Switching mechanism

A reciprocatory device comprising:

• a piston and a body defining a cylinder within which the piston can reciprocate along an axis, the piston having a shoulder in fluid-tight contact with the cylinder, and the cylinder defining first and second ports spaced from one another in the axial direction;
• a first fluid enclosure capable of being maintained at a relatively high pressure on one side of the shoulder and a second fluid enclosure capable of being maintained at a relatively low pressure on the other side of the shoulder, each enclosure being defined by the piston and cylinder, the arrangement of ports and shoulder being such that each of the ports can communicate, at one relative position of the piston and cylinder, with the first fluid enclosure only, and at a different relative position of the piston and cylinder, with the second fluid enclosure only;
• a signal line in fluid communication with the first port through a first fluid passage which includes a non-return valve arranged to prevent flow of fluid along the passage to the first port, and being in fluid communication with the second port through a second fluid passage which includes a non-return valve arranged to prevent flow of fluid along the passage away from the second port;
• and
• a main control valve having a supply inlet for connection to a high pressure fluid source and a supply outlet for draining fluid at low pressure, and a drive connection isolated in fluid tight manner from the first and second enclosures, the said main control valve having a signal input to which the signal line is connected and being actuable by changes in fluid pressure in the signal line to connect alternately the supply inlet and the supply outlet to the drive connection;
• the piston having a blind bore into which a part of the body projects, the said projecting part of the body having a duct through which the force generated by the presence of high pressure fluid in the drive connection can be transmitted to act on the end of the blind bore.