Indexing mechanism and apparatus incorporating the same

Within a longitudinally-extending main housing having a longitudinally-extending duct, an indexing mechanism which comprises:

• a mandrel within the duct and capable of longitudinal movement relative to the main housing;
• a cam sleeve mounted for rotation on, and with respect to, the mandrel but constrained in terms of longitudinal movement with respect to the mandrel, the cam sleeve having an external surface provided with a groove in the form of a closed loop;
• a cam follower mounted relative to the main housing and resiliently urged into the groove of the cam sleeve;
• main restoring means tending to urge the mandrel in a first direction along the duct;
• and
• a fluid abutment face, on which, in use, fluid under pressure can act to tend to urge the mandrel in a second direction opposite to the first direction;
• wherein the groove has ramps and associated steps thereby restricting relative movement of the cam follower with respect to the groove, to one direction progressively around the closed loop;
• and
• wherein the groove has a plurality of rest positions at which the cam follower can rest, depending on which of the force of the main restoring means and the force of the fluid pressure dominates the other:
• the arrangement of the indexing mechanism being such that, with the main restoring means initially dominating the force of fluid pressure, the mandrel is urged in the first direction and carries the cam sleeve in the first direction until the cam follower has travelled a first leg of the closed loop and comes to a first rest position in the groove;
• but when the fluid pressure is increased so that the force of the fluid pressure exceeds the force of the main restoring means, the mandrel is moved in the second direction, carrying the cam sleeve with it, the cam follower moving along the next leg of the closed loop of the groove until a second rest position is reached, and remaining there until the fluid pressure is reduced sufficiently such that the force of the main restoring means exceeds again the force of the fluid pressure, whereupon the mandrel moves again in the first direction, carrying the cam sleeve with it, with the cam follower travelling along a third leg of the closed loop to a third rest position.