A tub of grease costs next to nothing. A seized motor can bring a production line to a standstill, resulting in enormous accumulating losses. Magda Bartosova says that remembering to lubricate electric motors will probably get the company accountant whining about trickle-spending, but...
Electric motors are wonderful. They are highly efficient and so robust that they run and run and run. But they often drive critical processes, so a motor breakdown can be a very big issue. A good plant engineer will therefore want to make sure his motors get enough TLC to ensure a long, trouble-free working life.
Modern motors are almost exclusively lubricated with grease, which is what this discussion will focus on. However, some older motors use oil-bearing felt and their maintenance programmes will need to be adapted accordingly.
Grease can look messy and even harbour particles of dirt and debris, so the layman can be tempted to ‘clear it out and clean it up’. But, in fact, grease serves several functions including trapping dirt and debris so that it does not make its way into the interior of the motor and cause damage.
Grease also reduces friction between moving parts, typically the components within the ball or roller bearings supporting the motor shaft. Perhaps less obviously, grease also serves the key purposes of transferring heat away from hot spots to help the motor run cool, and to reduce/eliminate corrosion of the metal surfaces.
With all these functions to perform, it is not surprising that there are many different formulations of grease and choosing the right one for a particular application requires some thought. For instance, if you had four identical motors, one to be dispatched to the hot dry Atacama Desert, another to the humid Amazon basin, the third to Antarctica, and the last to a mild coastal climate for harbour-side duties, you will want to be looking at four very different lubricants.
Some motors are ‘greased for life’. These are typically used in domestic appliances, computer peripherals and light duty applications. Industrial motors work harder in rougher environments, so need regular maintenance, including re-greasing. It is worth noting here that motors can suffer from over-greasing, indeed many industrial motor failures can be put down to this.
So maintenance engineers need to know the lubrication requirements for each motor, and to keep a record of the details so that colleagues can look them up.
Motor grease is a manufactured product, there being many different formulations so that different performance characteristics are available for different situations. While many motors will need an average sort of grease, a significant number will need something more suited to their particular circumstances
And sometimes there is an extra consideration or two. For instance, the food industry insists upon greases that are not poisonous, that are tasteless, colourless and won’t react with the foodstuff in anyway, while motors destined for the space station will require a thick grease that will not decay in micro-gravity (or even solid graphite lubrication).
Maintenance engineers also need to be aware that some greases are incompatible with one another. For instance, lithium-based greases tend to leach the oil out of polyurea greases. However, calcium-based greases do not have this effect. Best practise is probably to use as few different greases in a plant as possible, perhaps only use one manufacturer for all your different greases and make sure the manufacturer(s) provides you with a simple, clear chart on intercompatibility.
Spot the difference
It is also worth noting that there may be counterfeit greases in the market – you think you are buying a respected brand made to exacting standards, but that may be far from the truth. The lesson here is to buy from a reputable supplier and beware cheap deals that appear to be too good to be true!
The different types of grease have different properties, and plant engineers must select the correct type for each application. Polyurea-based grease is perhaps the most common for use with electric motors; it is formulated to remain stable at the raised temperatures typically found in an industrial motor and at the sort of speeds a high-speed motor turns (which could easily be 10,000rpm or higher).
Other greases have other properties. It is always worth discussing a motor’s operating parameters with a tribology expert, and to be careful of rules of thumb, habit and non-expert but well-meant advice.
Regular maintenance
Every engineer will know the value of regular maintenance of plant and equipment including the electric motor, and many plants have a commendable maintenance regime. However, some industries are less organised; elsewhere budget cutbacks can result in squeezed maintenance resources.
A regular schedule is a simple way to run a maintenance operation – basically walking around the plant with a checklist of tasks to do at set intervals. Alternatively, instruments can be brought onto the shop floor to analyse vibration, ultrasound and thermography, or sensors can be permanently installed at key points around the plant and monitored by computer.
A word of warning: motors need greasing, as do a lot of other equipment, but if you lubricate after a problem has started to develop, the grease will tend to mask the noise and hide the developing problem. Therefore, it is better to take a pre-emptive approach and lubricate regularly, and to thoroughly investigate any new noise within the plant’s regular ‘sound signature’.
The maintenance schedule for any plant has to be developed as a bespoke programme, depending upon factors such as operating hours, loads (steady and shock), temperatures and speed. There is also a need to consider environmental conditions, design details (eg, size and type of bearings), process characteristics, age of plant, etc. A good maintenance schedule will be flexible to allow for ‘issues arising’, spot checks, operatives’ observations, extreme weather, etc.
Maintenance is a mixture of high-tech and low-tech, intuition and analysis, gut feel and precision data, experience and knowledge. In fact the most important thing to know about plant maintenance is the rule ‘If it can go wrong, sooner or later it will go wrong’ so maintenance is about trying to stay ahead of the game with preventive and predictive techniques.
Lubrication of motors is an essential part of any maintenance programme. Many motors fail prematurely because of poor lubrication. Using the correct grease and applying it on schedule is vital, but be prepared for a bit of early re-lubrication while being wary of over-greasing and incompatible greases. And – if you send a motor off for rewinding, send a lubrication strategy too!
Magda is Bartosova, Technical Advisor with Rotor UK, Wellingborough, UK.
