With so many access hardware choices available to design engineers — in terms of latch mechanics, ergonomic fit and aesthetic integration — finding a suitable solution for a proper latch fit would seem to be a relatively simple task. However, because there are so many choices overall, there are many opportunities for a design engineer to choose the wrong solution. With that in mind, here are several characteristics of access hardware considerations and several specific latch styles that can make it quicker and easier for a designer to identify an optimum choice among potentially acceptable product solutions.
Important Dimension Considerations – Regardless of latch style, any application can have one or more significant dimensions for the designer to include as early considerations in latch selection. One of those is the “grip” dimension. (The grip dimension is the distance between the face of the door panel and the point at which the latch engages the door frame or keeper, as illustrated. See figure 1, left). Some latch designs are more grip-tolerant than others, including some that provide a degree of adjustment in the final installation. Others demand careful consideration, especially when a precise fit is desired for environmentally sealed performance or rattle-free operation in a high-vibration environment. Additional critical considerations, regardless of latch style, can include the distance from the latch centerline to the door frame or keeper, the available space for the latch mechanism to protrude or rotate behind the door and available clearances and surface area for operator handhold in the final application.
Cam Latches – These are often the simplest and most economical of latch designs, typically easy to install in single-hole mounting. They can fit well in basic applications without particularly demanding performance or dimensional concerns. But even in the simplest applications, there are considerations that can make a difference in the final selection. Consider the consistency of the fit, whether due to enclosure manufacturing tolerances or to potential changes in the field. This could make the difference between specifying a fixed-grip cam latch or an adjustable-grip design that can minimize latch inventories by allowing for user adjustment. The designer can also ensure a relatively snug grip by choosing a cam latch with a slightly curved (or “ramped”) cam or a spring-loaded design to draw and hold the door in close contact with the frame. And to control or restrict access, the designer can choose among numerous tool-actuated or keylocking styles.
Compression Latches – When a good tight closure is critical — to reduce rattle and noise in high-vibration environments or to ensure a satisfactory seal for gasketed applications, whether for dust, moisture or EMI/RFI sealing — compression latches are an obviously good choice for the designer. But even within the compression-latch category, there are still wide ranges of options appropriate to different needs. Many versions provide secure, positive engagement with built-in “pull-up” that keeps the latch handle in the closed position, even during periods of vibration. Where consistent closure forces are critical, perhaps for reasons of even gasket compression, the designer should look for latches with pre-set pull-up compression. For less critical installations, a self-adjusting compression latch can minimize the need to inventory excessive latch styles and sizes. Many compression latches also offer the convenience of latch and handle capability in one assembly for ergonomic opening, where the operator can release the compression and open the door with one continuous motion (See figure 2, above left).
Push-to-Close Latches – As one of the most convenient-to-use latch styles, push-to-close latches (also called slam latches) are one of the most frequently preferred latch options. Certainly the ease of use on the part of the end user — as simple as a push on the door to secure it — is a desirable trait for user-friendly designs. However, there are certain considerations, that, if ignored, could quickly turn user satisfaction into user frustration. One important factor is these types of latches can be particularly sensitive to the grip dimension of the application, requiring them to be specified according to a narrow dimension range. Improperly specified latches, or applications where that grip dimension could change over time, can cause several types of problems. If the grip dimension is less than ideal for the latch, it can cause loose-fitting doors that rattle. Conversely, if the grip dimension becomes too great for the selected latch, it can lead to improper latch engagement, requiring extra effort to get doors to close and engage, and in the worst case, cause doors to open unexpectedly or become unable to be closed or secured.
Draw Latches – A unique attribute of draw latches is that they are designed for applications where the closing force is designed to draw together two co-planar panels (See figure 3, above right). Both lever-style and rotary-cam designs provide mechanical advantages to create the draw action. Flip-handle draw latches also offer the advantage of quick-release operation, while surface-mounted installation minimizes the need for protrusions into the interior of the enclosure. Curved claw or wire bail designs provide resilience under shock conditions and over-center action helps to keep handles closed in spite of vibration. While many draw latches have a basic utilitarian design, there are also more stylish designs for better aesthetics in high-profile applications. They are available in metal, plastic, elastomer and rubber handles for use in indoor, outdoor and potentially corrosive environments. Specialized styles are also available for butt-joint and right-angle applications.
Multi-Point Latches – While multi-point latch configurations are basically extensions of other latch styles (i.e. cam, compression or push-to-close latches), the fact they engage with the enclosure frame in multiple locations gives them added benefits for certain applications. For relatively longer door spans — especially those with a high degree of flexure in the door itself — a multi-point latch can add stability, security and a quieting effect to your installation. Perhaps the most common multi-point latching application is a three-point latching system with points of engagement at the middle, top, and bottom of the door (See figure 4, right). This anchors the door along all three of the unhinged sides to reduce potential for rattle, to minimize potential pry points for vandals and to ensure even closure around the entire perimeter of the door (an important consideration in gasketed door applications). Two-point latching (top and bottom) is also available for applications where a solid latching point is not available in the middle of the door. Multi-point latching with four or more adjustable points of engagement is also available for particularly long doors or where a consistently tight seal is especially important.
A Few Final Thoughts – Of course, in any application, there are always considerations of load ratings, dimensional compatibility and corrosion-resistant or cosmetic finishes. Some of those decision points might be flexible. Others are mandated by the installation environment. For example, in applications where shock and vibration are common, latches with over-center action can help prevent the latch from popping loose in a particularly stressful situation.
Having More Choices – In terms of latch styles, sizes, materials or finishes — increases the likelihood of being able to satisfy the designer’s hardware selection with the ideal choice. That’s why it is usually better to ask an experienced access hardware supplier to show new ideas related to the application requirements, rather than automatically settling for latches used in the past.
Finally, don’t leave evaluation of latches and other access hardware until the end of the product design cycle. Doing so can severely limit ergonomic or aesthetic options, prevent the designed enclosure from working with optimum efficiency or worst of all, can force the redesign of the product in order to have the necessary latch work properly. While this could be problematic in even the simplest enclosure, it can be especially costly in gasket-sealed enclosures or in other tight-tolerance limited-space applications.