STRETCHING a point

In an industry dominated by fast-moving technology and computer-aided everything, it's refreshing to find that there is still a place for craftsmen within the process sector. And that place is elbow-deep in sheets of rubber.

Rubber lining is an old and 'traditional' process. The benefits of applying a vulcanised coating of rubber to metal surfaces have been known for many years, generally to protect against corrosion and wear.

The process requires skills and knowledge developed over the years. Rubber covering of complex shapes requires great skill, especially where complete coverage is essential. There are no books on the subject; the craftsmen have learned their trade over the years. Using flat sheets, they can now line the most complicated castings or fabrications.

Liner trainees are generally expected to complete 18 months to two years' training before they are judged to be fully proficient. Initial training would commence with cutting and preparing rubber, and lining plain flat surfaces with a 3mm coat of soft natural or hard ebonite rubber. The final stage of the training after a year to 18 months and taking an additional six months to a year incorporates lining inside enclosed vessels, and working with very awkward materials, such as EPDM and white food quality rubber. One of the most difficult jobs is the lining of pipework, reserved for fully experienced liners.

The benefits of rubber lining in the process industries are many and varied. It provides resistance to wear, abrasion and corrosion; electrical and noise insulation; and product protection.

Several manufacturers of valves and pumps make extensive use of rubber linings soft natural rubber for chemical and wear resistance, and hard ebonite rubber for chemical resistance in more corrosive environments.

Vulcanised rubber linings for pump and valve components offer many advantages, both economically and practically. For example, rubber coatings can be bonded to mild steel, stainless steel, cast iron and aluminium, and the materials can be selected to optimise performance in a huge range of process industry applications. These vary from hot acids and mining slurries, hazardous materials like sewages and effluents; and foods, beverages and drinking water. Some of these require special lining materials, such as neoprene, butyl, EPDM and food quality material.

One major advantage is the ability to vary the coating thickness. Using thicknesses of 2mm upwards, the designer can specify a thicker coating in areas of greater wear, erosion, or corrosion, or use the different thicknesses to achieve the required shapes in particular areas.

The ability to vary coating thickness allows machining of the surface. Vulcanised ebonite rubber can be machined to precise tolerances, which gives great flexibility of design. The component receives a thick lining, which is then machined to give, for example, a precise interior diameter; or a taper or groove to mate with another component.

Linings can also help cut costs. Many applications might call for 'exotic' materials, such as bronze, titanium, gun metal or thermoplastics. All of these have advantages for specific applications, but are very expensive. Rubber coatings, on the other hand, allow manufacturers to use cheaper materials, like cast iron and mild steel, without losing out on durability and performance.

Pumps and valves are not the only equipment to benefit from rubber linings. They are often useful in large mixers and agitators, which need to operate in a range of environments. Again, rubber-coated mild steel will be cheaper than more exotic materials or coatings, while varying thicknesses can provide extra protection in the areas subject to the greatest wear. For example, a large agitator for an abrasive slurry might have 6mm thick rubber on the shaft, 10mm thick on the blades, and 15mm thick on the leading edges and tips.

Mr Punter is technical marketing manager of Applied Rubber Linings