Print wheel selection type compact printer

Abstract: In a print wheel selection type compact printer print wheels (3) are mounted on a print wheel shaft (1) to be ro­ tated into specific angular positions for type selection. A print wheel spring (2) is arranged between each print wheel (3) and the print wheel shaft (1) and forms part of a clutch mechanism to either couple the respective print wheel (3) to or release it from the print wheel shaft (1). One or more indentations (1a, 1e) are provided in the print wheel shaft (1) for each print wheel (3) and adapted to be engaged by or disengaged from a protrusion (2a) of the print wheel spring (2). The leading surface in the direc­ tion of rotation of said print wheel shaft (1) of the pro­ file of one or both of the indentation (1a) of the print wheel shaft (1) and the protrusion (2a) of the print wheel spring (2) comprises at least two surfaces of different slants.

Description: The present invention relates to a print wheel selection type compact printer. Such printers are provided on a print wheel shaft with one or more print wheels each carrying a plurality of types on their circumferential surface. For selecting specified types to be printed the print wheels are rotated by means of the print wheel shaft until a desired type of each print wheel is in a predetermined print position. See for instance GB-A-2 108 431. To facilitate the understanding of the background of the invention reference is first made to FIG. 2(a) and 2(b) showing a print wheel and associated means for type selection. In FIG. 2(a) a print wheel 3 is mounted on a print wheel shaft 1 rotatable in the direction of arrow A. The print wheel 3 has a number of types (characters, numerals etc.) on its outer circumference, and on its side it is equipped with ratchet teeth 3b, each corresponding to one of the types 3a, and a reset claw 3c extending outside of the path of rotation of the ratchet teeth 3b. A print wheel spring 2 is part of a clutch system used for either engaging or disengaging the print wheel 3 with the print wheel shaft 1. The print wheel shaft 1 is provided with an indentation 1a cooperating with a protrusion 2a of the print wheel spring 2. If the protrusion 2a of the print wheel spring 2 engages the indentation 1a of the print wheel shaft 1 the print wheel 3 will be coupled to the print wheel shaft 1 so that with a rotation of the print wheel shaft 1 the print wheel 3 will be rotated until the print wheel spring 2 is disengaged from the indentation 1a of the print wheel shaft 1. A selection claw 4 is rotatably supported on a selection claw shaft 5. For selecting one of the types 3a to be printed the print wheel shaft 1 and the print wheel 3 are rotated in the direction of arrow A. When during this selection process a trigger coil 6 is energized, the selection claw 4 is rotated in the direction of arrow B to engage a ratchet tooth 3b and to stop the print wheel 3. In the resetting process the selection claw 4 is rotated in the direction of arrow C not to engage the ratchet teeth 3b and the reset claw 3c. When the printer begins operation in response to a print command starting from the standby condition shown in FIG. 2(a) the selection claw 4 is rotated in the direction of arrow C, the engagement of the end 4a of the selection claw 4 and the reset claw 3c of the print wheel is released, and then as the print wheel 3 immediately begins rotating in the direction of arrow A together with the print wheel shaft 1 the end 4a of the selection claw 4 rides over the reset claw 3c. The selection claw 4 then rotates back to the standby position and the type selection process begins. The print wheel 3 keeps rotating together with the print wheel shaft 1 until the trigger coil 6 is energized when the desired type 3a on the print wheel comes to the printing position. This causes the selection claw 4 to rotate in the direction of arrow B and to engage ratchet tooth 3b of the print wheel 3. As a result, the print wheel 3 stops. Since, however, the print wheel shaft 1 continues to rotate, the protrusion 2a of the print wheel spring 2 comes out of the indentation 1a of the print wheel shaft 1 thereby releasing the print wheel 3 from the print wheel shaft 1. This condition is shown in FIG. 2(b). The print wheel shaft 1 rotates until all of the desired types 3a of all print wheels 3 arranged in parallel on the print wheel shaft 1, are at the printing position. Then the printing process begins. When the printing process is completed the resetting process begins. In the resetting process, the engagement of the selection claw 4 with the ratchet tooth 3b is released and the print wheel shaft 1 begins to rotate again. Due to the frictional force exerted by the spring force of the print wheel spring 2 the print wheel 3 follows the rotation of the print wheel shaft 1 in the direction of arrow A until the reset claw 3c of the print wheel 3 engages the selection claw end 4a. Then the print wheel 3 is stopped while the print wheel shaft 1 continues to rotate until the protrusion 2a of the print wheel spring 2 falls into the indentation 1a again, thus returning to the standby condition of FIG. 2(a). FIG. 4(a) and FIG. 4(b) show conventional arrangements and correspond to FIG. 2(a) with the selection