GB2080424A - Gear pump - Google Patents

Abstract

A gear pump of the externally- meshing type has the housing inner peripheral walls coated with a bearing material (36) on which the tips of the pumping gear-wheels (18, 20) slide. Preferably, the bearing material is cadmium and each coating is 0.1 to 0 &cirf& 4 mm thick and subtends an angle of about 240 DEG at the axis of the respective gear-wheel. The coatings may extend axially beyond the gear- wheels so as to be contiguous with bearing bushes for the gear-wheel shafts, Fig. 2 (not shown). <IMAGE>

Description

SPECIFICATION Gear pump The invention relates to gear pumps in which intermeshing, externally toothed gear wheels mounted on rotary bearings in a housing are rotated to pump fluid between inlet and outlet, openings of the housing.

When such gear pumps are operated at a relatively high delivery pressure of, for example, 1 50 to 300 bars, then, as a result of bearing play and/or elastic deformation of the shafts or bearing journals by means of which they are mounted in the bearings, the gear wheels are moved away from the delivery side of the pump and are pressed with their tooth tips against regions of the inner wall of the housing which are adjacent to the suction side, so that excessive wear or even signs of seizing can occur on the faces pressed against one another. This is especially true when the housing is made of steel, cast steel or spheroidal graphite cast iron, as might be required by a high delivery pressure and correspondingly high strength requirements.To overcome these disadvantages gear pumps, particularly with housings of cast steel, are conventionally constructed with such a large amount of clearance between the housing and gear wheels that contact between the housing and gear wheels is prevented. However, this clearance causes a leakage flow between the suction and delivery sides of the pump, as a result of which the voiumetric efficiency decreases according to the amount of clearance.

Attempts have been made to prevent such wear, and leakage resulting therefrom, by limiting the sealing contact with the periphery of the gear wheels to cylindrically rounded sheet-metal strips the rear faces of which facing away from the gear wheels are connected to the delivery side of the pump (German Offenlegungsschrift 2 239 727).

Such sheet-metal strips, which have to bridge the entire pressure differential between the delivery side and the suction side of the pump over a relatively narrow peripheral region of the gear wheels, are however easily subject to vibrations which are propagated in the pump housing and in the pumping medium. These vibrations cause considerable wear on the sheet-metal strips themselves and, if appropriate, on the bearing bodies and are transmitted via the pump housing and via the pumping medium to other machine elements where they also have a troublesome effect, as a result of which the seal provided by such sheet-metal strips has not proved successful.

According to the present invention, there is provided a gear pump having a housing in which two intermeshing, externally toothed gear wheels are located to be enclosed sealingly by an inner wall of the housing over a part of their peripheries lying between inlet and outlet connections of the housing, said gear wheels being mounted in bearings sealing against their end faces, said inner wall of the housing being coated with a bearing layer at least in the regions in which said wall encloses the gear wheels sealingly.

The provision of the bearing layer is able to ensure that the tooth tips of the gear wheels slide along on the inner wall of the housing with low friction. When, as a result of bending of the shaft or bearing journals of the gear wheels and/or as a result of bearing play in the case of a high pressure gradient between the delivery and suction sides of the pump, the tooth tips are pressed against the coated inner wall of the housing with an increased surface pressure, then the edges of the tooth tips scrape the surface of the bearing material without furrows forming in the bearing metal. It is assumed in this that the faces'of the tooth tips are ground in the conventional way.

The invention is preferably applied to gear pumps in which in known manner the housing has a continuous recess containing the gear wheels and bearings. In this case, according to a preferred feature of the invention the inner wall of the housing is coated with the bearing material also in the regions in which this inner wall surrounds the bearing bodies. The coating therefore extends preferably over the entire axial length of the housing, thus on the one hand, simplifying the production of the coating and on the other hand ensuring that when the gear pump is assembled the bearing bodies can be pressed sealingly against the end faces of the gear wheels, without being obstructed in the axial displacement necessary for this purpose by any step in the coating of bearing material.

