Gear Efficiency

Knowing about efficiency of different kind of gears as a function of gear ratios could help us to select the right gearmotor for our needs. The graph 1 and table 1 show a comparison of the efficiencies for different gear types. Keep in mind that these efficiencies values are for general guideline and for more accurate values, please consult the manufacturers catalogue

1. Spur and Helical Gear

Spur and Helical gearing is a parallel shaft arrangement, and these gears can achieve high efficiencies compared to others gear types due the prevalent rolling movement. Its efficiency varies from 94% to 98,5% with gear ratios from 1,25 up to 10,00.

2. Straight Bevel Gear

Straight bevel gearing is similar to spur gearing with intersecting shaft arrangement, most frequently at 90°, the normal range of gear ratios is from 1,50 up to 5,00 with efficiency from 93% up to 97%.    

3. Spiral Bevel Gear

         Compared to straight bevel gears it is smoother with advantage of more gradual teeth engagement, this way not only the noise level goes down but also the impact stress on the teeth. Because of tooth shape and the contact ratio of the spiral bevel gears, the efficiency is similar to helical gearing (94% to 98,5%).   

4. Worm Gear

Worm gear have crossing position of the shafts with mainly sliding movement; and the efficiency varies according to lead angle and friction factor. As an estimation you can use the equation 1 to calculate efficiency of worm gears “η”; where μ is the coefficient of friction (for worm gears normally ranges from 0.02 to 0.06), γ the lead angle and αn the pressure angle.

5. Spiroid Gear                                                     

Similar to worm gear, spiroid gearing have crossing position of the shafts and the efficiency is among 40-94% with gear ratio ranging normally from 5 up to 80 for a single stage. You can include a cylindrical gear pair to improve the “gearmotor” efficiency because combining a cylindrical gear pair with a spiroid gear pair, the spiroid gear ratio can be smaller (the smaller is the spiroid gear ratio the better is the efficiency) and with this two-stage geamotor combination it will be possible to reach higher efficiency, i.e. from 70 up to 94% 

Gear Technology Magazine section “Ask the Expert” published in May of 2012; section “Letters to the Editor” of the same magazine Gear Technology published in June of 2012 and the technical article “Six Key Elements of Gearmotor Optimization” of the Power Transmission Magazine published in august of 2017 provide us important information about efficiency of spiroid gearing; and help us to conclude that right angle gearmotors with just one stage spiroid gearing provide the efficiency just a little bit higher than one-stage worm gear, in the other hand Spiroid gearing with a cylindrical pair of gear in the first stage, the efficiency is improved. Another conception, Helical-bevel two-stage can reach efficiency 96% and three-stage helical-bevel gear units the efficiency is close to 94%. 

We know that the efficiency is resulting of the movements rolling and sliding while teeth are meshing; i.e. it is defined by the combination of rolling/sliding movements; the higher the sliding the worst is the efficiency and inversely; this is shown by the figure 1, which we can see when we go from intersecting position of the shafts (bevel gear) to cross position of the shafts (worm gear) the percentage of sliding movement go from 20% up to 90% and the efficiency goes down; the inferior section of the figure show us a cylindrical gear pair which have uppermost rolling contact and a sliding percentage varying from 10 up to 30% confirming the good efficiency of this kind of gears.