Common pipe fittings for liquid (or steam) have internal threads on the inside surface of the pipe-receiving end of the fitting head, and external threads on the outside surface of the matching end of said pipe. Then, said threaded pipe end is screwed into said pipe fitting with the engagement of said internal thread of the said pipe and the external thread of said pipe. Since any thread has depth, the mechanical strength of said threaded end will be greatly weakened. Since the thread on the end of pipe fittings are typically coarsely finished, screwing a liquid (or steam) pipe into a fitting is met with resistance. A common wrench is not useful to tighten common pipe fittings. Due to the smooth and round pipe body. Therefore, a pipe wrench with specially made chops should be used for biting on the pipe body. In doing so, the pipe can be screwed into the fitting. After assembling, scars and deformations caused by said wrench chops remain on the pipe body.
Furthermore, when the assembled pipe, used for a considerable time is disassembled, the process is generally more difficult than assembling because, both threaded portions of the pipe and of the fitting bind tightly together due to rust, deformation and gluing etc. Therefore, after disassembling, the scars and deformations caused by said wrench chops will be made more serious. At the same time, it is possible the whole pipe will be broken at the threaded portion.
Overtime, different piping materials have been used such as: wrought iron (zinc plated), plastics (hard PVC), brass, brass with plastic coating, wrought iron with inner plastic coating, copper with outer nickel-plating and alloy. Even if the external surface of the pipes is plated or coated, they are still not suitable to be bitten by pipe wrench because none of the plating or coating layers can bear the terrible biting of the wrench chops and any layer will be destroyed after the biting of wrench chops. Even copper pipes or alloy pipes with hardnesses lower than the hardness of wrench chops, will exhibit ugly scars and deformations on the pipe surface after a pipe wrench is used on the pipes.
As a result, many fittings that do not match by threading have been developed. One such pipe is provided with an integral flange on the pipe end. This flange method is only suitable for pipes made of soft materials such as brass. Another such pipe utilizes a plastic pressing ring located on the external surface of the pipe end. This method is usually used on indoor drain systems. Though the above mentioned methods have eliminated threads from pipe ends, the connection between the pipe and the fitting is still achieved by threading. The difference lies in that the external thread is made on the external surface of the fitting, and the internal thread is made on the inner surface of a casing head which sleeved on the pipe. Although these methods can solve the problem of pipe surface being damaged by wrench chops, but they have limited applications since they are not suitable for wrought iron pipes, and because the construction of threaded casing heads is complex. Furthermore, the process for flanging and screwing are troublesome.
Another solution was to replace the internal thread of prior art fittings with an internal teethed lock ring so that pipe without end threads can be directly inserted through the internal teethed lock ring integrated on the fitting and can be kept in position. Once it is in position, it is impossible to disassemble the pipe. But in practice, pipe pre-assembly is absolutely necessary. If an assembled set can not be disassembled, it means there can be no pre-assembly and no repair work is permissible. Therefore, nobody uses this method nowadays.