claw 4 and the trigger coil 6 being omitted. As shown in the cross sectional view of FIG. 4(a) the shape of the indentation 1a is complementary to that of the protrusion 2a of the print wheel spring 2 with the leading face in the direction of arrow A extending substantially radially. With this configuration, when the protrusion 2a falls into the indentation 1a a large impact and accompanying noise result because the protrusion 2a falls the full depth of the indentation 1a all at once. This also causes much wear of the print wheel spring. In the case of FIG. 4(b) the situation is different since the leading surface of the indentation 1a does not extend radially so that the protrusion 2a of the print wheel spring rides on the slanted leading surface of the indentation 1a to be gradually guided into the fully engaged condition shown in FIG. 4(b) while the print wheel shaft 1 and the print wheel 3 are moving relative to each other. In this case, however, positioning is performed by the bottom radius of the indentation 1a and the end radius of the protrusion 2a of the print wheel spring 2, thus preventing an accurate positioning and degrading the selection function. As explained above, in these prior art compact printers the slant of the leading surface in the direction of rotation of the print wheel shaft of the indentation in the cross sectional view of the print wheel shaft is fixed irrespective of the depth of the indentation and the slant of the protrusion of the print wheel spring cooperating with said indentation is also fixed. Further, for each print wheel there is only one indentation in the print wheel shaft of the prior art to couple the print wheel to the print wheel shaft in the selection process. In order to ensure a sufficient frictional force between the print wheel shaft and the print wheel during the resetting process necessary to bring the print wheel into its standby position quickly to allow high-speed printing, the print wheel spring had to be made strong. Thereby the noise occurring when the print wheel spring enters into the indentation of the print wheel shaft is further increased. The strong print wheel spring additionally requires a larger drive force for the print wheel shaft which, in turn, increases the power consumption. It is an object of the present invention to remedy these drawbacks of the prior art and to provide a compact printer with low noise, a superior selection function and lower power consumption. This object is achieved with a compact printer as claimed. Ways of carrying out the invention will be explained in detail below with reference to the drawings, in which: FIG. 1 is an enlarged cross sectional view of a portion of the print wheel shaft and the print wheel spring according to one embodiment of the invention, FIG. 2(a) to 2(e) are cross sections for explaining both, the general operation of the type selection in a print wheel selection type compact printer and, specifically, the operation of a compact printer according to the first embodiment of the invention, FIG. 3 is a view corresponding to FIG. 1 of a second embodiment of the invention, and FIG. 4(a) and 4(b) are cross sectional views of conventional arrangements. Fig. 1 shows in detail the structure of the indentation of the print wheel shaft according to one embodiment of the invention. The figure shows the engaged condition of the protrusion 2a of the print wheel spring 2 and the indentation 1a of the print wheel shaft 1 by which the print wheel 3 is coupled to the print wheel shaft 1 during the selection process for both parts to be moved as a unit. The leading side of the indentation 1a comprises two slanted surfaces 1b and 1c. The surface 1b which is nearer to the center of the print wheel shaft 1 forms an angle Θ1 with respect to the trailing surface 1d of the indentation 1a. The surface 1c which is nearer to the outer circumference of the print wheel shaft 1 forms an angle Θ2 with respect to the trailing surface 1d. The angle Θ2 is larger than the angle Θ1. In addition to the indentation 1a the print wheel shaft 1 has an auxiliary indentation 1e positioned at an angle Θ5 ahead of the indentation 1a in the direction of rotation (A) of the print wheel shaft 1. Since the selection process and the resetting process in general have been described above the following description will focus on the effect of the specific construction of the indentations according to the invention. As mentioned before, in the resetting process the print wheel 3 tends to follow the rotation of the print wheel shaft 1 due to the frictional force generated by the spring force of the print wheel spring 2. In order to achieve a high-speed printing the resetting process is fast and due to its inertia the print wheel 3 cannot follow the rotation of the print wheel shaft 1 by only the frictional force so that its phase gradually becomes delayed with respect to the print wheel shaft. Therefore, a relative rotation between the print wheel 3 and the print wheel shaft 1 occurs until the protrusion 2a of the print wheel spring 2 falls into the auxiliary indentation 1e of the print wheel shaft 1. When the protrusion 2a has fallen into the indentation 1e, while the print wheel shaft continues to rotate, the force of the