Mechanical piston retention for free wheeling

A fixed displacement fluid pump or motor having a plurality of pistons reciprocable in a plurality of cylinders, means normally in driving or driven contact with said pistons, valve means operable normally to cycle pressure fluid to said cylinders to operate said pistons and to prevent the pistons from being actuated out of contact with said driving or driven means, outwardly tapered wall means formed in each of said cylinders axially inwardly of the normal inner ends of the strokes of the respective pistons, locking means located on the respective pistons adapted to lockingly engage said tapered wall means in a free-wheeling condition of operation in which said pistons are actuated by inward momentum thereof only under certain conditions of relatively low fluid operating pressure in said cylinders to positions inwardly of one ends of the normal piston strokes wherein said locking means engage said tapered wall means, the pressure fluid conducted to said cylinders by said valve means in normal operation preventing the locking means from engagement with said tapered wall means, said pistons being actuated normally by said driving or driven means into positions closely spaced from the tapered wall means, said locked condition of said pistons under said condition of relatively low fluid operating pressure in said cylinders causing said pistons to be held continuously in positions out of contact with said driving or driven means.