TRANSPORT DEVICE FOR THE PRINT UNIT OF PRINTING MACHINES

Abstract: The device comprises an electric motor (6) whose rotor is capable of rotating selectively in either direction and a trans­ mission unit (21) comprises a sleeve (31) carrying a gear (19) at an input end in mesh with a pinion (18) on the motor shaft. At the output end of the sleeve (31) a toothed pulley (28) is en­ gaged with a tensioned, toothed belt (29) for moving the print unit back and forth. The sleeve (31) is journalled on a shaft (33) which can shift laterally at the input end and is urged by a spring (44) and lever (42) to shift in the sense forcing the gear (19) into mesh with the pinion (18), thereby to minimise the radial clearance between the pinion and the gear.

Description: The present invention relates to a transport device for the print unit of printing machines, in particular for typewriters, comprising a selector motor for displacement of the print unit in two directions. In a known transport device, the transport motor comprises a pinion engaged with a toothed gear of a transmission unit. The transmission unit comprises a toothed pulley which is coaxial with the gear and which engages with a toothed belt for transmitting the movement to the print unit. The toothed pulley and the gear are rotatable on a shaft which is fixed to the frame of the machine and the stator of the transport motor is mounted on an eccentric in order to take up clearances between the pinion and the gear. That arrangement suffers from the disadvantage that, as a result of wear, clearance between the pinion and the gear progressively increases and thus the pinion-gear coupling means becomes noisy and suffers from a lack of precision. Periodic adjustment of the eccentric is therefore necessary in order to reduce the clearances. DE-A-2 926 962 discloses a transport device in which the transport motor is mounted on a flange which is attached in a rocking manner to a stud on a fixed support and is guided by two pins engaging in slots on the fixed support. A spring urges the transport motor so as to bring the pinion into engagement with the gear of the transmission unit. A somewhat similar arrangement is disclosed in Japanese Laid-Open Patent Application No. 58-29690, in which a cam is also provided against which the rocking flange bears. These prior arrangements have relatively complex structures and suffer from a disadvantage, because every time the motor starts and rotates in one direction its stator is subject to a reaction in the opposite direction, and this reaction produces unevenness and jumping between the pinion and the gear. In one sense of rotation this is allowed by the spring and produces asymmetric wear and surging (movement that is not smooth). The present invention is defined in claim 1 below to which reference should now be made. A preferred embodiment of the invention is set forth in the following description which is given by way of non-limiting example and with reference to the accompanying drawings, in which: Figure 1 is a partial diagrammatic view of a typewriter on which a transport device embodying the invention is mounted, Figure 2 is a partly sectional plan view of the device shown in Figure 1, Figure 3 is a partial front view of some details of the device shown in Figure 2, Figure 4 is a partial front view of other details of the device shown in Figure 2, Figure 5 shows a detail of the device of Figure 2 on an enlarged scale, and Figure 6 shows an element for comparison in respect of the detail shown in Figure 5. Referring to Figure 1, a transport device according to the invention is generally indicated by reference numeral 10 and is applied to a typewriter comprising a platen roller 11 on which a sheet of paper 12 is carried. A print unit 13 comprising for example a character-carrying daisywheel of the type described in our published European patent application EP 0 118 277 is movable in two directions on two cylindrical guides 14 which are parallel to the platen roller 11. The transport device 10 (see Figure 2) comprises a d.c. transport motor 16 having a rotor with a shaft 17 on which a pinion 18 is fixed; the pinion 18 can mesh with the teeth of a gear 19 of a transmission unit 21. The motor 16 comprises a stator provided with a coupling portion 22 by means of which it is engaged and guided in a seat 23 of a support 24 and is fixed to the support 24 by means of three screws 26 (see Figure 4) received in respective slots 27. The slots 27 in the support 24 permit centering and adjustment of the angular position of the stator of the motor 16 with respect to the support 24, in the assembly phase. The transmission unit 21 (see Figure 2) comprises the gear 19 and a toothed pulley 28 which is coaxial with the gear 19 and which is continuously engaged with a toothed belt 29 which in turn is connected to the print unit 13 (see Figure 1) for displacement thereof in both directions. The gear 19 (see Figure 2) and the toothed pulley 28 are rigidly connected by a hollow sleeve 31. The transmission unit 21 is rotatable by means of two roller bearing assemblies 32 on a shaft 33. One end 34 of the shaft 33 is fixed to a fixed support 36 and another end 37 passes through an opening 38 in the support 24 and is fixed by means of a screw 39 (see Figure 4) to an arm 41 of a lever 42 which in turn is pivoted on a pin 43 on the support 24. A spring 44 rotates the lever 42 in the clockwise direction and urges the shaft 33 and thus the transmission unit 21 (see Figure 3) towards the shaft 17 of the motor 16, holding the gear 19 engaged with the pinion 18. The toothed belt 29, besides being engaged with the toothed pulley 28, is also engaged with a second toothed pulley 51 (see Figure 1) which is rotatable on a pin 52 on a bridge lever 53. The lever 53 comprises two projections 54 which are guided and housed in corresponding openings 56 in a support 57. An adjusting screw 58 with a lock nut 59 positions the lever 53 away from or close to the fixed support 57 so as to produce a predetermined tension, for example about 900 grams, in the toothed belt 29. The tension in the toothed belt 29 around the pulley 28 contributes to positioning the transmission unit 21 (see Figure 2) with respect to the motor 16. In particular, the tension in the toothed belt 29, which is reduced in accordance with the short lever arm acting thereon, and the force of the spring 44, corresponding to about 200 grams, generate a force such as to hold the transmission unit 22 constantly urged towards the motor 16 so that the gear 19 is always engaged, without play, with the pinion 18. That provides a self-regulating device which permits the radial clearances between the pinion 18 and the gear 19 to be automatically taken up. That structure makes it possible to achieve positioning with a high degree of accuracy and in a repetitive manner of the print unit 13 in front of the point of printing on the platen roller 11, even at high speed and after a long period of use. The transmission ratio between the pinion 18 and the gear 19 is 18/96 and that makes it possible to use a d.c. motor 16 of reduced power, with the movement being transmitted silently, even at high speed. The foregoing structure can be easily provided since the teeth of the pinion 18 and the gear 19 have been so designed as to permit the clearances between the teeth of the two gear systems to be taken up even in the event of wear on the teeth themselves. As is known, in standardised tooth configurations (see Figure 6), a tooth of a height H comprises an addendum, that is to say the part which is between the pitch circle which is shown by the dash-dotted line in Figure 6, and the tip of the tooth, which is equal to m, and a dedendum, that is to say the part between the base of the tooth and the pitch circle, which is equal to 7/6 m. The complete height is thus: H=m+7/6m , wherein the modulus 'm' denotes the ratio between the pitch circle diameter and the number z of teeth of the gear and is thus: m=dp/z. In the case of the teeth on the pinion 18 (see Figure 5) and the gear 19, the addendum according to the invention is in this case a fraction of the modulus and is always less by about 1/10 than the value of the modulus, while the dedendum retains its value which is equal to seven sixths of the modulus. Therefore the value of H is reduced by at least 1/10 with respect to the standardised value. In addition, between the tip of a tooth as indicated at 71 and the base between two opposite teeth 72 and 73, there is always a gap which is greater than that prescribed in meshing between standardised teeth. That ensures that the teeth of the two gears 18 and 19 will always operate with the side of the teeth and there will no longer be engagement between the tip of the tooth and the base of the tooth, even with a substantial amount of wear. The foregoing can be clearly seen from Figure 5 in which, with the addendum reduced in both of the gears 18 and 19, there is a clearly visible gap between the tip and the base of the meshing teeth. It will be appreciated that the transport device 10 for the print unit 13 may be the subject of modifications, and improvements within the scope of the claims.

