Latching onto good methods

The door latch may not appear to be the most important feature of an electronic enclosure, but unless an appropriate component is chosen the enclosure’s performance can suffer.

Enclosures house electronics or electrical components at specific conditions, including temperature and humidity. They protect components from vandalism, spills, dust and moisture. Often they are required to shield against electromagnetic interference.

The challenge for the designer is to address all these considerations in a functional and economical design.

Whether dealing with environmental or EMI (electromagnetic interference) sealing, the designer must incorporate gasketing and a latching system into the enclosure design early in the design process.

Compression is important: a gasket must be compressed to a specific height to be effective. Before designing gasketing onto a door or a panel, there are two key points to be addressed: How to provide the compression force required to close the gasketed door or panel? And how to guarantee an adequate deflection of the gaskets to obtain proper sealing?

Although answers can be provided by a number of latching systems, gasket implementation is critical and will help assure that the most effective latching system is selected for the design.

OPTIMISE GASKET INSTALLATION

For flat gaskets, the compression force is directly related to the mating area. Reducing the mating area by compressing a door edge instead of a flat surface against the gasket will significantly reduce the compression force requirements.

When both environmental and EMI gaskets are required, it is always a good idea to install them on different planes. Where possible, install the EMI gasket so that the compression applied is perpendicular to the face of the door, and install the environmental gaskets so that the compression applied is parallel to the face of the door.

There are two main types of latching systems for enclosures: pawl latches and compression latches. A pawl latch utilises a pawl ramping against the enclosure frame (or a special keeper) to provide some level of compression. The pull up provided by this system is limited by the geometry of the pawl (typically between 1.5 and 2.5mm). The only mechanical advantage lies in the handle (or key) used as a lever arm.

A compression latch utilizes a mechanism built into the latch which allows the positioning of the pawl behind the frame (or keeper) and then provides a translation of the pawl to compress the gasket. There is no limitation to the pull-up distance other than the overall dimensioning of the latch. The mechanical advantage is built into the latch; a lever actuator will only increase it.

Beyond the latch’s required closing force, consider the force the operator must apply to position the door prior to its latching.

With pawl latches, the operator needs to position the door 1.5 to 2.5mm away from the latched position to engage the pawl behind the frame. Very often this translates into the operator compressing the gasket prior to the latching system doing it. If a gasket needs to be compressed 3mm to realise effective sealing, and the pawl latch only provides 1.5mm compression, the operator will have to compress the gasket 1.5mm and then the latch will compress the remaining 1.5mm. With a compression latch providing 5mm of pull-up however, the operator does not have to compress the gasket directly. At 5mm away from the latched position, the pawl goes behind the frame and brings the door in to compress the gasket by 3mm. The operator fully benefits from the latch mechanical advantage.

Another consideration in choosing between pawl latches or compression latches is the potential scratching of the coating on the frame which is inherent to the function of a pawl latch. This will not occur with a compression latch.

COPING WITH DOOR WARPAGE

To guarantee an adequate deflection of the gaskets, the designer must consider not only the pull-up distance which will be provided by the latching system, but also door warpage. Typically, gasket manufacturers provide a gasket deflection range expressed as a percentage of the gasket height. To compensate for any door warpage, it is important to pick a latching system that will provide a compression in the highest range. The door must also be designed with adequate stiffness so that the gasket compression force does not warp the door in between attachment points beyond the minimum required gasket deflection.

While there are a variety of latching systems to choose from, seek the assistance of an access hardware manufacturer that offers the widest range of products combined with a high level of technical expertise.

HOW MANY LATCHES?

The number of latching points is determined by the overall compression force requirement:

N = (L x C x Sf) / (2 x WL)

N is the number of latching points needed.

L is the overall gasket length needed.

C is the gasket closure force.

(L x C) is the enclosure’s total closure force. Consider that the hinge side handles provide half of this force, the other half being left to the latching points. (L x C) / 2 is therefore the compression force that needs to be provided by the latch(es).

Sf is a safety factor.

WL is the latch working load.

Example:

On a 600mm by 1,800mm door, the gasket length is: 2 x (1,800 + 600) = 4,800mm = 480cm.

If the gasket has a closure force of 0.3kg/cm, the overall closure force would be:

480 x 0.3 = 144kg, that is 72kg on the latching side (half).

If the latch available is rated for 50kg with a safety factor of 1.1, then use:

N = 72 / (50/1.1) = 1.58 latches, i.e., Two latches are needed.

Southco. Tel: 01452 715400