Staying Connected - December 2012

Retention Force for Non-Locking Medical Connectors

Medical connectors can be divided into two broad groups based on how they are retained by the devices to which they are connected: locking, also referred to as latching and non-locking.  Each offers advantages and disadvantages.  This article addresses some of the issues surrounding non-locking connectors.

Locking vs. Non-Locking

Custom locking medical connector
with latch mechanism

USB connectors are common examples
of non-locking connectors

Early medical cable assemblies often used connectors adopted from industrial and military applications.  These “MS” or “mil-spec” metal connectors had knurled, threaded couplers.  Once screwed down to the receptacle, this connector would not come loose inadvertently.  While this type of locking connection met the requirement that the connector not disconnect unintentionally, they were typically difficult to connect and reconnect.  Additionally, this type of connector provided no safety disconnect feature which is often desirable for medical applications.

Example of locking mil-spec type connector often used in early
medical devices

RJ connector locking mechanism
is easily broken requiring the cable
to be replaced

A more up-to-date example of a locking connector often used in medical devices and computer connections is the “RJ” type plug.  Because of the design and materials typically used, the locking mechanism is not very robust and is easily broken.  Once the locking mechanism breaks, the plug is poorly retained and the cable must be replaced.

Medical cables, particularly those that connect between a patient and a stationary device, often require a safety mechanism that will allow disconnection without harm to the patient or damage to the device or cable.  The need to ensure a reliable connection to the device; yet allow for safe, inadvertent disconnect is a design requirement the Affinity engineering team has experience implementing.

Non-Locking Connectors

The interface and friction between
a pin and mating socket typically
plays a large role in establishing the
retention force of a connector

Medical cables require a positive connection between the plug and receptacle.  Any looseness in the connection will usually cause intermittent contact, resulting in unwanted noise or poor signal quality which may make diagnosis or therapy difficult, if not impossible.

How firmly the plug is held by the receptacle is referred to as retention force and is controllable in the design process.  Pin and socket selection, as well as the physical design of the plug and receptacle, allow control over both insertion and retention force.

Custom connector with retention force made
up of friction from 11 pins and sockets

Retention Force

Retention force of a connector pair - plug and receptacle - is nominally made up of the sum of the retention force of each socket and pin as well as any friction between the plug insulator and the receptacle walls.  For connector pairs with few contacts, friction between the insulator and receptacle wall may be the largest factor in determining the total retention force.  For units with a larger number of contacts, little or no friction may be needed between the insulator and receptacle wall.

Custom connector uses friction
between molded part and receptacle
housing to increase retention force

An additional factor to be considered is that in a connector with more than a few contacts, the total retention force is greater than the sum of each pin to socket retention force.  This phenomenon is detailed in a paper by Robert S. Mroczkowski, Sc.D “The Mating Game” in “Connector Specifier”, December, 2001.  Mroczkowski states that “mating force will always be greater than that value (if all contacts mate at the same time) because of tolerance and housing interaction effects.”

Retention Force Specification

One of the specifications that should be established early in any medical cable or connector project is the retention force of the plug to receptacle.  The amount of retention force as well the required number of mate and un-mate cycles are factors considered in electrical contact and material selection.

Once mating and retention forces are established and documented, molds are designed in a “tool safe” state.  That is, the tooling is designed to produce plastic parts that have retention force below the desired level.  By removing metal from the tool, the part becomes larger and retention force is increased.  Done in very small increments, retention force can be “dialed in.”  Sharing mold trial parts with project team members allows insertion and retention force to be evaluated and adjusted before tools are heat treated and production parts are manufactured.

Connector Retention Force Testing

Connector mate and un-mate cycling is
done by hand in the Affinity lab to replicate
actual use prior to retention force testing

Affinity engineering technician David
Moreno performs retention force testing

Once production parts have been manufactured DVT testing will confirm that all specifications, including connector retention force, are met.  Affinity performs mate and un-mate cycling by hand to closely replicate actual use.  Lab testing will include measuring retention force before, during, and after the specified number of mate and un-mate cycles.


The type of connector – locking or non-locking - and the desired mating and retention force are specifications that should be established early in the design process.  The Affinity engineering team has the experience and expertise to help guide your design for either locking or non-locking custom connectors.  And, when a custom connector is the not the most appropriate choice, the Affinity team can help select an off-the-shelf connector or create a hybrid connector to meet your specific requirements.  For additional information, contact Affinity at +1 949-477-9495 or email to

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Meet Bob Frank - Affinity General Manager

General Manager, Bob Frank

Bob Frank was promoted from Director of Engineering to General Manager as part of the acquisition of Affinity by Molex in October 2012.  Taking over leadership of the company from Mary Phillipp is the culmination of Bob’s nearly 30 years’ experience in the medical interconnect industry.

