EP0082929B1 - Print hammer coil with a device for screening the magnetic field and for carrying off the heat - Google Patents

Description

The invention relates to a pressure hammer coil for a hammer bank according to the preamble of claim 1.

It can be used in particular in print hammer banks for high-performance line printers.

A large number of print hammers, which are actuated by electromagnets, are used in high-performance line printers.

The print hammers are necessarily arranged close to one another in print hammer banks.

However, the requirement for compactness of the design creates problems of overheating and magnetic interaction which can affect the reliability of the print and the print quality.

Existing solutions for cooling the electromagnetic coils and for preventing magnetic interactions are not sufficiently effective thermally, are complex to manufacture and require too much space. However, this reduces the compactness and / or the efficiency of the hammer units.

US Pat. No. 4,033,255 describes a print hammer bank for a dot matrix printer in which heat exchange elements made of aluminum or the like are used. These heat exchange elements are connected to the outer part of the electromagnetic coils via a concave hemispherical base. The coils are wound on an insulating bobbin, which is applied to a magnetic pole. On the outside of the coil there is a layer of thermally conductive plastic material, which causes the heat conduction between the coil and the heat exchange elements attached outside. The latter are arranged in a special structure. An adhesive is used between the coil and the coil body in order to prevent twisting that would otherwise be caused by the additional mass of the heat exchange elements. The coils are relatively far apart. They all have in common a magnetic circuit with a permanent magnet. The magnetic interaction has not been encountered.

US Pat. No. 3,196,783 describes a printing hammer for high-speed printers with an improved magnetic core. This prevents the magnetic interaction if several print hammers are close together. The E-shaped core has a coil wound in grooves on the middle leg of the E. The grooves are formed between the middle leg and the outer legs. The outer legs represent extensions of the core material with a reduced cross section and extend over the ends of the middle leg, so that a recess is formed for the anchor part of the hammer element. The extensions prevent stray fields. This structure is not only expensive to manufacture, but also unsuitable for assemblies in which the hammers are arranged very close together. While some shielding is provided, the structure has a low cooling efficiency, which is disadvantageous when the hammers are operated at a very high speed.

US Pat. No. 4,269,117 describes a pressure hammer coil for a hammer bank with an arrangement for magnetic field shielding and heat dissipation, in which an electromagnetic yoke protrudes into the coil body from one side and an extension of magnetically soft material of a pivotable pressure hammer projects between the two and between the two mutually facing ends of the electromagnetic yoke and the extension, a magnetic working gap is formed and the shielding plates used between the coils are made of magnetic field shielding material, there being heat conduction between the coil former and the electromagnetic yoke, and a holder with contact pins for the coil is provided for the coil former.

. It is an object of the present invention to provide an electromagnetic pressure hammer coil for a pressure hammer bank with a simple production possibility with improved heat dissipation.

This object of the invention is achieved by the measures specified in the characterizing part of claim 1.

Advantageous developments of the invention can be found in the subclaims.

In one embodiment of the invention, the internal heat exchange takes place through a metallic coil core, which does not concentrate the magnetic flux, and onto which the winding is wound in several turns and layers so that heat transfer takes place. The flat, parallel plates shielding the magnetic field serve as external heat exchange elements. The sealing compound sticks the coil core and the coil between the shielding plates so that heat transfer takes place. The shielding plates can consist of several layers with different saturation and permeability characteristics. The outer shielding plates can protrude beyond the coil and coil core; they also serve to radiate heat into the ambient air for indirect cooling of the outer surface of the coil.

The magnetic cores of all hammers are attached to a block (or frame) that is used for heat dissipation. A blower or the like supplies cooling air that flows over the shielding plates and the electromagnetic yokes, thereby improving the efficiency of the coil cooling.

Exemplary embodiments of the invention are shown in the drawings and are described in more detail below:

• Figur 1 eine perspektivische Ansicht einer mehrere Druckhämmer umfassenden Druckhammerbank,
• Figuren 2 bis 5 eine Folge perspektivischer Ansichten verschiedener Elemente für den Zusammenbau einer Spuleneinheit für einen der in Fig. 1 gezeigten Druckhämmer,
• Figur 6 eine Schnittansicht entlang der Linie 6-6 für das in Fig. 5 gezeigte Spulenelement und
• Figur 7 ein weiteres Ausführungsbeispiel einer erfindungsgemäßen Spuleneinheit.