Claims: A transport device (10) for the print unit (13) of a printing machine, comprising a platen (11), a transport motor (16) having a pinion (18) for movement of the print unit (13) in two directions along the platen (11), a toothed belt (29) connected to the print unit, a transmission unit (21) including a toothed pulley (28) engaged with the belt and having a pinion-gear coupling means (18,19) between a gear (19) and the said pinion (18) and an elongate member (31) interconnecting the said toothed pulley (28) with the said gear (19), and a self-regulating device (42,44) for automatically taking up the radial clearance between the pinion (18) and the gear (19), characterized by a first fixed support (24) on which is rigidly fixed the transport motor (16) and supporting one extremity of the elongate member (31) adjacent to the gear (19) permitting its rotation and a movement transversal with respect to the pinion (18); and a second fixed support (36) spaced apart from the first fixed support (24) and supporting other extremity of the elongate member (31) adjacent to the toothed pulley (28) permitting its rotation and a tilting movement of the elongate member; and wherein the self-regulating device is operative on the said one extremity of the elongate member (31) so as to move the said one extremity towards the motor (16) and to bring the gear (19) into engagement and in mesh with the pinion (18); and wherein the belt (29) is subject to a tension around the toothed pulley (28) which is concurrent with the action of the self-regulating device on the pinion-gear coupling means. A transport device according the claim 1, wherein the elongate member (31) comprises a hollow sleeve rotatably supported on a support shaft (33) having a free end (37) adjacent to the said one extremity and another end (34) adjacent to the said other extremity and fixed to the second support, the first fixed support (24) comprises a fixed pin (43), and the self-regulating device comprises a lever (42) which is pivoted on the fixed pin (43), and the free end (37) of the support shaft (33) being fixed on said lever (42); and the self-regulating device comprises a spring (44) applied to the lever (42) for biasing the free end (37) of the support shaft (33) towards the transport motor (16). A transport device according to claim 2, wherein the hollow sleeve (31) connects the gear (19) and the toothed pulley (28), and two roller bearing assemblies (32) are mounted on the support shaft (33), and wherein the hollow sleeve (31) is rotatable by means of the two roller bearing assemblies (32) on the support shaft (33). A transport device according to claim 1, 2 or 3, including a third fixed support (57) to support a bridge lever (53) and another toothed pulley (51) on the bridge lever (53); and an adjusting screw (58) for positioning the bridge lever (53) away from or closer to the third fixed support (57) so as to produce tension in said toothed belt (29) which contributes to biasing the free end (37) of the support shaft (33) towards said transport motor (16). A transport device according to claim 4, including a lock nut (59) engaged with the adjusting screw (58) to lock the adjusting screw (58) in a predetermined position with respect to the third support (57). A transport device according to any preceding claim, wherein the pinion (18) and the gear (19) have teeth which have a reduced addendum with respect to the standard value of the modulus of the teeth, the reduced addendum of the teeth enabling the self-regulating device to automatically take-up the radial clearance between the pinion (18) and the gear (19) even in the event of wear on the teeth.

Inventors: MUSSO, PIETRO
Applicants: ING. C. OLIVETTI & C., S.P.A.
Application year: 1986
Main classification: B41J19
IPC codes: B41J19