There are always situations in manufacturing and construction where welds just can’t do the job. When assembling products for automotive, container manufacture, aerospace, railway, mining, and so on, you need a reliable, fail-proof joint that can withstand stress and vibrations. That’s when it’s time to look at lockbolts.
Before delving into the details of lockbolt installation, let’s define what we mean by a lockbolt fastener. In brief, a lockbolt is pin that is fitted with a collar and inserted through a pre-drilled hole. A collar is used instead of a nut to tighten the bolt into a permanent bond. Depending on the lockbolt installation, assembly can be quick and the lockbolts can be tightened by either manual or hydraulic tools.
Proper lockbolt installation can deliver more strength than welding, but you have to understand the tolerances and limitations of lockbolt installation. The lockbolts themselves can be rated for superior strength, but fastener failure occurs when the lockbolt isn’t properly installed, the top or bottom of the grip thresholds are not in spec, or the lockbolts are installed using too much or too little pressure. To minimize lockbolt installation problems, we offer six tips for lockbolt installation:
1. The lockbolt uses locking threads that are shallower than conventional nuts and bolts. This means there is more metal in the actual lockbolt to increase fatigue strength, typically to five times that of a nut and bolt. Using shallower threads also allows you to use narrower lockbolts or fewer lockbolts to achieve the desired connection strength.
2. The diameter of the hole is important as well. Matching the width of the lockbolt to the clearance hole diameter of the holes will make a tighter connection that is more resistant to movement, especially in applications where there is vibration. The more movement there is, the sooner the lockbolt will fail.
3. It’s important to match the power of the swaging tool to the lockbolt. Smaller lockbolts often can be installed using hand tools, while larger diameter lockbolts will require hydraulic or power tools. Matching the torque of the tool to the tensile strength of the lockbolt will give you a stronger connection. If the press is set too low, the fastener will be too loose, but if it is set to high, it could strip the lockbolt. Matching the right installation pressure to the lockbolt will also help your swaging tools last longer.
4. When installing a lockbolt, you want to be sure to use a swaging tool that delivers little to no lateral motion. A smooth installation makes for a stronger connection.
5. The objective is to maximize both tensile and shear strength so all lockbolt installations should be uniform, with a consistent preload on every fastener. A tight fastener will have greater tensile strength (i.e., strength perpendicular to the application surface or axial load) and shear strength (i.e., strength against a load parallel to the surface or a transverse load). The tensile strength is dictated by the shear resistance of the collar and the number of grooves the collar fills. The shear strength of a lockbolt will vary based on the material strength and the minimal diameter of the lockbolt.
Ultimately, the strength of a lockbolt or any fastener is a matter of the geometry of the collar and the metallurgy of the steel it’s made from. The pin is made from stronger material than the collar, so when the collar is swaged or locked using progressive pressure, it is forced into the grooves of the harder pin. The more metal-on-metal contact, the stronger the connection and the less prone the lockbolt will be to fatigue from vibration. Therefore, any steps you can take to ensure the pins match the hole size and the right pressure is applied by the swaging tool will lead to a stronger connection.
From our experience, we see Huck Alcoa as a leader in lockbolt technology and engineering. Stanley Advel also has a wide range of lockbolts for various applications. You want to make sure you have the right lockbolts and the right tools for the job. The Bay Supply fastener experts are always here to help you find what you need, whether it’s lockbolts or some other type of fastener.