Releasable fastening without fasteners
FIGURE 1. In this snap-fit assembly example, the elastic latch in the part to be assembled is deflected and snaps back behind the hook in the base as the parts slide together. The insert can be released with pressure applied to the sides.
Snap-fit connections in plastics need no introduction. The art of their design has, over many decades, been turned into well-established engineering practice. The design and use of such connections in sheet metal, however, is still in its infancy despite obvious benefits such as rapid assembly or disassembly and the elimination of fasteners.
Sheet metal assemblies using hooks or slot-and-tabs to hold parts together are not new. Hooks or equivalent structures remain involved in a snap-fit connection, holding the parts together in all degrees of freedom but one. A hook-and-latch connection is then integrated, preventing back-out, thereby locking the part in place. Once conceived, this method enabled the development of a family of fastening solutions among different combinations of sheet metal, thin-walled tubes, and components fabricated by other means. Following are several examples to provide a visual reference.
In Figure 1, the elastic latch in the part to be assembled is deflected and snaps back behind the hook in the base as the parts slide together. Disengagement or directional forces on the part load the latch in tension, eliminating any concerns about buckling. Optional finger buttons formed in the latches can be pressed for manual release.
Figure 2 has an elastic latch in the base that is deflected by a “short edge” in the part when pushed together. After sliding into the engaged position, the latch snaps back behind the edge, locking the part in place.
In Figure 3, a standard unlocked bayonet connection between a tube and sheet has been modified and is used to lock the two pieces into place. This can be released with a custom tool, also made of sheet metal.
Figure 4 showcases a couple of snap-fit connections. The elasticity of the flange itself can be used to form a latch. In this example, a U-shaped section functions as a base plate for a flat plate and is itself inserted into another base plate. The rectangular extension and hole at the bottom of the inserted flat plate serve as a tool grip to allow pull-in using a screwdriver. The U section is inserted into a base plate by squeezing the flanges together.
Figure 5 highlights the ability to incorporate injection-molded parts into a snap-fit design. In this case, elastic latches can be used to attach the dissimilar materials. This also can be used for machined parts.
Many more connections are possible than those pictured. They are possible between any combinations of sheet and thin-walled round or rectangular tube.
Assembly can be accomplished through sliding translation, sliding rotation, manual latch activation, and straight insertion. Connections can be designed to be easily accessible and releasable by hand, releasable with tools, hidden from reach, or inaccessible (permanent).
Disassembly may be discouraged in a connection as in Figure 1, for instance, by merely mirroring the orientation of the hook in the base, making the latch release movement (push) relatively inaccessible, as it would have to come from inside the assembled part.
FIGURE 2. This assembly has an elastic latch in the base that is deflected by a short edge in the part when pushed together. After sliding the part, the assembly is locked into place.
Elastic latches can be incorporated into the base or into the part to be assembled. The hooks on the elastic latches can be left open (see Figure 1) or be closed slots (see Figure 4). The required elasticity might be provided through the bending of the sheet plane itself or, with careful design, along/within the plane of the material.
The play necessary for easy assembly can be reduced or eliminated entirely. Proper engagement often is signaled with a satisfying “clang.”
In most cases when incorporating these snap-fit connections, assembly times are reduced dramatically when compared with using fasteners. This is most certainly true for sliding engagement with latch deflection designed into the parts. A slide-in production part based on the design in Figure 1 resulted in a bend count reduction from four to two, approximately 10% in material savings, and an estimated 90% savings in assembly time. If the latch or flange is manually activated before assembly, as in Figure 4, more care and strength are required when assembling, leading to relatively slower assembly times.
The flexibility of laser cutting allows the inclusion of extra details to add functionality or aid assembly. Some examples are motion limiters to prevent plastic deformation of latches and tool grip features where tool-aided assembly or disassembly makes sense. Simple helper tools for releasing parts may also be made using sheet metal (see Figure 3). Although the examples presented here have all been laser-cut, the same principles could be applied to parts cut with other methods or to stampings.
The author’s use of these types of connections in actual products has mainly been in attaching components to a cow-milking robot, which led to large reductions in the number of fasteners used. No tab-and-slot connections needed to be replaced. (The tab-and-slot connections are the closest living relative of the snap-fit connections discussed in this article.) For a comparison of tab-and-slot designs to snap-fit connections, see Figure 6.
Although prototype testing was done to ensure function, dynamic and stress testing were not necessary for the application. Most designs used 1.5- to 5-mm-thick stainless steel plate. Finite element analysis was only done in a few cases when the latches deflected in the plane of the material or in the case of some tube/tube connections.
More upfront design time is required for snap-fits than with conventional fasteners. The 3D information can be recycled, however, speeding up subsequent work. When parts are designed or redesigned to integrate snap-fits, they often can be simplified further for an even greater cost reduction.
Besides the obvious industrial use of these connections, they also make sense for consumer-oriented assembly and flat-pack products because of their simplicity and tool-free actuation. Think IKEA-type consumer applications.
Easily releasable mechanical fastening of sheet metal without fasteners is now a reality. The snap-fit connections make that possible. But be aware that new possibilities open up when plastic deformation is allowed or even included as a feature and releasability is not required.
Hopefully someone will be inspired to put these types of designs on a more solid engineering foundation to benefit all of industry. This could include strength and dynamic testing, as well as optimization of hook and latch dimensions for various sheet thicknesses and uses. Some standardization also would pave the way for incorporation into CAD/CAM libraries for clickable or drag-and-drop placement in new designs. (Ask your CAD application supplier for this.) That way, they can take their place in the engineer’s toolbox alongside the conventional nuts, bolts, rivets, and welds.
For more snap-fit connection examples, check out the Newton Innovations YouTube channel or website. Three models are available as a free STEP file download in the web shop.