Th400 basic operation

here’s a reader’s digest version of how in normal operation a th400 will shift:

 1st gear- forward clutch engages to drive the planetary gearset/all other clutches disengaged/ one way low roller sprag in the rear planetary holds the gearset to create a 2.48:1 ratio. in low 1 the rear band would also apply for extra holding power. at this point due to planetary action the forward drum rotates clockwise but the direct drum spind counter clockwise at this point.

 2nd gear- forward clutch engaged. the intermediate clutch applies to stop the direct drum dead in it’s tracks. this creates a 1.48:1 ratio. the low roller sprag that was holding then is renderred useless as the carrier switches direction. so you still have only 1 holding member.

 3rd gear- forward clutch engaged. the intermediate clutch dissengages and the internal clutch of the direct drum engages making a solid unit…..1:1 ratio. at this point the guts of the tranny spin as a unit. there is no gear reduction therefore no holding member in use

 reverse- forward clutch dissengaged. direct drum then engages and is driven off an outer hub of the forward clutch. now we have a problem we need a holding member but now if you are following this at all you guessed it, the low roller clutch won’t hold it either. this is where the rear band engages to hold the carrier all on it’s own. in normal operation this is fine but for derby this is a major source of problems.

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