Show it :

• FIG. 1 shows a perspective view of a printing hammer bank comprising several printing hammers,
• FIGS. 2 to 5 show a sequence of perspective views of different elements for the assembly of a coil unit for one of the print hammers shown in FIG. 1,
• Figure 6 is a sectional view taken along line 6-6 for the coil element shown in Fig. 5 and
• Figure 7 shows another embodiment of a coil unit according to the invention.

The assembly shown in Fig. 1 contains several print hammers. Each of the equidistant printing positions is assigned a U-shaped electromagnetic yoke with a coil unit and a one-piece hammer element (12). The hammer element is preferably of the type described in U.S. Patent 4,269,117. It has an extension 13. In the rest position of the hammer element, a working gap is formed between the extension 13 and the pole face 14 of the yoke leg 15 of the magnetic core 10. The working gap lies in the interior of the coil element 11. The hammer elements 12 are each rotatably arranged around the pin 16, which runs in a recess 17 in the legs 18 of the block 19. The cover plate, not shown, is attached to the block 19 to hold the coil elements 11 and the pin 16 in place. Further details can be found in the cited patent.

According to the embodiment of the invention, the U-shaped magnet yokes 10 are held in a block 20 made of plastic material or another material that does not concentrate the magnetic flux. Block 19 is in turn attached to block 20. This assembly is by suitable means, for. B. fixed by screws on a base plate 21. The yokes 10 are cast into the block 20 so that the upper leg 15 of each yoke protrudes from this block so far that it can receive and carry the coil elements 11. The base plate 21 is preferably made of metal and is in direct thermal contact with the edges of the magnet yokes 10. In this way, the base plate 21 serves to dissipate heat from all the magnet yokes 10. The heat generation is caused by the coil excitation.

As can be seen from FIGS. 2-6, the coil elements are preformed individual parts which can be quickly and easily applied to or removed from the yoke leg 15. Each coil element 11 (FIG. 5) consists of a coil and a coil body 22 (FIG. 3) and a magnetic field cutoff / heat dissipation structure 23 (FIG. 4). A complete coil body 24 (FIG. 2) consists of a metal coil body 25 which does not concentrate the magnetic flux and a cast plastic holder 26. The metal coil body 25 has a very thin wall and at one end an outwardly flared flange 27. A slot 28 over the entire length the coil body 25 and the flange 27 is provided to prevent the formation of eddy currents. The thin-walled construction has several advantages.

The cross-sectional area available for winding the coil 32 onto the bobbin 25 is increased; this increases the electromagnetic energy that can be generated by the coil 32 without the distance between adjacent yokes 10 having to be increased. Furthermore, the magnetic coupling between the coil 32, the yoke leg 15 of the yoke 10 and the extension 13 of the hammer element 12 is increased, which also reduces the leakage flux.

Of great importance is the fact that the metal coil body 25 serves as an excellent heat transfer medium for the internal conduction of the heat generated by the coil 32 to the yoke 10, which transfers the heat to the base plate 21. The heat is dissipated there by conduction, radiation or convection. In order to further improve the degree of heat transfer of the coil former 25, the latter preferably has the shape of a rectangular tube, so as to correspond as closely as possible to the rectangular cross section of the yoke leg 15 of the yoke 10.

In this way, the inner surfaces of the coil body 25 are in close thermal contact with the outer surfaces of the yoke leg 15, while at the same time a relatively easy assembly and separation of the coil element 11 from the yoke leg 15 is made possible.

According to the invention, the coil body 25 is preferably provided with a thin layer of a dielectric material in order to avoid a short circuit of the coil turns 32.

In the exemplary embodiment, the coil former 25 consists of anodized aluminum with a layer (5-10... M) of aluminum oxide made of dielectric material.