After graduating from high school, Bob joined Tronomed, a large medical cable manufacturer, as a draftsman. Bob quickly learned what went into designing robust, high quality medical cable assemblies.  Because of his enthusiasm he quickly moved from the engineering side of the business into sales.  Within a few years, Bob was again promoted, directing both inside and outside sales at Tronomed.

While at Tronomed, Bob worked closely with Mary Phillipp, the President of the company. Working together for many years, they established a strong professional relationship and a long term friendship.  When the company was acquired and moved to the east coast, Bob and Mary took the bold step to leave Tronomed and found Affinity Medical in 1997.

Affinity founders Bob Frank and
Mary Phillipp encounter unexpected
snow when visiting with an OEM partner

During the early years of Affinity, Bob “wore many hats,” including working in production, making sales calls, and designing cable assemblies.  As Affinity grew, Bob concentrated on engineering while Mary handled finances.  They both handled sales and to this day, Bob likes nothing better than visiting in-person with Affinity’s OEM partners.

Within a year of founding, Affinity landed its first major OEM partner and had a huge order to fill.  Bob, Mary and their spouses joined Affinity’s production team members working long hours to get cables manufactured.  Bob recalled the time he and Dave Johnson (Mary’s husband) were running molding machines almost around the clock. “We’d run parts until we were too tired to continue, retreat to our cars and catch a couple of hours of sleep, wake up and start the process all over again!  It was worth it.  We shipped the order on time and made our first large OEM partner very happy.”

Fifteen years later, Bob is still enthusiastic about Affinity.  The atmosphere is highly charged and a lot of that can be attributed to Bob.  His imposing stature - he’s 6’6” tall - hearty laugh and genuine enthusiasm help keep the energy level at Affinity very high.  Bob says, “We all work hard at Affinity, but we also have a lot of fun.”

Bob’s technical knowledge and reputation within the industry led him to be included on the committee that established the ANSI/AAMI EC53 standard.  EC53 is the only standard to specifically address the technical and safety requirements for medical cables and lead wires.  Bob’s intimate knowledge of the standard and how the various elements of the standard are applied to medical cable design benefit Affinity’s OEM partners.

Bob speaking to the Affinity team when
the acquisition by Molex was announced

“Bob has a tremendous amount of knowledge regarding virtually of every aspect of medical cable design and manufacturing.  He is respected for his technical knowledge, hard work and high ethical standards,” said Hank Mancini, Affinity Business Development Manager.  “Bob is unique in that he has both a strong technical background and many years of experience in sales.  This will help him successfully lead Affinity as the company continues to grow as part of Molex.  Bob is a wonderful person to work with a great person to work for.”

When asked what he enjoys most about his career at Affinity, Bob said, “I enjoy and value the relationships I have developed over the years with customers, suppliers and our team members.  It is great to be able to do business and work with people that I like, respect and consider friends.  I was honored to be offered the General Manager’s position and to lead Affinity.”

Besides work, Bob is an avid golfer with a single-digit handicap.  He also is a Formula 1 race fan.  He says, “I love engineering, technology and competition, which are all exhibited at the highest level by Formula 1 racing.”

Bob and his wife Joanne live in nearby Mission Viejo.  During the winter, when not working at Affinity, Bob enjoys snowboarding at Mammoth Mountain in California where he and his wife have a second home.

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PCB Lead Interface

While the Affinity engineering team takes pride in designing custom connectors, in some instances eliminating the connector all together is the best option!

One of Affinity’s OEM partners asked the Affinity engineering team to design a new interface.  The connection was intended to be semi-permanent with the cable being changed infrequently.

Custom interface designed to be
captured by top and bottom halves
of the device case

A Flexible circuit with a low-cost pin
header plugs directly into the PCB

An over molded assembly was designed with groves to be captured and held securely by the top and bottom halves of the device case.  Instead of using a pair of connectors – plug and receptacle - the interface was simplified and the cost reduced by using a small flexible circuit and a pin header.  The four-pin connection on the end of the flex circuit is plugged directly into a PCB.

The device pictured is custom and proprietary to one of Affinity’s OEM partners.  However unique interconnects like this can be designed to meet your specific requirements.

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Announcements, Information and Trivia

New Year’s Trivia

Gregorian calendar - Most countries in the world have adopted and use the Gregorian calendar, in which New Year’s Day is celebrated on January first.

Carrying a Suitcase – In Argentina, Bolivia, Mexico and Venezuela those who dream of graveling in the coming year carry a suitcase around the house at midnight.

New Year’s Baby – The tradition of a baby signifying the New Year was started around 600 BC by the Greeks.  To honor Dionysus, the God of Fertility, a baby was carried around in a basket at the start of the New Year.

Times Square Ball – Every year over a million people gather in Times Square to watch a Waterford Crystal ball drop.  The ball does not actually drop; rather it descends 77 feet in one minute stopping at the bottom at exactly 12:00 A.M., the start of the New Year.

January Holidays

Affinity will be open on Monday, December 31st and closed on Tuesday January 1, 2013 in observance of New Year’s Day.