Patent GB-1239772-A (GB)
Title: Impact-printing mechanism, particularly for price-computing shop counter weighing machines
IPC: B41K3/04
Filed: 1968-09-13
Abstract: 1,239,772. Selective printing. WIRTH GALLO & CO. 13 Sept., 1968 [11 Jan., 1968], No. 43680/68. Heading B6F. Hammer arrangement; noise prevention.-A hammer mechanism in an impact printer, particularly for price-computing shop counter weighing machines, comprises two movable elements mounted in a frame, the two elements each having one degree of freedom of movement and being positively coupled, the impression being produced by impact of one of the elements on a printing table, and the algebraic sum of the momenta of the elements and the moments of the momenta at the instant of impact being nil. In Fig. 1, the two elements 2 and 3 of equal mass are each formed with a projection 4 provided with rack teeth 5. Bores 6 and 8 have compression springs 7 and the racks 5 mesh with a pinion 9. The centres of gravity of the two elements move along a straight line A-A intersecting the shaft 10 of the pinion 9. The left-hand end of the frame 1 forms a printing table 20 supporting a type block 21, and the tape to be printed on is inserted into a slot 22. The mechanism is rocked by rotating the pinion 9 until the elements 2 and 3 reach the positions 2a and 3a, whereupon a tooth 18 of a spring- loaded lever 17 snaps into engagement with element 2. When the elements are released by depressing the lever 17, the springs 7 accelerate the two elements 2 and 3 and the end face of element 2 strikes the type block 21. A weak spring 15 at the end face of element 3 slightly retracts the two elements after impact to permit the tape to be withdrawn. At the instant of impact the sum of the momenta of the elements and the sum of the moments of the momenta is zero respectively, and therefore reactive moments likely to cause noise and vibration do not arise. In an alternative (Fig. 2), the two elements have the form of bell-crank levers pivoted at (23), the shorter lever arms (24) of the two elements having toothed sectors (25) in permanent mesh. One end of a leaf spring (26) engages the element 3, whereas its other end engages a notch (28) in the machine frame. The mechanism is rocked by deflecting the elements in contrary directions against the force of the leaf spring (26) to positions 2a, 3a, whereupon a pawl (31) engages element 3. At the same time type wheels (34) are set according to the object being weighed. When the pawl (31) releases the elements, they accelerate apart. At the point of impact, the centres of gravity (38) of the respective elements 2 and 3 are moving in opposite directions on a straight line. In (Fig. 3) a third embodiment is shown with co-rotating elements 2, 3, and in (Fig. 4) is shown a fourth embodiment, wherein the elements 2, 3 rotate about their respective centres of gravity. Hammer arrangement, toggle action.-In Fig. 5, the two elements 2, 3 are deflectably mounted on their respective shafts 39, and their masses are equal and equally distributed. Type faces are provided on a bar 44 which is adjusted according to a particular character. For creating the impression, the bar is impacted simultaneously from each side by the two impact faces 43.