Cadmium with a purity of at least 98% has proved especially successful as a bearing material.

The thickness of the bearing layer is preferably 0.1 to 0.4 mm.

An embodiment of the invention is described below by way of example with reference to the accompanying diagrammatic drawing in which: Figure 1 shows a gear pump according to the invention in cross-section along the line I-I in Figure 2, and Figure 2 shows the gear pump in longitudinal section along the line Il-Il in Figure 1.

The gear pump illustrated has a housing 12 made of cast steel, which is closed in a leak-tight manner on each of its two end faces by means of screwed-on covers 1 4 and 1 6 respectively.

Located within the housing 1 2 are two intermeshing, externally toothed gear wheels 1 8 and 20, the gear wheel 1 8 being fastened on a drive shaft 22 or made in one piece with the latter.

The drive shaft 22 extends through the cover 1 6 and ends in a projecting stub 24 on which a wheel or coupling part (not shown) for driving the gear pump can be fastened. The gear wheel 20 has two bearing journals 26. The shaft 22 and the bearing journals 26 are mounted on both sides of the gear wheels 1 8 and 20 in bearing bodies 28 made of bearing metal. The bearing body 28, in which the part of the shaft 22 projecting through the cover 1 6 is mounted, is sealed off from this shaft and from the cover 16 by means of a seal 30. The shaft 22 is, moreover, sealed off from the cover 1 6 by means of a lip seal 32 located on the outer side of said cover.

The housing 12 is coated with a metallic bearing layer 36, in particular cadmium of at least 98% purity, in the regions 34 of its inner wall which enclose the gear wheels 1 8 and 20. The layer thickness of the bearing metal 36 is shown greatly exaggerated in Figures 1 and 2; it amounts, in actual fact, to approximately 0.2 mm and is selected so that the gear wheels 1 8 and 20 which have ground tooth tip faces are accommodated in the housing 12 with a fit corresponding to a sliding fit, that is to say substantially free of play. The coating of bearing metal 36 extends over the entire axial length of the housing 12 and therefore also into the regions 38 in which the housing 12 surrounds the bearing bodies 28. The bearing bodies 28 are received into the coating of bearing metal 36 with a play-free sliding fit or a light press fit.

The housing 12 has two connections 40 and 42 for fluid conduits (not shown), these connections being arranged diametrically opposite one another in the plane of symmetry between the two gear wheels 1 8 and 20. In the direction of rotation of the gear wheels indicated in Figure 1 by arrows, the connection 40 is on the suction side and the connection 42 is on the delivery side.

Because of these connections 40 and 42 and because of the intermeshing of the two gear wheels 18 and 20, the housing 12 leaves each of the gear wheels 18 and 20 free over an angular extent of approximately 1200; the regions 34, in which the inner wall, coated with the bearing metal 36, of the housing 12 surrounds the gear wheels 18 and 20 sealingly, each extend, therefore, over an angle of approximately 2400.

To ensure an axial seal between the gear wheels 1 8, 20 and the bearing bodies 28, the bearing bodies can be pressed against the gear wheels by means of fluid pressure or springs.

Claims (5)

1. Gear pump having a housing in which two intermeshing, externally toothed gear wheels are located to be enclosed sealingly by an inner wall of the housing over a part of their peripheries lying between inlet and outlet connections of the housing, said gear wheels being mounted in bearings sealing against their end faces, said inner wall of the housing being coated with a bearing layer at least in the regions in which said wall encloses the gear wheels sealingly.

2. Gear pump according to claim 1 wherein the housing has a continuous recess containing the gear wheels and bearings and the housing inner wall is coated with the bearing layer also in the regions in which said inner wall surrounds the bearings.

3. Gear pump according to claim 1 or claim 2, wherein the bearing layer is of cadmium with e purity of at least 98%

4. Gear pump according to any one of claims 1 to 3 wherein the bearing layer thickness is 0.1 to 0.4 mm.

5. Gear pump constructed and arranged for use and operation substantially as described herein with reference to the accompanying drawing.