Designing For Manufacturing: Don't Forget the access hardware

By Michael Fahy
Supervisor, Customer Application and Development Center
Robert Bishop
Senior Application Engineer Southco Philadelphia, Pa.

EDITED BY STEPHEN J. MRAZ

Widespread concur-rent-engineering practices have made it routine for design teams to decide early in the design process which types of latches, hinges, screws, and other access components will be used. This helps en-sure final designs will actually work. But different manufacturing considerations for each type of access hardware are just as vital as the overall design and should also be examined as early as possible in the product-development cycle.

COMPRESSION LATCH BASICS
Compression latches provide enough closing force to engage their internal gaskets, thus damping vibrations and protecting the enclosure against dust, water, and other contaminants. Compression latches are often used in applications that require EMI/RFI shielding, environmental gasket compression, and complete sealing against contaminating or corrosive environments.

There are two types of compression latches. Fixed-compression versions let users apply a preset force when closing the latch. Adjustable-compression models let users regulate the amount of compressive force applied against the door, the frame, and the gasket.

When choosing compression latches, determine which features are important. Easy grip adjustment? Consistent compression? Strong leverage to open sticky doors? Simple installation? Then look for the latch that meets those functional requirements.

FIXED-COMPRESSION LATCHES
Fixed-compression latches securely close doors and panels, and then provide a strong, preset compressive force to seal out potential contamination. Turning the latch, either by a knob, handle, or tool driver, rotates the pawl behind the frame to hold the door closed. Turning the latch further moves the pawl up the shaft to pull the door and frame tightly together. The latch provides the same amount of compressive force each time it is used. Since the compression remains constant, these latches can help support flimsy doors by holding them tightly to their frames.

With fixed-compression latches, mechanical advantage is designed into the latch — users do not need to apply force to latch the door closed. The "pull-up" action of the pawl applies all the necessary force. Many fixed-compression latches let engineers adjust the grip range to allow for panels and gaskets of different thickness.

There are a few important manufacturing considerations specific to fixed-compression latches. Most must be assembled from the back of the panel, requiring complete access to that panel during installation. Fixed-compression latches are generally screw mounted, although a few styles may be welded.

Lever-style, fixed-compression latches are almost always installed on top of the outer panel, and do not latch behind the frame. Actually, these latches do not protrude at all inside the enclosure and do not require mounting holes. They do, however, require a special keeper installed on the outside of the frame for the latch to catch on.

Fixed-compression latches can be specified with many different grip-range options. In many cases, one latch can accommodate a wide grip range, which affects how the latch is assembled and installed. These latches have reversible pawls. Turned one way, the pawl has a short grip range. Reversed, the pawl will handle wider grips and thicker panels. Preassembled latches offer faster installation, because its components are already put together, but they may limit the options for grip range and panel thickness.

ADJUSTABLE-COMPRESSION LATCHES
Adjustable-compression latches operate similarly to fixed compression models in that turning the latch swings the pawl behind the frame to securely latch the door. But with adjustable latches, the user then continues to turn the latch to apply exactly the amount of compression needed. As the latch is turned, the internal pawl moves up the latch shaft and compresses an internal spring. Unlike fixed-compression latches that apply the same amount of compression each time, adjustable-compression versions apply only the amount of compression users stipulate. And it can change each time the latch is used. This can be an advantage in applications where changing environmental conditions may change the compression requirements.

Adjustable-compression latches are generally installed from the rear of the panel, like fixed-compression latches. But there are more styles of adjustable latches that can be welded to a panel, which speeds assembly. And although different sized adjustable-compression latches will fit a wide variety of grip ranges and panel thickness, each particular model fits just one specific range and usually, one maximum panel thickness. Therefore, these latches are preassembled. Assemblers need only prepare a hole in the door, pass the latch through the hole, and tighten the mounting screws or weld the latch to the door.

While each type of compression latch has different installation and assembly requirements, a few general rules can help select the best one. For example, panels should be finished before installing the latches. This includes heat treating, painting, and other finishing activities. If you decide to paint the latch to match the panel, take care to mask any driver recess, threaded components, and exposed features that might affect latch operation. Once installed, however, spot finishing or sanding can take care of final details, and should not affect the latch.

Screw mounting these latches provides a lot of flexibility for installation, mainly because it does not require bonding any materials. And although compression latches are generally installed into sheet metal, they can also be mounted on panels of fiberglass, wood, softer steels, and other materials.

Generally, keylocks and other special tool-driver recesses, which are used as an additional layer of security, are integral to the latch and do not require special panel preparation or installation requirements. Padlock bracket accessories, however, may require additional assembly steps.

Although there are a few different methods of installing compression latches, automated assembly is generally not a viable option because of the components involved. The application as well as the latch selected will generally dictate when in the assembly process to install the latch.

CAPTIVE SCREWS
Captive screws, like latches, are used to fasten two panels together. They are typically used in the computer and electronics industries to fasten computer panels, removable power supplies, and other components to a chassis. One of their major features is that they prevent access hardware from being dropped, lost, or misplaced during assembly, operation, and servicing. With their relatively small size and low profiles, they make ideal choices when space is limited.

