shear pin diameter

Hi I am wanting to build a large clock winding motor using a shear pin(dowel) as the 3rd and last resort safety feature, I am a clock restorer with only a little engineering theory. All the online formula I find assume you know the diameter of the pin. I am trying to find the pin diameter that will shear at 1550 nm. Thank you Symon.

It is a 4rpm motor with a 12mm diameter shaft which I want to mount a 32mm boss with a 32mm long dowel pin fitted through the shaft and boss that will shear at 1550 nm

1550 Nm on a 12 mm shaft is a huge torque. The stress in the shaft will be enough to break it.

Pick the material for your shear pin. Something softer than the shaft. Find the shear strength from a table or experiment.

Calculate the shear force by dividing your torque by the 6 mm radius. This force acts on the cross sectional area of the pin in two locations. (Each side of the shaft)

Divide the force by the shear strength to get the shear area which you expect to fail. Divide the area by two and determine the diameter from that value.

Thank you I shall start by checking my figures