How to Select Strain Reliefs

Image credit:  SAB North America | Fastenal | Digi-Key Corp.

Strain reliefs protect the lifeline of electrical products by absorbing the forces of push and pull that may be exerted on the flexible wires, cables, or tubing.

How Strain Reliefs Work

Strain reliefs are manufactured in a variety of sizes and shapes, and using various materials.  All strain reliefs serve one or more of three main purposes:

• Extra protection against overbending, which extends the life of the cable.  This protection is also useful against cutting or fraying by sharp edges of nearby equipment.

• Reinforcement of the cord's electrical connection.  These products keep the cord in a stationary position to prevent the conductors from twisting or pulling.

• Providing a liquid-tight seal on the cord's connection area.  This prevents moisture from entering and possibly damaging the component or machine the cord is connected to.

Strain reliefs can also be classified by style, depending on their flexibility or rigidity.  Dome style products are fitted to the cable at the connection point and prevent the cable from rubbing against a rough or sharp edge where the cable meets the device or electrical panel.  Flex style strain reliefs create a flexible bend at the connection point, protecting the cable against kinking or breakage.

Specifications

The GlobalSpec SpecSearch database contains information about numerous strain relief specifications, including type, material, and size.

Type

Strain relief types include cable bushings, cable glands, heat shrink tubing, romex connectors, sleeving, and wire ties. 

Cable Bushings

Bushings are hollow liners that encircle conductors or cable to provide electrical isolation. 

Cable Glands

Cable glands are also known as cord grips.  They serve to secure the end of a cable to a piece of equipment or device.  Cable glands include a provision for making an electrical connection from the cable to the device.

A variety of cable glands.  Image credit:  1-1111

This video provides an excellent overview of cable gland construction and installation.

Video credit:  Hawke International

Heat Shrink Tubing

Heat shrink tubing consists of a plastic tube that shrinks when heated.  When used with cables or wires, it can be used to bundle them together, to protect against abrasion, or seal entry points to electrical devices or bulkheads.

This video demonstrates the application of heat shrink tubing with a metal pipe and heat gun.

Video credit:  heatshrink

Romex Connectors

Romex connectors attach to romex cables to seal and support it as the cable enters a junction box or enclosure.

A romex connector.  Image credit:  Installation Parts Supply

Sleeving

Sleeving is a length of tube that fits over the outer diameter of a hose or a group of wires or cables.  Depending on the material and application, sleeving can provide protection from abrasion and/or high temperatures.

Wire Ties

As their name implies, wire ties are plastic devices designed to bundle wires, cables, or tubing.

Material

Strain relief products may be manufactured using different materials, each with its own unique characteristics.

• Aluminum is a light and malleable metal that features excellent conductivity, reflectivity, and oxidation resistance.
• Brass is an alloy of copper and zinc, and features superior strength, ductility, conductivity, and environmental resistance.  It typically costs more than other materials.
• Nylon is a synthetic polymer that has similar qualities to brass but is cheaper to buy and manufacture. 
• Various types of polymer are used as strain relief materials, including polyester, polyolefin, PTFE, and PVC.  While each polymer has different characteristics, most have good chemical resistance, flexibility, and versatility.
• Steel and stainless steel have excellent corrosion resistance and pressure ratings.

Size

Buyers of strain reliefs should carefully consider specifications related to the product's size.

Diameter

A strain relief's diameter refers to the diameter of the cable designed to pass through the product.

Mounting Size

Mounting size refers to the size of a strain relief's threads, which are used to connect the strain relief to a junction box or other device.  Thread sizes are standardized by the National Pipe Thread (NPT) in North America and metric sizes elsewhere. 

NPT sizes are determined by the nominal size of the pipe or tube.  For example, a 1/4" NPT standard describes the thread size on a 1/4" pipe.  Metric sizes all share the common format [M][number] where [number] is the nominal pipe size in millimeters, such that an M6 pipe specifies that the nominal pipe size is 6 mm.     

The following tables detail common NPT and metric thread sizes.  In both tables, OD refers to the pipe's outer diameter, and pitch refers to the distance between threads.

Table credits:  Wikipedia | Interpower

References

Interpower - How to choose a strain relief

• Choosing a Strain Relief for Any Application (.pdf)
  of the strain relief. · The method of mounting to the chassis. · Specialized body styles. · Cable or wire size. In this article, we will explore these topics and discuss a number of the various types of strain. reliefs available on the market, in order to help you select the appropriate component...
• Overmold Protection for Sensors, Cables, and Connectors Allows Special Terminations, Strain Relief and Protection
  and Connectors. Take advantage of the on-site molding and tooling design capabilities of Smith Systems, Inc. to incorporate custom overmold protection in your designs. Overmolding on sensors, connectors, cables and harnesses provide special termination configurations as well as strain relief...
• Strain and Flex Relief for I/O Interconnects
  Strain and Flex Relief for I/O Interconnects Strain and flex relief as part of the overall interconnect design should be a major consideration for designers. Strain relief is often an overlooked area of design as it crosses the boundary between the assembly of the interconnect components...