Advanced Grease Lubrication:  Keeping Electric Motors Online

  

In electric motors, more than 90% of bearing failures are due to over-greasing which causes seals to burst, allowing the grease to bypass the bearing and go straight into the motor windings. Another frequent cause of electric motor failure is misalignment.

 

There are many methods used to align the components, but what is most important is to align to tight tolerances. By choosing the appropriate grease, companies may significantly reduce operating costs, improve bearing performance, increase protection for long electric motor life and increase overall productivity.

 

Selecting grease

Grease is a semi-solid lubricant composed of a base oil, thickener and additives, and is frequently used as an electric motor bearing lubricant because of its ease of application and specific characteristics. Not only can it reduce friction and prevent everyday wear and tear, but the right grease can protect against corrosion and act as a seal to keep out dust, dirt, waste and other atmospheric contaminants.

 

Selecting the correct viscosity of grease for an application is the most important factor influencing electric motor bearing lubrication. Once the appropriate viscosity and type of grease have been selected, ensuring the correct level of lubricant release is the next challenge. Any factor that affects a grease’s ability to provide lubricating oil to an application in a controlled manner will negatively impact the grease’s ability to provide effective lubrication.  This, in turn, can lead to lubrication failure.

 

Grease can be affected by factors such as excessive mechanical shear, low and high temperatures, thermal-oxidative degradation of thickener and lubricating oil, as well as water ingress and other contaminations that can inhibit the ability to provide optimum lubrication and peak performance.

 

When selecting a grease to protect electric motor bearings, operators should consider a lubricant which offers the following benefits:

 

•   High temperature performance and low oil release: Electric motor bearings, especially the outboard bearing, often operate at higher temperatures than other equipment.  This is why high temperature performance and low oil release are so important to performance. Even a totally enclosed, fan cooled motor can run at a steady 95°C / 200°F

•   Corrosion protection: Electric motors are 'wet' by exception but not by design, and by running at, often, high temperatures, water can evaporate more quickly leaving the bearings potentially exposed to corrosion. If the use of the motor is intermittent, a grease offering corrosion protection can be beneficial to protect against possible condensation when the bearing cools down in a humid atmosphere

•   Low temperature performance: A grease designed to operate at low temperatures can be beneficial during the start-up of electric motors which are exposed to winter conditions as the resistant torque can damage seals

 

The base oil used in electric motor greases can be mineral or synthetic with most mineral oils providing sufficient protection for most electric motor bearing applications. However, more advanced synthetic base oils may be needed where motors are exposed to extreme temperatures or where longer re-greasing intervals are desired. The thickener in the synthetic oil acts as a carrier and prevents it from leaking out of the application.

 

A thickener increasingly used to protect electric motor bearings is polyuria and this is a key part of the make-up of the Mobil Polyrex EM Series, a range of greases engineered for use in electric motor bearings and able to withstand degradation under high temperatures and speed. The range can operate at temperatures from -20°C to 160°C.

 

Reducing downtime

One case that demonstrates the potential value of using an advanced grease such as Mobil Polyrex EM is that of a steel manufacturer in Russia where the use of this high-performance grease significantly improved the service life of bearings.

 

The company used a 185kW blower electric motor for hot aluminising aggregate, reaching speeds of 2900-4500 rpm. The combination of these speeds and temperatures to 120°C resulted in bearings beginning to fail after a short life span. On closer inspection by an ExxonMobil field engineer, a recommendation was made to use Mobil Polyrex EM. The previous grease lasted only one and a half months before the bearing failed, but Mobil Polyrex EM performed for nine months, demonstrating a six times longer service life*.

 

By using the right grease and ensuring electric motors are cared for and well maintained, companies can reduce downtime, cut repair costs and increase productivity.

* This proof of performance is based on the experience of a single customer. Results can vary depending upon the type of equipment used and its maintenance, operating conditions and environment, and any prior lubricant used.