fig4 comprises a threaded stud 27 secured at one end to universal joint 20 by a fastener 30 and unitary with a male coupling member 31 having a convex conical surface which tapers outwardly from a small tip end 32 fig1 and is unitary with a first external ring gear 33 a peripheral groove 34 is formed around cone 31 at a selected axial distance from end 32 and has the cross section of a portion less than half of a circle whose center lies just outside of the surface of said conical surface and on a line normal to said surface secured on stud 27 as by nuts 35 and 36 and a lock washer 37 is a flanged wheel or drum 40 having a flange or rim portion 41 assembly 21 is capable of moving axially with respect to tongue 14 in the direction of arrow 42 by reason of the telescopic action of shaft 17 driving assembly 22 comprises a female coupling member 43 having a concave conical surface and secured to or unitary with pto shaft 13 and a locking collar 44 assembled to a drive collar 45 by fasteners 46 the collar assembly is slidable axially on the outer surface 47 of member 43 by a driving fork 50 having pins 51 52 riding in an external groove 53 in collar 44 between a first position shown in fig1 3 and a second position shown in fig4 member 43 is formed at its outer end with a second external gear 54 like gear 33 and drive collar 45 has an internal ring gear 55 configured to mate with gears 33 and 54 and having a face wide enough to engage both the external gears at the.