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Subject Area: Mechanical & Materials Engineering
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Article citation: , (2011) "Gear Heat Treatment for Toughening Up Your Teeth", Anti-Corrosion Methods and Materials, Vol. 58 Iss: 4, pp. -
Gears, particularly load-carrying power transmission gears, play a vital role in every field of industry, from aerospace, agriculture and automotive, to winching and wind generation. Each gear wheel’s performance characteristics, with regard to its load carrying capacity, operating environment, applied stresses and design life, are determined by the microstructure and grade of ferrous-alloy typically employed, the type of gear and its mechanical specification, the actual gear cutting process and the choice of heat treatment method to achieve the required physical properties.
According to experts, heat treatment is a core competency that represents around 30 per cent of typical gear manufacturing overheads, so selecting the right heating and quenching process is vitally important for durability, reliability, cost control and the avoidance of over-engineering. Keighley Laboratories of West Yorkshire, which offers advanced analysis, testing and heat treatment of metals on a single site, provides a range of case hardening, induction hardening, Tufftride ferritic nitrocarburising and other processes for high performance gears and believe it can assist customers by engineering out cost and engineering in performance.
“Ideally, we like to be involved as early as possible in the gear design process, because our metallurgical experts can advise on the optimum material, our heat treatment professionals can identify how to achieve the correct hardness profile, at the right price, and our practical experience of heat treating hundreds of thousands of gears and gear teeth means we can often foresee technical issues down the line and avoid them,” says Heat Treatment Commercial Manager, Michael Emmott.
“We pride ourselves on keeping gear wheel distortion to a minimum and processing materials according to their metallurgy and technical requirements, giving the correct strength characteristics for the identified torque transmission,” he continues. “Keighley Labs offers different types of jigging to cope with different gear geometries and assist with distortion control, as well as various quenching techniques like press quenching for cooling thin or complex parts and molten salt baths for processes such as austempering and martempering, which keep distortion to an absolute minimum. A variety of gear shapes and sizes demands not only flexibility in the heat treatment process, but also in the quenching, and that’s what we can offer.”
Case hardening generally and carburising and carbonitriding in particular are the most commonly used techniques for high performance gears, producing a softer, tougher core to avoid tooth breakage, while achieving a hard wear-resistant case to prevent pitting. It lends itself to both high and low volume requirements and a wide variety of gear shapes and sizes, up to around 1 m in diameter and 2 tonnes in weight at the Keighley Labs facility. Case depths between 0.1 and 5 mm are achievable, with PLCs and oxygen probes enabling temperatures and cycle times to be accurately controlled. Test pieces are also highly representative of processed batches, thanks to excellent repeatability.
For larger gears up to 3.5 m in diameter and 4 tonnes in weight, or where deeper hardening to 10 mm is required, Keighley Labs offers induction hardening, which uses high, medium or low frequency alternating current to heat the surface of a gear tooth. Both spin hardening and tooth-by-tooth or contour induction hardening methods are available, with the tooth-by-tooth techniques especially suitable for larger gear wheels. Spin hardening is usually a rapid process, often completed in a matter of minutes, whereas tooth-by-tooth, also known as progressive or zone hardening, is more time consuming, although both methods ensure low distortion.
For relatively fragile gear geometries, Tufftride ferritic nitrocarburising is a further option. A comparatively low temperature, low distortion salt bath process, it produces a thin, iron nitride compound layer that is particularly resistant to wear and prized for its excellent sliding and running properties, ideal for gear wheels. Other specialist heat treatment techniques are available, as well as pre- and post-hardening processes such as annealing, normalising, stress relieving and tempering.
Internationally certificated for all heat treatment processes, with the options of metallurgical inspection by its UKAS accredited in-house testing service, Keighley Laboratories has experience of processing gears of all types and all sizes from several millimetres to several metres, although most fall in the mid-range sizes for mainstream power transmission applications. It can handle volume contracts and special one-offs for a single gear set, although there is naturally a premium for the smallest jobs, since setting up times for a one-off are much the same as for a batch.
“A customer once said, half-joking I assume, surely you just throw my small job into the furnace with lots of other stuff,” says Michael. “But we always endeavour to supply a gear wheel that’s fit for purpose, which means that heat treating a left hand and a right hand helical gear, for example, demands as much individual attention as a batch of monster gear wheels destined for crushing plant on the other side of the world. In either case, if the teeth start dropping off, that would be a catastrophe and that’s exactly what we’re paid to avoid” (Figure 2).
More information is available from: www.keighleylabs.co.uk