In automatic transmissions some parts need to be moving with the engine in one moment and in the other they need to be stopped. Why don't transmisisons then use detent clutches to accomplish this?

  • $\begingroup$ Is this an across-the-pond terminology thing? It sounds like you're talking about what's called a dog clutch in the US. $\endgroup$ – TimWescott Jul 30 '19 at 23:39

Detent clutches use mechanical interference to force the halves of the clutch into hard engagement. The wear surfaces in this case are metal-on-metal which works well when the engagement duty cycle is low, as for example in the overrunning-clutch mechanism used in motorcycle starters.

However, for purposes of shifting gear ratios in a car transmission, you do not want hard engagement because it jolts the car whenever a shift is made. Furthermore, the duty cycle of engagement and disengagement is high in a car transmission, which means the engagement surfaces will wear themselves out of tolerance before the desired service lifetime of the transmission as a whole has been met.

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  • $\begingroup$ That overrunning clutch mechanism is used in most car starters as well... $\endgroup$ – Solar Mike Jul 28 '19 at 4:12
  • $\begingroup$ Thank you for reply niels. I have another question then. Why not use electromagnetic clutches? Is it the same reason or something else? $\endgroup$ – Dominik Serbinek Jul 28 '19 at 8:46
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    $\begingroup$ Some transmissions do use electric clutches, but the thing is that a transmission runs immersed in oil and gets hot, so "wet disc clutches" get used instead- and the mechanism that activates the clutch (an electric solenoid, for example) is outside the transmission where it doesn't have to deal with hot oil. This is in contrast to true "electric clutches" in which the activating coils are inside the clutch housing. $\endgroup$ – niels nielsen Jul 28 '19 at 16:51
  • $\begingroup$ Makes sense. Thank you. $\endgroup$ – Dominik Serbinek Aug 1 '19 at 5:22

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