BEARING

 

Types of bearing ;

Rolling bearings, i.e ball and roller bearings, are reliable, inexpensive and easy to maintain. They are the only type of bearing used in small and medium size three phase motors. However, they have an upper size limit that is a functions of rotationsal speed. This limit depends on bearing type, size, load, method of lubrication etc. some times sleeve bearings are preferred for large motor sizes, even where rolling bearings could be used.

           

Bearing life of rolling bearing in motors is normally 25 000 to 100 000 hours L 10 to ISO R 281. Nominal life is the number of running hours at given speed for wich the bearing can rotate before signs of fatigue – scaling  - appear on the rings or rolling elements.

ISO definition L 10 means the length of life that 90 % of a large number of identifical bearings are expected to reach or exceed. Half of the bearing achieve as times the L 10 life.

VIBRATIONS

VIBRATIONS

The International Standardisation Organitation, ISO, has issued international standard covering balancing and vibration characteristics. ISO 2373 is of particular interest for electric motors. It governs permitted vibration level on delivery and applies to motors with shaft heights in the range 80 t0 400 mm. The vibration level is expressed in mm/s rms ( milimeteres per second root mean squared ) and must be measured at no load with the motor on elastic mounting. ISO 2373 requires the shaft extension to be fited with a full – size key during vibraton measurement. The requirements apply in the measurement range 10 to 1000 Hz.

Grade of	Speed	Maximum Vibration Velocity in mm/s rms
quality	r / min	at shaft height, mm
		8 - 132	160 - 225	250 - 400
N	600   ≤   3 600	1.8	2.8	4.5
Normal
R	600   ≤   1800	0.71	1.12	1.8
Reduced	>  1 800  ≤  3 600	1.12	1.8	2.8
S	600   ≤   1800	0.45	0.71	1.12
Special	>  1 800  ≤  3 600	1.71	1.12	1.8

The corresponding standard for large machines has not yet been issued, but a figure of 2.8 mm/s can be taken as guide, at least for squirrel – cage motors. Measurement with the motor bolted fast to a solid base may occur, as may measurement with a half key fitted to the shaft extension, the smooth shaft method.

Rotor balancing is a relatively simple opration and the balance is easy to check. However, the final vibration resistance is also iinfluenced by other factors, mainly the nature of base on wich he motor is mounted, although the method of clamping, the aligment and the electromagnetic forces also play part.