Effective Strain Relief Design
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 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
slip-on strain relief
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.
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
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.
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 email@example.com.
On April 11th, the Affinity team said good bye to former company president and founder Mary Phillipp. After a six-month transition period since the acquisition of Affinity by Molex, Mary’s day-to-day role at Affinity has ended.
“While Mary’s day-to-day role has ended, she will continue to be my mentor,” said Affinity Co-Founder and General Manager, Bob Frank. “Mary has been my business partner, mentor and friend and I will continue to look to her for support and guidance. And, of course, our friendship will continue.”
“Mary established and maintained a culture at Affinity where we treated each other and our customers as partners,” said Affinity Marketing Manager, Hank Mancini. “Even though we are now part of a large global corporation, we will continue to work and operate based on the principals upon which Affinity was founded. And, after six months as part of Molex, the same culture seems to be well shared within the Molex organization!
Mariachi in lobby just before starting
to play and enter Mary’s office
Mary, at her desk, is surprised by Mariachis
About noon on the 11th, while Mary was in a meeting, she was totally surprised when she heard Mariachi playing in the lobby! It was the start of a fun afternoon for the entire Affinity team.
For the party, second shift came in early so all team members could be present. The company provided a full lunch featuring Mexican food and Affinity team members did the serving and clean-up.
Affinity employees pooled their money to buy Mary several gifts, one of which was a silver Tiffany bracelet engraved on one side with Affinity and on the other side with “Staying Connected,” Affinity’s motto. While the bracelet itself was small, it was packed inside several boxes with the outer box being about 2 feet square. Mary had to work to reach the real gift. “We want Mary to remember us always and stay connected with Affinity,” said Affinity Quality Coordinator, Janett Lopez, who helped organize the party.
Many Affinity team members were surprised that one of their own, Genaro Zaragoza, was a Mariachi and played trumpet. The Mariachis played throughout lunch and several Affinity employees joined the band singing traditional songs.
After lunch tears started to flow as Affinity team members got up and talked about the positive influence Mary had on their lives. Once the party was officially over, there was a long line of people waiting to greet and hug Mary and thank her for the opportunities that working for Affinity had provided.
While Mary’s role at Affinity is ending, she will continue to consult for Molex. “Mary’s official role had ended, but she will always be part of Affinity,” commented Bob.
Medical cables and wires require labeling and marking for identification and to comply with regulatory standards. Electrode snap receptacles, also commonly referred to as snaps, are the most common way for a medical cable or lead to attach to a monitoring electrode. A number of effective methods are used to mark snap leads.
Individually Molded Colors
Molding snap assemblies in colors matching either AHA or IEC requirements is probably the simplest way to achieve compliance with marking requirements. Wires may all be the same color – black is the most common – or individually colored to match the snap assembly.
Another common method to meet marking requirements is to mold either AHA or IEC designations into the snap assembly. This method is both cost-effective and permanent. However, because the designations are molded at the same time the snap head is formed, the markings are the same color as the snap head and can be more difficult to read than a contrasting marking.
Printed Colored Inserts
The easiest-to-read markings consist of high-quality printed polycarbonate labels that are affixed to the snap assembly using a very aggressive, permanent adhesive. To make accidental removal of the label nearly impossible, the labels are commonly inserted into a recess molded into the snap assembly. This method allows the highest quality printing, but because it involves an additional component, it adds a small amount to the cost.
Affinity has tooling to produce electrode snap assemblies in a variety of forms. The Affinity engineering team can assist in designing snap assemblies that are easy for clinicians to use and meet all regulatory requirements including lead designations.
Tokyo Trivia Part 2
Tokyo Skytree – At a height of 634 meters (2,080) feet, Tokyo Skytree is the tallest tower in the world and the second tallest structure in the world. It was completed in May 2012 and has become the number one tourist attraction in Japan.
No Smoking – About a third of adult Japanese men smoke, but smoking rates in Japan are falling rapidly. One of the reasons is that in many areas, including the streets of Tokyo, smoking while walking is prohibited! Marked areas with ash boxes are available for smokers and are often crowded.
170,000 Trees – Meiji Shrine, located in Tokyo, is a natural haven surrounded by the largest metropolitan area in the world (United Nations Revised Population Database). Within the shrine’s 170 acres, there are approximately 170,000 trees representing 365 different species. The trees were donated by the Japanese people when the shrine was established in 1920.
Public Transportation – The Tokyo metropolitan area has the most extensive public transportation network in the world including rail lines, subways and buses! During the Monday to Friday workweek, the public transportation system is used more than 280 million times! There are nine railway lines and 14 subway lines in Tokyo alone!