More than likely the torque on the bearings is too low or too high. The grease turning dark is caused by heat. So if the torque on the nut is not correct it will cause excess heat because the bearing is not spinning straight or it is too tight and causing excess friction.
I attached the info about replacing them and adjusting them. Let me know if you find they are not torqued properly. Also, if the spindle has signs of wear it may need to be replaced as well.
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Monday, January 6th, 2020 AT 6:50 PM