The concepts of sustainability and energy efficiency are gaining importance in power transmission engineering. More than ever, manufacturers are looking for ways to reduce raw material consumption and attain a lower energy consumption and hence a better CO2 balance. This can be done by making machines more efficient, extending the lifetime of components as well as maintenance intervals. The added benefit of such measures is a reduction of operating costs.
The increase of gear efficiency harbors a frequently overlooked potential for increasing the efficiency of a machine. A very direct and effective way of increasing power transmission efficiency – which goes along with excellent wear protection – is a changeover from mineral-oil-based to synthetic lubricants. Synthetic lubricants, e.g. based on polyalphaolefin, ester or polyglycol oils, have proven to reduce energy costs while also extending the service life of gears. The possible extent of efficiency increase depends on the type of gears. While gears featuring a low percentage of sliding friction, such as spur or bevel gears, offer a relatively low potential, gears with a high percentage of sliding friction enable considerable improvements.
Worm gears show the highest potential for efficiency savings, when switching over to a polyglycol oil efficiency increases of up to 35 percent are possible. In addition to efficiency savings, their lifetime can be extended up to tenfold. A conversion to synthetic oils offers an enormous potential for savings especially in facilities where many gears are operated – for example in logistics centers, filling stations, breweries, or airports.
Tribological factors are decisive in attaining the maximum performance of a machine and its components. When choosing a lubricant for a gearbox or machine design engineers should be aware of the characteristics of the various types of lubricants and know how to use them. While, as a rule, synthetic special lubricants tend to be more expensive than mineral oils in terms of the sales price, they pay off after a short time when considering efficiency, oil change intervals, oil consumption, and extended lifetime of lubricated components. With such lubricants, gear manufacturers offer their customers the added benefit of lower operating costs.
Tested and proven
Applied to the effect a lubricant provides in worm gears, the aspects of sustainability and energy efficiency can be translated into the wear behavior and the efficiency of a gearbox. Reduced wear means longer service life of components, which has a consequential effect on the exploitation of resources as less raw material is required to make new components to replace the old ones. For example, polyglycol oils provide a better wear behavior a longer lifetime of worm wheels made from brass, that are getting more expensive due to raising copper prices.
Higher gear efficiency has a direct effect on the amount of energy consumed. On a worm gear test rig, the influence gear oils have on the wear and efficiency behavior in heavily loaded worm gears is examined under real-life conditions. Both the speed and the torque of the worm can be measured on this test rig. This is correlated to the worm wheel output torque to calculate the total efficiency of the gear unit. Wear on the worm wheel is measured by determining the weight loss and the abrasion of the tooth flanks occurring during operation. Various temperature values are also measured in the standard version of this test, namely oil sump temperature, mass temperature of the worm shaft, casing temperature and ambient temperature.
A hint that polyglycol oils offer the best wear protection to worm gears is already included in DIN 3996 on the design of cylindrical worm gears. They can help to extend the lifetime of a worm gearbox significantly compared with a mineral oil. In Figure 1 the wear behavior of three polyglycols with ISO viscosity grade 460 is estimated by the comparison of the weight loss of the worm wheels before and after the test run whereas the wear rate is measured by abrasion of the tooth flanks.
The examination of the wear behavior of the polyglycols shows highest wear for Polyglycol A, elevated wear for Polyglycol B and no measurable wear for Klübersynth GH 6.
Gear description: Test conditions:
Test gearbox: Flender CUW 63 Input speed: 350 rpm
Center distance: 63 mm Output torque: 300 Nm
Transmission ratio: 1:39 Runtime: 300 h
Viscosity of tested lubricants: ISO VG: 460
(Fig. 1 – Weight loss and wear rates)
This shows that the use of a high-performance polyglycol oil rather than a standard polyglycol oil enables an even more significant reduction of wear. Consequently, the worm wheel survives longer with the same load, or the output torque can be increased without dimensional changes. Additional benefits for machine operators are cost savings due to longer maintenance intervals, a lower risk of equipment failure and minimized downtime.