print wheel spring 2 trying to come out of the auxiliary indentation 1e cooperates with the frictional force of the print wheel spring 2 between the print wheel shaft 1 and the print wheel 3. Thereby, the print wheel 3 is again coupled to the print wheel shaft 1 to rotate with the latter in unity. The print wheel 3 being again coupled to the print wheel shaft 1 rotates until the reset claw 3c of the print wheel 3 engages the selection claw end 4a and then stops as shown in FIG. 2(d). Since, however, the print wheel shaft 1 continues to rotate, the protrusion 2a of the print wheel spring 2 comes out of the auxiliary indentation 1e and engages the indentation 1a again, thus returning to the standby condition. At this time, the protrusion of the print wheel spring 2 does not fall the full depth of the indentation 1a all at once, but rather, as shown in FIG. 2(e) it gradually slides down the slanted surface 1c of the indentation 1a near the circumference of the print wheel shaft 1 and then falls to the bottom of the indentation 1a, whereby its rate of fall is slowed and the impact lessened. In the present embodiment, the slanted surfaces 1c and 1b with angles Θ1 = 55° and Θ2 = 120° each control one half of the amount of fall of the print wheel spring 2 and as a result the noise is reduced approximately bY 3 dB and the positioning accuracy of the print wheel and the print wheel shaft is improved. A similar result is obtained with other values of angles Θ1 and Θ2 as long as the condition Θ1 < Θ2 is met. Further, since the depth of the indentation 1e is approximately one third that of the indentation 1a and the angle Θ5 formed between the indentations 1e and 1a is approximately 30°, the printer was able to withstand printing speeds approximately twice those of the prior art. However, the shape, the angle Θ5 and the number of auxiliary indentations 1e can be freely set, i.e. there may be more than one auxiliary indentation. FIG. 3 is a view corresponding to that of FIG. 1 for another embodiment of the present invention. In this case the trailing slanted surface 2d, i.e. the lower surface in FIG. 3, of the protrusion 2a forms an angle Θ4 with the leading surface 2b of the protrusion 2a and an angle Θ3 with the base 2c of the protrusion 2a, where Θ4 > Θ3. The leading surface 2b of the protrusion 2a is the surface sliding on the leading edge of the indentation 1a to have the protrusion 2a gradually entering the indentation 1a. In other words, the surface 2b of the second embodiment fulfills the function of the surface 1c of the first embodiment which, therefore, is not needed in the second embodiment. Except for the difference explained above the second embodiment is identical to the first embodiment so that further explanations can be omitted. The invention described above has the following effects. The speed with which the print wheel spring falls into the print wheel shaft indentation is decreased and the positioning accuracy of the print wheel shaft and the print wheel in the selection process improved, whereby noise is reduced, durability improved and the selection function improved. By using multiple indentations in the print wheel shaft the ability of the print wheel to follow the print wheel shaft is improved, so that high-speed operation of the printer can be achieved with a smaller print wheel spring force resulting in low noise and low power consumption.

Claims: A print wheel selection type compact printer comprising: a print wheel shaft (1), at least one print wheel (3) having a plurality of types (3a) on its outer circumference and ratchet teeth (3b) corresponding to said types, the print wheel (3) being rotatably mounted on said print wheel shaft (1), a print wheel spring (2) arranged between the print wheel shaft (1) and the print wheel (3), said print wheel spring having a protrusion (2a) adapted to be engaged with or disengaged from an indentation (1a) in the print wheel shaft such that in the engaged condition the print wheel (3) rotates together with the print wheel shaft (1) whereas in the disengaged condition the print wheel and the print wheel shaft are rotatable relative to each other, and a selection claw (4) having a part (4a) to be engaged with or disengaged from said ratchet teeth (3b), wherein a desired type (3a) of the print wheel (3) is selected by rotating said print wheel (3) in the engaged condition of said print wheel spring (2) and engaging said selection claw (4) with the ratchet tooth (3b) corresponding to the desired type (3a) to stop said print wheel and disengage the protrusion (2a) of the print wheel spring (2) from the indentation (1a) of the print wheel shaft, characterised in that the leading surface (1c, 1b; 2b, 2c) in the direction of rotation of said print wheel shaft (1) of the profile of one or both of the indentation (1a) of the print wheel shaft (1) and the protrusion (2a) of the print wheel spring (2) comprises at least two surfaces of different slants. Printer according to claim 1, wherein said print wheel shaft comprises multiple indentations (1a, 1e) for each print wheel (3).

Inventors: HARADA YASUHIRO, NAKATA TOSHIO, HARADA, YASUHIRO, NAKATA, TOSHIO
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1990
Main classification: B41J7
IPC codes: B41J7