This double sprocket is typically attached to the shaft using a "keyway and two set screws".
These sprockets are listed with their "rough stock bores" but can have other shaft sizes machined into them by either Power-Link or the end user.
This sprocket is designed to accept a double strand roller chain where the chain travels in one direction, offering more strength than a single chain could provide.
If the application requires one chain to travel in one direction and another in the opposite direction (i.e. powered conveyor line with multiple rollers), a double single sprocket would be required instead. Power-Link Catalog