Why does bush in bush pin coupling sit only in one of the couplings as shown in the following figure?

The rubber bush sits only in one of the two couplings to achieve a tight fit in one of the couplings for torque transmission as well as centering the shafts and a looser fit on the other coupling to accommodate misalignment. Here is why.

1. Centering and Torque Transmissions: The rubber bush acts as a spacer and centering element between the two connected shafts. It typically has a slightly larger outer diameter than the inner diameter of one of the couplings. This creates a tight fit that centres the shafts and transmits torque efficiently.
2. Accommodates Misalignment: The other half of the coupling accommodates the bolt only with no bush. The standard hole diameter is typically 1 millimetre larger than the bolt diameter. This diametrical clearance between the hole and the bolt of another half of the coupling provides some flexibility without compromising the connection’s strength.