EVEN FASTENERS CAN BE FUN

Contributing editor Michael Sibley finds that fastening can still be an exciting area of technology

Fasteners cover a wealth of ideas, technologies and materials. They are used in every industry. Despite the designer’s preference for tried and tested methods, new ideas and improvements to long standing techniques are constantly being introduced to meet new needs, overcome old problems and to cut costs.

The grab ring

A grab ring is a grab ring – a traditional fastener that is easy to put on and almost impossible to get off. Typically, it is stamped from thin sheet steel and essentially is a spring mechanism with an inverse Poisson ratio. This simply means that the mechanism becomes larger in diameter under an axial load instead of smaller, as it would if a direct Poisson’s ratio applied.

A major limitation of grab rings generally is that, being made from thin springy steel, current types can only carry fairly light loads. Thus they have been limited to being used to join low pressure, small diameter components typically in automotive applications, domestic plumbing and soft drinks dispensers.

When a load is applied to the grab ring, either by pressurising the pipework or by applying an end load, the spring resists the axial movement and the teeth dig in to the outer wall of the pipe. All the load is now carried on the grab ring.

In use, the grab ring performs two functions in the one unit; it acts as both a spring and as a load bearing member. To carry heavier loads, the rings need to be made thicker and therefore stiffer, but this makes them more difficult to assemble and can damage the surface of the pipe reducing their ability to seal.

Now a new design, Figure 1, which has been patented by engineers at UMIST, looks set to overcome this limitation enabling them to be used for much heavier applications.

The new design separates the two functions. Pairs of parallel slots are cut radially in the ring, enabling thicker spring steel to be used while retaining the vital springy characteristics. The thicker steel between the pairs of slots enables the ring to carry high loads making it suitable for larger and more highly stressed applications than currently possible.

Prototypes have been demonstrated on pipe up to 40mm diameter sealing a steel pipe pressurised to 400bar.

UMIST Ventures, where the fastener has been developed, claims that the coupling will revolutionise the joining of pipes, ropes and cables where high forces and pressures are involved. It is planning to make the fastener in a range of materials, including ferrous, non-ferrous, plastic and composite materials. By adapting the design a variant can be made to be dismantled and re-used.

A licensing agreement for the new grab ring is expected to be signed with a manufacturing company shortly.

Rotation resistant bolts

Another modification to an standard fastener builds on existing practice while overcoming the inherent problem of damage to factory finished parts. It concerns rotation resistant bolts which are widely used in constructing large liquid storage tanks and silos from factory finished steel plates.

The bolts are designed for joining together large rolled steel panels which have punched holes around the edges, and are bolted and sealed together to form cylindrical tanks and silos. Current practice is that an operator inserts a row of bolts from inside the vessel and then tightens the nuts from outside the vessel using a powered nut runner.

The bolt has a domed head for minimum protrusion into the vessel which is protected by a plastic cover to resist corrosion by the vessel’s contents and so cannot be gripped to prevent it turning.

Conventionally, the bolts are forged with three or four nibs on the underside of the head which bite into the inner diameter of the punched hole to resist turning as they are tightened. Typically, the steel sheets are protected from corrosion with a high quality factory applied stove enamel or similar finish. This is a hard, brittle material that flakes as the nibs grip the steel during tightening which can lead to corrosion of the exposed steel.

The new design of bolt by Permastore, Figure 2, has no nibs so there is no tendency to crack the coating. Instead, it is forged with an elliptical cross section non-threaded shank which matches elliptical holes punched in the steel sheets. The elliptical form automatically prevents the bolt from turning as it is tightened. Now under evaluation, the patented bolt will be used on large diameter tanks for the water industry and for holding corrosive agricultural wastes.

Bolts in rocks

Securing bolts into rocks is an important aspect of making mining as safe as possible. This is particularly true in mines where the roadway conditions impose vertical pressures which can cause the supporting ribs to buckle. The bolts are set into the rock to improve the stability of these supporting ribs. Unfortunately, standard grip rockbolts cannot always provide the necessary end load resistance to sideways movement often leading to expensive remedial action.

A new type of rockbolt from Carrington Weldgrip hopes to solve the problem. The Weldgrip High End Loading Fibrebolt, as it is called, Figure 3, consists of a combined high-pressure moulded nonmetallic nut and wedge arrangement used in conjunction with a steel collett on a standard 380kN rated Fibregrip rockbolt. The threaded section of the bolt is a standard buttress thread cut into the grip.

The assembly works on the principle of the load on the end plate forcing the collett onto the plastic nut and wedge. Increased loading from ground pressures increases the tightening effect of the collett on the wedge giving an end load capability of 200kN, a similar end loading rating to steel bolts.

