Abstract: A dog clutch having a sliding internally toothed annular power transmitting collar gear (41) which can reciprocatingly slide coaxially to engage and disengage a driving shaft portion (22) from a load shaft (9). The temporarily engaged shaft (22) and collar gear (41) have cooperative teeth (42,36) in which there is a first set of teeth having a given axial length and a second interleaved set having a shorter length. The collar gear is sandwich between a pair of abutting ring-shaped pistons (51,52) within a housing cylinder. Pressurized fluid in either chamber of the cylinder drives the pistons to slide axially. Such a dog clutch mechanism is particularly suited to disengagingly couple a power takeoff (PTO) to an hydraulic pump on a commercial vehicle such as a waste disposal truck.