In an assertive move, Howmet Fastening Systems (HFS), a brand of Howmet Aerospace, has ordered a cold-forging machine to produce large lockbolts in the UK, as it sees customers moving away from conventional joining methods such as bolts, studs and welding.
Currently in build, the Sacma 670AL (pictured below) is a high-speed, 33 mm, six-station cold-forging header with the capability of making parts at up to 80 pieces per minute in lockbolt diameters between 12-30 mm. It will produce the company’s Huck lockbolt pins and collars (pictured), a range of two-piece vibration-proof fasteners used in safety-critical applications across various industries. The machine is the first of its type in the UK, potentially heralding a step-change in the fastener market. So what precisely is so special about lockbolts?
Jonathan Craven, operations director at Howmet, says: “Our DIBt-approved range of grade 10.9 lockbolts offer maintenance-free vibration-resistant joints for the full service life of the joint. Other benefits include fast and easy installation, and an installed dynamic performance providing up to five times greater fatigue life than conventional HV bolts.”
THE DESIGN
Naturally, the secret behind the success of lockbolts lies in their design, as Andrew Smith, Howmet manufacturing manager, explains: “Lockbolts work in a similar way to a regular nut and bolt, but there are some crucial differences. The male part is the hardened metal pin, similar to a bolt, which inserts through the materials that require joining. The metal collar, similar to a nut, goes over the exposed end of the pin.”
A special installation tool grips the pintail of the lockbolt, pulling it to close any gaps in the parent materials. Inside the nose assembly, a hardened-steel, conical-shaped anvil forces its way down the collar to squeeze, or swage, the collar material progressively into the grooves in typically less than two seconds. The direct metal-to-metal contact of the collar swaged into the grooves of the pin eliminates the loosening effects of transverse vibration. This capability provides differentiation from conventional nuts and bolts, which may require other special components like washers, locking wires or thread-locking adhesive to prevent nuts from vibrating loose.
“With a lockbolt, because of the swaging process, the collar will not come off the pin,” confirms Smith. “Furthermore, the threadform on the lockbolt pin is far shallower than that of a regular nut and bolt. This design enables the collar material to swage into the lock grooves of the pin. The result is greater cross-sectional area through the pin, giving five times more fatigue strength than a conventional nut and bolt, as well as higher shear strength.”
Mechanically speaking, during a lockbolt installation, the squeezing action reduces the diameter of the collar, increasing its length. This, in turn, stretches the pin, generating a clamp force over the joint. Shear strength will vary according to material strength and fastener diameter. By increasing the diameter or grade of material, it is possible to improve shear strength. Notably, the tensile strength of lockbolts is dependent on the shear resistance of the collar material and the number of grooves it fills. The collar material and heat treatment are also critical to its capability as a vibration-proof fastener after swaging. Here, the major facilitating factor is 100% contact between the collar and pin.
“Another lockbolt advantage is a precise clamp,” explains Smith. “With Huck lockbolts, users can normally achieve a ±6% variation in clamp, compared with 30% for a conventional nut and bolt. Engineers may therefore be able to reduce the number of fasteners, or the diameter. In addition, they can achieve a more uniform load share between grouped fasteners in any given structure.”
SNUB LOAD
The collar also generates ‘snub’ load – the load put through the fastener before it starts to swage.
Says Smith: “Snub load allows gaps to be pulled out of structures that are not readily aligned. If you have a gap, particularly in large structures, it’s useful to pull those pieces of material together.”
Pintail-style lockbolts allow a very simple check for correct installation: the pintail will snap off when the load rises above the swage load. Once the pintail snaps, the user knows the installation is good. With pintail-less lockbolts, the collars feature swage indicators. Periodic inspection with swage gauges and inspection of the installation tooling can provide further guarantees of correct installation.
“Lockbolts are easy to install using battery, air or hydraulic tools [air tool pictured above right], depending on the diameter of the fastener and the volume throughput of the application,” says Smith. “None of the specialist training or certification required with welding is necessary. With only a few hours’ training, people are installing lockbolts.”
Howmet manufacturing engineer Toby Whittingham adds: “With specialist tooling, it’s also possible to remove lockbolts in seconds if required, which compares favourably to grinding off welds.”
COMPOSITES
Aside from growing demand for larger diameter lockbolts (driving Howmet’s investment in the Sacma cold-forging machine), another interesting development is applications involving composite materials.
“Our history is embedded in sectors such as mining, commercial vehicles and steel construction, but design philosophies are changing,” explains Craven. “We have to develop products that keep pace. In essence we look at the characteristics of our existing fasteners to develop solutions for evolving materials such as CFRP (carbon fibre reinforced plastic) and resin-transfer mouldings.”
Howmet often finds that the fastening solution is a latecomer to the party, after the OEM has decided on the material, design and structure. Among the typical composites that Howmet encounters features a honeycomb structure at its core.
Here, the company has set about developing a new type of Huck fastener from two existing solutions. “It allows a reasonably flush joint on the front, like a low-profile dome head, while on the blind side the fastener splits into multiple sections to give a very wide footprint,” says Craven. “It allows the joining of things to the composite material that would otherwise not be possible. A conventional fastener would pull through due to the thin-skin nature of the honeycomb material.”
Users obviously have to put a hole in the material to install Huck fasteners. With composites, the integrity of the hole is paramount.
“Above and beyond that, we’re able to develop lockbolts that do not impart any unnecessary damage to the joint,” concludes Craven. “Whereas a lot of fasteners expand aggressively inside the joint, we have a range of solutions that avoid this effect, ensuring no hoop stress and reducing the risk of premature material failure.”
BOX: An alternative system
Available from Stanley Engineered Fastening is Avdel NeoBolt, a high-strength, vibration-resistant lockbolt. This two-piece, non-breakstem steel lockbolt fastening system features annular locking grooves and shock-less installation for use in heavy-duty structural applications. Like the Huck product, NeoBolts require no re-torquing.
Available in many fastener diameters and lengths, NeoBolt lockbolts are applicable with a range of installation tools featuring fastener-specific nose assemblies.
According to Stanley Engineered Fastening, assembly applications where NeoBolt fasteners are found include railway, truck and trailer, commercial vehicles, solar and wind energy, bridge building, mining equipment, screening equipment, fencing and security screens, construction, overland infrastructure, and power and utility services.
