Fastening solutions evolve

Tom Shelley reports on the latest developments in fastening and joining.

Fasteners continue to evolve, not only to reduce costs, but also to meet the needs of the new Machinery Directive, which requires all guards and enclosures to have captive nuts, and to go even further to meet safety and reliability issues. To meet such needs, Eureka readers may recall a description of how the Japanese 'Hard Lock' vibration-resistant nut system is expected to reduce the risk of nuts coming undone in railway points (Eureka, April 2010). But a British development that is said to work even better is being taken up by the rail industry in The Netherlands. 'Wheelsure' and 'Tracksure' nuts are the result of a British Inventor coming up with an improvement to a century-old idea: Two nuts on a bolt, one of which runs on a main, right handed thread, and the other of which runs on a left-handed thread on a narrowed diameter section of the outermost end. The key to making it work effectively, according to Wheelsure CEO Gerhard Dodl, is a push on cap to keep the hexagonal protuberances of the two nuts in alignment, and a spring clip in either the cap or on one of the nuts, to hold the cap in place. Any vibration causing the main, inner nut to start to undo makes the two nuts lock together. The combination performs well in the Junkers vibration test, retaining 95% of clamp torque over the maximum 120s, and target initial markets are heavy goods vehicle wheel studs and the rail industry. There has been a significant order for use in a marshalling yard braking system for handling freight trains in Rotterdam. It is also approved in Italy and there is talk of trials on the London Underground. On the road, it is fitted to various goods vehicles, including more than 300 aggregate trucks owned by building products supplier, Cemex. The studs come in two sizes, M18 and M22, which fit the majority of wheel hub assemblies. For non-critical applications, most design engineers continue to specify threadlocking compounds. Henkel Loctite says that it has recently reformulated its 243 and 270 threadlocking compounds to make them function better on a wider range of metal surfaces, including passivated and trivalent chrome and to be more tolerant of oil traces. Service temperatures are now up to 180ºC instead of only 150ºC. A number of suppliers claim to have been doing good business supplying captive fasteners to meet the new guarding requirements arising from the Machinery Directive. Graham Leo and Roy Adkins of Specialty Fasteners and Components showed a new design in which bolts are retained by a thread that is of larger diameter than the bolt shank. The thread is rolled on after a spacer has been swaged on that attaches to the guard. Unifast, on the other hand, has chunky push and turn Italian made fasteners retained by an elastomeric, pushed on washer and Camloc has a quarter turn 'CAM-PT10' fastener (pictured) that is claimed to have a grip range '7.5 times wider than other quick release fasteners'. It too is retained by a plastic washer. David Antrobus, operations director of Binder Fastener Systems (UK) highlights the company's 'Klettostar' hook pads coated with adhesive activated by ultrasonic or hot air heating, that could be fixed to mats to make them 'Engage' to carpets and their 'Klettoplast' arrowhead equipped adhesive backed strip, that engages even more strongly. European customers include BMW. Lohmann Tapes has a hook and loop tape combination, with adhesive on one side that is only 0.5mm thick that is currently used for closures on medical products and trim within cars. The company also has a new pure acrylic adhesive strip, designated CPT 500F, that is optically clear, and can be stretched by a couple of hundred percent, returning to its former length when released. Developed for glazing to allow movement, it is used in the automotive industries. The company has also developed an acrylic adhesive tape that is particularly sticky, or as it puts it 'aggressive', with an adherence sufficient to allow it to be used to repair bouncy castles and HGV semi-trailer side curtains, and also non-slip, ribbed, adhesive pads for uses ranging from toothbrush grips to office furniture. Alfatex also makes hook and loop tapes at its factory in Belgium, and sales director Alan Rogers claims that it can produce products that are anti-microbial for medical use, and products that are anti-static. For permanent metal-to-metal bonding, Tox Pressotechnik's clinching technology allows thin sheets of metal to be joined by pressing a punch into metal, with a die on the reverse side that allows the formed cup to expand sideways. Advantages over spot welding are the lack of weakened, heat affected zones, and the ability to fasten different metals together, such as steel and aluminium. BTM (UK) Automation Products has what appears to be very similar technology, although managing director Mark Firmin says: "What sets us apart is our use of an expanding die, which requires less force to form the clinch, resulting in longer tool life." Böllhoff, on the other hand extols the virtues of its Rivtac system, in which fastening is achieved by forcing in 'tacks', which have the advantage that they only require access from one side. Various companies, notably Henrob, offer versions of technologies based on self piercing rivets . Henrob offers rivets with standoff pins and studs in addition to plain tops while other companies offer heads with nuts and even ball joints. Arnold Umformtechnik have 'Tripress' (ATP) fasteners with a trilobular cross section that when pressed into a round hole in ductile light metals and plastics, deform the hole and lock the fastener in place. The combination of shapes used minimises pressing force, while maximising the force required to dislodge it, usually increasing it beyond the yield stress of conventional fastener. The first part of the fastener to be pressed into the hole is designed to prevent it rotating, while the second part has a set of tapered circumferential ridges to prevent it being pulled out. The system is said to be particularly suited for securing electric terminal studs in motor vehicles. The same company has recently announced Remform screws for fastening detachable plastic parts. Although, at first sight, they look like conventional self tapping screws, they have threads with asymmetric flanks to optimise material flow and to increase pull out force. They have recently been employed in the assembly of a two piece washing machine outer drum. Requirements included maintaining the residual clamp force for the lifetime of the washing machine, vibration resistance, an operating range of -10 to 65ºC and an assembly time of less than 1s per screw.