How to test self-clinching fasteners for reliability

By Ron Dise, Corporate Senior Technology Specialist, PennEngineering

Self-clinching fasteners are a good way to attach thin metal assemblies. Here are tips on how to ensure proper installation.

Self-clinching fasteners deliver time-tested solutions for attaching thin metal assemblies. Regardless of type, these fasteners install permanently in thin ductile metal sheets when pressed into place in a properly sized hole with sufficient squeezing force. The fastener’s serrated clinching ring, knurl, ribs, or hex head is embedded into the panel surface, displacing sheet material into a specially designed annular recess in the shank or pilot of the fastener, known as an undercut. The metal displaced into the undercut secures the fastener against axial movement, while a non-round displacer secures the fastener against rotation, resulting in the fastener’s permanent installation.

Self-clinching fastener nuts have thread strengths that are greater than milled steel screws, and are commonly used wherever strong internal threads are needed for component attachment or fabrication assembly.

Upon installation, self-clinching fasteners provide permanent and reusable load-bearing threads to accept mating hardware in metal sheets too thin to be tapped or where extruded or stamped threads would be impractical. They become integral parts of an assembly, will not loosen or fall out (even when the mating thread is removed), never have to be restrained from rotation with a tool, and never have to be handled again.

While each attachment application will present particular requirements or challenges for designers when considering and evaluating self-clinching fastener options, two noteworthy measures of mechanical strength can help predict a fastener’s reliability in service: torque-out and push-out (expressed in in.-lb or N•m). Fastener manufacturers usually will conduct their own fastener tests and publish the resulting values as guidelines to help designers determine suitability for a given application.

Torque-out strength
Torque-out reflects the torsional holding power of a fastener’s clinch feature, or the amount of torque necessary to spin the fastener out of the host metal sheet. This test often is made at the head of the fastener with values usually exceeding the ultimate torsional strength of a mating screw or nut.

The measure of torque-out strength becomes a valuable predictor of anticipated reliability, because a fastener’s torque-out value must be adequately high to resist the highest torque that the fastener will encounter in service. If the fastener cannot offer such resistance, failure in service can follow.

When testing for torque-out, two potential modes of failure may arise. In the first, the fastener’s displacer will shear metal sheet material out of its way and rotate in the panel with little or no axial movement. This failure mode is typical for installations involving soft aluminum panels. The second failure mode, typically described as “cam-out,” occurs when angled portions of the fastener’s displacer exert an axial force in the push-out direction, causing some axial movement of the fastener in the sheet, thus allowing the displacer to rotate.

Another type of self-clinching fastener is the standoff. These fasteners are usually used where it is necessary to stack or space components away from a panel.

One common misconception is that 100% of the tightening torque applied to a mating threaded component will be applied to the self-clinching fastener as well, but this is not the case. A threaded connection is (at best) about 15% efficient, meaning that at least 85% of the tightening torque will be consumed by friction (divided almost equally between the mating threads and the loaded face of the turned part).

As a result, this fundamental rule of thumb applies (with few exceptions): tightening torque for normal assembly can be as high as two times the torque-out strength of the self-clinching fastener. (One notable exception is the abnormal condition of cross-threading, in which case the fastener will see 100% of the applied torque.)

Push-out values
Push-out values will indicate the axial resistance of a fastener to remove it from the metal sheet opposite to the direction from which it was installed – in short, assessing the axial holding power of the fastener’s clinch feature. In general, the axial resistance of a fastener should be approximately 5% to 10% of the force used to install it.

In a typical installation, the self-clinching fastener’s undercut is filled (or nearly filled) with material from the host metal panel, which allows the panel material to make contact with the back taper of the fastener’s shank. Optimum push-out performance will be created when the force to dislodge the fastener’s displacer combines with the force required to clear the panel material from the undercut.

Conversely, reduced push-out will occur if the hardness differential between the fastener and host metal panel is insufficient – resulting in deformation of the fastener’s displacer and only partial filling of the undercut. In this case, the force to clear the panel material from the undercut will peak after the force to dislodge the embedded displacer, resulting in lower push-out.

Push-out strength ultimately must be adequate to resist any force applied during the engagement of the mating threaded part.

For threaded self-clinching fasteners, another useful test to help determine a self-clinching fastener’s reliability in service is the measure of pull-through – the resistance of a fastener to pulling through the host metal sheet when the mating fastener is tightened, creating clamp load. (Notably, pull-through generally is published only for self-clinching studs and standoffs.)

In addition to these performance tests, designers will also want to ensure that a fastener can be expected to meet other relevant application requirements, such as vibration resistance, thread locking, heat and electrical characteristics.

Another type of self-clinching fastener is the stud. These fasteners suit applications where the attachment must be positioned before being fastened.

Other considerations
Testing for suitable torque-out and push-out strength can shed significant light on how reliable a fastener will perform in an application. But a fastener’s reliability additionally will be influenced by a variety of application-specific factors and, if any one of these factors is not within the required range, performance may be compromised.

Among them:
• The fastener must always be harder than the sheet in which it will be installed. Regardless of type, self-clinching fasteners install permanently in thin ductile metal sheets by pressing them into place in a properly sized hole and then applying sufficient squeezing force. The fastener’s serrated clinching ring, knurl, ribs, or hex head is consequently embedded into the panel surface, displacing sheet material into a specially designed annular recess in the shank or pilot of the fastener, known as an undercut. The metal displaced into the undercut secures the fastener against axial movement, while a non-round displacer secures the fastener against rotation to result in the fastener’s permanent installation.

Due to the requirements of this process, metal sheets into which the fastener will be installed must exhibit adequate ductility to allow the displaced sheet material to cold flow into the undercut without fracturing. In addition, the host metal sheet must always be sufficiently softer (typically 20 points on the HRB or HRC scale) than the fastener to prevent fastener deformation during installation and to promote reliability in service.
• Holes in the host metal sheet must be properly sized and distanced from edges and bends.
• Host sheets must meet the minimum thickness recommended for the fastener. If the host sheet is too thin to accommodate the fastener, failure can follow. Most legacy self-clinching fastener product families can be installed reliably into sheets as thin as 0.76 mm, with several traditional families going down to 0.51 mm. Trends toward thinner sheets have prompted new product families to satisfy those applications with sheets as thin as 0.3 mm (and, in some cases, even thinner). Typically, there will be no limitation on the maximum thickness of a sheet.
• The clinch feature of the fastener must be designed correctly.
• The clinch feature of the fastener must be manufactured to tight tolerances.
• Fasteners must not have been embrittled by any acid cleaning or electroplating.

Designers have an array of options in the world of self-clinching fastener technology, especially since dozens of fastener types and thousands of variations (steel, stainless steel, or aluminum) have been engineered over the years. As a big assist, besides testing for reliability, best practices can be advanced when designers consult with an established and experienced fastener manufacturer early in the design process on the road toward successful outcomes.

Reprint info >>

PennEngineering
www.pemnet.com