So you’re designing an enclosure. You look through your old notes, but it seems you slept through the enclosuredesign component of Engineering 101. You assume it can’t be that hard, until you start looking at all the available hinge, latch, handle, and support options. And you know that designing it right the first time saves time and money, not to mention your reputation, so let’s take a minute to look at the design considerations and hardware choices that will result in the best enclosure for the money.
It all hinges on this
Hinges are the first piece of hardware you might think of. After all, a hinge is essential for access to whatever is inside the enclosure. But even though hinges appear relatively simple, there are a wide range of issues to consider.
Edited by Jessica Shapiro
First is the weight of the door or lid. Turn to force diagrams and moment calculations to best support the weight of the door, and anything suspended from it, with one or more hinges.
More detailed mechanics-of-materials analyses may be needed to determine the best hinge type and material. Keep in mind that you can specify different materials for hinge plates and pins, although it’s often not economical to do so. For instance, lighter aluminum plates might be augmented with stainless-steel pins for better strength.
Pins can be secured by staking them in place. This is done by slightly indenting one or more of the knuckles with a pointed tool to hold the pin in place. Other choices include coined pins and spun ends. In both cases, pins are formed to make the head larger than the pin diameter but smaller than the knuckle OD. Also consider pins that are welded, crimped, bent, or splined. (See sidebar.)
It’s also important to think about where the enclosure will be installed and how the door opens. Which way will the door swing? Is there room for external hinges to operate freely? Should the door open 90 or 180°? Should the hinge hold the door at a predetermined angle? Stop hinges maintain a set door position.
Also choose the amount of offsetting or swaging. Swaging is the distance a leaf is offset in relation to the centerline of the pin. The right offset gives you the desired open angle. Swaging also controls the distance between the two surfaces joined by the hinge. (See sidebar.)
Security is another consideration. When properly mounted, weld-on hinges, invisible, and concealed hinges make door removal harder. Consider the durability of external hinges that might be prone to either accidental damage or vandalism; bent hinges can compromise the function and life of the enclosure.
Finally, don’t neglect aesthetic considerations. You can specify various finishes, including different colors of anodizing, a brushed surface, or polished surfaces for many hinges.
Continuous hinges come in a variety of stock lengths, widths, and thicknesses. Manufacturers can also customize hole count and placement, offsets, fastener type, springs, and surface finish. Simple twoknuckle, male-female hinges ease door removal and are often called slip hinges. Butt hinges have a three, four, or fiveknuckle configuration.
Latch hinges and unhinges have grips on their pins and springs to ease pin retraction for door removal. Some have automatic holdback features to keep pins from springing back into position until the door is installed.
Concealed hinges mount inside the enclosure, a configuration with both aesthetic and security advantages. Invisible hinges mount into the door’s thickness rather than on its surface so when the door closes the hinge cannot be seen, accessed, or tampered with. This also eliminates the door-to-jamb gap seen with most hinge designs.
Depending on production timeline, budget, and number of units, you might choose to modify a stock hinge and perform secondary operations or custom order the entire hinge assembly.
What’s the catch?
So now that you’ve chosen the right hinge for the application, it’s time to look at some other hardware for your enclosure door. Because you have a door, you’ll need a latch or catch to keep it closed and a handle to help open it. Although the two hardware types have opposite functions, they need to work together.
Temperature, humidity, corrosive environments, and cleaning fluids can affect hinge, latch, and handle function. Tight-tolerance enclosures can become difficult to open if latches corrode. Handles that are too thin could either corrode through in harsh environments or fail in fatigue from repeated use.
When it comes to handles, also take a look at ergonomics. Who is opening the door and from what angle? Can they use one hand or two? Many types of latches can be closed with one hand. Slam latches spring into place when the door is pushed shut and are designed to withstand the door being slammed. Magnetic catches have no moving parts, so they are reliable over the long term.
For applications where magnets could damage sensitive electronics, you might prefer nonmagnetic catches. The male part of the latch, mounted on the door, inserts between spring-mounted balls on the mating part. Technicians can adjust the springs to make the latch easier or harder to open. Magnetic and nonmagnetic catches usually mount inside the enclosure; slam latches mount on the outside.
Compression-spring, or pulldown, catch-and-strike sets have a loop on the door-mounted part that hooks into the frame-mounted part. Pressing the catch flush against the door, with or without the aid of springs, puts tension on the loop and holds the door shut.
A hasp-and-staple latch permits secondary locks. The hasp, a metal tab with a slot, mounts on the outside of the door. The staple, a metal part that extends from the enclosure frame or an adjacent door, fits through the hasp’s slot. There it can be secured by turning the hasp or by fitting a lock or pin through the staple.
When you discuss hinge options with an experienced supplier, it helps to have your terms straight. Use these diagrams to distinguish knuckle length from pitch, full-swaging from half-swaging, and staked pins from welded ones.
While latches keep doors closed, pulls, knobs, and handles let users open it again. Your handle hardware choice will be largely dictated by aesthetics, but other factors come into play as well. In tight quarters you might choose a flush pull where the handle swings into a recess when not in use. Harsh environments or washdown requirements may affect the material choice.
Hinges, latches, and handles are fundamental parts of enclosures, but other simple devices like gas springs can assist in door operations. A gas spring uses the properties of pressurized nitrogen to help support and hold a heavy door open and help keep it from slamming closed.
Design the enclosure so the gas spring installs rod end down at rest and through most of its stroke. This lets oil in the shaft cushion the piston at the end of its stroke and permits lubrication with every stroke. Rodend- down orientation also staves off seal failure.
Pressurization of gas springs means ambient temperature affects their output force and the rate at which the spring lets the door close. At high temperatures, rubber seals become more permeable and gas molecules diffuse through the seal more quickly. On the other hand, cold temperature stiffens rubber compounds and can create seal problems as well. So make sure seal material and design can tolerate the entire range of temperatures you expect, keeping in mind that gas springs are typically safe to use between 0 and 150°F.
Gas springs will constantly lose pressure over time. Without actually testing the application, it’s hard to know how long a gas spring will last, so plan on regular enclosure inspections. Factors affecting the rate of loss include size, orientation, number of cycles, ambient temperature, and vibration.
You’ll need to do an engineering work-up to determine where best to place the gas spring on the door, spring length, and force or internal pressure requirements. Ergonomics come into play again here. Some gas springs come with a shaft lock that prevents spring compression until the lock is intentionally released.
To keep a heavy door from banging open or slamming shut, look into dampers using the same considerations as above. Instead of supplying force as gas springs do, dampers are force-absorbing devices. Small orifices in the damper’s piston limit the movement of hydraulic fluid to control the damper’s speed and force.
Extension dampers should mount shaft-down like gas springs. Compression dampers should mount shaft-up for the same reason. Dampers mounted in the wrong orientation may work weakly or not at all. Dampers contain oil so that lubrication concerns are moot.
You may also choose doubledamped dampers, which work in either direction. Self-centering dampers control motion in compression and extension and return to a center position.
Once the door opens, workers may want to prop it open. This is especially true for lids. If you think workers will need more than momentary access to the cabinet, consider installing a lid support or using a gas cylinder with a shaft lock.
Lid supports are usually two-piece units. One or both ends should have a pivoting feature to accommodate the changing angle between the door and the enclosure. The supports fold on themselves when the door closes and unfold to lock in place when the door reaches a certain opening angle. The two parts can be joined at a single pivot point or one part can travel via a slot in the other.
When in doubt, a good hardware supplier can walk you through choosing hinges, latches, supports, and other hardware that all work together.