01 April 2013
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Design challenges and the benefits of captive panel screws

There are many benefits in using captive fasteners but there are important design considerations to be made before specification. In this special report, FAST takes an in-depth look at some of the main concerns.

Captive screws will often be the ideal solution for a designer looking how best to equip panels, covers, drawers, racks and other access points with fastening hardware. These types of fastener provide secure attachment and controlled access and facilitate easy and efficient handling and installation by keeping parts to a minimum. Captive screw assemblies can be mounted permanently, which eliminates the risks that components may be damaged if loose hardware falls out, gets lost or misplaced.

There are universal benefits to captive screws, but within this product category there are a number of options and opportunities which can be bewildering for designers. Understanding the features and benefits of the various options allows designers to focus on their requirements and decide which mounting solution is best for their application.

Mounting style

Self-clinching, flare or float/flare mounted technologies are employed as standard mounting styles for captive panel screws. The key benefit of a self-clinching fastener is that it will install flush to the backside of the panel when using the shortest screw length, ensuring that the panel opposite the installation will require minimal preparation. The method also eliminates secondary operations, thus maximising productivity on the production line. The fastener is inserted into a properly-sized hole in the mounting sheet and becomes a permanent part of the assembly by applying sufficient squeezing force from a press. As the sheet material surrounding the hole is forced into the fastener's undercut, it secures against axial movement and against rotation by a displacer.

There are three basic requirements which must be met regarding the relationship between the fastener and the sheet material.

Metal sheets into which the captive panel fastener will be installed must have adequate ductility to allow the displaced sheet material to cold flow into the undercut. Metal sheets into which
the fastener will be installed must be sufficiently softer than the fastener so that the fastener itself does not deform during the installation process. And sheets must meet the minimum thickness required by the particular fastener. Self-clinching hardware is not always the correct choice, however; this is particularly true when considering stainless steel panels due to the relative fastener to panel hardness issue. There are compatible solutions available: a captive panel fastener with a retainer made from 400 series stainless, which will reliably install into a stainless sheet.

Stainless applications will often employ flare-mounted panel fasteners with captive screws, which are suitable for any thin material with sufficient hardness. A flare-mounted fastener's shank
deforms – in contrast to a self-clinching fastener where this occurs to the panel – during installation. The technique requires minimum force, and will install flush on the backside of a panel similar to the self-clinching option. This option can also be used for centreline-to-edge applications and, since paint thickness will not hinder attachment, it can also be used for painted panels without marring the surface during installation. There is, however, an extra step in preparation is required as the mounting hole will require a countersink.

Panel hardness is not an issue for floating/flaring style fasteners, which can be installed into stainless and any thin panel material, including PC boards. Like flaremounted products, these deform on installation, require minimal squeezing force with punch and anvil to flare their retainer and serve as practical hardware for close centreline-to-edge applications.

These products also offer an added benefit, because they can compensate for mating thread misalignment, offering up to 2.06mm if float within the mounting hole. Features and Functions
Following on from mounting style considerations, designers can look at the inherent features and functions to determine the best solution for their application.

Both standard and optional features often serve to provide designers with potential solutions which advance their objectives and address primary concerns. There are several key issues addressed by fastener selection. Is it possible to avoid off-angle thread engagement and potential thread damage during assembly?

Patented anti-cross threading technology can be engineered into self-clinching, flare-mounted and floating captive panel screws. This technology can correct offangle installations, align components and slide through clogged internal threads. It is therefore possible to eliminate the failures, repairs, scrap and downtime associated with thread damage by specifying this feature. The anti-cross threading feature begins to cam over the female thread during the initial engagement when threads come into contact. The fasteners will then drive normally with reduced effort as the two thread helices align perfectly.

Can tool-only access be delivered with captive panel screws? There are captive panel screw products which will suit applications where security or safety issues can be expected to arise. Several of these fasteners have been UL approved for tool-only operation and benefit from fully recessed heads and Phillips drives. A recent (European) EC machinery directive imposing strict
safety requirements for machine guarding and the ergonomics of operator equipment has made tool only access a vital issue.

Machinery Directive 200642/EC requires that fasteners remain attached to fixed guards or machinery when guards are removed – loose screws are no longer acceptable – and tool-only products can meet these requirements.

Can over-installation of a fastener be prevented? Over-installation is prevented by a shoulder on the retainer of the captive panel screw, which provides a unique positive stop during installation. During off-angle installations, the shoulder can also help to "square up" the fastener. Which products work best when an application requires equipment to be repositioned?

It is possible to move racks, drawer slides, access panels and equipment consoles to new positions without requiring tools with a product which acts as a positioning pin, as can be found with self-clinching spring loaded plunger assemblies. A unique lockout feature can keep the fastener's springloaded plunger retracted until the plunger pin drops into the final position – the retracted locking and unlocking actions can be achieved with a simple quarter-turn of the fastener.

These types of captive panel screws – also available without the lockout feature – promote both fast installation and the swift removal of components. Can captive panel screws be protected against corrosion?

The most viable safe guard in this instance is to ensure that captive panel screw components are manufactured from 300 series stainless steel for applications where parts will be exposed
to corrosive elements.

What are the differences and advantages within the range of caps on these fasteners?

There are a range of differences within cap design for self-clinching, flaring or floating fastener mounting styles. For example, metal version with large knurled heads promote easier hand operation; smooth metal caps without knurls would be more appropriate for limited manual access and metal caps with optional DuraBlack finish (also applied to the screw component) will resist scratches and enhance the cosmetic appearance of the assembly.

Plastic (PC/ABS) caps have been developed as an alternative to metal caps; these solutions advance maintenance, functional and branding requirements. To support aesthetic requirements within the design, the plastic caps can be specified in black, red, orange, yellow, green, blue, violet or metallic as standard. Custom colours can also be delivered by providing the Pantone number, L.A.B. number or a swatch. These colour options offer the capability to designate service access levels for equipment, colour-reference operating and maintenance instructions as well as allowing designs to confirm to aesthetic branding requirements. What driver types are available for captive panel screws?

Designers can choose from Phillips/slot combinations, Phillips only and slotonly drives. As with all features, the designer's selection will be influenced by the application of the product. Other custom driver styles are also available. Cost If cost is key, designers have a range of options for how to address this concern. or example, panel fasteners with captive screws have largely been engineered as spring loaded assemblies, but there are also patented types available without springs which offer low-cost design solutions.

The patented spinning clinch bolt with one-piece captive screw can provide a lower cost alternative to multi-component fastener assemblies. These fasteners benefit from controlled clinching action during installation; this captivates the screw permanently while allowing it to spin freely in the host metal sheet. This product for metal panels provides a reliable means of attachment without use of a retainer or washer and can be supplied with an unthreaded neck for axial float with a jacking feature or spring for selfretracting, which all contributes to lowering manufacturing costs. These products also address concerns related to centreline-to-edge applications.

Technological innovation and creative design ensures that the range of possibilities when specifying captive panel screws continues to evolve. A designer can enlist a supplier's support and resources early in the design process in order to find a solution which is fit for purpose. Given a comprehensive understanding of the application, the supplier can also address new assembly challenges or changes to the end-product design as and when they arise, working in partnership with the manufacturer to achieve creative and effective solutions. As the working partnership between manufacturer and supplier grows, so too will the choices become clearer and the application will benefit from the latest, cutting edge fastener solutions.

Author
Paul Gay

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