1. Field of the Invention
The present invention relates to a pump for a printing press which is configured so as to axially reciprocate a plunger, while rotating the same, by means of a motor, to thereby supply ink to an inking unit of the printing press.
2. Description of the Related Art
A conventional pump for an offset printing press is disclosed in, for example, Japanese Patent No. 2864447. In the disclosed pump, a plunger accommodated within a main bore is reciprocated and rotated by a variable-speed motor in such a manner that the plunger undergoes a single cycle of reciprocating motion for each turn; and such operation is repeated in order to supply ink to an inking unit.
As shown in FIG. 8, the conventional pump consists of a body 101, a cylinder 102, a plunger 103, a variable-speed motor 104, and a transmission mechanism 105. The body 101 is formed by a block portion 111 and a hollow housing portion 112 connected to the block portion 111. The cylinder 102 is fitted into a through hole 113 of the block portion 111 in such a manner that its outer end projects from the outer side surface of the block portion 111, and its inner end reaches a hollow space 112a formed inside the hollow housing portion 112. The plunger 103 is accommodated within a main bore 120 of the cylinder 102 to be rotatable and receiprocatable along the axial direction. A base end portion of the plunger 103 projects into the hollow space 112a of the hollow housing portion 112. The variable-speed motor 104 is attached to an outer wall 112b of the hollow housing portion 112 in such a manner that an output shaft 140 of the variable-speed motor 104 projects into the hollow space 112a of the hollow housing portion 112. The transmission mechanism 105 is disposed in the hollow space 112a in order to connect the output shaft 104 to the plunger 103, to thereby transmit rotation of the output shaft 140 to the plunger 103 while converting the rotational motion of the output shaft 140 to rotation and reciprocative motion of the plunger 103.
The through hole 113 of the block portion 111 for receiving the cylinder 102 is formed to extend from the side of the hollow housing portion 112 toward the outer side surface of the block portion 111, while inclining downward at a certain inclination angle. Accordingly, the cylinder 102 also inclines downward at the same inclination angle. An outer-end opening portion of the main bore 120 of the cylinder 102 is closed by means of a plug. At an axial position which is a predetermined distance away from the outer end of the cylinder 102, an intake hole 121 and a discharge hole 122 are provided in the cylinder 102 at diametrically opposite positions such that the intake hole 121 and the discharge hole 122 penetrate the cylinder 102 upward and downward, respectively, along a common center axis. In other words, the intake hole 121 and the discharge hole 122 are opened to the main bore 120 with a phase difference of 180xc2x0 therebetween.
A distal-end portion of the plunger 103, which is accommodated within the main bore 120 to be rotatable and receiprocatable along the axial direction, has a cutaway portion 131. The cutaway portion 131 is formed through removal of a portion having a substantially semicircular cross section and extends axially over a predetermined distance from the distal end surface.
Specifically, the cutaway portion 131 has a length such that the base end of the cutaway portion 131 is located on the side toward the open end of the main bore 120 with respect to the intake hole 121 and the discharge hole 122, when the plunger 103 is moved to the deepest point within the main bore 120, as will be described later.
More specifically, the cutaway portion 131 is formed in such a manner that, when the plunger 103 is moved to the deepest point within the main bore 120 in the course of its reciprocating movement, the base end of the cutaway portion 131 is located at a position which is offset toward the open end of the main bore 120 with respect to axial positions at which the intake hole 121 and the discharge hole 122 communicate with the main bore 120, respectively, and a cylindrical surface of the cutaway portion 131 can close the intake hole 121 and the discharge hole 122 simultaneously, and that the distal end of the plunger 103 is located between the plugged end of the main bore 120 and the points at which the intake hole 121 and the discharge hole 122 communicate with the main bore 120, respectively. Therefore, upon rotation of the plunger 103, the intake hole 121 and the discharge hole 122 are closed simultaneously by the cutaway portion 131 at phase angles having an angular difference of 180xc2x0 therebetween, and only one of the intake hole 121 and the discharge hole 122 is closed by the cutaway portion 131 at other phase angles.
The rotational axis of the plunger 103 and the rotational axis of the output shaft 140 form an angle therebetween; and a tip end portion of the output shaft 140 is connected to the base end portion of the plunger 103 by means of the transmission mechanism 105.
Moreover, fluid passages 114 and 115 communicating with the intake hole 121 and the discharge hole 122, respectively, of the cylinder 102 are formed in the block portion 111.
The fluid passage 114 is connected to an unillustrated ink supply source. The fluid passage 115 is connected to an inking unit of a printing press. Therefore, ink is supplied to the main bore 120 of the cylinder 102 via the intake hole 121; and ink pressurized to a predetermined pressure is fed from the main bore 120 to the inking unit via the discharge hole 122.
The transmission mechanism 105, which connects the output shaft 140 of the variable-speed motor 104 to the plunger 103, includes a connection member 151 and an arm 150. The connection member 151 projects radially from the base end portion of the plunger 103, which projects into the hollow space 112a. The arm 150 is attached to the output shaft 140 and has a protrusion which projects toward the plunger 103 at an eccentric position with respect to the rotational axis of the output shaft 140. A spherical bearing 152 is attached to the tip end of the projection of the arm 150, and the tip end portion of the connection member 151 attached to the plunger 103 is fitted into the inner ring of the bearing 152 to be movable along the axis thereof.
The above-described conventional pump for a printing press involves the following problems.
Since the variable-speed motor (hereinafter referred to as the xe2x80x9cmotorxe2x80x9d) 104 is disposed in such a manner that the rotational axis of the output shaft 140 of the motor 104 intersects the rotational axis of the plunger 103 at a predetermined intersecting angle, when the output shaft 140 of the motor 104 rotates 180xc2x0 (a half turn) during a single rotation, the plunger 103 rotates 180xc2x0 and moves toward the open end of the main bore 120 to thereby take in ink.
When the output shaft 140 rotates a further 180xc2x0 (the remaining half turn), the plunger 103 rotates another 180xc2x0 and moves toward the closed end of the main bore 120 to thereby discharge ink. The relationship between angular displacement of the output shaft 140 of the motor 104 and that of the plunger 103 is maintained constant at all times; and the angular displacement of the output shaft 140 for the intake stage of the operation cycle of the plunger 103 is identical with that for the discharge stage of the operation cycle. Therefore, intake and discharge of ink are performed alternately over respective periods of equal length.
Therefore, when the plunger 103 is in the intake stage, supply of ink to the inking unit stops even when a paper surface undergoing printing requires ink. When the plunger 103 enters the discharge stage, an increased amount of ink is discharged in order to compensate for the insufficient supply in the intake stage.
Therefore, a large quantity of ink is supplied at one time to the inking unit, so that the amount of ink supplied from the inking unit to the surface of a printing plate changes greatly when the plunger 103 enters the discharge stage, thereby producing an unevenness in density of ink on the printed paper surface, leading to deteriorated printing quality.
Depending on a matter to be printed on a paper surface, a large amount of ink may be demanded. In such a case, the pump must be operated to perform the discharge operation at a frequency higher than the ordinary frequency, in order to discharge a larger amount of ink within a short period of time. Therefore, a larger amount of ink as compared to that involved in the ordinary case is fed to the discharge passage, and the pressure within the discharge passage increases, with a resultant tendency of increased discharge pressure of ink.
In other words, high pressure is generated within the main bore and the ink discharge passage in the discharge stage, and due to this high pressure, ink is likely to leak from the very small clearance between the main bore 120 and the plunger 103. The leaked ink may adhere to the outer circumferential surface of the portion of the plunger 103 projecting into the hollow space 112a and the transmission mechanism and harden there, possibly hindering smooth operation of the pump. Therefore, maintenance work for disassembling and cleaning the pump must be performed frequently.
Further, since the above-mentioned increase in discharge pressure and adhesion of ink increase the load imposed on the pump, a motor of relatively large output torque must be employed in order to cope with the increased load, thereby making it difficult to reduce the size of the pump and increasing consumption of electric power and heat generation.
The present invention has been accomplished in order to simultaneously solve the problems involved in the conventional techniques, and an object of the present invention is to provide a pump for a printing press which can eliminate unevenness of ink density on a printed paper surface; which suppresses an increase in discharge pressure of ink so as to prevent ink leakage, to thereby facilitate maintenance work; and which can reduce the size of the pump and conserve energy.
The present invention provides a pump for a printing press which includes a cylinder having a main bore which extends along an axis of the cylinder and is closed at one end, and intake and discharge holes which are opened to an inner surface of the cylinder at different circumferential positions. A plunger is fitted into the main bore to be rotatable about the axis and reciprocatable along the axis. The plunger closes the intake and discharge holes simultaneously at certain angular positions and closes only one of the intake and discharge holes at other angular positions. A motor for rotating and reciprocating the plunger is disposed in such a manner that an output shaft of the motor faces a base end portion of the plunger projecting from an open end of the cylinder, that a space is present between the base end portion and the output shaft, and that a rotational axis of the output shaft is radially offset, by a certain eccentricity distance, from a rotational axis of the plunger and forms an angle with the rotational axis of the plunger. A transmission mechanism is disposed in the space and includes a first member fixed to the base end portion of the plunger, and a second member fixed to the output shaft of the motor. The first and second members are connected with each other at an eccentric position in such manner that the first member and the second member can move relative to each other in the radial direction of the output shaft or the plunger and can change an intersecting angle therebetween.
Preferably, the first member is an arm attached to the base end portion of the plunger, and the second member is a connection member attached to the output shaft of the motor; the arm has a projection extending toward the output shaft of the motor at an eccentric position with respect to the rotational axis of the plunger, the projection supporting a spherical bearing; and the connection member radially extends from the output shaft and is slidably received by an inner ring of the spherical bearing, wherein the distance between the eccentric position and the rotational axis of the plunger is greater than the eccentricity distance.
Alternatively, the first member is a connection member attached to the base end portion of the plunger, and the second member is an arm attached to the output shaft of the motor; the arm has a projection extending toward the base end portion of the plunger at an eccentric position with respect to the rotational axis of the output shaft, the projection supporting a spherical bearing; and the connection member radially extends from the base end portion of the plunger and is slidably received by an inner ring of the spherical bearing, wherein the distance between the eccentric position and the rotational axis of the output shaft is greater than the eccentricity distance.
In the present invention, the rotational axis of the output shaft of the motor is disposed to be offset from and angled with respect to the rotational axis of the plunger. In each operation period of the plunger, the output shaft rotates one turn; and the plunger, which receives the rotation of the output shaft via the transmission mechanism, rotates one turn and reciprocates through one cycle in order to effect intake and discharge of ink. In the operation period, the angular displacement of the output shaft corresponding to the intake stroke of the plunger is smaller than the angular displacement of the output shaft corresponding to the discharge stroke of the plunger, whereby intake of ink is completed quickly, and ink is discharged slowly at a substantially constant rate. Therefore, the following effects are achieved.
(1) The intake stage in which no ink is supplied to the inking unit becomes shorter, and the discharge stage in which ink is supplied to the inking unit becomes longer. Further, ink can be discharged at a substantially constant rate. Therefore, variation in ink density on the surface of printed paper can be eliminated in order to improve print quality.
(2) Since in the discharge stage the plunger can be operated slowly as compared with the intake stage, the discharge pressure of ink can be reduced to a low level as compared to conventional pumps, and leakage of ink from a clearance between the cylinder and the plunger can be prevented. As a result, it becomes possible to eliminate adhesion of ink to the outer circumferential surface of the plunger and the transmission mechanism to thereby secure smooth operation. Therefore, the frequency of malfunctions decreases, and the frequency of cleaning can be reduced. Accordingly, maintenance work is facilitated considerably.
(3) Since the discharge pressure of ink is reduced, ink does not leak and does not adhere to the plunger, and the torque required to move the plunger becomes smaller, thereby enabling reduction in size of the motor and energy savings.