Definition - What does Spigot Joint mean?
A spigot joint is a type of pipe fitting that is inserted into another pipe fitting. The spigot end typically has the same outer diameter as the pipe and is usually fitted into another larger-diameter joint called a bell. Together, these two elements form what is commonly known as a bell and spigot. The spigot end always fits into the bell end.
The joint is then completed by sealing it with a caulking compound or a compressible rubber ring to make it watertight. In situations where the spigot end needs to connect to a bell end that is too small for it to be inserted into, a reducer bushing can be used to scale down the size of the spigot end.
In the field, the spigot end of the pipe fitting is commonly referred to as the ‘male’ end, and the bell is the equivalent of the ‘female’ end.
Trenchlesspedia explains Spigot Joint
Figure 1: Illustration of a spigot joint inserted into a bell joint (source)
The first bell and spigot joint was introduced in 1785 by Thomas Simpson, an engineer in the Chelsea Water Company. At the time, the joint was caulked with hemp rope impregnated with pine resin and sealed in place with molten lead.
One of the main benefits of spigot joints is their flexibility. These joints allow for some degree of lateral movement and rotation, making them ideal for achieving deflections in ductile iron pipes. This flexibility is also useful for situations where pipes are installed on soils that are subject to movement. As the soil shifts (due to settlement or heaving), the joint rotates slightly, allowing stresses in the pipe to be relieved.
Why Do Some Spigot Joints Have Stripes?
The spigot ends of some ductile iron pipes are often painted with two thin lines or stripes. These stripes are applied during the pipe coating process. Each pipe is rotated as a paint applicator creates a double stripe on the circumference of the pipe’s exterior.
Figure 2: Dual stripes on the spigot end of a ductile iron pipe (source)
These stripes serve two main purposes. Firstly, the spigot stripes correspond to the depth of the bell. This helps the installer gauge the full insertion of the spigot end into the bell of an adjoining pipe.
If the stripes are parallel with the bell around the pipe's circumference, it is safe to insert the spigot end into the other pipe without risk of damaging the gaskets or other critical pipe components. When properly installed, the first line (or stripe) disappears, while the second stripe remains flush with the edge of the bell end of the pipe.
The spigot stripes also serve as a deflection check during assembly. Ductile iron pipes can be tilted in various directions during pipe alignment. The stripes help installers to gauge how much the pipe is deflected from the parallel direction. This, in turn, can help installers determine if a pipe is deflected beyond the safe allowable limit.