Maximum energy efficiency of a gear unit means that it produces the highest possible output power for a given input power. The energy lost in the process manifests itself in the form of heat, for example in bearings, O-ring seals or gear wheels. As gear efficiency increases, its temperature will go down. This has several positive effects: a decreasing temperature not only extends the oil lifetime, but the service life of seals as well. This in turn reduces the risk of leakage. Another benefit is that fans or air conditioners in production facilities might be switched off, which is another contributor to lower energy costs and a better CO2 balance.
Figure 2 contains the efficiency and temperature measurements for the polyglycols from Figure 1 for the same test run. The examination of the efficiency correlates with the wear results with the expected lowest efficiency for Polyglycol A, slightly higher efficiency for Polyglycol B and highest efficiency for Klübersynth GH 6. Higher efficiency causes less heat generation due to friction lossen what is underlined by the low oil sump temperature of 61°C of Klübersynth GH6. Polyglycol A and B have more than 10°C higher oil sump temperatures and Polyglycol B with the higher efficiency value shows an unexpected higher oil sump temperature.
Test gearbox: Flender CUW 63
Center distance: 63 mm
Transmission ratio: 1:39
Input speed: 350 rpm
Output torque: 300 Nm
Runtime: 300 h
Viscosity of tested lubricants: ISO VG: 460
(Fig. 2 – Temperatures and efficiency ratings)
To visualize the great differences in temperature the thermographic images in Figure 3 have been taken. The measured point in the images fits the location of the PT100 temperature sensor to measure the housing temperature. Thermographic images of the gear housing and the oil sump temperatures were monitored simultaneously. These measurements confirm the correlation of the sump temperature from Figure 2 to the housing temperature in Figure 3. The lowest oil sump temperature of 61°C causes a housing temperature of 50.6°C. The higher oil sump temperatures of Polyglycol A and B cause housing temperatures of ca. 59.7°C and respectively 63.2°C.
(Fig.3 – Thermographic images)
Per DIN 3996, the efficiency of gears in mesh is influenced by, among other factors, the oil’s basic friction coefficient. Consequently, oils with a low friction coefficient offer potential for increasing gear efficiency. Similar to their wear characteristics, polyglycol oils show a lower friction coefficient than other base oils. Suitable additives can help to further improve the friction coefficient of a polyglycol. Figure 4 shows a comparison of two polyglycol oils. The basic friction coefficients determined for Klübersynth GH 6 is clearly below the figures to be assumed for polyglycol oils per DIN 3996. The high efficiency of the gear oil Klübersynth GH 6- 460 could not just be proven at the worm gear test rig rather at the Gear Research Center (FZG) of TU Munich as well. According to the investigation report FVA 503 II the gear oil could obtain the enormous efficiency of 96% at a large sized worm gear test rig with center distance 315 mm and reduction of 10.25.
(Fig. 4 – Basic friction coefficient)
The described effects of higher efficiency make themselves strongly felt in the energy balance of facilities operating several hundred gearboxes.
The changeover from mineral-oil-based to synthetic gear oils is a simple and highly effective way of minimizing wear and improving energy efficiency. The extent of optimization possible depends on the individual gear type. Best results are obtained where polyglycol oils are used in gear boxes with a higher percentage of sliding. Worm gears show the largest savings potential. Additional potential for improvement is offered by polyglycol oils based on special formulations and containing specific additives: such lubricants enable even longer gear and machine life as well as a lower energy consumption for a given output power.
The result is savings both in terms of financial resources and raw materials. Besides these savings, operators enjoy the benefit of a much better CO2 balance in their operations.
This article first appeared in the May 2011 issue of Gear Technology (http://www.geartechnology.com/issues/0511x/siebert.pdf). It was updated in 2017 by Matthias Pfadt, Craig Desrochers and Michael Hochman at Klüber.
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