Segmented girth gears have a number of advantages, including simpler manufacturing, ease of transport and straightforward installation. Buy why has this technology come to the fore in recent years? Here, SEW-Eurodrive discusses how austempered ductile iron (ADI) has enabled the revolution.
Segmented girth gears are used to drive large, rotating systems such as dryers, rotary kilns or horizontal mills. They are installed around the circumference of these systems, and transfer the drive torque from the gear motor to the rotary cylinder.
If necessary, the rotation of the cylinder can be induced by several drives, to distribute the load. When the girth gears are used in this manner high torques can be achieved by smaller, more efficient units.
Traditional girth gears generally consist of two-to-four segments that are assembled during manufacturing for further processing. They require large and expensive machines for casting, handling and heat treatment.
In contrast, SEW-Eurodrive segmented girth gears are split into several identical segments in order to keep the component parts short and easy to handle.
“This is made possible by the use of austempered ductile iron when casting the girth gears,” according to David Atkinson, technical sales representative, SEW-Eurodrive. “The tensile strength of ADI is superior to that of other materials.”
Segmented girth gears made from ADI are lighter than their traditional counterparts and feature an above-average contact fatigue strength, thanks to the cold work-hardening properties of ADI. The decreased size of the girth-gear segments reduces the cost of scrapping the blanks used for casting the segments. The blanks for the segmented gears can be used without additional welding or oversizing.
Because of the physical properties of ADI, segmented girth gears can be constructed with a thinner face width on the gears. This has the advantage of reducing wear and noise, as well as contributing to the lighter weight and reduced material cost.
And, in addition to the physical advantages, ADI enables fast manufacturing times compared to traditional methods.
More quenching
The key distinguishing characteristics of ADI are introduced during the tempering stage of the iron production process. For ADI, the ductile iron is held at the quenching temperature for an extended period. This produces ausferrite, which is a matrix (or mixture) of acicular ferrite and austenite, stabilised with about two per cent carbon. Named after the British metallurgist, Sir William Chandler Roberts-Austen, austenite is a non-magnetic solid solution of iron and another alloying element.
The first step in this process is to make a casting from ductile iron, which consists of traditional cast iron with the addition of magnesium and silicon. The casting is immersed in liquid salt, sometimes described by engineers as “corrosive lava”. This immersion leads to the matrix changing to austenite. The casting is then quenched, or cooled rapidly. The temperature is now reduced, and the austempering step begins. Finally, the casting is left to cool. It is then ready for use.
Enhanced properties
The austempering style of quenching leads to a material renowned for its low cost, design flexibility, good machinability, high strength-to-weight ratio and good toughness, wear resistance and fatigue strength. According to the Ductile Iron Society, ADI delivers twice the strength for a given level of elongation when compared with conventional grades of ductile iron. In addition, ADI offers exceptional wear resistance and fatigue strength.
For a typical component, ADI costs 20 per cent less per unit weight than steel.
During manufacture, ADI gears require around 50 per cent less energy than those made from conventional materials. Other advantages include increased machine shop productivity, reduction in weight of up to 10 per cent, reduced gear noise, rapid ‘break in’ of new gears and improved resistance to scoring.
It is the cold work-hardening properties of ADI, when combined with an appropriate girth gear size, that allow for a more compact and lighter gear design than traditional solutions. This low weight is important for the handling and assembly of the girth gear, as well as for achieving circumferential velocity.
Small segments
SEW-Eurodrive launched its first ADI segmented girth gears in 2008. The segmented design of the these girth gears simplifies handling at the construction site. Similarly, there is no need for special transportation. The design also guarantees an initial pitch accuracy of ISO 8, which minimises the vibrations of the girth gears. (ISO 8 is a standard that refers to the Q-number, also known as gear quality number or gear quality grade. It indicates the geometric accuracy level of teeth on a gear).
Mr Atkinson says a further benefit of the design is that segments can be replaced easily, without the need for dismantling the entire ring. In addition, with the correct dimensioning, alignment, load and lubrication, an ADI girth gear is almost wearfree.
This also means that spare holdings can be simplified and reduced. “The ADI girth gears manufactured by SEW-Eurodrive are suitable for use in a variety of sectors. These include the energy sector, the pulp and paper industry, mining, the steel sector and the cement industry,” he says.
The largest of these girth gears, typically used in mills, are up to 16 metres in diameter and deliver power of up to approximately 15 megawatts. In these applications, the rotation speed can be up to 20 rpm. Those used in rotary kilns are usually smaller, with diameters up to nine metres and power typically up to one megawatt. The rotation speed of these smaller girth gears is slower, typically up to two rpm.
Industry is clearly taking notice of the developments in girth-gear technology. The number of ADI segmented girthgear projects in Australia has increased markedly over the last 12 months, particularly in the mining sector. The trend looks set to continue.
Technical enquiries: Mr David Atkinson, (08) 9251 4900, david.atkinson@sew-eurodrive.com.au
Media enquiries: Ms Sally Perry, (03) 9933 1000, sally.perry@sew-eurodrive.com.au
For more information: http://www.sew-eurodrive.com.au