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Tests show gearbox efficiency ratings are out by up to 25%

03 February, 2016

A series of tests of industrial gearbox efficiencies conducted by a team of Belgian researchers have cast doubt on the accuracy of efficiency figures given in gearbox manufacturers' catalogues. The researchers, from the University of Ghent, tested more than a dozen gearboxes and found that the measured efficiencies ranged from 25% worse to 11% better than those given by the manufacturers in their catalogues.

The gearboxes tested came from six unnamed manufacturers and covered a variety of technologies including right-angled worm, helical-bevel, helical-worm and helical-spiroid boxes, as well as a straight helical box. The boxes had up to three stages, power ratings from 0.34–5.58kW, torque ratings from 167–505Nm, and ratios from 11.41–87.65.

According to the researchers – who reported their findings to the Eemods conference in Helsinki last year ­– there is no standardisation in the measurement of gearbox efficiencies. Also, manufacturers do not usually give any information about the conditions under which they test their gearboxes, so efficiency figures from different manufacturers cannot be compared directly.

Gearbox inefficiencies are caused by a combination of losses including those in the seals and bearings, as well the churning of lubricants. They are affected by factors such as the design, power and ratio of the gearbox, as well as the number of stages involved.

For their tests, the researchers used a 1,000Nm, 3,000 rpm rig, rated at 0.12–15kW. They fitted torque sensors on the input and output of the test gearbox, which was driven by one motor, and used to drive a second motor (the load) via a reducer gearbox. They claim that their results are accurate to ±1%.

One of their findings was that efficiencies drop off at part-loads, with the effect being most severe for low-power gearboxes with high ratios. They also compared three gearboxes with different technologies from the same manufacturer and found that a right-angled helical-bevel gear had to same efficiency (95.5%) as a straight helical box – but cost almost three times as much. A right-angled helical-worm box from the same supplier had an efficiency of 91.5%.

Two of the Belgian researchers with their 15kW gearbox test rig

The researchers also found that the efficiency of a three-stage helical-bevel gear from another manufacturer was 95% – up to 17% more than a two-stage helical-worm gear from the same company (both rated at 0.34–0.39Nm and 180–190Nm). The researchers acknowledge that there are occasions when the self-locking characteristics of a worm gear make it preferable.

Another factor that can affect gearbox efficiencies is the temperature and type of lubricant used. The Belgian researchers found that using an “energy-efficient” lubricant with added Teflon powder increased the efficiency of a helical-bevel gear by 2% (compared to standard lubricating oil) but decreased the efficiency of a worm gear by 15%. They say that more work is needed in this area.

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