Dual-line pump unit, lubrication system, and related apparatus and method

A dual-line pump unit for supplying lubricant, comprising:

• a reservoir having an interior for holding lubricant,
• a first pump for pumping lubricant from the reservoir through a first main line to a lubricant distribution system;
• and
• a second pump for pumping lubricant from the reservoir through a second main line to the lubricant distribution system, each of said pumps comprising:
• a cylinder bore;
• a cylinder inlet in communication with the interior of the reservoir for flow of lubricant from the reservoir into the cylinder bore;
• a cylinder outlet;
• a piston movable in the cylinder bore;
• a vent passage communicating with the interior of the reservoir;
• a linear position drive mechanism for moving the piston (i) in a forward direction in the cylinder bore through a pumping stroke for pumping lubricant through the cylinder outlet to a respective main line for supply of lubricant to the lubricant distribution system, (ii) in a rearward direction through a non-venting return stroke in which the vent passage does not communicate with the interior of the reservoir, and (iii) in a rearward direction through a venting return stroke in which the vent passage communicates with the interior of the reservoir;
• and
• a controller for controlling operation of the linear position drive mechanism, the controller operating the drive mechanism in a first stage of operation to move the piston of the first pump through at least one pumping stroke to pump lubricant through the first main line to the lubrication distribution system while the second main line vents back to the reservoir via the vent passage of the second pump, and the controller operating the drive mechanism in a second stage of operation to move the piston of the second pump through at least one pumping stroke to pump lubricant through the second main line to the lubrication distribution system while the first main line vents back to the reservoir via the vent passage of the first pump.