NSK bearing developments help maximize the performance and energy efficiency of industrial motors
The energy consumption of the motors used in machinery is thought to represent 40-50% of global electricity consumption, making these industry-critical devices a key energy reduction target. With this in mind, the market is seeing a proliferation of industrial motors claiming to offer better energy efficiency. The latest inverter motors also help reduce energy consumption as they adjust their rotation speed according to operating conditions. Additionally, demand is growing for next-generation servo motors, which typically find use in industrial robots because they can rotate forward and backward for precise positioning. These three types of motors (high efficiency, inverter and servo) all rely on a common component to optimize performance and minimize energy consumption: bearings.
Global efforts to reduce energy consumption have led to regulations requiring motor efficiency improvements. The energy loss of a motor includes three factors: iron loss (heat production from the iron core); copper loss (heat production from the winding); and mechanical losses (due to the rotation of bearings, for example). Although mechanical losses can be as low as 1% of motor energy consumption, they represent 0.4-0.5% of global electricity consumption, meaning that the global impact of any reduction would be significant.
Low torque bearings for high efficiency motors
NSK has made an effort to assess bearing mechanical losses using actual high-efficiency motors and actual operating conditions. The company sought to identify potential reductions in energy consumption by developing a test method that allows the direct assessment of mechanical losses in bearings. The results of these tests would feed into NSK’s bearing development program for high-efficiency motors. For example, evaluation of a high-efficiency bipolar motor running at 3000 rpm revealed that the cause of about 80% of mechanical losses was resistance to lubrication, i.e. resistance to shear and pressure. stirring of the grease between the bearing parts. NSK therefore launched a project to optimize the type and quantity of grease, as well as the shape of the bearing cage.
Subsequent tests showed that optimizing the amount of grease reduced mechanical losses by 60% compared to conventional bearings. Another point to note is that reducing the grease content in conventional bearings tends to break up the oil film and shorten the life of the grease seizing. In contrast, the low-torque bearing developed by NSK uses a special grease which, in tests, has been shown to increase grease seizure time by more than 2.7 times. Further tests have shown that NSK’s specially developed plastic cage reduces mechanical losses by half compared to steel cages, mainly because the plastic cage removes resistance to grease agitation between the cage and the balls.
NSK’s low-torque bearings, which reduce mechanical losses and extend the life of grease seizure in high-efficiency motors, are available in sizes ranging from 16 to 170 mm outside diameter.
Ceramic coated bearings for inverter motors
Inverter motors, controllable with optimum rotation frequency, are useful for energy saving in pumps and blowers. The general trend of development is that the frequency to be controlled (called the carrier frequency) increases so that the motor can operate with a higher degree of precision. However, as the carrier frequency increases, electrolytic corrosion can occur due to the high frequency current in the bearing.
Electrolytic corrosion is a phenomenon by which sparks occur across the film of lubricating oil between the surface of the raceway and the rolling elements, causing local melting and irregularities. This effect also leads to early abnormal running noises and seizures.
Some bearings for small motors feature ceramic balls which do not carry current to protect against electrolytic corrosion, but there are productivity issues with larger diameter ceramic balls required for medium size inverter motors and big. To remedy this problem, NSK has developed an electrolytic anti-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 isolation, with tests showing approximately ten times more isolation (using DC power) than general ceramic coated bearings. Note that the NSK bearing also demonstrates equivalent or slightly better insulation with an AC supply, satisfying an impedance of 100? or more at a frequency of 1 MHz.
In terms of mechanical performance, the shock resistance of the coating is about three times that of a general ceramic coated bearing. Heat dissipation, which is a disadvantage of ceramic coatings, can be eliminated by using a relatively dense coating compared to that used on conventional bearings, extending both lubricant and motor life. In testing, the temperature rise during bearing rotation was approximately 10°C less than that exhibited by a general ceramic coated bearing. The dense coating also means fewer voids and better durability.
NSK’s electrolytic anti-corrosion ceramic coated bearings are available in sizes from 130mm to 230mm outside diameter for medium and large inverter motors.
Low particle emission bearings for servo motors
The precise positioning capability of a servo motor derives from the transmission or reflection of the light emission signal from the LED in the etched pattern on the encoder plate and the transmission of the received signal to the motor controller. However, contamination of the encoder plate surface with oil or other material disables signal reception and feedback of position information to the motor controller, preventing smooth operation.
In a robot servo motor, it is necessary to stop the arm and the workpiece via an electromagnetic brake. Contamination of the brake plate with oil or other material causes the brakes to slip. Servo motors used in industrial robots and collaborative robots (cobots) must be very reliable, so the bearings must have low particle emission to avoid contaminating encoders or brakes.
NSK has developed a bearing for servomotors which uses a low particle emission grease with an optimized composition. The bearing also takes advantage of the company’s light contact DW seal, which provides excellent contamination protection when torque levels are considered.
A series of simulation tests investigated the relationship between bearings and encoder brake plates with respect to the dispersion of grease contamination. Tests have confirmed that the amount of scatter decreases when using bearings with a non-contact seal and NSK’s low-emission LGU grease (compared to bearings with a non-contact seal and conventional grease). Additionally, the bearings with LGU grease and the light contact DW seal showed virtually no grease scatter, a result which also represents an effective countermeasure against encoder corrosion.
NSK’s low-particle bearings, featuring LGU grease and DW light contact seals, are available in sizes from 26mm to 120mm OD to suit all common servo motors.
1) Display of mechanical loss rate of different cage materials used on NSK low torque ball bearings for high efficiency motors
2) NSK electrolytic corrosion resistant ceramic coated bearings for inverter motors
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