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Bearings - 2020-07-13

Many sections of papermaking machinery are subject to high temperatures which present specific challenges to bearings. Photo: Moreno Soppelsa/Shutterstock

NSK spherical roller bearings with TL technology are being increasingly adopted in the papermaking industry as more machinery manufacturers and paper mills seek to maximise operating life and uptime. The unique attributes of NSK’s TL (tough and long life) specification is proven to provide greater resistance to hoop stress, subsequently reducing inner ring fractures and delivering service life more than twice that of conventional bearings with standard heat treatment.

TL series spherical roller bearings are specifically designed for sections of papermaking machinery where elevated temperatures prevail, including dryer rolls, calender rolls, canvas rolls and PV rolls. The bearings were developed as a solution to a specific problem: inner ring fractures. This problem is common on dryer cylinders (including Yankee dryer rolls) and calender rolls, where steam is passed through the hollow axis of the roll to heat it. Here, the journal expands faster than the bearing, making the fit tighter. As the hoop stress (force exerted circumferentially) of the inner ring increases, cracks can begin to appear.

High temperatures in papermaking machinery also present further challenges to bearings. For instance, the presence of heat lowers the viscosity of lubricant, reducing bearing life, while unwanted creep can have a negative impact on dimensional stability.

Although adopting a slow start-up procedure can prevent such problems – by introducing temperature gradually – it can take several hours and paper mills cannot afford the lost production and revenue. Instead, the solution lies in astute bearing selection.

To counter the effects of temperature, NSK analysed the mechanism of inner ring fracture and established a strength evaluation method that subsequently led to the development of the proprietary TL specification. The type of steel used in these bearings is case-carburised using a special method of heat treatment that attains both high raceway surface hardness and dimensional stability under high temperatures. Importantly, the strength of the inner ring is increased sufficiently to better resist hoop stress, even at temperatures up to 200°C.

The resistance of inner rings that meet the TL specification is higher than the resistance of inner rings made from bainitic steel or bearing steel, while the surface hardness of the rolling surface is greater than the hardness of rolling surfaces made from bainitic steel or case-hardened steel. These factors underpin the ability of TL bearings to provide longer service life when used on calenders, guide rolls and smoothing rolls, for example.

A case in point involves a major paper mill that was experiencing problems with its calender rolls, where multiple cracks were developing on the inner rings of the roll bearings. Frustrated by up to 12 failures annually, which led to excessive downtime and maintenance, the mill invited NSK’s team of expert engineers to investigate and resolve the issue. As part of NSK’s Added Value Programme AIP, a comprehensive application review was commenced.

The NSK specialists soon discovered that the standard spherical bearings being used in the calender roll did not feature carburised inner rings, leaving them susceptible to the high heat of the application, and cracking of the inner rings. It was therefore recommended that the paper mill switch to NSK spherical roller bearings with TL technology. The innovative chemical steel composition and greater resistance to hoop stress and cracks, doubled operating life and reduced the cost of replacement bearings and downtime. Over a five-year period since the change was made, the paper mill has saved around €430,000.

NSK spherical roller bearings with TL technology have been in operation on dryer cylinders and other papermaking machinery rolls since 1994, with no problems reported. Today, over 100 different TL bearing sizes/types are produced, helping a wide range of customers achieve better and more cost-effective performance. In addition, to accommodate bearings for Yankee dryer rolls, NSK can offer large TL series self-aligning roller bearings with diameters up to 1,360 mm.


Picture 1): Many sections of papermaking machinery are subject to high temperatures which present specific challenges to bearings. Photo: Moreno Soppelsa/Shutterstock

Picture 2): Example of a dryer cylinder roll structure showing the typical temperatures present.
1 Steam, 2 Lubrication oil, 3 Oil return

Picture 3): Fracture evident on the through-hardened inner ring of a spherical roller bearing due to hoop stress

Picture 4): NSK spherical roller bearings with TL technology deliver twice the service life of conventional counterparts

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