You are correct that a larger disc made from similar material will have a larger moment of inertia (when I studied this stuff 20 odd years ago, we used a different expression for "moment of inertia", but I cannot remember right now what is was.).
The moment if inertia will only have an effect when there is angular acceleration (RPM changes), if I understand the current question, it is to do with a pulley pulling on a rope at a constant speed, no acceleration involved.
The "simplification" I was referring to was the various forces on the pulley/crankshaft.
As well as the torque component, the force due to the rope also creates a downward force on the pulley/shaft mounting.
This is probably not directly relevant in this example, but for a full analysis of the forces using a "free body diagram" it is essential.
JimB
The moment if inertia will only have an effect when there is angular acceleration (RPM changes), if I understand the current question, it is to do with a pulley pulling on a rope at a constant speed, no acceleration involved.
The "simplification" I was referring to was the various forces on the pulley/crankshaft.
As well as the torque component, the force due to the rope also creates a downward force on the pulley/shaft mounting.
This is probably not directly relevant in this example, but for a full analysis of the forces using a "free body diagram" it is essential.
JimB