So the screw in my reading lamp fell out and disappeared. I propped it up but from time to time it would let loose and crash down on my face. A trip to the hardware store was clearly in order.  Replacing the screw was easier said than done.  There are so many types. Different threads, head styles, lengths, drives and other criteria.  So what was that screw?

 

The Simple Screw

The screw is one of the six classical simple machines. The other five are the wheel and axle,

incline plane, wedge, lever, and pulley.

 

Lead and Pitch


 

 

 

 

 

 

 

 

 

 

 

 

A screw is defined as having a coarse or fine thread depending on the lead and the pitch. The lead is defined as the axial distance a screw travels in one complete revolution (360°) of the shaft. The lead determines the mechanical advantage of the screw; the smaller the lead, the greater the mechanical advantage. The pitch, however, is defined as the axial distance between the crests of adjacent threads.

Single start screws have a single helical thread wrapped round them and the lead and the pitch are equal.   Multiple start screws have several intertwining threads.  In multiple start screws the lead is equal to the pitch multiplied by the number of starts. Examples would be bottle screw caps or ball point pens where the linear motion for a given rotation is required.

 

Types of Thread

In threaded fasteners, there can be large amounts of friction. Fasteners usually have large 60° thread angles. This is to ensure the screw does not unwind or creep.  There are seven types of screw threads: 1.) V, 2) American National, 3) Whitworth or British Standard, 4) Square Thread, 5) Acme Thread, 6) Buttress Thread, and 7) Knuckle Thread.

 

Handedness of a Thread

Handedness is the ability of the helix of a screw’s thread to twist in two possible directions. Most screws are made so that when viewed from above, the screw shaft moves away from the viewer.  Thus the screw is tightened when turned in a clockwise direction. This is known as right-handedness.  Screws oriented in the opposite direction are known as left-handed screws

Right-handed screws became the standard because most people are right-handed. It is easier for a right-handed person to tighten a right-handed screw using a screw driver.  This is because it uses the supinator muscle of the arm rather than the weaker pronator muscle. But left-handed screws are used in some machines where the rotation of a shaft would cause a common right-handed nut to loosen rather than to tighten.  Examples include left-handed screws in pedals on bicycles as well as left-handed screws holding circular saw blades in place.  And it is said that the lids of coffins traditionally are held in place with left-handed screws.

 

 

 

 

 

 

Head Styles of Machine Screws


 

 

 

a pan head                                       d truss head

b binding head                                e countersunk/flat head

c round head                                   f oval head

 

There are many, many head styles on screws. Above are the common ones for machine screws.

So what is a machine screw you may wonder. It is a threaded fastener, either used with a nut or driven into a tapped hole, usually having a diameter of about 1/4 inch (6.4 mm) or less. The drive is either a slot or a Phillips or one of many other drives. Slotted or Phillips are the most common.

There are thousands of screw types. Without sending the screw from the reading lamp to a lab or calling the manufacturer the task of identifying it was too complex to be successful. But with the help of the hardware store clerk it was identified as an M4 by 6mm long Phillips truss head.

Craftech Industries, Inc. in Hudson NY makes machine screws in more than 100 different high performance plastics. For more information call our office.

 

 

 

 

 

 

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