Strain Reliefs [Shapes, Types & Usage]
Strain relief is a mechanical fixture that is used to protect various flexible products (cables, cords, or wires) against their longitudinal motion and thus increasing the lifeline of these flexible products.
Various flexible hardware products may experience forces of push or pull during which are absorbed by the strain reliefs and these products are protected from generated mechanical stresses.
Strain reliefs are designed in different shapes, sizes and are manufactured by using various types of materials.
Strain relief is mainly used to protect electrical cables against stresses and tensions which could break the connection between cable and connector or plug.
These relief devices also prevent the conductor inside electrical cables. All the hardware products which experience any type of tensions or stresses during their lifetime are equipped with metal or plastic strain relief devices.
The hardware products which require strain relief instruments include electrical cables, wires or cords, tubes, and various types of accessories used in medical equipment.
Why Strain Relief is necessary
High stresses and fatigue is produced in the cable-connector region due to tight bends and a combination of stiff-flexible part of that device or simply cable.
Failure increases as bend become tighter because of the inverse relation of strain with a radius of curvature. The major purpose of strain reliefs includes extra protection against over bending of the flexible cords or cables or protection against cutting due to sharp edges of other attached devices.
Another objective of using strain relief devices is the reinforcement of the electrical connection of cord or wire which protects the conductors against twisting or pulling.
These devices may be used to provide the liquid-tight seal at the connection region.
Strain Reliefs Shapes and Types
The classification of strain reliefs depend on rigidity or flexibility and is in accordance with their styles. They may be dome style or flex style.
The dome-style device is generally used where high protection thus high rigidity is required to protect the cable against cutting due to sharp edges of attached device or electrical panel and it is fitted at the point of connection.
Flex style is generally used to protect the cables from breakage by providing extra bending relief to them.
Both styles are very important and equally used in various hardware products to increases their life cycle and also increases the quality as well as safety of electrical cables or wires.
Both types of shapes can be made up of aluminum, brass, nylon, steel, stainless steel, and various polymer materials.
Types of strain reliefs include wire ties, cable glands, cable bushings, and Romex connectors. They all are manufactured according to the size of required fittings and in accordance with the required shape.
Strain Relief Designs
As discussed earlier, strain reliefs are designed in different shapes, sizes and are manufactured by using various types of materials. Generally, they are made up of aluminum, brass (copper and zinc alloy), nylon, steel, stainless steel, and various polymer materials (polyester, PTFE or PVC).
Depending on the type of materials used for strain relief devices, it is divided into two types of designs which include metal strain relief design and plastic strain relief design.
A specific type of design is selected based on customer requirements or required installation features. Some strain reliefs can be made by both plastic and metal without reducing their effectiveness.
The electrical conduits are made of metal as well as plastic but serve the same purposes. Generally, due to safety reasons, plastic relief designs are more preferred.
Metal Strain Relief Design
As the name indicates, this type of strain relief fitting is designed based on the plastic material. This is the older design of strain reliefs and manufactured in the form of spring which surrounds the cable at the connection point.
This metal spring is installed at the junction of cable and connector to serve the above-mentioned purposes.
Metal strain reliefs provide extra protection against over bending of the flexible cables and also protect them against cutting due to sharp edges of other attached devices.
It is also installed for reinforcement of the electrical connection of the cord which in fact protects the conductors against twisting or pulling. Metals used for the manufacturing of this type include steel, aluminum, stainless steel or metal alloy brass.
Some metal strain reliefs may be made of plated steel and nickel-plated material. The classic look of the metal strain relief design makes it very appealing on the consumer end.
But nowadays, they have been almost replaced by plastic designs which are more safe and beneficial for all types of required purposes of the strain reliefs.
Plastic Strain Relief Design
Nowadays, plastic strain relief designs are very common which are made of flexible plastic-type. There is an infinite number of plastic designs depending on shapes, sizes, and variety of plastic materials which is selected based on the customer requirements or required installation features.
Generally, rubber or good quality is used in various electrical cable manufacturing. This type of design is specified by the use of nylon and various polymer materials including polyester or PVC in the manufacturing of strain relief plugs, connectors, cable glands, and wire ties.
Plastic strain relief designs are classified into three different categories including one-direction strain relief plastic design, multi-direction strain relief plastic design, and solid strain relief plastic design.
All the hardware products including electrical cables, wires or cords, tubes, and various types of accessories used in medical equipment that experience any type of tensions or stresses during their lifetime are mostly equipped with plastic strain relief designs of any above-mentioned category.
The design category for industrial applications is selected based on the required quality of the device or the necessary features required by customers. Some industrial applications use metal springs while some find solid design more effective.
All categories are described with respect to the protection of different hardware products especially electrical cables.
One-Direction Plastic Strain Relief Design
As the name indicates that this type of design provides relief of bending in just one direction while other direction/s are open to experience explained stresses or tensions.
In a single direction, it is fitted with a number of connected plastic segments creating the minimum value of the bend radius.
These types of fittings are very effective in one direction as strain is prevented in just a single direction. The applications of one-direction plastic strain relief design are infinite in robotics manufacturing.
Multi-Direction Plastic Strain Relief Design
This is the most widely used design due to its design flexibility in more than one direction.
The protection of cables inside the equipment in all directions is very essential for increasing its quality as well as the life cycle. In this design, multi-direction plastic segments are connected with each other around the cable with the same design consideration of the minimum bend radius.
It is also installed at the connector-cable interface and allows the motion in two or more than two directions. As this design is more effective and flexible, so it is largely used in many applications of electronics, medical equipment, and electrical cables or devices.
Solid Plastic Strain Relief Design
This is the most famous design and it can be seen everywhere on all types of related hardware products especially electrical cables and cords.
All the devices from the vacuum cleaner to the simplest lightning cables are being manufactured with this design. Because of its solid design, its cleaning is easy which appeals to the customers as well.
As this design is getting more and more popular, at the same time highly complex design considerations are involved in its formation.
It should be designed in such a way that strain must not be concentrated in between the cable and strain relief as in this design strain relief region is too stiff.
The important design consideration includes that it should allow only bending of cable in between the strain relief area but not exactly at the junction.
To achieve this design component, strain relief must not be too stiff or too flexible, although it should be flexible enough to allow bending inside the mentioned area.
This design has many applications in the beverage, food, electronics, and medical industries. Special requirements of the food and medical industry need a perfect material selection for meeting demands. All the new designs are tested by making a prototype to avoid high business losses.
The prototype is tested and then the final design is completed, according to which all the production is made.
Design Size Considerations
One of the very important factors in designing any type of strain relief is perfect size manufacturing in accordance with the use of strain relief on the specific hardware product.
Thus the size of strain relief must be according to the size of the target product or tool.
Size is the key to success for any design. If the size does not conform to the required size of the object, then the whole design gets failed until rectified again.
Consume should take this consideration into account according to his/her product’s size conformance. The proper size of the diameter of wire ties, cable glands, cable bushings, and the Romex connectors should be applied in order to confirm the product.
The size of threads of strain relief must be in accordance with junction box size and it is also called the mounting size. These sizes have been specified by National Pipe Thread (NPT) in North America and metric sizes elsewhere.
Properly designed strain reliefs are necessary for extra protection against over bending of the flexible cords or cables or protection against cutting due to sharp edges of other attached device,
for reinforcement of the electrical connection of cord or wire which protects the conductors against twisting or pulling and for providing the liquid-tight seal at the connection region.