The gearbox of a geared turbofan engine has 4 reducing gear small wheels being spun by a larger wheel or gear, simultaneously spinning the shaft that is attached to the fan.

That is 8 points of contact, two for each small wheel/gear meaning 8 the points where friction and subsequent heat would result. I gather the use of 4 small wheels/gears instead of one is for reducing the load on the teeth at each given point.

Would increasing the smaller gear wheels (i.e. the "planets" ) from 4 to 8 or 12 or 16 for a gearbox designed for an engine the size of a Trent 1000 or more render the gear teeth less likely to break or fail?

With some reinforcement on the teeth, would it be possible to make a gearbox for this large engine?

As for the extra heat, cooling and rotating the oil through a cooler or along the length of the aircraft several times over, exposed to the atmosphere at cruising altitude using a high pressure oil pump passing oil through a sealed gear box would take care of cooling.

With the ambient energy loses to heat would this render the engine a lot more fuel efficient and possible?

  • $\begingroup$ how do you increase the points of contact? increasing the number of teeth? but then you have teeth that are way small. then you have to increase the diameter of the gears, and you end up with an gearbox that's way too big. $\endgroup$
    – Federico
    Jan 19, 2018 at 12:57
  • $\begingroup$ The problem with gearbox cooling is not cooling the oil afterwards, but circulating enough oil in an efficient manner to cool the gearbox in the first place. Perhaps this can help you focus your question towards what you really want to ask. $\endgroup$
    – Sanchises
    Jan 19, 2018 at 13:08
  • $\begingroup$ Smaller teeth means it can't hold as much torque, so I think adding teeth will make it more likely to fail. $\endgroup$
    – Ron Beyer
    Jan 19, 2018 at 16:42
  • 1
    $\begingroup$ While you mention 4 satellites, the PW1100 has 5 ones. Epicyclic gearing is a crazy technology and efficient gearboxes are difficult to built due to the complex plays to catch in spite of wear if you want all teeth to transfer the load. $\endgroup$
    – mins
    Jan 20, 2018 at 20:37

1 Answer 1


The fundamental question you seem to be asking is whether adding more planets to a planetary gear improves the load carrying capability of the gearbox. The answer to that question is yes... to a point.

Take a look at a picture of the PW geared turbofan gearbox. You'll note that there are five planets in there, and they are packed in pretty tightly. There's really no room to add any more. Although adding more planets would distribute the load such that each planet takes less load, it's impossible in this configuration because there is no space. Let's say that you really really wanted to make it six planets instead of five. The only way to make enough room would be make the planets smaller. As you make the planets smaller, then either the ring gear has to get smaller too, or the sun gear has to get larger. Either way, that is going to change the gear ratio (see formulas here). Specifically, the gear ratio is going to get closer to 1, which means the gearbox no longer does the job it was intended to do (namely reduce speed and increase torque).

So you can't have your cake and eat it too. If you want 16 planets in a planetary gear, you can do it, but the gear ratio will be something like 1.02:1 which is not useful. You might as well forget the gearbox and just do direct drive at that point. If you need a decently high reduction ratio like 2:1, you can do it, but you will be limited on the number of planets that you can fit in.

PW GTF gearbox

  • $\begingroup$ The PW gearbox of the geared turbo fan was designed for an engine designed for an aircraft the size of an A320. One would expect the gearbox of an engine desinged for an A380 to be larger, hence a large sun. Reducing the size of planets and increase the size of the sun would result in a larger gearing ratio. Reducing the size of the planets would increase the space to add more planets and increasing th size of the sun would increase space for more planets hence more smaller planets and a larger sun would increase gearing ratio. if planets were the size of the sun gearing ratio would be 1:1 $\endgroup$ Jan 25, 2018 at 12:58
  • $\begingroup$ “Specifically, the gear ratio is going to get closer to 1” – this is not true. In the limit $N_\mathrm{s} \approx N_\mathrm{r}$, $N_\mathrm{p} \ll N_\mathrm{s}$, the transmission ratio $\frac{\omega_\mathrm{c}}{\omega_\mathrm{s}}$ approaches $\frac12$. I.e., it is possible to have a planetary gearbox with arbitrarily many planet gears, while still getting a useful torque conversion. It just probably doesn't offer as good a tradeoff between power, weight, reliability and efficiency-gain as the 5-planets configuration does. $\endgroup$ Dec 15, 2019 at 17:38
  • $\begingroup$ @leftaroundabout I stand corrected. Not sure what I was thinking when I wrote the answer but you are right. $\endgroup$
    – Daniel K
    Dec 15, 2019 at 18:12

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