GB2274489A - Gear pumps - Google Patents

Abstract

A gear pump comprises two pairs of intermeshing spur gears 3, 4, 5, 6 mounted between hearing members 10, 11 and a pressure plate 12 on first and second shafts 1, 2. The two pairs of gears are mounted in such a way that pulsations caused by the operation of the two pairs are out of phase with each other. The first pair of gears 3, 4 is supported by bearings 13, 14 on both sides, whereas the second pair of gears is supported by bearings 14 on only one side, the pressure plate on its other side being a hydraulically balanced pressure plate but incorporating no bearings for the shafts. <IMAGE>

Description

TITLE Gear Pumps Technical Field The invention relates to gear pumps which minimize pulsating pressure in the fluid being pumped.

Background Art Gear pumps are very commonly used in the pumping of fluids, and there is great interest in making improvements to the basic design. One of the major problems with traditional gear pumps relates to the generation of a pulsating pressure in the fluid being pumped.

The intermeshing of the rotating gears causes the formation of fluid filled cavities as individual gear teeth rotate out of contact with one another. As the gearwheels rotate, the cavities and their contents are carried around the circumferences vr^ the gearwheels to the other side of the pump. Since it is not possible for fluid to pass between the gearwheels at their point of intermeshing, the transfer of fluid is in one direction only.

Because the fluid is moved in discrete packets, the rotation of the gearwheels causes pulsations in the output pressure of the gear pump. The pulsations produce noise as well as vibrations which eventually degrade machinery.

One way of reducing vibrations is to mount two pairs of gears alongside one another in such a way that the pulsations caused by the release of fluid in the two sets of gears occur at mutually different times. However, such gears suffer from serious manufacturing difficulties.

One method of mounting the two pairs of gear wheels on the shafts in a two gear-pair pump is tc use three sets of bearing members, one on each side of the gearwheels and one in the middle between the gearwheels. The difficulties associated with mounting a shaft within three bearings are very great. If a shaft is mounted in two bearings which are minimally misaligned, the shaft is able to shift slightly in order to compensate for this misalignment. Where three bearings are used however, any movement of the shaft to align with two of the bearings will almost inevitably bring it out of line with the third. One bearing invariably ends up being put under excessive stress, or barely acting at all. Either way the efficiency of the gear pump is reduced.

Even if it were possible to align the bearings, the cost of using three rather than two bearing members, and installing them, particularly to such high tolerances, would be prohibitive.

An alternative way of mounting the two pairs of gearwheels on the shaft is to provide only two sets of bearing members, one on either side of the gearwheels, and a pressure plate positioned between the gearwheels.

This central pressure plate includes no bearings and provides no support for the shaft. This arrangement eliminates the alignment problems discussed above, but introduces other difficulties, caused by the lack of support of the shaft in its central region. Each pair of gearwheels is effectively supported by both of the two bearing members, one of these bearing members being positioned adjacent to the gearwheels and one being at a distance from the gearwheel dictated by the total width of the central pressure plate and the other gearwheel.

The central shaft region therefore acts as a cantilever providing support for each pair of gearwheels from its opposite bearing member. This arrangement causes severe stresses in the shaft, which can lead to poor performance of the gear pump.

The Invention The invention provides a gear pump comprising a first pair of intermeshing spur gears on first and second shafts respectively, and a second pair of intermeshing spur gears on the same shafts, the gears of at least the second pair being axially slidable relative to their shafts and the gears being aligned such that in use the pressure pulsations produced by the first and second pairs of gears are mutually out of phase; a first end bearing member axially movable relative to the outer side faces of the gears of the first pair; an intermediate bearing member axially movable relative to the inwardly directed side faces of the gears of the first and second pairs; and a second end pressure plate axially movable relative to the outer side faces of the gears of the second pair; wherein first journal bearings are provided between the first end bearing member and the portions of the first and second shafts passing therethrough; second journal bearings are provided between the intermediate bearing member and the portions of the first and second shafts passing therethrough; but no journal bearings are provided between the second end pressure plate and the first and second shafts.

Such a gear pump overcomes the problems associated with unwanted pulsations, without introducing the difficulties associated with prior art double gear-pair pumps.

Instead of including three sets of bearing members, to mount fully each shaft and support both first and second gearwheels from both sides, a gear pump according to the invention uses only two bearings around each shaft. The gear wheels of the first pair are each supported by bearings on either side, but the gearwheels of the second pair are each supported by just one bearing in a cantilevered fashion. This cantilever is much shorter than that resulting from the above described prior art two-bearing arrangement, and does not produce excessive stresses in the shaft. The arrangement therefore provides sufficient support from the second pair of gearwheels while eliminating the problems associated with attempting to mount a shaft on three bearings.

It is a particularly advantageous feature of the invention that the pump can be manufactured as a high pressure pump. For such a pump to function efficiently, pressures within it must be kept high and balanced. It is therefore necessary that at least one of the sets of gearwheels be allowed some freedom of movement in an axial direction. In a preferred embodiment, the gearwheels of the first pair are axially fixed relative to their respective shafts, while the gearwheels of the second pair are permitted at least a certain amount of axial movement on their shafts.

