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Product category: Bearings
News Release from: NSK UK | Subject: EM Series
Edited by the Engineeringtalk Editorial Team on 03 May 2004

Novel bearings cut motor noise

A new design of quiet-running cylindrical roller bearings for electric motors can reduce noise levels by as much as 7-8dB compared with conventional cylindrical roller bearings.

With almost two-thirds of industrial electricity consumption accountable to the running of electric motors much pressure has been exerted on motor manufacturers to produce more energy efficient machines However, this is not the only operating consideration

Increasingly, electric motors are required to run more quietly and smoothly to reduce their impact on the environment.

Bearing manufacturer, NSK, has taken a major step in enabling motor manufacturers to achieve these objectives.

The company has developed the EM Series, a new design of quiet running cylindrical roller bearing that reduces noise levels by as much as 7-8dB compared with conventional CRB bearings and by as much as 3-4dB compared with conventional low-noise bearings.

As major components in electric motors, bearings are a particular area of focus in achieving noise and vibration reduction.

Recent developments have concentrated on the role of cylindrical roller bearings (CRBs) used in large and medium size electric motors.

In certain speed ranges, and under operating conditions of high radial loads, these bearings (and sometimes ball bearings also) can produce an unpleasant intermittent or high-pitched squeal that may increase running noise by up to 20dB, making it quite audible at times and unpleasant to the ear.

The cause of this noise has been determined by NSK in tests.

It is influenced by the type of lubrication used - usually grease - and is the result of increasing vibration between the outer ring and rolling elements of the bearing.

In its test programme NSK identified a close relationship between squeal noise and the unstable behaviour of the bearing rolling elements in the unloaded regions.

To address this problem NSK investigated several factors, both individually and in combination.

These included bearing clearance, dimensional accuracy, roller shape and the material of the bearing rings, rollers and cage.

The result of these investigations is a new design specification which achieves major improvements in running noise as a result of: reprofiling of the rolling contact surfaces; and improving guide width and reducing guide clearance in relation to the cage to outer ring rib contact.

The effect of these design initiatives is threefold.

Reprofiling prevents noise by stabilising the position of the roller in the bearing.

In addition, improved flow of both grease and oil lubricants is facilitated between the cage and the outer ring rib.

Finally, cage noise is attenuated while running speed fluctuations of the cage are reduced, allowing smoother roller movement.

The effectiveness of these design improvements has been proven by NSK in tests against conventional low noise bearings and conventional standard cylindrical roller bearings.

The tests were conducted using CM and C3 radial clearances, CM, which is less than C3, being the clearance used almost exclusively in electric motors.

Noise levels of the newly developed quiet running bearings stabilised at a low level irrespective of the clearance.

They also represent a marked improvement in quiet operation against both the conventional CRB and low-noise bearings, being quieter by 7-8 and 3-4dB, respectively.

As grease is also a factor in the squeal noise produced with CRBs, further tests were conducted using the various sizes of quiet running and conventional bearings with three typical greases whose base oil viscosity was in the range between 26 and 130mm2/s (at 40C).

These confirmed the silent running capabilities of NSK's new design, very little additional noise being produced with any of the grease types.

Finally, to determine the effectiveness the new quiet running design over time, NSK conducted a durability test of 1kh.

For this test, a machine that simulated an actual motor was employed, and included housings for the bearings with front and rear caps to contain grease.

Two designs of quiet running bearings were used in the test, one that employed a shallow-groove in its outer ring raceway, which is traditionally used to eliminate squeal noise.

The second employed a conventional outer ring.

The test revealed that bearing housing vibrations remained stable at levels of about 20m/s2, and that no squeal noise was produced by either of the bearings.

Despite their different designs, the outer ring temperatures of both bearings were also similar, although the temperature of the grooved outer ring was slightly higher.

Taken together, these results show that with noise prevention efforts, bearings can remain relatively quiet for at least 1kh, even on designs that have an oil retaining effect due to an outer ring groove.

There is no doubt that NSK's development of the EM Series provides a real advantage for designers who are under increasing pressure to reduce noise emissions and vibration from their machines in order to ensure a friendlier working environment.

Moreover, the fact that the EM Series has been designed for use in electric motors, which are one of the most widely used pieces of equipment in industry, means that the possibilities for overall noise reduction - and, hence, more efficient working - are that much greater. Request a free brochure from NSK UK ...

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