Technical background article from WEG Electric Motors (UK)

Bearing relubrication is key to longer motor life

Relubricating motor bearings is a low-cost way to improve productivity through reduced downtime, says Marek Lukaszczyk of WEG Electric Motors (UK).

The increasing importance accorded to the maintenance of electric motors across all industry sectors is being driven by the requirement to guarantee trouble-free operation in what is a highly pressurized climate for manufacturing.

Because of this, many manufacturers have invested in motor monitoring and protection systems whose function is to reduce the frequency of maintenance intervals for electric motors, and also the incidence of motor failures.

These monitoring and protection measures, whilst effective, are not cheap.
Moreover, they do not provide a total solution, more basic initiatives, such as the planned relubrication of motor bearings at regular intervals, being far more effective over the life of the motor.

Relubrication intervals are the function of a number of bearing factors including:
bearing type, operating speed and temperature, type of grease and type of bearing housing. 
Helpfully, in most instances the respective interval may be obtained with the use of graphs supplied by bearing suppliers. 

These use lithium-based soap grease as a reference (reference temperature is 70C) and recommend that the relubrication interval be reduced by half for every 15C of temperature increase - this in respect of the maximum temperature limit allowed by the grease. 
As an example, WEG uses Polyrex EM grease in its IEC 225 to 355 frame motors (NEMA 364T to 5877) with a temperature reference grease of 85C. Therefore, when a bearing is operating at 100C, the lubrication interval indicated on the maintenance manual supplied with the motors must be reduced by half. 
It is important to know that relubrication intervals are defined based on tests that do not allow for any ingress of water and/or solid contaminants into the motor. In cases where motors are subject to such contaminants then careful periodic cheeks must be carried out and the grease replaced more often to maintain its efficiency. 

The process of replacing and relubricating is a critical one, requiring special care to achieve the levels of bearing performance that extend bearing life.
In general four key recommendations need to be observed.

1. The grease should be stored in a proper room to avoid penetration of contaminants. Additionally, before relubricating a motor, clean the grease nipple. Whenever possible, motors fitted with a grease fitting must be relubricated with the motor running.
   If not, pump in half of the grease fill recommended in the motor manual, run the motor for a minute, switch it off and then pump in the remaining grease. Motors not designed with a grease fitting must have the bearing carefully removed for relubrication.
2. All grease must be removed and the bearing housing carefully cleaned with the application of kerosene or diesel. When regreasing, force the grease to penetrate into bearing races and all other internal orifices.
3. It is quite important to spin the bearing while relubricating it so as to ensure proper grease penetration, hence avoiding noisy operation. The heating temperature when mounting the bearings cannot exceed 90C, as this will affect the grease adversely resulting in a reduction of bearing life.
4. In cases where the bearing has been removed then heated and refitted to a shaft, it is important to avoid incorrect alignment by checking to determine if the internal bearing cap making a correct fit with the shaft.

If these recommendations are observed as part of a planned maintenance schedule, reliable motor operation will be ensured and also extended bearing life. As a result, maintenance costs will be reduced and unexpected - and costly - production failures will be avoided.