Novel grease keeps bearings running quietly
A
NSK UK product story
Edited by the Engineeringtalk editorial team
Dec 2, 2004
A new formulation of low-noise bearing grease answers environmental and consumer demands for quieter-running domestic appliances.
The consistent rise in living standards over the last five decades means that the average home in the developed world is now stocked with a range of appliances that provide greater comfort in our everyday lives.
Unfortunately, many of these appliances generate levels of noise that are incompatible with a quiet environment.
One of the sources of this noise is ball bearings incorporated in equipment such as vacuum cleaners, washing machines, computers, air conditioners and cooking range hood ventilators.
Until recently, such bearing products were able to meet sound performance and low friction torque requirements using conventional lithium soap based greases.
However, recent market trends have demanded greater sound durability performance from greases used in domestic bearing products, together with a wider operating temperature range and higher efficiency performance.
In light of these demands, and growing environmental concerns, NSK has developed its NSA low noise grease.
This product is superior to conventional lithium soap based greases in a number of areas, providing increased sound durability performance at high temperatures and improved resistance to fretting (relative oscillatory motion, usually of small amplitude, that can occur between the bearing balls and raceway).
The demand for the development of NSA grease originated from the Chinese market, which is the largest for fan motor bearings in the world.
Due in part to lower production costs, the output of air conditioner fan motors has been increasing sharply in China, production levels having now reached a level of 15 million units per year, or approximately 65% of the gross demand in the world.
The acceptance criteria of motor manufacturers for rolling bearings, and the gross acceptance inspection criteria of air conditioner assembly plants for motors are very rigid in China compared with those in other countries.
Under these demanding inspection regimes, bearing noise, even if otherwise inaudible under the dominating hum of a running fan, is often singled out for further evaluation.
Unfortunately, the process of eradicating noise is often complicated by the rugged roads on which air conditioner and fan motors are transported.
Bearings in motors or air conditioners that are transported in this way may suffer from shock load and excessive vibrations, resulting in fretting of raceway surfaces, which, in turn, may cause bearing noise.
The task for any new bearing grease is to provide improved durability to overcome this problem, at the same time removing one of the main causes of noise.
Both these objectives have been achieved with low noise NSA grease.
The product is a lithium soap grease, but one based on a high viscosity mixture of synthetic hydrocarbon oil and polyolester oil.
This composition enables the grease to reduce frictional torque within the fan bearings despite the high viscosity of its base oil.
To determine the initial sound performance of the new grease, the noise levels of deep groove ball bearings were measured in a soundproof chamber.
A vibration transducer was placed in direct contact with the outer ring of a bearing, the objective being to arrive at an Anderon value for the bearings (there being a direct relationship between noise levels and vibration values).
NSA grease had better sound properties than conventional grease, and comparable performance with that of NSK's previous generation, NS7 grease, which is also based on low-viscosity ester oil.
The sound-minimising performance of NSA grease in comparison with NS7 was also measured.
Two fan motors equipped with bearings packed with NSA grease and NS7 grease were operated in a thermostatic bath at 105C and motor noise levels were measured at certain time intervals.
Although both motors produced increasing noise over time, the motor with NSA grease proved to be much quieter than the other motor, even after a year of continuous operation.
NSK's test programme also proved that NSA was the most effective grease in terms of its resistance to fretting.
NSA grease outperformed NS7 grease in a long-term oscillating test under an atmospheric temperature of 40C, corresponding to the hottest of summers where fan motors might be put to use.
Its anti-fretting performance was also better than that of conventionally used general-purpose greases.
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