What causes an A/T to heat up?

Teach me something.

I do not have an issue with this but was wondering the root cause for the heat in an automatic transmission. Don't confuse this with the practice that heats it up like heavy towing. I want to know WHY towing or other uses adds heat to a A/T. In my mind I see the only areas of friction would be a clutch pack or band. If they are slipping or are engaging frequently that would cause heat. Other than that what really causes the heat. The gears are practically submerged in oil. There is no combustion going on. The torque converter is basically 2 turbines with oil between them so no physical hard surface contact. Some heat would transfer to the A/T from the engine block. I don't understand where the heat comes from.

 

I have some info posted here that may help you figure out why a transmission gets hot . Most of the issues are caused by friction .
http://www.tacomaworld.com/forum/te...hift-kit-shift-recalibration-kit-2nd-gen.html

 

Shifting, shifting, more shifting. Under load, like vehicle weight, and towing. Oh and are you aware of how many clutch packs are in the A750F? 7 these packs are engaging and disengaging with each shift=HEAT

 

So if I get this right just having the fluid moving around inside a torque converter creates friction and heat? If that's the case then I can see how towing and other heavy use can cause heat. There would be more resistances on the fluid from the engine side to the drive side of the torque converter. I wouldn't think fluid movement would generate much friction.

 

CLUTCH PACKS
http://www.lastchanceautorepairs.co...Automatic-Transmission-Clutch-Replacement.jpg
Along with fluid under pressure, shearing forces in the torque converter among other places. And just like the engine the transmission has a optimum operating temperature. That's why there is a transmission warmer. The warmer is a heat exchanger in the radiator to warm the ATF.