The big challenges for todays gearbox designers fall into three broad categories, observed Thomas Lohner of FZG, the Gear Research Centre at the Technical University of Munich. One is to eliminate the noise, vibration and harshness that plague many operations. Another is to satisfy the never-ending quest for greater power density so that smaller, lighter gearboxes can take the place of bigger ones. And the third is the growing demand for resource efficiency and loss reduction.

The last of these is gaining importance, Lohner told the 21st International Colloquium Tribology in January, because gearboxes offer several starting points for reducing CO2 emissions. There are possibilities for loss reduction in gears, bearings, seals and auxiliary units. More efficient gears are needed to meet environmental requirements, preserve scarce resources and usher in the era of long-range electric vehicles.

Professor Jorge Seabra, of the University of Porto in Portugal, told the gathering he feels the same urgency. Improving the efficiency of gearboxes is one of the biggest challenges we face, which isnt surprising when you consider all of the types of motion involved. Loads, speeds and pressure points are constantly changing in a gearbox, as teeth slide, mesh and grip each other.

Efficiency Drive

Not just automotive vehicles but also industrial equipment, wind turbines and other applications are clamoring for greater efficiency, Seabra stressed. The optimal results will come when gearbox designers and builders seek early input from gear oil and additive suppliers, he said, rather than bringing them late into the process after the gearbox design is complete and then expecting them to lubricate it successfully.

Kai D. Kreiskother, chief of production engineering for e-mobility components at RWTH Aachen University, also predicted at the meeting that the pressure to boost gear efficiency will grow, especially after 2020, as the adoption of electric powertrains begins to pick up speed. Two distinct segments appear to be emerging: pure electrics used as city vehicles for short trip commuting and city deliveries, and hybrids with real electric powertrains and longer driving ranges.

The key moving component in both cases will be gears. In the all-electric vehicles, Kreiskother reminded, the transmission and gearbox are critical for transferring power from the motor or battery to the wheels. Hybrids, whether serial or parallel, are also highly dependent on their gearbox and clutch.

Researchers already are hard at work on the problem of load-dependent gear losses. One strategy is to pinpoint the exact source of such losses – gears, bearings, seals, friction from oil – and then address each in a step-wise fashion to rack up a tidy gain in efficiency.

That was the aim of FZG alumnus Michael Hinterstoisser, whose 2014 doctoral work was the framework for Lohners presentation. Hinterstoisser posited that losses in spur gears depend principally on gear geometry, the flank surface finish, lubricant type and lubricant supply. Optimizing each of these factors, he estimated, could result in saving over 70 percent of the power losses across all operating conditions.

Start with gear geometry: Does it help to flatten the gear teeth so they mesh less deeply? In recent years, Lohner noted, designers have done just that, creating moderate low-loss and extreme low-loss gears having shallower teeth than the high protrusions seen on conventional equipment. This configuration reduces the transverse contact ratio of the gear and concentrates the gear contact zone around the pitch point. However, it also results in extremely high pressures and high loads being concentrated right at the contact point rather than across a wider zone. And it changes the nature of dip lubrication, where the gearboxs oil is picked up and splashed about by the turning teeth.

Other researchers had theorized that moderate low-loss gear designs might supply up to 69 percent efficiency gains, compared to typical gear geometry under loaded conditions, Lohner said, and that the improvement might rise to 85 percent when extreme low-loss gears are used.

Measuring Success

To quantify the actual improvements, the research center experimented with various gear geometries and lubricants, using a modified FZG back-to-back gear efficiency test rig. This rig pairs a test gearbox and a slave gearbox, connected by two shafts to form a closed power circuit. The symmetrical setup means the total power loss of the circle can be measured and divided equally between the test and slave gear pairs (after first subtracting any losses attributable to the rigs bearings).

These tests were carried out on both moderate low-loss gears having a transverse contact ratio of one-to-one, and on an extreme low-loss geometry with a ratio well below one. As a reference design, the researchers chose the sixth gear from a passenger car manual transmission – a conventional setup with high teeth and a contact ratio of just over two.

Losses dont only come in the loaded operating regime, Lohner said, and as much as 25 percent of the power losses happen under no-load conditions. However, the centers tests essentially confirmed those earlier calculations regarding gear geometry and load-dependent losses. Using the moderate low-loss gears, the experimental results showed an average power loss saving of about 61 percent versus the conventional design and about 79 percent using the extreme low-loss design.

Additional tests indicated that oil supply plays a critical role in power losses, and further is affected by the fluids viscosity and type; that is, synthetic versus mineral oil, said Lohner, who heads the centers Elastohydrodynamic Lubrication, Tribological Contact and Efficiency Department. Because oil type makes such a difference, the selection needs to be carefully matched to the gearbox type as well as the operating conditions, he explained to the colloquium, which was held at the Technische Akademie Esslingen.

FZG wanted to see if oils generally can be ranked according to their contribution to efficiency. In its next round of tests, it used the standard FZG gear efficiency rig and scrutinized three popular types of gear oil: a mineral oil with an extreme pressure additive, a polyalphaolefin and a polyether synthetic (also known as polyalkylene glycol).

Although the oils all had the same ISO viscosity at 100 degrees Celsius (10 centiStoke), they did not perform the same. As measured by the power consumed in the FZG rig test, the PAO based oil offered a 35 percent reduction in the gears mean coefficient of friction versus the mineral oil, Lohner said. The polyether product did even better and cut that by another 12 percent, meaning the CoF was trimmed by a whopping 47 percent versus the mineral oil product.

The volume of oil in the gearbox also had a measurable influence on efficiency, he went on. In fact, its possible to have too much of a good thing. While a good supply of oil is needed to help manage thermal effects in the gearbox, too much oil contributes to internal friction and slows down the action. This becomes yet more evident with those shallow-toothed, low-loss gears. All tests so far show that low-loss gears can allow designers to decrease the level of oil, to the point where even the pinion doesnt dip anymore, Lohner said.

When the outcomes of all the FZG gear efficiency rig tests were lined up in ranking order, they did offer some insights on how designers can prioritize the factors that deliver the best efficiency gains, he continued. First, establish a baseline performance such as conventional gears lubricated with a mineral oil based gear oil. The first giant step forward from that would be to switch to a moderate low-loss gear design, while still using mineral-based gear oil.

The next gain in efficiency can be captured by applying superfinishing to the gear flanks and then implementing a special run-in procedure so that an optimal surface roughness is achieved.

For their next shot of efficiency, Lohner recommended that equipment designers should specify polyether lubricants, and lastly design low-loss components that can operate with a reduced level of this synthetic oil in the gearbox.

All told, these combined measures can deliver 87 percent maximum and 93 percent mean efficiencies in the high-speed and high-torque operating regime, Lohner declared. Need more than that? Additional potential gains in efficiency can come via additives, DLC coatings and applying better thermal management, he said.