In failure mode above 200kN, the load starts to overcome the friction between the threaded sections of the nut and the fibrebolt, causing the end arrangement to slide along the bolt in a controlled manner. This results in a decreasing residual load effect until the assembly is pushed off the end of the bolt.

The rockbolts are manufactured by pultruding glass-fibre yarns around a central core made either from solid steel or carbon or steel yarns. The yarns are continuously resin bonded onto the core by heat applied as the yarns emerge from a forming die.

Securing high voltage lines

Fastening costs can often be substantial. One new method which combines improved design with better manufacturing methods is cutting the cost of ties used to secure high-voltage power lines.

Insulator ties and ‘dead-ends’ have traditionally been made from a number of circular cross-sectional wires clamped and bonded around the conductor wires with a combination of grit and adhesive to hold the strands of the wires together. But this method suffers from the possibility of built-in stresses and non-uniformities forming when the component parts are joined, and there is only point contact between the bonding wires and the conductors. Also, the method used for making these ties involves some six operations with skilled labour at each stage.

Now with a new patented technique from Dulmison, the tie, Figure 4, is fabricated from flat, usually aluminium strip, formed into a helical shape which fits round the central conductor. The tie’s inner surface, called a Spirastrip, has a knurled inside surface to give grip and the necessary strength and stiffness. The change from a round to a flat section ensures a better connection with the conductor as it dispenses with the combined glue and grit which can become dislodged reducing the effectiveness of the fitting.

An added advantage is that the components, being simpler in design, can be formed continuously from a single strip, avoiding the need to weld or clamp the parts together. The new ties are made from a homogeneous material which virtually eliminates corrosion problems. They can be made from aluminium, zinc coated steel or even stainless steel.

Besides producing an improved and more reliable product, the helically formed strip can be made on a single CNC machine requiring only a single operator. The expected output rate is up to twice that of the previous method.

The new ties, which may also be used to secure overhead fibre optic cables, have been tested for uplift and slip and 100 million vibration cycles.

Self pierce rivetting

As a replacement for spot welding, a pierce-and-roll rivetting system is now being used for the first time in an assembly cell for the new BMW 5 Series car, Figure 5. This development takes traditional self-pierce rivetting a step forward by fastening two or more sheets together without the rivet breaking through the lower sheet. As with self-piercing, there is still no need for a pre-pierced hole in the parts being joined.

The move to lightweight materials, pre-finished components and composites is posing major problems for spot-welders. Aluminium alloys are inherently more difficult to weld than steel, and the call for improved corrosion resistance of finished products requires protective coatings which are difficult or impossible to weld. Glass and carbon reinforced composites cannot be welded.

Rivetting has been the traditional response to these problems, but this requires holes which are costly and accurate alignment. What is more, the technique damages both surfaces of the assembly. Self-pierce rivets are used to join dissimilar materials, but pierce through both components being joined.

Pierce and roll, from Ariel Industries, offers a solution for applications using protective coated sheet and the lower surface of the assembly. Although changed in shape, it retains the original surface finish. Also, by retaining the integrity of the lower sheet, the join has better physical properties than if fully pierced.

In action, the cold-formed steel rivet pierces the upper sheet and rolling begins immediately it enters the lower sheet. The process is precisely controlled to prevent the rivet piercing too far and cutting right through the assembly. If the rolling action starts too soon it will fail to enter the lower sheet and no fixing will result. The rolling action spreads the rivet into the deformed area of the sheet produced by the rivet, thus securing two or more sheets together.

Figure 1: The grab ring. Pairs of parallel slots are cut radially in the ring, enabling thicker spring steel to be used while retaining the vital springy characteristics. The thicker steel between the pairs of slots enables the ring to carry high loads making it suitable for larger and more highly stressed applications than currently possible

Figure 2: (a) Typical bolted tanks

Figure 2: (b) Elliptical shank matches elliptical hole to prevent turning during tightening

Figure 2: (c) Current bolt showing nibs under head

Figure 3: Glass fibre yarns are pultruded and bonded onto a central steel core

Figure 4: New ties from Dulmison are fabricated from flat, usually aluminium strip, formed into a helical shape which fits round the central conductors

Figure 5: A pierce-and-roll rivetting system is being used in an assembly cell for the new BMW 5 Series car. This development, by Ariel Fasteners, fastens two or more sheets together without the rivet breaking through the lower sheet

{{UMIST TechnologiesTel: 0161-200 3838Enter 550

PermastoreTel: Eye (01359) 870723Enter 551

Carrington WeldgripTel: Barnsley (01226) 283438Enter 552

DulmisonTel: Corby (01536) 260701Enter 553

Ariel IndustriesTel: Leicester (01162) 736541Enter 554}}