw machinery are designed with self-locking properties and will stay still as long as no moment is applied on the axle. However, some kinds of screw machinery with long thread pitch and good lubrication are not provided with self-locking functions.
The Makeblock robotic gripper used this screw mechanism, which is perfect for all your gripping needs. Made from a heavy duty but lightweight PVC, it comes with four standard M4 thread holes on the bottom for easy assembly. And the main features are Self-locking fingers, Extra anti-slip material on the inside of two fingers and A built-in resettable fuse protects the N20 screw motor from short-circuits and over-current
Check the showcase video here: