Piston mounting and balancing system
A compact 180° opposed piston pump/compressor minimizes axial spacing between its pistons on the drive shaft and has mass-balanced pistons designed to reduce the shaking couple and noise from reciprocation of the pistons. The piston assemblies can have members, such as cup retainers, that are of different masses selected to compensate for the difference in piston masses and thereby equalize the masses of the pistons. Each piston assembly can be mounted to the drive shaft at eccentrics that occupy minimal or no space between the pistons.
1 . A piston and drive shaft assembly for a pump, comprising at least two piston assemblies having:
first and second pistons each having a head and a connecting rod, the connecting rods defining respective first and second openings;
first and second bearings disposed in the first and second openings and having open centers;
first and second eccentric elements disposed in the centers of the respective first and second bearings; and
first and second mass members coupled to the respective first and second pistons having different masses essentially equal to a mass difference of the first and second pistons so as to essentially equalize the total mass of each piston assembly.
2 . The assembly of claim 1 , wherein the eccentric elements are disk shaped.
3 . The assembly of claim 1 , wherein the first and second bearings each have an outer race rotatable with respect an inner race defining the center opening and wherein the outer races are press-fit in the first and second openings of the connecting rods and the eccentric elements are press-fit into the openings defined by the inner races.
4 . The assembly of claim 1 , wherein the first and second mass members are retainers mounted to the heads of the respective first and second pistons, wherein the first piston has a greater mass than the second piston and the first retainer has a lesser mass than the second retainer.
5 . The assembly of claim 4 , wherein the first retainer is made of a different material than the second retainer.
6 . The assembly of claim 5 , wherein the first retainer is zinc and the second retainer is magnesium.
7 . The assembly of claim 5 , wherein the first retainer is zinc and the second retainer is aluminum.
8 . The assembly of claim 1 , wherein the connecting rods of the first and second pistons are mounted to the drive shaft spaced apart no more than {fraction (1/16)} inch.
9 . The assembly of claim 1 , wherein the first eccentric element has an axial dimension no more than substantially the axial dimension of the first piston connecting rod and the second eccentric element has an axial dimension no more than substantially the axial dimension of the second piston connecting rod.
10 . The assembly of claim 1 , wherein the first and second eccentric elements each have an axial through bore and extend axially to one side substantially no further than a face of the corresponding piston connecting rod.
11 . The assembly of claim 1 , wherein at least on of the first and second retainers is plastic.
12 . A pump, comprising;
a motor having a drive shaft;
a crankcase housing the drive shaft and having a pair of cylinders;
at least two piston assemblies including:
two pistons each having a head disposed in one of the cylinders and a connecting rod extending from the head to the drive shaft;
two bearings disposed in openings in the connecting rods axially offset along the drive shaft;
two eccentric elements disposed in the bearings; and
two mass members coupled to the pistons having different masses essentially equal to a mass difference of the pistons so as to essentially equalize the total mass of each piston assembly.
13 . The pump of claim 12 , wherein the mass members are cup retainers attached to the piston heads so that the center of gravity is at essentially the same location of each piston.
14 . The pump of claim 12 , wherein the eccentric members have axial through bores receiving the drive shaft.
15 . A pump or compressor comprising a pair of piston assemblies, each assembly having a piston with a head and a connecting rod, which mounts a bearing that is rotatably mountable to a drive shaft by an eccentric element, and a mass member coupled to the piston, wherein the mass member of one assembly is different than that of the other assembly such that a mass difference of the mass members is essentially equal to a mass difference of the pistons.
16 . The pump or compressor of claim 15 , wherein each of the mass members is selected from the group of materials including metals and polymers.