A screw holds things together. Unlike a nail it will hold hold things together while there are forces pulling them away from each other.
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"an inclined plane wrapped helically around an axis"
Screws are specialized for different purposes. There are wood screws or screws designed to go through and hold metal.
Most screws have a "head". This is the opposite end from the tip or end which will go into the materials being held together first. The head can be flat or rounded. And the head has a slot or notched shape in it where the blade of a screwdriver will be placed to connect the two in order to turn the screw so that it connects the materials. Once the screw is placed a washer, nut and/or bolt may be placed on the tip end to keep it from unscrewing.
The shape and size of the slot may vary. The two most common shapes are a straight groove, called a slotted screw, or an x shaped or 4 pointed star, called a Phillips head. The diameter of the shaft of the screw can vary, and the size of the head can also vary. Specialized screws may have a different type shape. For instance, some end in a loop. The loop can then be used to anchor or tie other things.
The overall size of the screw can also vary. The density of the threading has different standards. For instance, the threads for metal screws are much tighter than for wood screws.
- Screw Head Types - A Guide to What Type of Screw is This? A visual guide and description of different screwhead types by Tanner Bold & Nut Corp.
- Bolt Depot.com Fastener Type Chart with pictures of different types of screws, bolts, nuts and washers.
- Curious Inventor (tm) has a Tutorial on Choosing Screw Type, Size Strength.