TYPEWRITER CARRIAGE ESCAPEMENT MECHANISM

Abstract: A typewriter carriage escapement mechanism comprises a rack (10) movable with the typewriter carriage (2) and having two sets of teeth (16 and 18) projecting in opposite directions. An escapement dog (30) has two separate teeth or sets of teeth (44 and 54) arranged to engage the two sets of rack teeth (16 and 18) alternately as the dog (30) moves at right angles to the rack (10), on operation of the keys (90) or space bar (92) of the typewriter. One of the sets of teeth (54) is mounted on a pawl (50) movable relative to the other tooth (44) or set of teeth, to enable both sets of teeth to be disengaged from the rack (10) to release the carriage (2). The pawl (50) is movable against a return spring (70) by means of an actuating rod (80) engaging a lug (62) on the pawl (50). The mechanism of the invention enables a single escape­ ment member (30) to effect both the stepwise movement of the carriage (2) and also the carriage release. The elements of the mechanism need only to be reciprocated in a straight line, so simplifying the construction of the mechanism.

Description: This invention relates to typewriters and in particular to typewriter carriage escapement mechanisms. In typewriters having a movable carriage on which the platen is supported, the carriage is usually spring-biased to move in one direction, and an escapement mechanism actuated by the typewriter keys allows the carriage to move in steps corresponding to the spacing between characters to be printed. The escapement usually consists of a rack fixed to, and extending, the length of the carriage and a dog engaging the rack. A carriage release mechanism is also usually provided, to withdraw the dog from the rack and allow the carriage to move freely on actuation of a lever on the carriage or of a tabulator key. Known carriage escapements are often complex mechanisms. This is a disadvantage particularly in the manufacture of inexpensive typewriters, such as toy typewriters, where low cost of manufacture and cost of assembly are important considerations. United States Patent US-A-1 041 557 describes a typewriter with a carriage escapement mechanism having a rack movable with the carriage and having two sets of teeth on opposite sides of the rack. The two sets of teeth are engaged alternately by two dogs mounted on the top of a pivoted arm. One of the dogs is pivotable on the arm about a vertical axis to allow stepwise movement of the rack after the dog has engaged the rack. This dog can also be pivoted about a horizontal axis to enable it to be withdrawn from the rack whilst the other dog is also out of engagement with the rack, to enable the rack to move freely. This mechanism suffers from the disadvantage of being relatively complex and difficult to manufacture. This invention consists in a typewriter having a carriage and driving means biasing the carriage to move in one direction, and a carriage escapement mechanism comprising a rack movable with the carriage, the rack having first and second sets of teeth projecting in opposite directions, an escapement member movable at right angles to the rack between first and second positions, the escapement member having a tooth arranged to engage the first set of teeth of the rack when the escapement member is in the second position, a pawl mounted on the escapement member and movable relative to the escapement member between first and second positions, means biasing the pawl to its first position, the pawl having a tooth arranged, when the pawl is in its first position and the escapement member is in its first position, to engage the second set of teeth of the rack, and means for moving the pawl to its second position when the escapement member is in its first position, to withdraw the tooth of the pawl from the rack to allow the rack to move freely, characterised in that the escapement member is constrained to slide in a straight line at right angles to the direction of movement of the rack, the pawl is slidable in the escapement member in a straight line at right angles to the direction of movement of the rack, and the tooth of the escapement member and the tooth of the pawl are so positioned relative to the teeth of the rack, in the direction of movement of the rack, that movement of the escapement member from its first to its second position allows the rack to move through a first predetermined distance and subsequent movement of the escapement member back to its first position allows the rack to move through a second predetermined distance. In one form of the invention, the two sets of teeth of the rack project away from one another and the rack moves between the tooth of the escapement member and the tooth of the pawl, the tooth of the escapement member and the tooth of the pawl, when the pawl is in its first position, being spaced apart from one another by a distance, measured in the direction of movement of the escapement member, less than the distance, measured in the same direction, between the crests of the teeth of the first and second sets of teeth of the rack. With the mechanism of this invention, the escapement member and pawl need only to be reciprocated in a straight line, so that mounting and actuation of the escape member and pawl can be made very simple, facilitating manufacture of the typewriter. