The reason I wondered, is that in years past I had serviced machines that used a "wrap spring clutch"[1] which I knew used the same principle. These are mentioned in passing in the Wpedia article on clutches[2] where it says they obey the "Capstan equation". That article[3] opens,
> The capstan equation or belt friction equation, also known as Eytelwein's formula, relates the hold-force to the load-force if a flexible line is wound around a cylinder (a bollard, a winch or a capstan)
So there's the generic idea, which can be viewed as a clutch or a bollard. The bollard is an "amplifier" in the same sense that a transistor is, where a small controlling voltage regulates a larger current flow. However the wrap spring clutch is on/off, not proportional, so is effectively a torque SWITCH.
I think a better transistor analogy than a FET is a BJT, where both the amplified quantity and control signal are electrical current.
The key difference between a clutch and torque amplifier is that, with a clutch, the output shaft power all comes from the input shaft. The torque amplifier's output shaft power mostly comes from the output drum, not the input shaft. The drums are constantly spinning, powered by some external source. https://www.youtube.com/watch?v=oNNkNc7mFJ4
> The capstan equation or belt friction equation, also known as Eytelwein's formula, relates the hold-force to the load-force if a flexible line is wound around a cylinder (a bollard, a winch or a capstan)
So there's the generic idea, which can be viewed as a clutch or a bollard. The bollard is an "amplifier" in the same sense that a transistor is, where a small controlling voltage regulates a larger current flow. However the wrap spring clutch is on/off, not proportional, so is effectively a torque SWITCH.
[1] https://www.warnerelectric.com/products/clutch-products/wrap...
[2] https://en.wikipedia.org/wiki/Clutch
[3] https://en.wikipedia.org/wiki/Capstan_equation