Strive to Improve Your Gearbox Performance
Todd R. Bobak
In industry, power consumption in the manufacturing environment accounts for approximately one-third of all energy consumed annually in the U.S. In the enclosed-gearing industry, questions, such as “how efficient is product X,” are becoming increasingly common. Several factors influence how efficiency is lost during the operation of the gearbox system. For more information on how you can get the most out of your gearbox, Sumitomo Drive Technologies offers its detailed “How Green is Your Gearbox” white paper. Let’s get started and first take a look at the effect oil seals and lubricants can have on gearbox efficiency.
Oil seals and efficiency
Virtually all speed reducers incorporate the use of oil seals within their assemblies. These seals can be found on both the input and output shafts, as well as internally within the unit. Their primary function is to retain the lubricant within the gearbox, while eliminating the ingress of dirt and water into it. There exist a variety of different types of seals for a variety of different applications (i.e. axial-shaft seals), but the most common type used in industrial gearboxes is the radial-shaft seal.
The performance of a radial seal is dependant upon an interference fit that provides pressure of the seal lip against the shaft or collar surface. Through operation, the seal lip will gradually wear so, in some cases, a garter spring is incorporated into the oil seal to maintain adequate seal-lip pressure against the shaft. Additionally, a secondary seal lip may be utilized on the seal to prevent the ingress of contaminants into the system.
Since these seal lip(s) are riding against a rotating shaft (or collar), friction at this interface is developed, hence, an energy loss (albeit small) is realized. The amount of this energy loss due to friction is dependent upon many factors that include shaft speed, shaft diameter and the surface finish/roughness against which the seal lip(s) are in contact. As an example, published data indicates that an oil seal riding on a 100-mm shaft (= four inches) that’s rotating at 500 RPM will generate frictional losses on the magnitude of 20 Watts.
While it’s true that this is a seemingly minuscule value, it’s common for some gearbox manufacturers to incorporate more than a single seal on a given shaft as an added feature to minimize, or eliminate, the possibility of lubrication leakage. Having said that, multiple seals within a single speed reducer may develop frictional losses exceeding 100 Watts (once again, depending on seal sizes and rotational speeds).
Effects of lubricant on efficiency
For internal gearing, the use of the appropriate lubricant is crucial to obtaining maximum service life and reliability of the gearbox. The function of the lubricant is two-fold: first it provides a thin film between the internal rotating components, as well as the gear teeth in mesh. This prevents direct metal-to-metal contact and, second, it provides a median in which heat-developed through normal unit operation-is dissipated.
As noted previously, the type of lubrication utilized in a gearbox plays a role in the overall efficiency of the unit. As the internal gearing moves through the lubricant, the lubricant is continuously displaced by the action of the gears striking it. This is typically known as churning loss, since power that could otherwise be used for the application is absorbed (or required) by this action of the gearing-striking, pumping or moving the lubricant.
For example, a gearbox lubricated with grease would be less efficient than if it were to be lubricated with oil. This makes sense intuitively, since grease is typically thicker than oil and it requires a greater amount of power to move the gearing through it. Imagine, for a moment, what it would be like to swim in syrup as opposed to swimming in water. Clearly, the thicker media (syrup) would require more personal power to “swim” thru.
Another avenue for loss in efficiency specifically related to gearing and lubricant is what is known as windage loss. As the gearing rotates through the lubricant, and then out of the sump, a certain amount of lubricant adheres to the surface of the gear itself. Since the gear is rotating, centrifugal forces cast the lubricant adhering to the gearing into the enclosed atmosphere of the speed-reducer casing. This action may serve to create lubrication “mist” through which the gearing must pass. In essence, this mist is another barrier for the gear to pass through, thereby requiring (or diverting) power that otherwise could have been utilized as usable output torque.
To quantify the effects of lubricants on speed-reducer efficiency, testing has been conducted by Sumitomo Drive Technologies on a planetary gearbox of a given size and reduction ratio (4:1). This efficiency testing was conducted twice: once with the gearbox lubricated with a grease of NLGI Grade #2 (a moderately soft grease with the approximate consistency of peanut butter) and again with a grease of NLGI Grade #00 (a semi-fluid grease with an approximate consistency of applesauce). Other than the lubricant, no other components within the test units were changed. Post-test results revealed that the speed reducer lubricated with the NLGI #00 grease had an efficiency of 92.1 percent, whereas the same unit lubricated with the NLGI #2 grease was 90.9 percent efficient.
This isn’t to say, however, that oil lubrication for a gearbox is distinctly preferred over grease. Grease has the advantage in that it may provide for universal mounting of the gearbox (i.e. output shaft vertical up or vertical down), it’s replenishment/replacement interval may be longer than a comparably sized oil lubricated unit, and grease is less likely to leak through the shaft seals of the unit.
As discussed, many components incorporated into the gearbox construction and its subsequent operation influence the overall efficiency of the speed reducer itself. While the greatest loss in efficiency is typically associated with the interaction of the gears in mesh, other factors and components also serve to influence the overall efficiency of the system. Utilization of high-quality gearing with superior surface finish on the gear teeth combined with the incorporation of low-friction seals and bearings-all serve to maximize the power efficiency of the enclosed gearing product lines.
From the point of view of the user (or potential user), perhaps one of the most important factors in selecting a unit to assure that its efficiency is being optimized for the application is unit size. In short, make sure that the gearbox is properly sized for the application. Prior to ordering the speed reducer from the manufacturer, it’s imperative that the power requirements for the application and demands are clearly understood. Utilization of the appropriate service factor for the speed reducer must be taken into consideration and applied.
If the gearbox is unnecessarily oversized (specifically, if the power capacity of the gearbox greatly exceeds the power of the applied motor combined with the application service factor), much of the motor power will be used to overcome the constant losses within the gearbox, thereby leaving little additional usable power/torque for the application itself. In short, this would be a situation where the speed reducer is yielding a very low efficiency. Conversely, however, a gearbox undersized for an application runs the risk of low life expectancy due to overload conditions despite a seemingly high efficiency.
Additionally, follow the manufacturer’s recommendation for the correct type of lubricant to be used within the speed reducer along with its recommended change interval. Be it oil or grease, over time, all lubricants lose their effective properties. As a result, overall gearbox efficiency also stands to decrease over time.
Todd R. Bobak is research and development project engineer with U.S.-based Sumitomo Drive Technologies, and this is an edited version of a more detailed “How Green is Your Gearbox” white paper