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a typewriter incorporating a carriage escapement mechanism, in accordance with the invention, Figure 2 is a plan view of the escapement mechanism, Figure 3 is an underneath plan view of the escapement mechanism, Figure 4 is a front elevation of a carriage slide forming part of the mechanism, Figure 5 is a section on line V-V of Figure 4, Figure 6 is a side elevation of an escapement dog forming part of the mechanism Figure 7 is a plan view of the body of the dog, with the pawl removed, Figure 8 is an end elevation of the dog, Figure 9 is a plan view of the pawl Figure 10 is a side elevation of the pawl, and Figure 11 is an elevation of the pawl, Figures 12 to 14 are diagrammatic views of the rack and dog, showing the dog in three different positions during operation of the mechanism. Referring to the drawings, a typewriter has a carriage 2 movable on a body 4. The carriage is mounted on a slide 6, which is movable longitudinally on a runner (not shown) fixed to the body of the typewriter, for example as shown in British patent specification No. 1,591,987. The slide 6 is biased to move from right to left, as seen in Figure 1, by a known form of drive mechanism consisting of a spring-loaded pinion (not shown) engaging a drive rack 8 on the slide. Movement of the slide 6, and therefore of the carriage 2, is controlled by an escapement mechanism comprising an escapement rack 10 on the slide 6 and an escapement dog 30, movable in the body of the typewriter. The escapement rack 10 is formed on the underside of a flange 12 extending forwards from the lower part of the slide 6. The rack 10 has two sets of teeth 16 and 18, extending forwards and backwards respectively on opposite sides of a central stem 14 which extends along the slide 6. Each of the teeth has a flat face 20 at right angles to the direction of movement of the slide and facing in the direction in which the slide 6 is biased, i.e. right-to-left as seen in Figure 1, and a face 22 inclined at 45° to the flat face 20. The escapement dog 30 consists of a body 32 and a pawl 50 slidable in the body 32. The body 32 is slidable in guides (5) formed in the body 4 of the typewriter so as to be movable horizontally in a direction at right angles to the direction of movement of the carriage slide 6. The pawl 50 is slidable in a recess 34 formed in the rear part of the body 32 of the dog 30. The pawl 50 is held against upward movement by engagement of flanges 52 on the pawl beneath flanges 36 at the rear of the body 32 and by a bridge member 38 on the body 32 beneath which the forward part of the pawl slides. A rearwardly directed tooth 40 is formed on the upper face of the body 32 of dog 30, on one side of the slot 34. The tooth 40 has a flat vertical face 42, facing in the direction opposite to the direction in which the carriage slide 6 is biased, and a face 44 inclined at 45° to the flat face 42. A pair of forwardly directed teeth 54 are formed on the upper face of the pawl 50. Each tooth 54 has a flat vertical face 56, facing in the same direction as the flat face 42 of the tooth 40, and a face 58 inclined at 45° to the flat face 56. The spacing between the flat faces 56 of the teeth 54 of the pawl 50 is equal to the interval between the teeth 16 and 18 of the escapement rack 10. The spacing, measured in the direction of movement of the carriage slide 6, between the flat face 42 of the tooth 40 on the body of the dog 30 and the flat face 56 of the nearest tooth 54 of the pawl, is equal to approximately half the interval between the teeth 16 and 18 of the escapement rack 10. When the pawl 50 is in its forwardmost position relative to the body 32 of the dog 30, determined by engagement of the end of the pawl 50 with the forward end of the recess 34, the distance between the teeth 54 of the pawl and the tooth 40 of the dog, measured in the direction of movement of the dog, is less than the overall distance betweeen the crests of the teeth 16 and the crests of the teeth 18 of the escapement rack 10. The pawl 50 is biased to its forwardmost position by means of a tension spring 70 acting between a downwardly extending post 60 fixed to the pawl and a downwardly extending post 46 on the body 32 of the dog 30. The pawl 50 can be moved rearwards relative to the body 32 of the dog 30 to a position in which the teeth 54 on the pawl are separated from the tooth 40 on the dog by a distance greater than the overall distance between the crests of the teeth 16 and 18 of the escapement rack. This movement is effected by means of an actuating rod 80 mounted in the carriage 2 so as to extend between the rack 10 and a lug 62 at the rear end of the pawl, in all positions of the carriage. The rod 80 is carried by upwardly extending portions 81 pivoted at their mid-points in the carriage so that movement of a carriage release knob 83 engaging the top of one of the portions 81 causes the rod 80 to move rearwardly. On such movement, the rod 80 engages the lug 62 to move the pawl 50 rearwardly. The dog 30 is positioned beneath the flange 12 of the carriage slide 6, so that the escapement rack 10 extends between the tooth 40 on the body 32 of the dog and the teeth 