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(11) | EP 0 149 538 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Improved water dampening apparatus for a lithographic press |
| (57) A dampening apparatus known in the art as a water stop for a printing roll of a lithographic
press said dampening apparatus being characterised in that it comprises a pan roll
(12) and a brush roll (10) having contact at a nip (11) along at least a part of which
a toothed blade is received that is able to move substantially tangentially of said
nip to allow said teeth to move directly into or out from said nip. |
[0002] It is well known that in the older art the plate cylinder of an offset printing machine generally is damped by apparatus comprising two, three or as a maximum four distributing rolls. In such known damping apparatus, a cloth-clad duct roll dips into a trough filled with water and is intermittently rotated, by an adjustable angular amount, through a pawl-and-ratchet gear. A supply roll, which also is cloth-clad, alternately contacts for a certain length of time the duct roll and then a swanskin roll, and thus transfers a certain quantity of water. The latter roll delivers the water to two brass rolls called brayers or mullers which, in addition to rotating perform an axially reciprocating movement and deliver water to the distributing rolls which finally dampen the plate or the printing cylinder.
[0003] The known construction described above has the following disadvantages:- (1) It requires much space and a great number of rolls, and is in consequence relatively expensive; (2) It requires the provision of an actuating mechanism for the duct roll and for the supply roll; which is again expensive. (3) It offers little opportunity to regulate the water supply over the width of the plate cylinder; and (4) It involves a considerable wear of the cloth covers that are used with such a mechanism.
[0004] It has been proposed to effect damping by spraying. In one such proposal, the plate cylinder is overcooled, and water vapour blown onto the cylinder where it condenses, thus keeping the cylinder moist.
[0005] More recently spraying has been effected from a circular brush rotating at a high speed when compared with the slower spped of the pan roller, the periphery of which the brush contacts. In this form of dampening apparatus the brush flicks a dampening film in a fine spray on to the plate roller per se and or on to rollers associated with the plate roller.
[0006] It has proved difficult to adjust the amount of spray over the width of the plate roll when the press is required to print less than its total width and reduce the flow of dampening solution to the unused part of the printing cylinders.
[0007] To overcome these difficulties we have found that it is necessary to use a water stop and according to the present invention there is provided a dampening apparatus for a printing roll of a lithographic press said apparatus comprising a pan roll and a brush roll having contact at a nip along at least a part of which a toothed blade is received that is able to move substantially tangentially of said nip to allow said teeth to move directly into or out from said nip.
[0008] Clearly the spacing and the form of the teeth of the toothed blade are critical to the flow pattern and volume of liquid supplied by the brush roll to the printing rolls. Equally critical is the position of the teeth within or to either side of the nip. To that end various teeth forms may be used and various diametral pitch (DP) systems used to advantage. Further the toothed blade may be moved by various accurate means, either electrically or mechanically:
[0009] The invention will be more fully understood from the following description given by way of example only with reference to the several figures of the accompanying drawing in which:-
Figure 1 is a schematic view in oblique perspective of a dampening aparatus of the present invention;
Figure 1A is a detail of the rolls in Figure 1 to show the nip;
Figures 2A and 2B respectively are part sectional views of detail parts of the apparatus of Figure 1; and
Figure 3A and 3B respectively are a side and end elevation of an adjustable toothed blade and an enlarged view of its tooth form.
[0010] In figures 1 and 1A a brush roll 10 has peripheral contact at nip 11 with a pan roll 12. The directions of rotation are shown by the arrows A and B. A dampening film (not shown) is formed on a plate or printing roll (not shown) by spray projected from the brush roll 10 as shown by the arrow C.
[0011] A toothed blade 13 is received at the nip 11 along at least a part of its length (AA1) only the total length of the rolls being shown at AB1 extended. The toothed blade 13 has teeth shown at edge 131 so that said teeth can be moved along the tangent line T1 T2 directly into and out of the line of the nip 11. The movement is effected with great accuracy by a pair of screw threaded bolts 141, 142 turning freely in blocks 151 152 and reciprocated by nuts 161 162 on bracket 17.
[0012] In figures 2A and 2B the toothed blade 13 has a toothed edge 131 and the individual tooth form is shown in Figure 3B. The teeth are uniform and the form per se is that of a truncated equilateral triangle its sides inclined at an angle of seventy degrees of arc to its base which has a width of 0.75 inch (19.05mm) and a vertical height of 0.562 inch (14.2748 mm) The tooth form has conventional root and crest radii respectively of 0.15 inch (3.81 mm) and 0.060 inch (1.254 mm).
[0013] In the one example shown the length of the toothed blade is 16.25 inches (412.75mm) and it has a thickness of 250 microns and a width of 3.062 inches (77.77 mm). It carries twenty two individual teeth and twenty pitches at 0.75 being equal to a linear distance of 15.0 inches (381 mm).
[0014] The linear length of the toothed blade is generally only a part of the total length of the rolls the nip of which it contacts and this length AAI is commensurate with the total length BBI such that often the length AA1 is one page wide and the brush roller 10 (figure 1) flicks a dampening solution onto the printing roll in proportion to the area of the pan roller exposed to the brush by the toothed blade. This gives the printer in charge of the apparatus a fine control of the volume of dampening solution to ensure an adequate lubrication and to prevent a progressive build-up of unwanted ink on the printing rolls.
[0015] In one example as stated above the blade is 16.25 inches (412.75 mm) long which is substantially the width of a newspaper page and will cooperate with a nip roll length of some 68 inches (1727 mm). The two roll, the brush roll and pan roll will be about 6 inches (152.4 mm) in diameter with rotational speeds of one thousand and nine revolutions per minute respectively.
1. A dampening apparatus for a printing roll of a lithographic press said apparatus
being characterised in that it comprises a pan roll (12) and a brush roll (10) having
contact at a nip (11) along at least a part of which a toothed blade is received that
is able to move substantially tangentially of said nip to allow said teeth to move
directly into or out from said nip.
2. The dampening apparatus as claimed in claim 1 wherein the teeth (131) are uniform.
3. The dampening apparatus claimed in claim 2 wherein the tooth form (Figure 3B) is
that of a truncated equilateral triangle its sides inclined at seventy degrees of
arc to its base which has a width of 0.75 inch (19.05 mm) and a vertical height of
0.562 inch (14.2748 mm).
4. A dampening apparatus as claimed in any preceding claim wherein the toothed blade
is moved into and out from the nip (11) by spaced apart screw threaded bolts (141, 142) cooperating with stationary nuts which bolts are able to be clamped (22).