To increase the *power transmitted* by a shaft, you need to change things *connected to* the shaft, not the shaft itself:
* Increase the power source: Use a more powerful motor, engine, or other prime mover. This directly increases the available torque and, depending on the gearing, the speed (rpm).
* Change the gearing: Gears can increase torque at the expense of speed (e.g., a low gear in a car), or increase speed at the expense of torque (e.g., a high gear in a car). This doesn't change the shaft's inherent capabilities, but changes how much power is delivered *through* the shaft.
* Use a stronger, larger diameter shaft: If the shaft is failing due to stress from the existing torque and rpm, a larger, stronger shaft (perhaps made of a higher-strength material) can handle greater power. This is a direct change to the shaft itself, but still doesn't *increase* the shaft's rpm or torque in a way that’s independent of the system connected to it; it merely increases its capacity.
In short, you can increase the *power* transferred *by* a shaft by modifying the system, but you cannot directly increase the intrinsic rpm or torque rating of the shaft itself. Think of it like a pipe; you can't make the pipe itself flow more water, but you can increase the water pressure and flow rate at the source to get more water through the pipe.
