Reciprocatory piston and cylinder machine

An ail free reciprocatory vacuum pump, comprising:

• a cylinder having a first portion closed at one end and a second portion contiguous with, but of smaller diameter than the first portion;
• a piston having a first hollow, metallic cylindrical head portion relatively slidable in the first cylinder portion and a second hollow, metallic piston portion relatively slidable in the second cylinder portion, said first piston head portion having a front face facing the closed cylinder end and an annular back face;
• a gas inlet in said cylinder for inlet of gas to the interior of the first cylinder portion between the front face of the piston head portion and the closed cylinder end on reciprocation of the piston with said piston acting as a valve member to control the flow of gas through said gas inlet;
• a first exhaust port for exhaustion of gas from the interior of the first cylinder portion ahead of the piston head portion by pumping action of the front face of the piston head portion;
• a first one-way valve in said first exhaust port operable to permit exhaustion of gas from the interior of the first cylinder portion ahead of the piston head portion;
• a second exhaust port for exhaustion of gas from the interior of the first cylinder portion behind the first piston head portion by pumping action of the back face of the first piston head portion;
• a second one-way valve in said second exhaust port;
• a passage by which gas may be passed from the interior of the first cylinder portion ahead of the piston head portion to the interior of the first cylinder portion behind the piston head portion;
• and,
• respective sealing means for substantially sealing the annular space between said cylindrical piston portions and the respective cylinder portions in which they are slidably reciprocable, in lieu of oil or other liquid lubricant;
• characterized in that said sealing means for the first piston head portion includes a first sleeve of a low friction material disposed on the cylindrical surface of the first piston head portion, such that, over the temperature range encountered during normal operation of the pump, a mean gap is sustained about the sleeve between the sleeve and the cylinder, which gap is of a maximum size at which leakage of gas past the sleeve is at a level acceptable for a vacuum to be sustained by the pump;
• said sealing means for the second piston portion includes a second sleeve of low friction material disposed on the cylindrical surface of the second piston portion, such that, over the temperature range encountered during normal operation of the pump, a mean gap is sustained about the sleeve between the sleeve and the cylinder over substantially the entire surface of the sleeve and a sealing ring element about said cylindrical surface of the second piston portion, at or adjacent an end of the second sleeve remote from said first piston portion, and means biasing the sealing ring element into sliding contact with the second cylinder portion.