54 on the pawl 50. The dog is movable between a first position, shown in Figure 12, in which the teeth 54 of the pawl are engaged between teeth 16 of the rack 10, and a second position, shown in Figure 13, in which the tooth 40 on the body 32 of the dog is engaged between teeth 18 of the rack 10. The dog 30 is biased to the first position by means of a tension spring 72 acting between the post 46 on the body 32 of the dog and a suitable anchorage point (not shown) on the body 4 of the typewriter. The dog 30 can be moved to the second position against the spring bias by means of a bail (not shown) which bears against she forward end of the dog 30 and which is engaged by the type bars of the typewriter so as to move the dog on depression of any of the corresponding keys 90. The construction and operation of the bail may be similar to that described in the above mentioned British patent specification No. 1,591,987. In addition, the dog 30 may be moved to the second position by depression of the space bar 92, through a lever (93) which is coupled to the space bar through a rod 94, On depression of the space bar 92, the lever 94 is pivoted and engages a lug 48 on the body 32 of the dog, to push the dog rearwardly. An arm 49 formed on the body 32 of the dog 30 is positioned to engage a conventional mechanism for lifting the typewriter ribbon to the appropriate position on each actuation of any of the character keys of the typewriter. In operation, the dog 30 is normally held by the springs 70 and 72 in the position shown in Figure 12, with the tooth 54 on the pawl 50 engaging between the teeth 18 on the escapement rack 10, so that the carriage slide 6 is held stationary by engagement of the faces 20 of the teeth 18 with the faces 56 of the teeth 54. On depression of a character key 90 or the space bar 92, the dog 30 is moved in the direction of arrow 25, as described above, to the position shown in Figure 13. As the dog 30 moves to this position, the teeth 54 of the pawl 50 move out of engagement with the teeth 18 of the escapement rack 10, so that the rack 10 is released and moves in the direction of the arrow 24. At the same time the tooth 40 on the dog 30 moves into the path of the teeth 16 of the rack 10, so that movement of the carriage slide 6 is arrested by engagement of the face 20 of one of the teeth 16 with the face 42 of the tooth 40. The carriage is thus held stationary as the corresponding type bar (when a character key is depressed) strikes the platen. On release of the character key or space bar, the dog 30 is moved back to its initial position by the spring 72 , the pawl 50 being held in position relative to the body 32 of the dog 30 by the spring 72. As the dog 30 moves back to its initial position, the rack 10 is again released as the tooth 40 is withdrawn from the rack and moves further to be arrested by engagement of two of the teeth 18 of the rack with the teeth 54 of the pawl. The reciprocation of the dog 30 therefore allows the carriage slide 6, and with it the carriage 2, to move through a distance corresponding to the spacing between the teeth 16 on the rack 10. Actuation of the carriage release knob 83, or a tabulator key (if one is provided), of the typewriter caused the pawl 50 to be moved relative to the body 32 of the dog 30 in the direction of arrows 26 to the position shown in Figure 14, against the bias of the spring 70. The spring 70 is weaker than the spring 72 acting on the body 32 of the dog, so that the body remains stationary during this movement of the pawl 50. Movement of the pawl 50 withdraws the teeth 54 from engagement with the teeth 18 of the rack 10, so that the rack is free to move between the teeth 54 and the tooth 40. Deactuation of the carriage release knob 83 or tabulator key allows the pawl 50 to be moved back to its initial position by the spring 70. The bevelled faces 22 on the teeth 16 and 18 of the escapement rack 10, and the correspondingly bevelled faces 58 on the teeth 54 of the pawl, allow the carriage to be moved manually in a direction opposite to that of the arrow 24 without the need to actuate the carriage release lever, the mutual engagement of the bevelled faces causing the pawl 50 to reciprocate as the teeth of the rack ride over the teeth of the pawl. It will be appreciated that changes could be made in the described embodiment within the scope of the appended claims. For example, the pawl could have one tooth, or more than the two teeth of the described embodiment and the body of the dog could similarly be provided with a number of teeth appropriately spaced. The spacing between the flat face of the tooth on the body of the dog and the flat face of the nearest tooth on the pawl may vary considerably from the spacing in the described embodiment (i.e. equal to half the interval between the teeth of the escapement member). This spacing determines the proportion of each stepwise movement of the carriage which is taken up by the initial movement, on depression of a character key, to the position at which the type bar strikes the platen. The spacing might, for example, be made larger than half the interval between teeth on the escapement member, so that the initial movement of the carriage is smaller than the final movement, on release of the key.

