Copyright ©2001 by Scott Greaves
The screw is an elegant and ingenious device, whose concept has been a part of many mechanical inventions over the past two thousand years. Think back to your third-grade physics lesson and remember a simple machine called an inclined plane. The inclined plane follows the theory that you expend less effort sliding an object up a ramp than by picking the object straight up from the ground. This same theory allows a wedge to split a piece of firewood or to lift one corner of your kitchen table. A screw is an inclined plane wrapped around a shaft. When the shaft is turned, the plane, in the form of a screw thread, exerts the same kind of force as a wedge.
The Greek mathematician Archimedes (287 to 212 BC) is credited with discovering the screw because he was the first to discover a use for it. Archimedes used the screw to lift water to a higher level. The circular motion of the screw would trap water in the threads and would continue to raise the water higher as the screw turned. Many areas that were considered arid were brought under cultivation as a result of Archimedes' screw. A similar device is still used today to transport some dry materials, such as grain.
Another early use for a screw was to gradually exert pressure in a press, for squeezing the juice from grapes and the oil from olives. Exerting pressure with the use of a screw is also the concept behind many types of clamps, and even some car jacks. Meat grinders use a screw mechanism to force the meat through the grinding plate. Your jar of peanut butter and bottle of soda both have screw-on lids. The propeller on ships and airplanes are another example of the inclined plane wrapped around a shaft.
As a fastener the screw is unsurpassed in its ability to hold. Depending on the kind of thread, there is many times more surface area on a screw as there is on an equivalent nail. But the increased surface area of the screw is only the beginning of its holding ability. Because the threads are an inclined plane, the force to pull a screw out is shifted into a lateral or circular motion, essentially deflecting the force trying to pull the screw out. When you pair the screw, in the form of a bolt, with an accompanying nut at the far end, the ability to compress and hold the material between the two ends is increased dramatically.
With all of this great history, you'd think there would be an easy way to figure out what kind of screw you need for a particular project. The screws we use in our homes today are categorized by their use, the shape of the head, length, and the type of driver used. There is also a distinction made between screws and bolts, although the practical differences are negligible at times. An easy way to remember the difference is that bolts are used with a nut, and screws do not require a nut. Here are some basic types of screws for use around the home:
Wood Screw - These are tapered screws with a fairly aggressive - wide threads at a steep angle - thread. They usually have a smooth, non-threaded shank just beneath the head that varies in length depending on the thickness of the piece of wood it will be going through first. The threaded portion of the screw is intended to be inserted into the second piece of wood, drawing it up to and tight with the first piece of wood. The first piece of wood would be pre-drilled with a larger hole to accommodate the smooth shank of the screw, and the second piece of wood would be pre-drilled with a smaller diameter hole so the threads will have something to bite into. They are usually seen with either a flat head or a round head.
Sheet Metal Screw - These are a tapered screw with a finer thread pattern to increase their Mechanical Advantage (MA). A higher MA would result from more threads per inch and a less pronounced thread profile, requiring less effort to turn the screw into the work surface. Because they are intended for use on sheet metal, a much harder material than wood, they require the higher MA. The shank is usually threaded all the way up to the head of the screw. These are often configured with a self-drilling tip, which is a screw tip shaped in a way that the screw itself will cut into the metal surface, essentially pre-drilling its own hole. They are usually seen with either a pan head, a round head or a hex head.
Machine Screw - These are straight screws with no taper in the shank. They usually have a finer thread pattern than even the sheet metal screw for increased MA. They are used when attaching an item to another piece that has already been drilled and threaded. Machine screws can be used with a nut, in which case they would qualify as a small bolt. These are used to connect parts in a machine or mechanism. They are usually seen with a round head or a flat head.
Lag Screw - These resemble a large wood screw in that they have a smooth non-threaded shank for a space below the head and the threaded part is tapered. The smallest lag screw is about the size of the largest wood screw, and there are lag screws well over a foot long. Lag screws are often used to connect timbers or framing members. If you are adding a deck to the back of your house, lag screws are often used to attach the deck to the wall of your house. They are usually seen with a hex head.
Drywall Screw - These are special-use screws that seem to be used for everything. Designed with a thinner shank and a more pronounced thread profile, they are intended to attach gypsum board (drywall, Sheetrock) to interior walls. They have a more aggressive thread pattern, because MA is of little concern when driving these with a modern screw gun. Some of these screws are designed with a higher thread count for times when the gypsum board is being attached to a harder framing member, such as steel studs. They are usually seen with a bugle head, similar to a flat head, that is designed to not break the paper surface of the board, while still sinking the head below the surface.
There are numerous other specialty screws designed for particular purposes. The types of screws shown above are just the most common. To determine what kind of screw you need for a project, look at what is required by the project: Wood screws for connecting wood pieces, sheet metal screws when sheet metal is involved, etc. Next determine the length needed. It is a more common mistake to use a screw that is too long instead of one that is too short. When attaching two thin pieces of sheet metal, a screw longer than a half an inch is unnecessary. But when attaching two pieces of wood that are each two inches thick, a three-and-a-half-inch wood screw would be required.
The type of head on the screw is also important. With wood screws, if it is important that the head be set below the surface of the wood, a flat head is needed. A round head would remain above the surface of the wood. A hex head is useful in sheet metal screws and in lag screws, but for different reasons. Hex head sheet metal screws are used with a socket bit in a screw gun, while lag screws are driven with a wrench. A pan head, one that will remain above the surface but is flat across the top, is often used in sheet metal screws. The bugle head, mentioned above in connection with drywall screws, is a flat head with sides that gently curve down to the diameter of the shank.
Last you will need to determine the type of driver to be used. The old-fashioned slot screw used to be the standard in the industry. Now the Phillips, resembling a cross, is the most widely used, because it is easier to hold the driver in the drive recess. There are a number of recent innovations in driver styles, of which the square drive, designed to be even more secure than the Phillips, is the most popular. Torx drivers, kind of a star shape, are also common. Be sure you have the kind of driver needed for the screw you are buying.
Depending on the use you intend for your screw, there are various coatings available and different metals. Screws for exterior uses are coated or galvanized to be resistant to corrosion. Drywall screws often have a black oxide coating to prevent rust, so they won't discolor a painted wall. Brass screws are used for their decorative appearance, as are nickel-plated screws. There are even stainless steel and aluminum screws.
One of the drawbacks of screws in the construction trades had been the length of time and the effort it took to drive the screw. Power screwdrivers and screw guns practically eliminated the problem, and the advent recently of auto-feed screw guns makes the process almost as fast as a nail gun. Because of their exceptional holding power, screws are particularly useful in fastening decking in new construction, and they have become the standard fastener for gypsum board. Special screws with a large washer head have become useful in stabilizing old plaster walls where the plaster may have come loose from the lath. It seems as though there is a screw made for whatever old-home repair need you may have.
The screw has helped irrigate arid lands, advanced the wine-making and olive oil industries in ancient times, and was an integral part of early printing presses. The Industrial Revolution wouldn't have happened if the technology for cheaply and accurately making screws hadn't led the way. In recent times screws transport grain, propel ships and airplanes, and can jack up your car. But the lowly screw is best known to old house owners as the most secure type of mechanical fastener available for construction and home repairs. What would Archimedes think of his simple inclined plane wound around a shaft if he could see it today?