The holder 26 is preferably a cast part and consists of a vertical rectangular frame part 29 and a horizontal connector 30 with melted-in contact pins 31 for connection to the ends of the coil 32 (FIG. 3). The coil is first wound onto the bobbin 25. The contact pins are used to insert the coil element into an external connector. A rectangular opening 33 in the frame part 29 receives the flange-free end of the bobbin 25, which is attached by expansion or another suitable measure. After the attachment of the holder 26 to the coil body 25, the part 24 (FIG. 2) can be placed on a spindle and the coil 32 can be wound onto the coil body 25 in the desired number of turns and layers between the frame part 29 and the flange 27. After winding the coil 32, its free ends are connected to the contact pins 31. The coil and bobbin 22 are now preferably in the coil area with a high temperature epoxy mixture. for example with «Thermoset 314» (brand name from Thermoset Plastic, Inc.). This is intended to prevent the wires of the coil 32 from rubbing against one another during printing hammer operation and also to improve the heat transfer from the coil 32 to the coil body 25.

As can be seen from FIG. 4, the magnetic field shielding / heat dissipation arrangement 23 contains parallel plates 34 and 35 which are connected to one another by the bending strip 36. The plate 34 is substantially rectangular. The U-shaped plate 35 has vertical legs 37 and 38 and a horizontal extension 39. The vertical legs 37 and 38 of the plate 35 are arranged parallel and at a certain distance from one another and form a substantially rectangular opening 40 which is somewhat larger than the surface of one side of the coil and the bobbin 22. The opening 40 serves to receive and align the coil and bobbin 22 to adapt to tolerances of the flange 27, the frame part 29 and the coil 32, without the width of the distance between the plates 34 and 35 to influence. The coil and bobbin 22 are preferably cast by injection molding using a plastic material 43 between the plates 34 and 35 of the arrangement 23, the plastic material also encapsulating the winding 32. Holes 41 are provided in the vertical legs 37, 38 and in the horizontal extension 39 so that the injected plastic material 43 receives a good positive connection with the part 35. Similar holes 42 (see Fig. 6) are provided in plate 34 to ensure the coil and bobbin 22 are secured to part 23. In addition, as can be seen in Figure 6, the plastic material 43 forms a heat transfer path from the flange 27 to the plates 34 and 35. In this way, the internal cooling for the coil 32 is specific to the part which is above the end 14 of the leg 15 of the yoke 10 is further improved. A suitable plastic material for this purpose with good thermal conductivity properties is “Polyset EMC-90” (brand name of Morton Corporation).

Basically, the plates 34 and 35 and the flex strip 36 are made from a piece of magnetic field shielding material such as silicon iron, then bent and positioned during the potting of the material 43 so that the bottom and side edges of the plate 34 and frame 35 are substantially parallel. Such a one piece construction is preferred over other designs because of its compactness as well as better magnetic shielding and heat transfer. The space between the plates 34 and 35 is made so wide that the coil and bobbin 22 of FIG. 3 fit into the opening 40, so that the plate 34 has good thermal contact with one side of the winding 32 and the frame 35 the ends of the complete bobbin 25 encloses and surrounds, so that the working gap is completely enclosed between the vertical legs 37 and 38 and the plate 34. This working gap is also shielded by the plate 34 of the neighboring coil unit. This ensures that the coil 32 is shielded from stray flux in the vicinity of the air gap.

5 and 6, the injected plastic material 43 fills the space around the coil 32 in the opening 40 of the plate 35, the space between the coil and the plate 34 and a space between the flange 27 of the metal bobbin 25 and plate 34 (with holes 42) and vertical leg 37 of plate 35 (with holes 41). The plastic material 43 thus forms an integrated external and partially internal conductive heat transfer path between the winding parts of the coil 32, the coil body 25 and the shielding plates 34 and 35.

From Fig. 6 it can be seen that the coil element 11 is very compact and that a good shielding of the coil 32 against magnetic leakage flux is achieved, as well as an effective external heat transfer from the sides of the coil 32 in a confined space. At the opening 40 in the Plate 35 may have coil 32 and metal bobbin 25 partially within the space between legs 37 and 38 of plate 35. As a result, the coil elements can be packed even more densely, so that a hammer bank according to FIG. 1 results in a greater packing density than in previous designs.

The internal heat transfer to the yoke leg 15 and the plates 34 and 35 given by the metallic coil formers also allows a compact design of a hammer bank with high pressure output.

1 shows that in the case of adjacent coil elements lying on the yoke legs 15 of different yokes 10, the plate 34 of a first magnetic field shielding / heat dissipation arrangement of the plate 35 is adjacent to the adjacent second arrangement. As a result, the corresponding coils 32 and yokes 10 are shielded against stray foot which could otherwise pass through the openings 40 in the plates 35. The stray magnetic flux between adjacent coils 32 is thus completely shielded.

