One time, I had in my hand an object, part of a tractor gearbox, and i know its Portuguese name:"carreto inversor".

The device has two cogs, one is the input and the other is the output. The function of it is to implement the Reverse moving of the tractor. Each movement done in the input wheel will produce a movement in the opposite sense on the output wheel.

So, what's the name of this device in English?


I think people who answered the question didn't know what I am asking. It is a single component who has an input reel, and output reel, and it is NOT an idler gear of a gearbox.


To be more accurate, in the concrete case I am speaking about, the visible parts where:

  • an input ring
  • a structure in the middle of the rings, which holds them both and is hollow in the middle
  • an output ring, which rotates in the opposite sense of the input ring.

I couldn't yet get a picture on the Internet, because the many searches I did seem to not match what I want to describe.

  • How about posting a photograph. From your words it could simply be the reverse gear or part of the PTO or various other things.
    – Chenmunka
    Mar 12, 2015 at 12:36
  • The reason it's called an idler gear is not because it is idle, it's called an idler gear because it is not used in the direct input nor output of power, but a standby. This is the only part in the transmission which does what you describe. Please explain further as this is the only answer which fits what you ask. Google translates "carreto inversor" into "drive sprocket", but any gear in the transmission could fit this description except the idler gear. Mar 12, 2015 at 13:54
  • @Paulster2: I tried Google Translator before and it is badly translated. Without knowing the exact mechanical terms for naming it, more correct translations would be: inverter cog/reel/sprocket/pinion
    – sergiol
    Mar 12, 2015 at 17:31
  • Are you talking about a Planetary gear like in this video? youtube.com/watch?v=bP9YyKOJoYs Mar 12, 2015 at 18:29

2 Answers 2


On wikipedia they are calling it an "idler gear" or "reverse idler". In this image they are calling it the "reverse idler gear":

enter image description here

  • see the Update.
    – sergiol
    Mar 12, 2015 at 12:34

In most manual transmission you have three shafts.

Input shaft (IS), connected to the clutch Output shaft (OS), connected to the drive shaft Counter shaft (CS),runs parallel to the input and output shafts.

You may be referring to the idler gear. That is just an extra gear between the counter shaft and output shaft used to reverse the output

The picture below is an example of one manual transmission.

enter image description here

Image Source

After some research with translating "carreto inversor" it looks like it translates to drive sprocket.

This site refers to "carreto inversor" in the following paragraph.

A última figura representa a marcha atrás. Para obtermos “marcha atrás” temos de inverter o movimento de rotação à saída da caixa de velocidades. Para isso recorre-se ao chamado “carreto inversor”. Trata-se tão simplesmente de um carreto (roda dentada) que, pelo facto de intermediar o par de carretos (rodas dentadas) que se encarregam de produzir o “rapport” da marcha atrás, inverte o movimento de saída – rotação de sentido inverso para impulsionar as rodas de tracção ao contrário do movimento normal (para a frente).

When translated by the Google, says

The last figure is the reverse. To obtain "reverse" we have to reverse the rotational movement out of the gearbox. For this refers to the so-called "drive gear". This is so simply from one sprocket (sprocket) that because of the pair of intermediate gears (gears) that are responsible for producing the "rapport" the reverse, reverses the output motion - reverse rotation to drive the traction wheels unlike the normal motion (forwards).

In this translation “carreto inversor” is "drive gear"

Just below that paragraph is an image that's in English calling it an idler gear.

enter image description here

The text referencing that picture follows.

Vejamos em pormenor este tipo de engrenagem (marcha atrás): Como se pode ver na figura ao lado, o carreto central “A” serve para inverter o sentido do movimento normal, à saída da caixa de velocidades. O rapport é fruto da relação de transmissão dos 3 carretos.

Translated by The Google

Let's see in detail this type of gear (reverse): As shown in the figure, the center reel "A" serves to reverse the direction of normal movement, out of the gearbox. The rapport is the result of the transmission ratio of 3 reels.

In this context it translates Carretos to reels, which really are gears.

Based on all the above I would say you are referring to what is know as the idler gear or reverse idler gear. It's sole purpose is to reverse the direction of the shaft.


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