NSK has used its extensive problem-solving capabilities and experience to provide
mines and quarries with the solution to a long-standing problem. Many conveyors
operating in the hostile environment of a mine suffer from the premature failure of
bearings on the head, tail, drive and tensioning pulley. There are numerous reasons
why overcoming this issue has proved challenging over the years, but NSK has
come up with a solution that improves service life by up to 20 times without
comprising any other requirements.
To allow important accessibility, the head, tail, drive and tensioning pulley on mining and
quarrying conveyors traditionally relies on open bearings. However, the short service life of
these bearings frequently threatens the uptime of operations.
Widespread problem
In Australia, mining is a pillar of the country’s economy, where downtime and costly repairs
represent a danger to bottom-line profitability. Like in many other regions of the world,
mining companies across Australia experience expensive maintenance and lengthy
conveyor downtime due to the failure of tensioning pulley bearings. These bearings typically
suffer from hard particles (dirt, dust) and water penetrating the housing seals and entering
the open bearings.
As well as improving the sealing situation, any new solution would have to avoid
compromising important user requirements, one of which is rapid mounting. As an industry standard procedure, each spherical roller bearing slides quickly and easily on to a tapered
sleeve. In contrast, bearings that mount on a regular cylindrical surface require heating and
cooling to ensure a suitable shrink fit, not only taking time but requiring special equipment.
Tapered bearing bores and corresponding tapered adapter sleeves provide a faster and
more convenient solution. The adapter sleeve has a cylindrical bore to slide over the end of
the conveyor belt roller shaft, while the outside diameter of the sleeve features a taper to
match that of the bearing bore. At the end of the adapter sleeve is a thread, where the
application of a nut serves to push the bearing on the taper.
To know how much the spherical roller bearing requires pushing on the tapered surface,
maintenance professionals use a feeler gauge to check the bearing’s internal clearance.
This strategy ensures there is no over-exertion of force that could push the bearing too far
up the tapered sleeve, causing too much reduction in internal clearance.
The beginning of change
The life expectancy of open spherical roller bearings for applications such as mining and
quarrying conveyors is relatively short, typically just months. However, in the absence of a
better solution, this somewhat unacceptable situation has prevailed for many years.
The first sign of a breakthrough arrived in 2012-13, when NSK Australia and its distributor
began working closely with major mining companies across the country to set up several
on-site trials in various problematic conveyor pulley locations where the mean time between
failures (MTBF) was as low as 6 months.
NSK set out to develop a type of sealed spherical roller bearing that could provide a useful
service life extension through highly effective seal design and the use of unique long-life
bearing material. However, among the major challenges was accommodating the need for
final radial clearance measurement. Only by using open bearings was it previously possible
for feeler gauges to gain access. Using a conventional sealed bearing would prevent this
critical operation from taking place.
Some bearing manufacturers have addressed this issue by offering standard sealed
spherical roller bearings (featuring a tapered bore) with a set of conversion tables. These
tables provide a correlation between the amount of axial push distance, a hydraulic nut oil
pump pressure and how much the internal clearance is shrinking. However, this is a
theoretical calculation that does not offer 100% certainty, leading to a percentage of
mounting errors.
A brand new concept
To improve upon this approach, NSK came up with a world-first idea: a detachable seal.
This concept centres on a seal mounted to a ring/holder that is fully detachable (via bolts),
thus providing access for clearance measurements using feeler gauges (as well as the end
user’s ability to select and pack their choice of grease at commissioning and re-grease if
required). The NSK design requires no heat-shrink equipment and continues to make use
of ISO-standard tapered adapter sleeves and locknuts, thus offering interchangeability with
existing solutions.
Importantly, despite the addition of the detachable seal, there is no extension of the ISOspecified boundary width dimension, therefore permitting the use of standard bearing
housings. NSK achieved this outcome by deploying its proprietary, independently certified
Hi-TF/Super-TF bearing material to mitigate the design constraints of the seal.
This material increases the load rating by up to 10% (depending on size) in comparison
with a standard open bearing, allowing the solution to remain within the same dimensional
envelope. Mining companies can therefore quickly and simply swap their existing open
bearings with the NSK solution; there is no requirement for a new housing.
As a point of note, Hi-TF and Super-TF bearing material provides twice the service life of
conventional bearing material when operating with clean lubrication, and up to 10 times
when operating with contaminated lubrication.
NSK’s solution also features proven heavy-duty nitrile (HNBR) seal technology capable of
operating up to 100°C., providing 30% more effective sealing than standard industry
concepts thanks to the use of spring-loaded lip technology. This design helps to deliver
protection from contaminants and long service life.
Service life up to 10 years
Some of the trial spherical roller bearings fitted to mining conveyors by NSK Australia in
2012-13, remain in service with problem-free running. However, NSK removed one set
(although working perfectly) after two years and sent it to the company’s R&D Centre in
Japan for X-ray fatigue analysis. The result indicated that a residual life of eight years
remained. So, together with the service time already achieved, NSK determined that a 10-
year service life is possible, which is far in excess of anything previously witnessed by
conveyor applications in the mining and quarrying industry.
NSK Australia has since supplied around 1600 long-life spherical roller bearings with
detachable seal technology across the country’s mining industry. A full range of standard
sizes from 120 to 480 mm bore diameter is now available worldwide, including in Europe.
Some installations have already taken place in the Nordic countries, for example.
