HOW GREEN IS YOUR GEARBOX

Rising energy costs and concerns about global warming are at the forefront of today?s news. Turn to local or national TV programming, browse the internet or read the paper and one can find numerous stories about the seemingly irreversible energy costs and the subsequent impact that these costs have on simply doing business. We are becoming increasingly aware of the cost of energy and are being introduced to a variety of methods and/or products that will minimize the impact of these costs.

In industry, power consumption in the manufacturing environment accounts for approximately 1/3 of all energy consumed annually within 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 operation of the gearbox system. Here we will address some of these factors and further provide recommendations/ideas for obtaining a highly efficient gearbox while still taking into consideration application constraints.

GEARBOX EFFICIENCY – DEFINED
As it relates to enclosed gearing, efficiency is simply the ratio of the output power (power transmitted through the gearbox as usable work) to the input power. As no mechanical device is 100 percent efficient, this numeric value of efficiency will always be less than 1. As an example, take a Paramax gear reducer, the type of which is shown in Figure 1.

If this speed reducer were 100 percent efficient, it would be concluded that the 3,600 in/lb of torque being applied to the input shaft would generate 540,000 in/lb of torque at the output shaft through the 150:1 gear reduction. It can be seen in this example that the output torque is less than the ?expected? value due to the internal losses. In using this example, the unit efficiency in can be determined using the equation found in Figure 2. Given this, it is calculated that the gearbox in this example is 85.8 percent efficient.

EFFICIENCY OF GEARING TYPES
The term speed reducer is a general term used to describe a device that increases torque while, at the same time, reduces the rotational speed of the prime mover (which is usually an AC motor). This is achieved through the interaction of gears within the speed reducer. Different gear types can be utilized to facilitate this reduction of speed/increase of torque. Each of these gear types has distinct advantages and disadvantages associated with them and, likewise, each of these gear types have different efficiencies associated with them.

Specifically, as it relates to efficiency, two gears in mesh incur losses in efficiency due to the sliding action of one gear tooth against the corresponding gear tooth of the mating gear. This sliding action reduces the overall efficiency of the gearset since useable power is converted to heat. It is not accurate to say that a specific gear type has a definite efficiency associated with it since factors such as reduction ratio, gear-manufacturing methods and lubricant (among others) all play a roll in the efficiency of a gearset. The table in Figure 3 details three common gear types along with their associated typical efficiencies.

It is not uncommon for speed reducers to incorporate more than one set of gears (called stages) to achieve the desired overall reduction ratio. In such cases, the overall efficiency of the gear train is the product of the individual efficiencies of each gear reduction stage.
Say, for example, that a Paramax gearbox incorporates three stages of Helical Gearing. Further say that each stage has an efficiency of 98.5 percent. The overall gear train efficiency can be calculated using the equation seen in Figure 4.

To continue this example, it is possible that a multistage gearbox utilizes different types of gearing for each of its reduction stages. A Right Angle gearbox (one where the output shaft is at a right angle to the input shaft) may utilize a Spiral Bevel gearset as its first reduction stage followed thereafter by a helical gearset as its second reduction stage. Using the typical gearing efficiencies detailed previously, the efficiency of the gear train in this example can be calculated as seen in Figure 5.

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 of seals used in industrial gearboxes are Radial Shaft Seals.

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 in order 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 ? refer to the following section view of a Cyclo® speed reducer which details an oil seal cut away for clarification (see Figure 6).

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 (≈ 4 inches) that is rotating at 500 rpm will generate frictional losses on magnitude of 20 W. While it is true that this is a seemingly miniscule value, it is 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 W (once again, depending on seal sizes and rotational speeds).

BEARINGS AND EFFICIENCY
As with the other components discussed thus far, bearings are another component common to all speed reducers. In its simplest form, a bearing is a device that allows for a smooth, low-friction, motion between two components. In the case of speed reducers, roller bearings are used to secure and support shafting and gearing within the unit. These roller bearings are intended to accept external loading (radial and axial) on the input and output shafts. Additionally, these bearings are also utilized to accommodate the internal forces generated by the gears in mesh.

To accept these forces, roller bearings rely on balls (spheres) or rollers retained between an inner and outer race ? refer to the following section of a Helical Buddybox which details a Deep Groove Ball Bearing for clarification (see Figure 7). As noted, a bearing is a low-friction device, it is not friction-free. As one race rotates about the other race, the balls (or rollers) likewise rotate/slide within the race. The rotating/sliding action of the balls (or rollers) creates friction between these bearing components thereby creating an additional avenue for energy loss. Like oil seals and gearing, the amount of energy consumed by a bearing is dependent on many different factors.

Mathematical formulas exist which can be used to calculate the Friction Torque of a given bearing. This value of friction torque is a function of the coefficient of friction between the rolling elements of the bearing, the bore diameter of the bearing itself and the load acting upon it and it is expressed as seen in Figure 8.
For example, using this equation, the Friction Torque for a ball bearing of size 6211 with a 4,250 lb radial load acting upon it is approximately 5.8 in/lb. Whereas, for a similarly sized Tapered Roller bearing (32211) operating under the same loading conditions, the Friction Torque is calculated to be 9.8 in/lb.

These ?required? torque values may seem relatively small in comparison to the overall requirements of the system, but these values are for one bearing only. Gearboxes incorporate the use of four or more bearings, each one of which has a Friction Torque associated to it.
It should be noted that some bearings contain integrated seals or shields, the purpose of which is to maintain lubricant within the bearing and/or to prevent the ingress of foreign matter into the race. Tapered Roller bearings may incorporate a Nilos Ring for the same purpose. Inclusion of such sealing devices further contributes to efficiency losses since these sealing devices are in direct contact with the rotating race(s) of the bearing.

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 thereby preventing 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. Intuitively, this makes sense 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 a 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 which otherwise could have been utilized as usable output torque. To quantify the effects of lubricants on speed reducer efficiency, testing has been conducted 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 is not 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.

CONCLUSIONS
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.

Speed reducer manufacturers should actively design to minimize efficiency losses within their product through a variety of means. 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 is imperative that the application power requirements 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 and, due to this, the overall gearbox efficiency stands to decrease over time as well.

Finally, consider the method by which the gearbox is attached to the driven shaft. Is it possible to directly couple the output shaft of the reducer directly to the driven shaft? This may be preferred, from an efficiency point-of-view, since the use of belts and/or chains generates friction or possibly slippage at their interface which, in-turn, leads to additional efficiency losses.

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Todd R. Bobak works for Sumitomo Drive Technologies, Sumitomo Machinery Corporation of America, 4200 Holland Boulevard, Chesapeake, VA 23323, 757-485-33535, Fax: 757-485-3193, www.sumitomodrive.com.