EUREKA JUNE 2005 COVER FEATURE STORY

Bearings adapt to the task

Tom Shelley reports on novel bearings that adjust themselves to take account of different load conditions and run unusually fast and cool in machine tool spindles

New bearings developed to support some of the rolls in paper mills use a combination of slightly pre-loaded balls to prevent slippage under low load, and rollers to support high loads

The same company has also developed a radically new concept in machine tool spindle bearings that improves lubrication to a point where running temperatures are reduced by 10 deg C, permitting higher speeds combined with longer lives.

In a business increasingly challenged by low cost standard bearings manufactured in the Far East, these are just two of the examples of the move among companies with well established technology bases to develop bearings to meet specific needs.

The Anti Slippage Spherical Roller (ASSR) Bearing was recently unveiled on the INA FAG stand at this year's Hannover Fair. They are specially designed to support typically 6m long rolls used in paper making machines. Calender rolls are used to smooth the rough surface of the paper and remove variations in web thickness. In the proprietary NIPCO technology, upper rolls are conventional but lower rolls consist of an inner roll, and large number of hydrostatic elements that compensate for roll flexing, and keep an outer sleeve pressed to give an exact same thickness to the web across its width. The rolls typically weigh around 4 tonnes each. In normal use, their bearings are only slightly loaded. However, if the paper breaks, or the line has to be stopped for maintenance, the bearings have to support the full weight of the rolls. If the bearings are of the normal spherical roller bearing type, there is a danger that under low or no load, the bearings will slip instead of roll, and break through the lubricant film resulting in damage and even cold welding. The ASSR bearings therefore incorporate alternate balls and rollers. The balls are oversized by a few microns to give a pre-load. These support the rolls at or below 0.5% nominal load rating. Full load, 6% of nominal load rating, leads to the balls distorting slightly to the point that full load is borne by the rollers.


When we asked, we were told that the new bearings do not represent a new range as much as a new capability, since the bearings apparently have to be designed and precisely manufactured to suit each particular application. However, the engineers said that the idea did have other possible applications, such as the support of very large fan rotors, where normal running only imposes very light loads, but bearings have to be rated to support large out of balance loads on some occasions.

Glorious mud

Equally specially designed and on a similarly large size scale, are bearings designed for National Oilwell's new hex mud pump, also shown on the stand. Mud pumps in the oil industry are used to circulate drilling fluid down through the drill string and up through the well bore. The new pump, while still large, is a third smaller and 30% less heavy than conventional three cylinder pumps and delivers more mud, up to 3900 l/min.

The core of the pump is a cast cam plate that rotates at up to 212 rpm. This acts on cam rollers mounted in forks, transforming the motion of the rotating cam plate contour into a linear movement of the stroke cylinder.

The forces on the supports for the cam plate are mainly axial and are borne by a thrust tapered roller bearing. A cylindrical roller bearing is responsible for radial guidance. The mating bearing is an angular contact thrust bearing. To prevent slip, this bearing is preloaded via a pressure ring with springs. To ease handling and mounting, all the large bearings are equipped with self retaining cages and have threads for eye bolts.

The cam rollers are mounted in forks. They transform the motion of the rolling cam plate contour into the linear cylinder movements. This task is undertaken by INA rail units RUE 55D and RUE 65 D-H. The outer ring of the track rollers has an optimised cross direction profile to prevent edge stress. The cage has been designed for high radial acceleration. A patent application has been submitted for a preloaded and pre-mounted tapered bearing unit that supports the required forces.

Finally, two cylindrical roller bearings in contact with a four point contact ball bearing form the pinion shaft bearing supports of the drive. The cylndrical roller bearings support radial forces, and the four point contact ball bearing supports axial forces. All the roller bearing supports and linear guides were developed using the company's 'Bearingx' software package. Three pumps are in service on three platforms off the coast of Angola and in the Gulf of Mexico.

Cool for customers

At the same exhibition, Dr Arbogast Grunau, who is in charge of development for the whole Shaeffler Group, also showed us the company's latest 'Low temperature' machine spindle bearings. These have improved lubrication that reduces running temperatures by approximately 10 deg C allowing speed increases of up to 10 per cent. This is described by the company as a "MiIestone towards reliably achieving speed indices of 2.5 million n x dm for grease lubricated bearings and setting a new standard for spindle bearings as a whole." (Note: for those who don't know, n is the limiting speed in rpm and dm is the mean bearing diameter, but the factor is roughly constant for bearings up to about 75mm diameter only)

Knowing that the exact mechanism is crucial, we will not reveal exactly how it is done, except to observe that there is a groove on the outside of the bearing, and a bore through the outer ring, at what Dr Grunau described as a "Special position of the cage." The cages are also noticeably tapered.

The design concept is said to be based on the idea that the cage needs to be positioned above the area in which the lubricant gathers as a result of centrifugal force. Lower friction inside the bearing means lower temperatures, and this helps maintain the viscosity of the lubricant. The danger of mixed friction and the resulting wear and tear on the bearing decreases as a result of the cooler temperatures. The range can be recognised by the designation 'TX' at the cage position in the bearing code.

INA Bearing Company

Eureka says: Despite the maturity of rolling element bearings as products, advances continue to be made in their development, especially with regard to special purpose designs to meet specific customer needs.

Pointers

* A combination of pre-loaded balls and spherical rollers allows bearings to avoid the possibility of slippage under zero or low load but still cope with high loads

* Special bearings have been developed for use in a novel mud pump for offshore oil applications allowing it to be made a third smaller and 30 per cent lighter

* New spindle bearings with improved lubricant flow run 10 deg C cooler under the same conditions of service

For more technical developments see www.eurekamagazine.co.uk

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