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Possible causes:  
1、 Improper assembly cause damage or deformation on the rolling tracks;
2、 The fit between bearing and shaft, and the bearing house is too tight or too loose. Especially, too tight will reduce the gap and produce too much heat due to the heat expansion of bearing;
3、 Machining finish grade is insufficient. For example, the eccentric between the front and back bearing bracket spigot, or the eccentric between the bearing house and bearing bracket spigot, or ellipse due to the poor machining will cause too much heat.
4、 Poor bearing quality, such as large air gap, dirty rolling track, corrosion, ellipse of rolling parts.
5、 Poor lubrication, such as too much or less of grease, dirt etc.