Forming a bond

4 min read

Engineers are increasingly turning to adhesive bonding for a permanent join.

The choices of fixing components together are numerous and varied. But while traditional methods such as screws, rivets, latches or spot welds still have a place, engineers are increasingly turning to adhesive bonding when they require a permanent join.

Modern adhesives are classified either by the way they are used or by their chemical type. The strongest ones solidify by a chemical reaction, while less strong types harden through physical change.

Adhesives include: anaerobics that harden when in contact with metal and air is excluded, often used as locking compounds; cyanoacrylates, a special type of acrylic that cures through reaction with moisture held on the surface to be bonded that is suited to small parts and rubber; epoxies, consisting of an epoxy resin plus a hardener that provides strong durable bonds with most materials; polyurethanes that provide strong resilient joints that are resistant to impacts and useful for bonding glass fibre-reinforced plastics; and modified phenolics, the first adhesives for metals which have a history of use for making high-strength metal-to-metal joints.

Adhesives have numerous advantages over mechanical fasteners. The bond is continuous which allows a more uniform distribution of stresses over the bonded area, eradicating the local concentrations of stresses present in mechanical bondings. This continuous nature also produces a stiffer structure; and if stiffness is not a design requisite then the weight can be reduced while maintaining the required stiffness. When sensitive materials are involved, bonding does not require high temperatures so the danger of distortion or a change in properties from heat is avoided.

Other advantages include reduced stress concentrations, improved vibration damping, reduced corrosion, better electrical insulation and ease of bonding dissimilar materials.

One company to benefit from the use of adhesive bonding is Pace Cycles in Yorkshire. Since the late 1980s, its engineers have developed a number of unique designs, including rear-mounted fork bridges and brakes, hollow crowns, grease ports, remote lockouts and the use of composites. One of its most recent innovations is the lightweight cross-country Hollowform fork range, which incorporates a hollow carbon fibre bridge-slider assembly.

‘All products are designed and assembled in-house’, explained Pace managing director Adrian Carter. ‘In every application safety is the key factor, with each joint being torque tested. We will only accept zero defects.’

That standard of excellence means all the products used in the manufacture of suspension units must also be exceptional. As far as the bonding of the joints is concerned, Pace Cycles has chosen a toughened, two-part epoxy adhesive from Loctite that is ideal for gap filling and vertical bonding of metals and rigid plastics. The firm has found it ideal to join the magnesium components to composite materials on the super lightweight cycle forks.

For some applications, such as the bonding of forged aluminium crown to the carbon fibre fork leg, the adhesive is applied through pre-drilled holes so it can wick into the joint, while for other jobs the epoxy is simply ‘pasted’ into the bond line. To ensure accurate wheel tracking and alignment, each part is clamped into a bonding jig.

‘We have used other adhesives, but found them to be expensive, not easy to handle and requiring a long cure time,’ said Carter.

He explained that the forks have to be lightweight to blend in with the rest of the cycles, yet at the same time sturdy enough to withstand all the extremes of temperature and environment experienced during racing. In addition to the external situations, these conditions include the considerable heat produced by hydraulic brake systems — temperatures that are transferred through the bonded joints to the carbon fibre frame.

As well as using the epoxy adhesive, Pace Cycles relies on Loctite anaerobic products for other bonding applications, in particular to bond internal threaded cylindrical parts, including fork dampers. Alongside the need for high-strength assemblies, the adhesive must work in an environment that includes hydraulic oils.

The NightStar flashlight is another example of innovative adhesive bonding. Manufactured by Applied Innovative Technologies, the flashlight produces more than 20 minutes of high-brightness light after being shaken for only 30 seconds. Bonding technology from Araldite adhesives has played a key role in its development. The magnetic charging system utilises zinc-plated, rare-earth ceramic magnets bonded in either end of a polycarbonate (PC) housing with structural epoxy adhesive that maintains its bond strength even when exposed to extreme temperatures, shocks and vibration.

Steve Vetorino, R&D director at Applied Innovative Technologies said: ‘The epoxy adhesive plays a significant role in the reliable performance of our flashlight, supporting its use in the most demanding environments and applications. During development we tried numerous different magnet and adhesive combinations. In fact, in shear tests, the ceramic magnets tore apart before bonds broke. The epoxy has the added benefit of being cost-effective and easy to handle.’

During assembly, a PC housing is placed in a holding fixture and a repulsion magnet in another jig to ensure its precise orientation. Bond surfaces require no pre-treatment, but care is taken to handle magnets with gloves to prevent fingerprint contamination.

Vetorino explained: ‘The magnet is pressed into the housing so that adhesive spreads across the bottom and up around the sides, increasing the bonded surface area for greater strength.’ Huntsman’s Araldite 2015 epoxy adhesive is well suited for this process because its viscosity allows the material to easily flow in and fill gaps.

Technicians follow the same process to bond the second magnet in the top of the flashlight. Sections then cure at room temperature for about eight hours before moving into final assembly. The cured epoxy has a lap shear strength of 218bar at 25°C and maintains its high bond strength even at elevated temperatures. It also exhibits good peel strength with a roller peel of 1.7bar.

Of course, not every application is suitable for adhesives, particularly when a fastening only provides a temporary closure. When Alstom Transport wanted latches for its trams and trains there were various considerations such as security, reliability and passenger comfort. The company eventually selected Vise Action Compression Latches from Southco.

‘Compared with the latches previously used on these vehicles, these are better suited to railway operating conditions,’ said Jean-Patrick Briand of Alstom Transport, in La Rochelle, France.

The latches are based on a patented mechanism that locks the latch in a position where it can be released only by rotation of a handle or use of a key, preventing panels opening accidentally or over time, due to vibration. With a single half turn, the latches complete two sequential motions.

First the latching pawl swings smoothly into place behind the doorframe. When the half turn is finished the latching pawl pulls up tightly against the doorframe, applying up to 6.4mm of vice-like compression against the gasket. This consistent compression makes it a more reliable, safe solution than traditional quarter-turn systems which use friction locking, which is often vulnerable to wear and over time can create a risk of accidental opening.

In addition, these latches help dampen vibration, which is an important benefit in terms of passenger comfort as it significantly reduces noise levels. They offer clear visual indicators of open/close positions and prevent wear between the pawl and the frame. From the security point of view, they restrict access by requiring a specific tool for opening and, with the option of a completely flush design, they eliminate pry points for vandals.