The plates 34 and 35 and the bending strips 36, like the coils 32, are swept by circulating cooling air. The opening 44 in the base plate 21 serves as an inlet or outlet opening for this cooling air flow.

In a second exemplary embodiment shown in FIG. 7, an improved magnetic field shielding / heat dissipation arrangement is provided. The plates 45 and 46 which conduct the magnetic flux are essentially U-shaped and have central openings 48 and 49, respectively. The plates 45 and 46 are connected by the strip 50 essentially in the same way as in the embodiment of FIGS. 1 to 6 Case is. Coil and bobbin 22 are essentially the same on openings 48 and 49 aligned as in the first embodiment. In the exemplary embodiment according to FIG. 7, a rectangular magnetic field shielding cover plate 47 is provided on at least one of the U-shaped plates 45 or 46. B. attached from silicon iron. In the exemplary embodiment in FIG. 7, the plates 45 and 46 preferably consist of laminated layers 51 and 52 made of magnetic flux-conducting material, such as low-carbon steel. The cover plate can be welded to the laminated layers 45, 46 or connected in such a way that a laminated structure results. Holes 53, 54 and 55 are provided in plates 45, 46 and 47 for fixing the plastic material with which the magnetic field shield is connected to coil and coil body 22 essentially in the same way as in the first exemplary embodiment.

Laminated structures are more effective in shielding against stray flux. Layers 51 and 52 serve as additional shunt paths for diverting a high-intensity flow that could otherwise reach shielding coils 32 through shielding 47. An arrangement of the type shown in FIG. 7 is particularly advantageous when the distance between the hammers in a printing hammer bank can be somewhat larger or when particularly high demands are placed on the magnetic field shielding.

Claims (5)

1. Print hammer coil for a hammer bank with an arrangement for magnetic field screening and for a heat sink, where there protrudes into the coil body (25) an electromagnetic yoke (15) from the one side, and from the other an extension of magnetically soft material of a pivotable print hammer (12), and where between the ends of the electromagnetic yoke and of the extension facing each other a magnetic operating gap is formed, and where the screen plates (34, 35; 45, 46) inserted betwen the coils (32) consist of magnetic field-screening material, a heat duct existing between coil body and electro magnetic yoke, and a holder (26) with conductor pins (31) for the coil (32) being provided for the coil body, characterised in that each coil body (25) with coil (32) has an associated separated pair of screen plates (34, 35 ; 45, 46), on screen plate (35) having an opening (40) which permits the alignment of the coil body (25) and of the coil (32) to be fixed by bonding between the two screen plates of the respective pair of plates, that in the coil units with associated screen plates provided one beside the other in the print hammer bank, apart from the screen plate (35) with the opening (40) of a first coil unit the screen plate (34) without opening of the second coil unit adjacent to the first one is arranged, that the surface of the coil windings and the coil body are coated with a highly heat- conductive synthetic material, and that the electromagnetic yoke (15) protruding into the coil body is closely surrounded by the coil body (22) (Fig. 1-6).

2. Arrangement as claimed in claim 1, characterized in that both screen plates (45, 46) of each pair of plates respectively comprise one opening (48, 49) permitting the alignment of the coil body (25) and the coil (32) respectively to be fixed by bonding between the two screen plates, that on a screen plate (46) of each pair of plates one further third screen plate (46) of a magnetic field-screening material is arranged, said plate covering the respective opening (49), and that in the coil units with screen plates placed one beside the other in the print hammer bank, apart from the screen plate (45) of a first coil unit with the opening (48) not covered by the third screen plate, the third screen plate of the second coil unit adjacent to said first coil unit is provided (Fig. 7).

3. Arrangement as claimed in claim 1, characterized in that the coil body (22) is coated with a thin layer of dielectric material.

4. Arrangement as claimed in any one of claims 1-3, characterized in that the coil body (22) and the electric magnetic yoke (15) have a rectangular cross-section.

5. Arrangement as claimed in any one of claims 1-4, characterized in that the screen plates (34, 35 ; 45, 46) of each pair of screen plates are interconnected by a connecting strap (36 ; 50).

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