I want to design a shaft for a motor test rig. The motor and generator are 48V and 1.5kW and the maximum torque produced by the motor is 18Nm. RPM varies from 2900-3100. What shall be my next steps? I also intend to use Lovejoy coupling.
Most engineers would just take the motor and generator shafts and use one or the other if the two need to be located farther apart than a coupling would provide. 2 HP is nothing from an industrial standpoint, but that is a fairly high speed, and vibration becomes an issue. The type of coupling will matter if we also have misalignment between the driver and driven units. If there are other design considerations (weight, vibration, etc.), we might get into the details of shaft design to build a hollow shaft, or a vibration-damping system. A long heavy shaft also starts to add radial stress to the bearings beyond rotor weights, so we might want to support the shaft with an intermediate bearing.
I realize this doesn't answer you question directly, but it's hard to design without detailed specifications.