Effective Strain Relief Design


Example of an effective,
overmolded strain relief

A strain relief, also commonly called a bend relief, is important to the electrical and mechanical integrity and overall performance of a medical cable assembly.

A strain relief is used to provide a transition from flexible cable to a rigid connector or connection point.  A properly designed strain relief will prevent mechanical force applied to the exterior of a cable from being transferred to the electrical terminations within the connector or device which could lead to failure.  An effective strain relief should prevent any load applied to the cable from being transferred to conductor terminations.


A combination strain relief and grommet
designed to be captured and held in
place by an opening in the enclosure

A grommet, with an integrated strain relief, is commonly used when a cable is permanently attached to a device.  Cables that pass through an opening into a device typically employ a grommet to anchor the cable to the device and provide protection against damage from an axially applied load.  If it is likely that the cable will be flexed, it is common to use a combination grommet and strain relief for through-hole applications.

Pre-manufactured and Custom Strain Reliefs


Pre-manufactured flexible
slip-on strain relief

Pre-manufactured metallic
spring-style strain relief

Strain reliefs can be either pre-manufactured or custom designed and manufactured for a specific application.  A custom strain relief can be designed as a separate unit to be slipped onto cable or can be designed to be molded over cable, commonly capturing a portion of the connector.


A custom strain relief pre-molded and designed
to fit snugly into the device housing.  A cable
clamp increases tensile strength

Pre-manufactured strain reliefs are typically slipped over the cable before the connector is terminated.  After the cable conductors are terminated to the connector, the strain relief is screwed or glued onto the connector assembly.  Most off-the-shelf connectors have corresponding pre-manufactured strain reliefs available to fit a variety of cable diameters and are often available in different colors.

In most instances, a custom designed and over molded strain relief offers superior flex life, tensile strength, and moisture protection performance than a pre-manufactured strain relief.

Flex Life and Tensile Strength Performance

A well designed strain relief can enhance both flex life performance and tensile strength.  Typically raw cable or wire will withstand a higher number of flex cycles and have a higher tensile strength than a cable assembly.  The point where the cable or wire is terminated is generally considered the most likely point of failure in terms of flex life and tensile strength.  A well designed strain relief should isolate electrical terminations, preventing force from being applied to these points.

Design considerations for a strain relief include the geometry of the part, the interaction of the strain relief and cable jacket material.  Tensile strength will be increased if the strain relief chemically bonds to both the cable jacket and connector body.  The strength of the union of the strain relief to the connector can be improved further by providing features that will allow the strain relief to physically bond to the connector body.


Features shown in yellow allow
inner mold material to form physical
bond with connector body

Flat surfaces show cable clamp that has been crimped onto the cable jacket increasing tensile strength when captured in overmold

Additional tensile strength can be achieved by incorporating a clamp which is crimped onto the cable jacket prior to molding.  When the strain relief is over molded, the clamp is embedded into the mold material which greatly increases the tensile strength of the assembly.

Segmented or Smooth Design

Strain reliefs can be solid and smooth or segmented.  A solid strain relief is easier to clean which can be an important consideration for many medical applications.  If the material, size and geometry are the same, a segmented strain relief typically offers greater flexibility than a solid strain relief, but with the trade-off of being more difficult to clean.


A segmented strain relief can be
more difficult to clean as compared
to a smooth strain relief

A smooth, solid strain relief is typically
less flexible but is easier to clean as
compared to a segmented strain relief

The bending action of a well-
designed segmented strain relief

Strain Relief Design

A well designed segmented strain relief will feature walls and spaces designed to allow the bend radius to increase a greater distance from the connector or connection point.  The size of the solid sections and the size of the gaps between solid portions are varied to achieve the desired bend radius.  Segmented strain reliefs are generally designed so that the segment closest to the fixed point closes first and the segment furthest from the fixed point closes last.  This provides the greatest amount of bend relief, protecting the electrical terminations within the connector.

Strain reliefs are typically designed to flex in either one axis – unidirectional or in two axes – multidirectional.   A unidirectional strain relief is typically used when the cable or wire exiting the strain relief is not round.  This is commonly the case with bonded cable or wire.  In this instance, the flex is limited to one axis by the cable design, not by the strain relief design.


Strain relief designed to flex in
only one axis

Wrapping a cable around a portable
device is a common storage method.
Anticipating how a cable will be
stored when not in use is an
important design consideration

The length of a strain relief also contributes to performance.  Generally, a longer the strain relief, will be more effective than a shorter strain relief.  However, consideration should be given to how the cable will be stored in clinical use.  Experience has shown that cables are often left attached and wound around portable devices, placing continuous strain on the assembly.  In this instance, a shorter strain relief may be more effective than a longer strain relief because the bend radius at the cable end of the strain relief may be larger

Custom Overmolded or Off-the-shelf Strain Relief

It is generally accepted that a strain relief molded over cable offers superior performance on several levels when compared to a pre-manufactured strain relief.

An over molded strain relief typically provides a higher degree of moisture resistance as compared to a pre-manufactured strain relief due to physical and possibly chemical bonding of the overmold to the cable jacket and connector.  This can be an important consideration for cables that are cleaned often or otherwise subjected to contact with liquids.  Similarly, an overmolded strain relief will typically offer improved tensile strength when compared to a pre-manufactured strain relief because of the bonding achieved by molding.

Two factors that may lead to selecting a pre-manufactured strain relief instead of a custom overmolded strain relief are lead-time to design and produce tooling and the cost to have the tooling fabricated.

Affinity has developed a variety of strain relief tools that can be used by our OEM partners.  Depending on the design of the connector and diameter of the cable material, some tooling can be used as-is, with no additional cost.  Other molds may require modification but with less cost and time required than designing and fabricating tooling from scratch.  The Affinity engineering team can suggest design paths which may allow the use of available tooling, saving both time and money.


The Affinity engineering team is can
help with all aspects of your medical
cable or connector design including
effective strain relief

Summary

By understanding performance and design requirements, the Affinity engineering team can help you choose the appropriate strain relief system, whether it is an off-the-shelf or custom molded solution.  A well designed strain relief can improve both the performance and useful life of a medical cable assembly.

For more information or to discuss any aspect of medical cable design, contact Affinity Customer Care at +1 949-477-9495 or email to customercare2@affinitymedical.com.