For many decades, welding was considered the only way to ensure joint integrity in demanding load-bearing or high-vibration structures. So, companies manufacturing heavy-duty equipment or fabricating large, metal structures only welded joints.
Today, there is a fastener alternative to welding: direct-tension, swaged lockbolts. These two-piece fasteners are part bolt and part blind rivet. They consist of a short, non-threaded cylindrical collar and a long, headed pin that has both non-helical threads and a pintail that breaks off during installation.
These fasteners are installed using a direct tension technique, in which the pin is pulled while the collar is simultaneously swaged into the locking grooves of the pin, deforming the collar into the grooves.
Lockbolts produce permanent joints with high clamp loads, stability and excellent resistance to vibration. Because the lockbolts’ installation process is faster and safer than welding, and does not require highly trained personnel, manufacturers are increasingly using them to assemble truck suspensions and chassis, railroad track crossings and heavy defense vehicles.
Over many years and millions of joints, welding has proven to be one of the surest processes for securing a joint. But, it does have several drawbacks. The welding process is time consuming, and it requires several costly and time-consuming quality control procedures. Welding also requires skilled, often highly paid, technicians, who, in industries such as oil and gas, are difficult to find and retain.
Another issue relating to welding is heat. The heat generated by welding can significantly reduce the strength of the metals being joined and damage critical surface coatings. In addition, welding dissimilar metals or pieces of varying sizes can be a challenge. Experienced welders also know that removing and reworking a weld can be problematic.
Finally, welding is dangerous and dirty. Safety is critical and must be practiced continually because a random spark can start a deadly fire. Equally important, spent rods and other remnants of the welding process must be handled properly to keep the work area and plant safe and clean.
Fastener Features and Benefits
Introduced during the mid-20th century, lockbolts are available in a wide range of pin diameters. Lockbolts are usually made from steel, stainless steel or titanium. However, they can also be made from aluminum because their non-helical threads do not gall during installation. When a material galls, some of it is pulled by the contacting surface. Helical threads on conventional fasteners often gall during run-down.
Pins can be made with round, brazier, truss, mushroom or countersunk heads. Collars come with or without flanges. Both components can be coated or plated to prevent corrosion.
The main benefit of a lockbolt is its long-term resistance to the effects of vibration. In a conventional bolted joint, there are always gaps between the threads of the nut and bolt. These spaces cause loosening in high-vibration applications. In contrast, an installed lockbolt has no gaps because the collar is fully swaged into the shallow locking grooves of the pin. These shallow grooves produce high joint strength—up to five times that of a conventional nut and bolt.
Other benefits include high shear and tensile strength. Data from Junker tests—a standard procedure that exposes fastened joints to transverse vibrations—indicate that lockbolts maintain consistent clamp load over time, without requiring secondary devices such as lock washers, wires or thread-locking compounds.
Most lockbolts can also be used with virtually any metal. Dissimilar metals with dissimilar coefficients of thermal expansion present no problems. Varying piece sizes in a joint are readily accommodated, and surface finishes are not harmed.
Installation: Quick and Easy
Lockbolts are installed using a direct tension technique. The pin of the standard lockbolt is inserted into a prepared hole, the collar is spun onto the pin, and the installation tool is placed over the grooves on the pin. When the tool is activated, a puller in its nose assembly draws the pin into the tool, causing the swaging anvil to press on the collar and draw up any sheet gap.
At a predetermined force, the anvil begins to swage the collar into the pin’s lock grooves. Continued swaging elongates the collar and pin to produce a precise clamp load. When swaging of the collar into the pin lock grooves is complete, the tool pulls on the pintail and snaps it. This completes installation.
A typical lockbolt (0.625-inch-diameter Grade 8 fastener) can be installed in as little as 2 seconds, not counting the time it takes to drill a hole. Cycle time is short because the tool grabs the pin very quickly.
Once the operator engages the trigger, the swage and eject sequence is programmed to complete the cycle without any additional installer input. A quick visual inspection is all that is required to confirm the fastener is properly installed. In addition, an operator requires minimal training to proficiently install lockbolts—making the use of lockbolt fasteners a cost-effective assembly process.