(57) A rotary piston vacuum pump is provided with an oil pump in the lubricating oil circuit,
a pressure relief vent connected to the pumping chamber from a higher pressure source,
and biased valve connected between the oil pump and the oil circuit as well as between
the pressure relief vent and the first pumping chamber for controlling the flow of
oil and pressure, respectively. The oil pump pressurizes the oil in the oil lubrication
circuit as a function of vacuum pump operation. The valve is responsive to the oil
pressure created by the oil pump. The valve is actuated to maintain the free flow
of oil from the oil pump to the oil circuit when the pump is operating. When the pump
is shut-off, the bias of the valve shuts off the flow of oil through the oil circuit
and opens the connection between the pressure relief vent and the first pumping chamber
to increase the pressure in the latter such that lubricating oil is not drawn into
that pumping chamber. This increased pressure from the pressure relief vent actuates
an inlet shut-off valve which isolates the first pressure chamber from the operating
chamber.
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