Focus Industry

Air spring actuators in the coal pulverizing mill as a wear-free alternative to metal cylinders

Working conditions in a coal pulverizing mill are very harsh. Coal dust and vibrations mean materials are subjected to extreme stresses (Fig. 1). The hydraulic compression cylinders used to apply pressure to the grinding gear wear quickly. The English manufacturer FTL Seals Technology recorded the lifetime of the hydraulic components in a coal pulverizing mill as just three months. Over ten years ago therefore, the engineers opted for a pneumatic solution: the piston rods are actuated by means of rolling lobe air springs, which boast an up to 10-times-longer lifetime compared to conventional cylinders and oil-free operation in coal pulverizing mills.

1 Principle of Operation
Air actuators are dimensionally stable components made from highly sophisticated elastomer grades with vulcanized textile reinforcements. ContiTech manufactures two basic designs: folding bellows and rolling lobe air springs. The air actuators adjusted to the pneumatic functions generate the stroke based on the flexing of the highly flexible outer shell. Piston rod and therefore guiding elements and gaskets are not necessary in air spring actuators. By means of air supply, the element produced by ContiTech can be used, for example, to lift loads, exactly like a pneumatic cylinder. The stroke can be effected in different ways: with oil-free or oil-containing compressed air or optionally with other gaseous media, e.g. nitrogen. In low-pressure ranges, hydraulic media such as water or glycol are possible.

2 Performance
The maximum pressure of the standard air actuators totals 8 bar. For higher requirements, strengthened models are available. With just 8 bar internal pressure, pressure and carrying forces from 0.5 to 440 kN can be achieved – standard products with 1000 mm diameter can then lift 44 t mass. Depending on the type of actuator, the stroke reaches up to 455 mm, and that without stick-slip effect: even with minimal pressure changes, the stroke remains jolt-free. Air actuators can also be tilted. Angles of 5° to 30° and a lateral offset up to 20 mm are possible. Connecting elements and articulated structures are therefore not necessary. A wide range of special models is available, e.g. low- or high-temperature actuators for applications to –60° C or 130° C peak temperature.

3 Durability and Lifetime
As they do not require any mechanically moving parts, air actuators are wear- and maintenance-free compared to conventional cylinders: working conditions such as dirt, dust, granulate or sludge have no adverse effect on the lifetime. Standard air actuators can easily withstand many chemicals. For special requirements, e.g. aggressive chemical compounds, special elastomers are available. The standard air actuators are designed for temperatures from –30° C to 70° C. Models are also available for other application conditions: for example as low-temperature actuators (–60° C) in the timber industry or as high-temperature actuators for the glass and paper industries (up to 115° C continuous and 130° C peak temperature).

4 Models
ContiTech produces many models of air actuators, which can be divided into two basic variants: folding bellows and rolling lobe air springs. Single-, double- and triple-convolution air springs have a very low minimum height (Fig. 2) and low return forces. Compared to folding bellows, rolling lobe air springs have a smaller diameter with the same maximum force and an almost constant force emission over the stroke. The standard range comprises:
• Single-convolution air springs for strokes to 164 mm and lifting forces to 440 kN
• Double-convolution air springs for strokes to 400 mm and lifting forces to 410 kN • Triple-convolution air springs for strokes to 455 mm and lifting forces to 440 kN
• Sleeve-type air springs for strokes to 105 mm and lifting forces to 11 kN
• Rolling lobe air springs for strokes to 280 mm and maximum lifting forces to 55 kN

5 Field Report: Coal Pulverizing Mill
For the company FTL Seals Technology, the use of air actuators proved the answer to the harsh working conditions in a coal pulverizing mill. FTL Seals Technology engineers cylinders for pressing the grinding gear of a mill with up to 20 bar against the mill feed. The cylinders ensure a constant adjustable pressure and also compensate for wear to the grinding balls. A degree of flexibility is also needed when the grinding balls roll over larger pieces of coal.

Up to the mid-1990s, coal pulverizing mills were generally equipped with classical metal cylinders with hydraulically actuated rams. The lifetime of the metal cylinders, however, proved unsatisfactory in the working conditions: coal dust settled and caked onto seals and piston rods, having the effect of emery paper. As a result of the constant vibrations, leaks sprung, regularly causing operating faults, from oil contamination to complete stoppage of the mills. The manufacturer documented an average lifetime of three months, then the cylinders finally succumbed to wear and had to be replaced. In the middle of the 1990s, the first cylinders were converted and the hydraulic replaced with a pneumatic solution. A rolling lobe air spring with reinforced casing generates the pressure within the pressure
cylinder. The cylindrical shell surface is fitted to the outer shell of the pressure cylinder; the stroke is effected by the abrasion-free rolling movement of the air spring lobe.

The nitrogen-filled air springs proved effective in adverse conditions. The product operated without sliding gaskets – and without oil. The nominal lifetime of the new RodasRam cylinders was lengthened tenfold. In an extreme application in a coal-fuelled power plant in South Africa, the cylinders have already in operation for more than five and a half years. In the meantime, the product with the integrated air spring actuator is supplying the right pressure to coal pulverizing mills (Fig. 3) and other mills in England, Mexico, Holland, India and South Africa.

Dr. Olaf Kluth,
ContiTech Air Spring Systems
ContiTech AG, Hannover (D),
+49 511-938-02,


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