Captive screws come in a wide variety of standard thread sizes, styles, and driver recesses, as well as in various installation styles, including press-in, flare-in, snap-in, and floating. And each type of screw has different manufacturing and assembly considerations, which may also depend on the application. The primary consideration when installing any captive screws is the panel. As with most access hardware, all finishing operations, including heat treating, painting, and plating should be complete before installing captive screws.

The next most important consideration is to allow clearance for the backup tool during installation. Press-in and flare-in installation styles require that the assembly person press a tool under-neath the screw to keep panels from dimpling or being damaged during installation.

Alignment is also a potential issue, so be careful of tolerance stack up. If panels are hinged, slide on tracks, or need to be fastened with anything other than captive screws, it is extremely important that each piece of hardware stay within tolerances. Otherwise, screw threads may not align prop-erly with mating threads, giving the screw a heavy side load. This often causes improper operation and somewhat diminished screw retention.

Installing captive fasteners also requires the assembly person have access to both front and back of the panel. This means captive screws must be installed relatively early in the assembly process. Designers must also provide threads for the screw to fasten into. You may need to drill threads into mating panels, or install separate threaded inserts. Through all of this, make sure panels will align properly when brought together.

Press-in and flare-in captive screws are widely used because they require only minimal installation work. For press-in screws, once a hole is prepared in the panel, the screw is pressed into the hole until its ferrule passes through the panel. For flare-in versions, the screw is placed in a prepared hole and an installation tool flares the ferrule from underneath the panel to hold it tightly to the panel.

Captive screws that must be pressed into panels call for some specific considerations. For example, before plating, painting, heat treating, or finishing the panel, the screw hole in the panel must be properly prepared with the correct dimensions. Don't worry about paint or plating getting in the hole. As the screw is pressed into the panel, it will displace any paint or plating. This lets the panel material "flow" into the groove in the screw's ferrule, making sure the screw is retained correctly in the panel.

Snap-in captive-screw assemblies simply snap into prepared holes in panels. This leaves a gap between the inner and outer panels, unless holes are counterbored. With thicker panels, the counter-bore is necessary to hold the screw in place. For a pair of panels too thin to counterbore, one of the panels might need to be dimpled to bring both panels flush with each other.

Floating captive-screw assemblies are good choices for applications in which it is difficult to hold tight alignment tolerances. Floating captive screws allow for up to 1.1 mm of radial float.

HINGES
Hinges come in several different styles, from standard piano-type to versions that offer added functions, such as door positioning, concealing the hinge, and easy lift-off or removal. They can become the "forgotten component" in access hardware, especially if your main consideration is the latch. However, hinges come with their own manufacturing considerations, especially for installation and assembly. Hinges are generally easy to install because it can be done with various methods, including screws, studs, rivets, welding, and adhesives, depending on the hinge.

Hinges, like compression latches and captive fasteners, generally require that the panel and door be painted, plated, and finished before installation, because the hinge materials may not tolerate all of the finishing operations.

It is also important to keep in mind manufacturing tolerances. Proper hinge alignment on the door and frame is critical to performance. One way to compensate for manufacturing variations is to make slotted holes in the panel. They let the hinge be positioned more finely during installation.

Door-positioning hinges combine two functions: a hinge and door stay. They can hold doors open without using secondary mechanical supports like door stays or gas springs. Some have a builtin negative detent to hold a door closed under vibration and prevent rattling. Door-positioning hinges let users position doors at preset detent angles for hands-free access. With friction-style positioning hinges, users can dampen or free up the door swing simply by turning a positioning screw.

When installing these hinges, the door must first be properly positioned in the frame to ensure everything aligns when the door is hung on the hinge.

Concealed hinges are installed inside the enclosure or cabinet, and are completely hidden, leaving a clean outer surface. Concealed hinges also allow for door removal. This makes them a good choice for cabinets that have little space around the outside, but require complete access to the interior.

Most concealed hinges can be welded to the door and frame. Depending on the enclosure design, you may need to add a return flange to the door or frame to completely conceal any installation hardware. This may mean an extra manufacturing step. There also has to be clearance for operators to install the hinge. Make sure there is enough space on both the door and frame to let the proper tool reach the hinge and complete the installation. And, of course, ensure there is enough clearance for the hardware itself.

Manufacturers who outsource assembly to different vendors, especially if doors and frames are manufactured at one company and assembled in another, should consider using concealed hinges that consist of two separate leaves and a hinge pin. It accommodates modular assembly, and each section can be installed separately.

Doors on lift-off hinges can be quickly and easily removed and replaced. Lift-off hinges are a good choice for most applications because they install easily on a variety of door and frame configurations. As with most other access hardware, it's important to finish the panel before installing hinges. It is also critical to allow enough clearance at the back of the panel for the assembler to put in the mounting hardware and to ensure each hinge leaf aligns properly.

Thinking about the manufacturing process early in the design cycle saves time and money by making sure the product will be manufactured and assembled cost effectively, and accommodate end-user needs.