Ceramic-coated, electrolytic-corrosion-resistant bearings for inverter motors

The power usage of motors used in machinery is said to make up 40-50% of the world´s electricity consumption, making these industry-essential devices a key energy reduction target. With this thought in mind, the market is seeing a proliferation of industrial motors claiming to offer improved energy efficiency. The latest inverter motors are also contributing to lower power consumption as they adjust their rotational speed according to operating conditions. 

Furthermore, demand is ramping up for new-generation servo motors, which typically find use in industrial robots as they can rotate forwards and backwards for accurate positioning. These three motor types (high-efficiency, inverter and servo) all rely on one common component to help maximise performance and minimise energy consumption: bearings. Worldwide endeavours to reduce power consumption have led to regulations requiring motor efficiency improvements. A motor´s energy loss comprises of three factors: iron loss (heat generation from the iron core); copper loss (heat generation from the winding); and mechanical loss (due to bearing rotation, for instance). Although mechanical loss can be as small as 1% of motor power consumption, it accounts for 0.4-0.5% of global electricity use, which means the worldwide impact of any reduction would be significant.

Low-torque bearings for high-efficiency motors

NSK has been evaluating mechanical loss from bearings using actual high-efficiency motors and real-life operating conditions. The company sought to identify potential reductions in power consumption by developing a test method that allows for the direct evaluation of mechanical loss in bearings. The results from these tests would feed into NSK´s bearing development programme for high-efficiency motors. For instance, the evaluation of a two-pole high-efficiency motor running at 3000 rpm revealed that the cause of around 80% of mechanical loss was lubrication resistance, namely the shearing and agitation resistance of grease between the bearing parts. NSK therefore began a project to optimize the type and amount of grease, as well as the bearing’s cage shape.

Subsequent tests showed that optimising the grease quantity reduced mechanical loss by 60% in comparison with conventional bearings. As a further point of note, reducing the grease content in conventional bearings tends to break the oil film and shorten grease seizure life. In contrast, the low-torque bearing developed by NSK uses special grease that, in tests, demonstrated an increase in grease seizure life of more than 2.7 times. Further tests showed that NSK´s specially developed plastic cage reduced mechanical loss by half that of steel cages, largely because the plastic cage supresses grease agitation resistance between the cage and balls.

NSK´s low-torque bearings, which reduce mechanical loss and allow for longer grease seizure life in high-efficiency motors, are available in sizes from 16 to 170mm outer diameter.

Ceramic-coated bearings for inverter motors

Inverter motors, controllable with optimal rotational frequency, are useful for delivering energy savings in pumps and blowers. The general development trend is that the frequency requiring control (known as the carrier frequency) is increasing so that the motor can operate with a higher degree of accuracy. However, as the carrier frequency increases, electrolytic corrosion may occur due to high-frequency current in the bearing.

Electrolytic corrosion is a phenomenon whereby sparks generate through the lubricating oil film between the raceway surface and rolling elements, causing local melting and unevenness. This effect also leads to early abnormal running noise and seizure.

Some bearings for small motors feature ceramic balls that do not pass a current to protect against electrolytic corrosion, but there are productivity issues with bigger diameter ceramic balls required for medium and large inverter motors. To remediate this issue, NSK has developed an anti-electrolytic-corrosion, ceramic-coated bearing specifically for use in these inverter motors, where the company applies a ceramic spray coating to the outer ring.

The newly developed bearing exhibits excellent electrical insulation, with tests showing around ten times more insulation (using a DC power supply) than general ceramic-coated bearings. As a point of note, the NSK bearing also demonstrates equivalent or slightly better insulation with an AC power supply, satisfying an impedance of 100Ω or more at 1MHz frequency.

In terms of mechanical performance, the impact resistance of the coating is about three time that of a general ceramic-coated bearing. Heat dissipation, which is a disadvantage of ceramic coatings, is suppressible using a relatively dense coating compared with that used on conventional bearings, extending both lubricant life and motor life. In tests, the temperature rise during bearing rotation was about 10°C lower than that exhibited by a general ceramic-coated bearing. The dense coating also means fewer voids and better durability.

NSK´s anti-electrolytic-corrosion, ceramic-coated bearings are available in sizes ranging from 130 to 230mm outer diameter for medium and large inverter motors.

Low particle emission bearings for servomotors

The precise positioning capability of a servo motor derives from transmitting or reflecting the LED’s light emission signal in the pattern engraved on the encoder plate and feeding the received signal back to the motor controller. However, contaminating the encoder plate’s surface with oil or other matter disables the signal reception and feedback of the position information to the motor controller, preventing regular operation.

In a servo motor for a robot, it is necessary to stop the arm and workpiece via an electromagnetic brake. Contaminating the brake plate with oil or other material causes brake slippage. Servo motors used in industrial robots and collaborative robots (cobots) must be highly reliable, so bearings should exhibit low particle emission to avoid contaminating the encoders or brakes.

NSK has developed a bearing for servo motors that uses a low particle emission grease with optimized composition. The bearing also takes advantage of the company’s light-contact DW seal, which provides excellent contamination protection when torque levels are a consideration.

A series of simulation tests investigated the relationship between bearings and encoder brake plates with regard to the scattering of grease contamination. The tests confirmed that the scatter amount decreases when using bearings with a non-contact seal and low-emission LGU grease from NSK (compared with bearings featuring a non-contact seal and conventional grease). Furthermore, bearings with LGU grease and the light-contact DW seal showed hardly any grease scattering, an outcome that also represents an effective countermeasure against encoder corrosion.

NSK´s low particle emission bearings, featuring LGU grease and DW light-contact seals, are available in sizes from 26 to 120mm outer diameter to suit all common servo motors.