Seals upgrade connectors

Circular plastic connectors (CPCs) predominate in industrial settings, but also find use in tractor trailers, locomotives, agricultural machinery, and off-road equipment. Occasionally, design engineers applying connectors in industrial and vehicular applications have retrofitted O-rings, die-cut gaskets, and gels as seals to connectors to minimize intrusion of moisture and other contaminants. But these do-it-yourself seals often meet with limited success.

Source of the problem. Connector seals must prevent contamination infusion from two sources. The first is from the area surrounding the individual wires leading into the contact interface. Wire grommet seals must surround each wire to obstruct moisture that might condense on the wire insulation and then trickle down into the contacts. If the wires are wrapped in a cable sheath, then one overall seal can sit between the cable sheath and the connector.

The second source of intrusion is the interface of the two connector halves. For a circular connector, the interfacial seal can be a resilient ring or a gasket cut to accommodate the connector shape.

Two interfaces on a CPC connector, the wires entering the connector and the interface between connector halves, must be sealed against particles and moisture.

CPCs consist of a plug and receptacle assembly (see figure). The connector halves may join two cables, or join a cable to a panel or PC board. The CPC housing is generally nylon, which resists most chemicals seen in industrial environments. With stainless steel springs providing a relatively high normal mating force, the contacts have high reliability. Stainless retains its spring properties under a wide range of temperature variation. The springs also keep the pin contact centered in the socket cavity. For high-vibration applications, higher performance springs and contact systems are available to increase normal force.

To assemble a connector, a technician inserts individual wires ranging in size from 24 to 14 AWG through the holes at the back of the connector (see figure). Prior to insertion, the assembler fits the wires with crimped pin or socket contacts. As they are inserted, the wire contacts lock into place in the housing. A threaded coupling ring on the plug assembly secures the two connector halves together, correctly mating the internal pins and sockets.

Sealing the deal. CPCs come in several sizes to accommodate up to 37 wires. Each position can handle a range of currents from low signal levels to power levels with 40A or more, depending on the type of loaded contacts. In many cases, the application calls for a CPC with a combination of many signal lines and a few power lines. Because a CPC connector housing can accommodate various wire sizes, different wire grommet seal glands are generally required. Manufacturers often provide color-coded wire entry seals to facilitate mating a narrow range of wire sizes to a proper seal.

In the CPC, a resilient silicone mask configured with a grid of holes corresponding to the contact positions serves as the set of wire's grommet seal. A plastic pressure plate with identical hole positions snaps into place to protect and retain the seal at the back of the connector.

Silicone and fluorosilicone serve as good resilient sealing materials for connectors because they resist most industrial chemicals, are highly elastic, and exhibit virtually no outgassing when properly cured. Residual volatiles from other seal materials can outgas within the connector and corrode its contacts.

The wire holes in the silicone mask may not extend completely through the seal. A thin membrane of resilient silicone remains to handle those applications that don't require a full complement of wire positions in the connector housing. In assembly, the technician pushes the wire contact completely through the hole, breaking through the membrane at the far end of the mask. If a wire contact is later removed, the connector manufacturer will provide a polypropylene plug to seal off the empty hole.

A peripheral seal within the CPC connector's threaded coupling ring seals the connector halves interface. The rear seal closing off wires entering the connector is often a series of three progressively smaller annular glands (inset).

As a rule of thumb, a reliable wire grommet seal design incorporates three annular glands in each silicone wire-entry seal cavity. The gland diameters get progressively smaller as the wire is pushed through, forming progressively tighter seals. Another seal design guideline is that the hole in the resilient material must be 10 to 30% smaller than the wire diameter, depending on the gauge.

Having to seal a connector may limit the wire size used because smaller wire gauges are difficult to seal. Thus, to attain a proper seal, connector-manufacturer engineers may recommend a larger gauge wire than required to carry the current in the application.

Perimeter seals. A resilient material positioned between the receptacle and plug connector halves constitutes a perimeter or interface seal. For CPC connectors, this is generally a silicone ring. In other cases, a cut gasket that follows the shape of the connector does the job. The connector housings are typically molded with a gland to retain the gasket and maintain a proper seal. In some cases, however, the seal is simply bonded to the plastic housing with adhesive. The overall effectiveness of sealing is subject to international standards and tests (see sidebar).

In high vibration environments, interfacial seals provide another advantage for CPCs. Without the seal, a small amount of float may exist between the two connector halves. Under conditions of high vibration, fretting caused by this relative motion may degrade the contacts. But the peripheral seal acts as a shock absorber, removing the float between the two housings.

Panel seals. Another kind of sealing occurs when the connector mounts in a panel and the designer simply wants to prevent the passage of dust or fluids between the two compartments separated by the panel. For example, wires from a dashboard pass through a bulkhead to the engine compartment. CPCs fitted with a square flange, resilient gasket, and mounting screws hold these connectors tight against the bulkhead. Maintenance people can spray down the engine compartment without water getting through the bulkhead into the cab. In refrigerated appliances, such as panel seal may be necessary to keep the polyurethane foam insulation from seeping into the refrigerator or freezer compartment during manufacture.

In many cases, to meet the requirements of a particular application, the connector manufacturer can retrofit panel seals. Or the manufacturer may provide an optional version of the connector (simply by adding a flange to the periphery) pre-configured to provide a panel seal, usually neoprene rubber.

To summarize, the International Electrotechnical Commission (IEC) sets standards for defining degrees of electrical connector seals. By studying the application, the design engineer can determine the IEC standard and choose an appropriate connector. In general, high sealing standards will raise connector costs and require more care during assembly. Responding to concerns over sealing CPCs, Tyco Electronics initiated a development program to re-engineer these connectors to incorporate the noted factory designed and installed seals.