Understanding Lemo and Fischer Push Pull Connectors

In the late 50’s and early 60’s Lemo and Fischer patented push pull connectors. For over 60 years these designs have remained essentially unchanged. The Lemo and Fischer designs allow the user to plug and unplug the connector with one hand, in a push and pull motion while also preventing accidental disconnection. This article outlines the major design differences between Lemo and Fischer and how these aspects impact connector performance.
 

Latch Design

An important design difference between Lemo and Fischer is how the latching mechanism works. When comparing Lemo and Fischer receptacles, how the latch works is almost identical. The real magic happens on the plugs. Lemo achieves this function with an outer shell and latching sleeve. The latching sleeve and outer sleeve work together to achieve the push-pull experience. Straight plug items 1 and 2 below.
 

Figure 1: Lemo B-series cross section: Lemo Unipole Multipole catalog page 13
 
The Fischer design achieves this same function by altering these two components. The outer sleeve and plug body can be seen below.
 

Figure 2: Fischer latch design: Fischer Technical Specification Volume 1 page A-5.
 
Although the design differences are subtle, the difference in connector performance is drastic. This latch design difference impacts two major connector performance areas; ingress protection and shell size options.
 

Ingress Protection

The Lemo latch sleeve goes from the inside to the outside of the connector, creating an opening for particles to ingress. These openings in the latch sleeve limit the Lemo B-series and P-series to a IP-64 maximum rating. On the other hand, Fischer plug body is essentially a tube that only needs to be sealed on each end to become IP-68 rated. In order to achieve IP-68 rated connectors Lemo created a separate product line, the K-series. The K-series is essentially a B series connector with a tube under the latch sleeve.
 
Figure 3: Lemo K-series cross section: Lemo Unipole Multipole catalog page 38
 

Shell Sizes

Lemo has many different connector series, each with different shell sizes. This was done because the latching mechanism did not allow for a “one size fits all” connector. The K series needed to increase in size because the inner tube was added. The P-series was created because the latch design could not be molded in plastic while maintaining envelope dimensions. Meanwhile the Fischer latching design can be molded in plastic and is IP-68 while maintaining backwards compatibility to the receptacle. Having the ability to pair up a metal receptacle with a plastic connector has many practical uses. For example, in many medical applications the device connector is disposable (plastic) and the console receptacle (metal) needs to be highly reliable.
 

Connector Marketplace

Over the years many competitors to Lemo and Fischer have surfaced. One of the most well-known companies is ODU Connectors. ODU has backward-compatible connectors to both Lemo and Fischer. The list goes on and on, there are hundreds of companies that have simply copied the designs and offered them at a lower price.
 
At Global Interconnect our engineers strive to innovate, not imitate. Our Purpose-Built™ connectors focus on simplified designs while maintaining quality and backwards compatibility with the industries push pull connectors. Our engineers work to understand your application and create a Purpose-Built™ solution for you, we don’t just select the closest match from a catalog. To learn more about how our connector solutions may be able to help you, check out Understanding Custom Medical Connectors.

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