A screw anchor is a piece of hardware that holds a screw in place. This hardware is usually used when the surface into which the screw is drilled is too thin, too light or too fragile to provide a secure grip for the screw threads. Anchors come in a wide variety of types, materials and price points and may be used for both light- and heavy-duty applications.
Screw anchors generally serve the same type of purpose as a nut attached to the back of a bolt or the back of an earring attached to an earring post. They keep the screw from coming out of the hole it had been screwed into. When the back part of the screw is located in a place that can't be reached, such as inside a wall, there is no way to use a nut or similar hardware piece. Screw anchors, however, can be applied from the outside of the wall or other surface.
Screw anchors are most commonly used when hanging items on walls, so they are often called wall anchors or wall plugs. The drywall used in most modern housing often crumbles too easily for the shallow threads of most screws to be effective. The addition of a screw anchor provides a firmer grip for the threads.
Some anchor types called "expansion anchors" also provide opposite-surface pressure that helps hold the screw in place. When this type of screw anchor is inserted, part of the anchor protrudes through the back side of the drywall or other surface. This portion of the anchor expands in one of several ways so it is larger than the hole through which it was placed. This keeps it from slipping back through the hole.
When a picture or other item is hung from the screw, expansion anchors also provide a surface area across which the pressure caused by the weight of the picture is distributed. This prevents the weight of the picture or other hanging from pulling the screw back through the hole. Expansion anchors may also be used on boats, in floors or in many other situations.
Self-drilling anchors are usually made of plastic or metal. They have a slotted head and taper to a point and can be screwed directly into the wall. These hollow anchors are formed with threads on the inside and on the outside. Once the anchor is in the wall, the screw can be threaded into the inner threads, creating a secure connection. As it is inserted, the size and pressure of the screw causes the end of the anchor inside the wall to expand so that it is larger than the hole.
When using other types of screw anchors, the user must first drill a hole appropriate to the anchor size. The anchor is inserted through the hole and either expands automatically or expands when a screw is threaded into it. Either type of screw anchor is usually sold based on the type of substrate into which it will be screwed and the weight of the item to be secured.