backshell

A backshell is a cylindrical metal or plastic sleeve that screws onto the rear threads of an electrical connector to seal and strain-relieve the cable or wire bundle emerging from it. It bridges the connector body to the cable jacket, protecting the termination point from mechanical stress, moisture, and vibration. The backshell is part of the complete connector assembly, not optional hardware, and its selection depends on cable diameter, connector type, and operating environment.

Backshells come in several material grades and thread patterns. Aluminum backshells are standard for general industrial use and offer light weight with adequate EMI shielding. Stainless steel variants resist corrosion in marine and chemical environments. Plastic backshells appear in some low-voltage signal applications. Thread sizes follow the connector standard: MIL-DTL-38999 circular connectors typically use metric fine threads (M20.5 x 0.75 or M27 x 1.0), while rectangular connectors may use UNC 37/64-32 threads. The backshell must match both the connector pattern and the cable diameter it will grip.

The backshell serves three practical functions. First, it holds the cable firmly in compression, preventing rotation and axial pull that could damage internal terminations. Second, it provides strain relief, allowing cable bending radius to increase gradually from the stiff connector body to the flexible cable jacket. Third, in shielded connector applications, the backshell creates a grounding continuity path, often through a bonding strap or shell, that maintains EMI shielding integrity from cable braid to connector shell. Without proper backshell installation, connector failures from vibration fatigue or water ingress increase sharply.

Backshells fail when they are under-tightened, over-tightened, or incorrectly sized. Under-tightening allows cable motion and water seepage around the cable gland entry. Over-tightening strips the internal threads or crushes the cable jacket. Incorrect size mismatch, such as using a backshell designed for 0.625-inch cable on a 0.75-inch cable, results in either poor sealing or incomplete engagement. Corrosion of aluminum backshells in saltwater or chemical environments can seize threads or reduce grounding conductivity. Metal backshells must sometimes be cadmium or nickel plated to resist corrosion in aerospace or marine service.

Backshells are ordered as part of a complete connector kit, specified by standard number and part code. The term 'backshell' does not always appear in consumer product documentation; instead you may see references to 'cable clamp', 'coupling nut', or 'boot', though these are technically separate components. In military and aerospace procurement, backshell selection is mandated by drawing and standard, not left to field choice. Field technicians should always verify thread pattern and cable fit before installation to avoid assembly delays or rework.