A wall plug (UK English) or screw anchor (US English), commonly called a dowel in US English, is a fiber or plastic (originally wood) dowel used to enable the attachment of a screw in material that is porous or brittle or that would otherwise not support the weight of the object attached with the screw. It is a type of fastener that for example allows screws to be fitted into masonry walls.
There are many forms of wall plug, but the most common principle is to use a tapered tube of soft material, such as plastic. This is inserted loosely into a drilled hole, then a screw is tightened into the centre. The screwing action wedges the plug firmly in place, the soft material conforming tightly to the masonry. Such fasteners can attach one object to another in situations where screws, nails, adhesives, or other simple fasteners are either impractical or ineffective. Different types have different levels of strength and can be used on different types of surfaces.
Before commercial wall plugs, fixings were made to brick or masonry walls by chiselling a groove into a soft mortar joint, hammering in a crude wooden plug and then attaching to the wooden plug. This was time consuming and required a large hole, thus more patching of the wall afterwards. It also limited the holes' location to the mortar joints.
The original wall plug was invented by John Joseph Rawlings in 1911, and marketed under the name Rawlplug. These plugs became popular after the First World War, when a demand for retro-fitting existing buildings with new electric lighting coincided with a shortage of labour, encouraging many new labour-saving innovations in the building trade. Rawlplug gained their prominence from their adoption in the British Museum.
Some early plugs, including Rawlings', were also made of soft metal strips, punched with raised perforations for grip.
Fibre and resin mixes
On crumbling walls it may be difficult to drill a clean hole, or the force of the expanding plug may be enough to cause cracking. In these cases, a hardening liquid or putty mixture may be used instead.
One of the first of these mixtures was produced by Rawlplug and was composed of dry white asbestos fibres, sold loose in a tin. The user wetted some into a ball (usually by spitting on them) and pushed this plug of putty into the hole. A small tamper and spike was supplied with the kit. This putty worked very well, but the hazard of the asbestos fibres means that the product is no longer available. However, another way to fix wall plugs is accomplished by the application of a cotton weaved pad which has been impregnated with a special formulated gypsum to bond into the wall. The pad is wetted and wrapped around the wall plug, and the two are inserted into the hole; after a short time it hardens and a strong bond is achieved and the wall fitting can be applied. It is used in combination with wall plugs in masonry, ceramic, wood and plasterboard walls.
There are special fasteners for hollow walls such as plasterboard partitions, which are not thick enough to take wall plugs. The fasteners have toggle arms, which either drop into place or expand within the cavity, and a fixing screw which is threaded through them.
These are often called "lead anchors" or "plastic anchors", and are used as follows:
- Drill a hole the same size as the anchor body.
- Push the anchor to its full depth in the hole.
- Insert the screw through the item being fastened and screw it into the anchor tightly.
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