Insulators cord bushings such as bushing, strain relief, friction bushing, insulation insert, end sleeve belong to the group of insulation materials.
Bushing is: a plastic or rubber ring that is placed where the lamp cord passes the metal housing as extra insulation to avoid electric shock.
Strain relief is: a form of insulating detail where it must also prevent the lamp cord from being pulled loose from its bracket in, for example, the lamp holder and avoid electric shock.
Friction bushing is: may not really belong to this group but we place it here anyway. The friction bushing can be used between the lamp holder and the lamp base to prevent slipping between the surfaces. Can also be used as a soft spacer for delicate materials such as porcelain.
Insulation insert is: similar to the bushing but is used inside a lamp holder large and small. It is recommended with high priority as it is common for abrasions on a lamp cord to occur just inside a lamp holder.
End sleeve is: a soft fine insulation that can replace the insulation insert. End sleeve has its name as it is used as the end sleeve at the ends of lamp tubes such as pipe nipples.
Strain relief is an insulator with locking of lamp cord or power cable. Strain relief in brass or other metal has an insulator built-in. In the picture of the reading lamp you see a strain relief. The strain reliefs are available in the outer thread in different lengths. This is because, as the picture shows, giving distance to the casing. For mounting strain relief directly to the lamp holder, the lengths of 3-5 mm are preferred.