Consider that this motor-gearbox combination is backdriven so that the motor acts as a generator. My goal is to try and make a relatively accurate simulation of the losses/damping that occur due to the fact that it's driven in reverse. From what I understand, the total inertia of the motor felt by the output of the gearbox (now used as an input) can be calculated by the equation below where $I_{gearbox}$ refers to the mass inertia of the gearbox.


I know that motor cogging torque needs to be considered as does the friction in the gearbox, but I am unsure as to how this is done. My impression is that the most resistance comes from the friction within the spur gearbox as it is occuring to the point that it takes grown man strength to get it turning at all.

Are there specially designed gearboxes which are designed to work somewhat equivalently in reverse as well? I know that "planetary gearheads up to two stages or spur gearheads" should work but the efficiency is still low. https://drive.tech/en/stream-content/dc-motors-as-generators

Or would I be better off making a simple gear train with two/three spur gears? The driving torque would be quite low.

Thanks in advance for any assistance

  • $\begingroup$ Hydraulics can provide that functionality... $\endgroup$
    – Solar Mike
    Dec 5, 2019 at 13:36

1 Answer 1


If you want a hight reduction ratio (such as 1:50 as mentioned in your other question), don't use spur gears. These are self-locking. Lock after cycloidal drives: https://en.wikipedia.org/wiki/Cycloidal_drive

If your rpm's are low, the unbalanced property might not hurt so much.

  • $\begingroup$ Thanks for the reply. The highest gear ratio I would actually want is around 20:1. Maybe I'm misunderstanding, but couldn't two spur gears be driven in either direction? How would they be self-locking? $\endgroup$
    – Inf_E
    Dec 9, 2019 at 11:00
  • $\begingroup$ I am not sure how official and well known "self-locking" is a good translation for the german "selbsthemmung" is. It's an effect, that comes with high ratios, is caused by friction and is worst in celical gears, but still noticable in spur gears. The wikipedia page for the cycloidal drives claim, they don't have this effect. And indeed, they're probably invented for this reason. You can also watch out for Wittenstein. A company that invented another kind of gears "Galaxie". It reminds me of the cycloidal gear, but is different. Look the properties and see how it fits for you. $\endgroup$
    – Ingo
    Dec 11, 2019 at 12:20
  • $\begingroup$ celical -> helical $\endgroup$
    – Ingo
    Dec 12, 2019 at 11:02

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