On the surface, the reasoning looks simple enough. All rotating machinery moves in bearings; bearings involve friction; friction is wasted energy; better lubrication reduces friction. Therefore: better lubrication will raise machine efficiency. Only carefully controlled lab tests, however, could ever evaluate the savings. The effect is insignificantly small.
Reduced friction can be an important energy saver in some machinery, particularly in gears. As much as half the total power transmitted through gearing can be lost to friction. To improve that, many special gear types and designs have been developed. (One simple change in gear configuration for fractional horsepower transmission assemblies can raise efficiency from 30% up to 50% or more, although the amount of power involved is not great.) Gears, though, are a special case. Power is transmitted only through friction; no other power loss exists.
An electric motor, in the 1-200 hp range equipped with standard ball bearings, is quite — a different matter. Motor losses include these five separate components:
|Type of loss||Typical percentage of total loss|
|Heating in stator conductors||
|Heating in rotor conductors||
|Stray load loss||
|Iron or core loss||
|Friction & windage
(“F & W”)
“Windage” loss comes from air movement through or over the motor –the spinning rotor, and any fan involved. None of that is affected by bearings or lubricant. How much of the 10% F & W component is windage alone? Neither calculation nor test can tell us. As for friction itself — ball bearings are called “anti-friction” bearings because they are almost friction-free. Most motor friction loss is really within the lubricating grease, caused by churning as the bearings rotate (all standard industrial motor bearings use grease, not oil, as the lubricant).
Assume that one-third of all F & W is in grease friction. Let’s generously assume further than some magic grease will halve that fraction. For a typical 20 hp motor, then:
Original losses 1.66 kW; original full-load efficiency 0.900
New losses (98.3% of original) 1.64 kW; new efficiency 0.901
Even this trivial change is highly unlikely. More important — it can’t be verified. And it will vary unpredictably as the grease deteriorates with age or contamination, and as relubrication changes the amount of it in the bearings. So don’t look to lubrication changes to cut motor energy usage.
Richard L. Nailen, P. E