Screw Selection

Screw Selection


Screws are amazingly versatile and powerful fasteners. The basic concept is a threaded cylinder that is used for holding all sorts of things together, including wood, plastic and metal. Different types of screws have been developed to maximize efficacy for particular applications. The various types of screws can be categorized according to driving methods, head shape, type of threads and the material and finish used to make the screw.

Drive Types

 

  • Slotted: Slotted screw heads are perhaps the oldest and most common variety; a linear slot in the head accepts a standard screwdriver, which is also called a flat-head screwdriver.
  • Phillips: The Phillips head is an improvement to the slotted screw; cross-shaped grooves that do not extend to the edge of the screw head accept a Phillips-head screwdriver. These heads, which have a slight circular shape where the slots cross, provide a larger mating surface between the screw and the screwdriver, which minimizes wear and helps prevent slipping. Although many people refer to any screw head that has cross-shaped grooves as a Phillips head, there are other types of screws that have similar slots. A cross head features two full-length slots that cross, which allows a flat-head screwdriver to be used and makes the screw usable even if one slot has been worn away. Another type that has two short slots that cross but does not have a small circular shape at the center is known as a Prince, Frearson or Reed head.
  • Hex: There are two types of screws that have hex heads — one that has a hexagonal recession in the head and one that does not have any grooves or slots at all. A hex socket screw or Allen screw requires the use of an Allen wrench, which has a hexagonal shaft that is inserted into the recession in the screw head. The other type of hex screw has an entire head is hexagonally shaped. A wrench set, socket wrench or adjustable wrench is required for driving hex screws.
  • Square: Also know as a Robertson screw head, it has a square indentation to minimize slipping. It also requires a special driver for tightening and loosening.
  • One way: One-way heads are a variation on the slotted screw. They can be tightened with a standard screwdriver, but they are tamper-resistant because they require special tools for removal.
  • Torx: A torx head has a recession in the shape of a six-pointed star to provide even more surface area for driving. It requires the less-common torx screwdriver and therefore can be considered to be tamper-resistant.
  • Others: There are many other, less-common driving methods that have been developed, with types of screws having heads that feature recessions of various shapes. Among these are polydrive, double hex, triple square and tri-wing screws.

Head Types

 

  • Pan: This is the standard screw head profile with an average diameter and average height.
  • Button: This is similar to a pan head but has a top that is more curved.
  • Round: A round head is even more pronounced than a button head.
  • Flat: This has a flat top and a tapered underside that is intended to be driven into a countersunk hole.
  • Oval: The underside is tapered like a flat-head screw, but it has a rounded top.
  • Truss: This is a large-diameter heads with a low profile.
  • Fillister: This has a thick profile with a slightly rounded top.
  • Others: As with driving methods, there are many less-common varieties as well.

Thread Varieties

 

  • Wood: The threads on wood screws usually are coarse and deep to help them grab the wood.
  • Machine: Machine screws have finer threads than wood screws. They are designed to be used in conjunction with a nut or tapped hole.
  • Sheet metal: These screws usually are short and have coarse threads that are designed to grab onto relatively thin sheet metal.
  • High-low: These screws have two sets of threads with alternating heights. High-low screws are specifically designed for certain plastics and other low-density materials.
  • Self-tapping: Self-tapping or thread-forming screws feature threads that are designed to tap their own holes. These work well in softer materials such as wood and plastic but are not suitable for harder materials.