Claims: A typewriter having a carriage (2) and driving means biasing the carriage to move in one direction, and a carriage escapement mechanism comprising a rack (10) movable with the carriage (2), the rack (10) having first and second sets of teeth (16,18) projecting in opposite directions, an escapement member (30) movable at right angles to the rack (10) between first and second positions, the escapement member (30) having a tooth (40) arranged to engage the first set of teeth (16) of the rack (10) when the escapement member (30) is in the second position, a pawl (50) mounted on the escapement member (30) and movable relative to the escapement member (30) between first and second positions, means (70) biasing the pawl (50) to its first position, the pawl (50) having a tooth (54), arranged when the pawl (50) is in its first position and the escapement member (30) is in its first position, to engage the second set of teeth (18) of the rack (10), and means (80) for moving the pawl (50) to its second position when the escapement member (30) is in its first position, to withdraw the tooth (54) of the pawl (50) from the rack (10) to allow the rack to move freely, characterised in that the escapement member (30) is constrained to slide in a straight line at right angles to the direction of movement of the rack (10), the pawl (50) is slidable in the escapement member (30) in a straight line at right angles to the direction of movement of the rack (10), and the tooth (40) of the escapement member and the tooth (54) of the pawl (50) are so positioned relative to the teeth (16,18) of the rack (10), in the direction of movement of the rack, that movement of the escapement member (30) from its first to its second position allows the rack (10) to move through a first predetermined distance and subsequent movement of the escapement member (30) back to its first position allows the rack (10) to move through a second predetermined distance. A typewriter as claimed in Claim 1, in which the escapement member (30) is resiliently biased to the first position, and means (93) are provided to move the escapement member (30) to the second position on actuation of a key or space bar of the typewriter. A typewriter as claimed in Claim 1 or Claim 2, in which the two sets of teeth (16,18) of the rack (10) project away from one another and the rack (10) moves between the tooth (40) of the escapement member (30) and the tooth (54) of the pawl (50), the tooth (40) of the escapement member (30) and the tooth (54) of the pawl (50), when the pawl (50) is in its first position, being spaced apart from one another by a distance, measured in the direction of movement of the escapement member (30), less than the distance, measured in the same direction, between the crests of the teeth (16,18) of the first and second sets of teeth of the rack (10). A typewriter as claimed in any preceding claim, in which the teeth (16,18) of the rack (10) have flat faces (20) which engage corresponding flat faces (42,56) on the tooth (40) of the escapement member (30) and the tooth (54) of the pawl (50) during stepwise movement of the rack (10), and bevelled faces (22) which engage corresponding bevelled faces (44,58) on the tooth (40) of the escapement member (30) and the tooth (54) of the pawl (50) when the carriage (2) is moved manually in a direction opposite to the said direction of movement, the bevelled faces (22;44,58) allowing the teeth (40,54) of the escapement member (30) and pawl (50) to ride over the teeth (16,18) of the rack (10) during such movement A typewriter as claimed in any preceding claim, in which the escapement member (30) is movable horizontally.

Inventors: GREAVES, PETER
Applicants: DOBSON PARK INDUSTRIES PLC
Application year: 1985
Main classification: B41J19
IPC codes: B41J19, B41J3