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For example in the picture below you can see an allison cd-850 planetary gearbox which was first designed in mid 1940s and used in patton tanks. We can see that it uses brake bands to hold the elements of the planetary geartrain stationary. In modern day the holding still of elements is done by multi plate clutch packs, why back in the old days they used a Servo activated by the hydraulic pressure in the valve body which the servo would then later squeeze the brake band and hold the elements stationary? Why not get rid of the servo and the brake band and replace with multi plate clutch pack instead? It also used a multi plate clutch pack for when transferring motion to a gearset which you can also see in the pictures below. Why use clutch packs for transferring motion but not when holding elements stationary? Also another interesting thing about the cd-850 is that the tanks it was used in such as m48 does not have disc or drum brakes. Its braking system is also a multi plate clutch pack and when that it applied it slows down the vehicle which you can also see in the schematic below. Interesting that even the service brakes of the vehicle is multi plate clutch pack but the way of holding couple of the planetary elements as stationary is done by servos and brake bands instead. enter image description here enter image description here enter image description here

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    Evolution of design over time. Reliability analysis at the time. Cost questions. Maintenance questions. Engineering tradeoffs. Why did they use carburetors instead of fuel injection in the 40's?
    – Jon Custer
    Commented Oct 4, 2023 at 12:49
  • okay but they used clutch packs for transferring motion to gearsets and even for the main brakes of the vehicle itself. carb vs injection example doesnt really make sense because multi plate clutch packs were already being used extensively even in this transmission. They already had it nailed down and already being used. Oh and when early 50s rolled around the cd500 from m41 had already ditched the brake bands for a full multi plate clutch pack setup. I guess the only explanation is cost. Commented Oct 4, 2023 at 16:07
  • @Timothy, cost is almost never the only explanation. It will be cost, engineering, tolerances, familiarity, availability, reliability, etc etc etc
    – Rory Alsop
    Commented Oct 4, 2023 at 16:19
  • In other words, Newton's first law of motion applies to vehicle manufacturing and nearly every other human endeavor.
    – MTA
    Commented Oct 4, 2023 at 20:36
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    Clutch wear is heavy too, and the brake bands may have been quicker and cheaper to replace (perhaps in the field). Commented Oct 5, 2023 at 17:29

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