An improved design of seal, developed by SKF, is helping to protect bearings against contamination and water ingress in extreme environments such as mining and cement manufacture, according to the company. “A dirty working environment is punishing for both humans and machines. Workers in industries like mining and cement production need protective gear and breathing equipment to protect against the effects of contamination. Machines in these industries need similar protection if they are to remain in good working order. For many pieces of machinery in the mining and cement industries, such as a conveyor pulley, the bearings are likely to fail before other components – meaning that production must be stopped in order to replace them.”
There will be costs to rebuild the pulley, while – in extreme circumstances – a seized bearing or blocked pulley can seriously damage or even destroy the conveyor belt and cause structural damage. This is very costly, in terms of both repairs and lost production. “The answer is to protect the machines, and the vital components within, from the rigours of these harsh environments. Gold standard Taconite seals are already the standard in protecting bearings from ingress in the mining industry.”
They were developed in the 1960s, and take their name from taconite – a fine-grained, abrasive iron ore that was proving too much for existing housing seals. Taconite dust is almost as fine as smoke particles: this makes it highly abrasive, very invasive, and harmful to windings and bearings. A heavy-duty seal was needed, capable of preventing ingress of this dust, as well as preventing grease and oil from escaping. This was realised in SKF’s Taconite seal, a cartridge seal machined from cast iron or steel. In time, taconite seals began to be used in other industries – especially those with very wet or dusty environments, including, pulp & paper, metals and marine. They are most commonly seen in conveyor pulleys, but also find use in many other applications, including grinding mill pinion housings, bucket elevators, hoists, winches, jack shafts and pulverisers.
Now, SKF says it has taken the design further, with a complete re-engineering of the Taconite seal so that it offers increased protection to bearings in split block housings. The new version was developed by SKF Australia for use in iron ore mines, as part of an ongoing effort to help protect bearings from the rigours of this tough working environment – in terms of both abrasive iron ore dust and high-pressure water wash-downs. Benefits of the new SKF Taconite seal, which is available globally, include compact axial size, easier installation, more efficient use of grease purge and, of course, superior sealing.
The new design is also able to accommodate misalignments of up to 0.5 degrees. Labyrinth rings At the heart of the new design are two labyrinth rings – one stationary and one rotating. Once mated, they create a multi-stage labyrinth with a very small clearance, which is hard to penetrate. The rotating ring carries a low-friction V-ring, which seals against the stationary ring to ensure effective grease purge and provide additional resistance to the ingress of contaminants. At the same time, it is sealed to the shaft by an O-ring in its bore. The V-ring and O-ring seal are both made from acrylonitrile-butadiene rubber (NBR). Water ingress is controlled by the design of the rotating ring, which faces the bearing housing. The high centrifugal force generated when it rotates at speed helps to ‘fling’ water away rather than letting it penetrate the bearing housing. Assembly and alignment are simplified, thanks to a circumferential groove on the outer surface of the stationary ring. As the rotating ring is slid over the shaft – towards the housing – it engages with the stationary ring until its inner side face aligns with the groove. Three grub or set screws on the rotating ring are then tightened against the shaft to secure it square to the shaft and drive it. The housing’s alignment is then adjusted if the inner face of the rotating ring is not aligned with the groove in the stationary ring. Using these particular seals – in combination with upgraded Sealed SKF Explorer spherical roller bearings can also lead to a significant reduction in grease consumption.
SKF recently modelled an example for a client, for 60 conveyor pulleys with 280 mm shafts. Pulleys that use traditional open bearings and taconite seals will each require 1.8 kg of grease per month – equating to a total of 108 kg per month and 1.27 t per year for all 60 pulleys. In comparison, the SKF alternative will consume just 0.625 kg of grease per month for each pulley. That is 37.5 kg per month, or a total of 450 kg for the year – a reduction of 65%. As well as reducing grease costs by two-thirds, the reduction will also mean less grease being discharged into the environment. The SKF Taconite seal can be grease purged in three ways: manually; using the SKF System 24; or via an SKF Automated Lubrication System (ALS). The new seal is available for most SKF metric and inch split block housings, from 50mm (1-15/16 in) shaft diameter and larger. It fits SKF metric housings SE, SNL, SNL 30, 31 and 32 series and SKF inch housings SAF, SAF B, SAW, SAFD and SDAF. Cases in point SKF’s traditional approach to heavy duty sealing of bearing housings is its ‘Three-Barrier Solution’; comprising a standard primary housing seal, a grease filling the housing cavity and a sealed SKF spherical roller bearing . Now, it says, it will offer the ‘SKF Three-Barrier Solution’ for extremely contaminated conditions, through a combination of an upgraded Sealed SKF Explorer roller bearing, new SKF Taconite Seal, and a grease barrier. The company has already demonstrated the power of the new system, in applications across a range of industries including marine, mining and metals. In one loading facility, a conveyor pulley using non-SKF bearings – mounted in split plummer block housings – was experiencing a Mean Time Between Failures (MTBF) of just 12 months. This was caused by iron ore contamination of the bearings within the housings, the grease showing contamination levels of up to 14,000 ppm.
“By replacing the existing seals with SKF Taconite Seals, contamination levels fell dramatically. After six months, grease sampling showed a level no higher than 60 ppm. The bearing MTBF is now greater than the pulley lagging life – which is typically four years. In a similar example, the ultimate SKF Three-Barrier Solution extended MTBF from 18 months to beyond five years. The system also proved invaluable in a gold mine, where the non-drive end spherical roller bearing on an ultra-fine grinding mill was failing every 15 months due to water ingress through the mill drum gland pack. In May 2012, SKF installed the ultimate three-barrier system, replaced the failing bearing – and, as a precaution, the drive-side bearing. Nearly three years later, the bearings are still in service and performing well.” Similar improvements have been seen on a range of equipment – mainly conveyors – that must withstand such tough conditions.