Each pair of gearwheels must comprise a driving gear and a driven gear. The torque path may be: (a) first shaft to first gear of first pair; first gear of first pair to second gear of first pair; first shaft to first gear of second pair; first gear of second pair to second gear of second pair, or (b) first shaft to first gear of first pair; first gear of first pair to second gear of first pair; second gear of first pair to second shaft; second shaft to second gear of second pair; second gear of second pair to first gear of second pair.

In the former case there is no torque path from the second shaft to the second gear of the second pair of gears, and in the latter case there is no torque path from the first shaft to the first gear of the second pair of gears.

In the first case, the first gear of the second pair is the driving gear of that pair, and in the latter case, the second gear of the second pair is the driving gear.

In either case, the driving gear is preferably secured to the shaft by means of keys or teeth, shaped to allow some axial movement of the gear on the shaft.

Depending on which of these two torque paths is used, the two pairs of gears are arranged differently with respect to each other on the shaft, to minimise unwanted pulsations.

If the former torque path (a) is used, the driving gear is the first gear of each pair. These two driving gears are mounted on the shaft so that their teeth are out of phase with each other preferably by one half tooth pitch.

This causes the pulsations resulting from the pumping actions of the two gear pairs to be out of phase, minimising outlet pressure pulsations and therefore noise.

If the latter torque path (b) is used, the situation is somewhat more complex. In this case the driving gears are the first gear of the first pair and the second gear of the second pair.

The engagement of the tooth flanks between gears in each pair therefore differs, because the leading flanks of the teeth on the driving gear always engage the trailing flanks of the teeth on the driven gear. This causes the line of action of the points of intersection of one gear set to lag that of the other, and means that the pulsations caused by each pair of gears are out of phase.

Therefore the effect of minimising pulsations by causing the two gear pairs to work out of phase may be achieved without the necessity to mount the gears physically out of phase on the shaft.

Drawings Fig. 1 is a cross-section of a prior art gear pump, Fig. 2 is a cross-section of a gear pump according to one embodiment of the invention, Fig. 3 is a view of the gear pump of Fig. 2 sectioned along line A-A, Fig. d is a view of the gear pump of Fig. 2 sectioned along line B-B to show the shape of the bearing members, Fig. 5 is a view of the gear pump of Fig. 2 sectioned along line C-C, to show the shape and locaticn of the gaskets, Fig. 6 is a view along the first shaft in Fig. 2, showing the relative positions of the gearwheels.

The gear pump of Figs. 2 to 6 operates according to the torque path (a) previously described.

Referring to Figs. 2 and 3, the gear pump comprises an input shaft 1 and an idler shaft 2 enclosed by a side housing 7, and end sealing plates 8 and 9.

Mounted on the input shaft 1 are a first input gearwheel 3 and a second input gearwheel 5 spaced apart from one another along the shaft 1. On the idler shaft 2 are mounted first and second idler gearwheels 4 and 6 respectively. The first input and idler gearwheels 3 and 4, intermesh, as do the second input and idler gearwheels 5 and 6, and the two pairs of intermeshing gears are out of phase with one another by one half tooth pitch. This can be seen from Fig. 6, which shows the the input gearwheels 3 and 5 viewed along the input shaft 1.

The first gearwheels 3 and 4 are firmly fixed to their respective shafts and cannot move axially. The second idler gearwheel 6 is not fixed on the shaft and is free to move along it. The second input gearwheel 5 is keyed to the shaft as can be seen clearly in Fig. 3. The four keys 15 fit into both the shaft and the gearwheel to prevent relative rotary movement between the two. The keys 15, however, are smaller in an axial direction than are the holes in which they are mounted. Thus a certain freedom of axial movement of the gearwheel 5 along the shaft 1 is permitted.

Referring to Fig. 3, the side housing 7 is internally shaped to enclose the gearwheels while leaving cavities on either side of their point of engagement. These are the second inlet and outlet cavities 17 and 18 through which the liquid to be pumped flows. They are fed by inlet and outlet bores, not shown in the drawing, and the input shaft is driven by the spline 16, to pump fluid from the input to the output cavity. Fig. 4 is a section along line B-B in Fig. 2 that does not pass through a pair of gearwheels, and shows the shape and position of ietermediate beczfiny loembes 11.The first end bearing member 10, intermediate bearing member 11 and second end pressure plate 12 are free to move axially in the direction of the shafts, and so can transmit applied pressure and maintain a balanced pressure along the length of the pump.

The pressure is applied to the first end bearing member 10 and second end pressure plate from hydraulic chambers defined between gaskets 19 and 20 as illustrated in Fig.

5, which is a cross-section along line C-C in Fig. 1.

Gaskets 20 are fully compressed, and form tight seals between the housing 7 and the end sealing plates 8 and 9.

Each gasket 19 forms a seal between the respective bearing member or end pressure plate 10 or 12 and the respective sealing plate 8 or 9 and by its resilience maintains that hydraulic seal during the range of axial movement of the pressure plates. The gasket 19 is so shaped because of the very high pressures induced in the outlet ports of the gear pump when it is in use. These pressures are far higher than those on the inlet side.

The shape of the gasket 19, and therefore of each hydraulic chamber, is such that a high pressure is exerted on an area of the adjacent bearing member or pressure plate corresponding to the high pressure outlet region of the gear pump. This is the unshaded area to the outside of the gasket 19 in Fig. 5. Because the bearing members, pressure plates and the second gearwheels have some freedom of axial movement, high pressures are maintained throughout the whole outlet side of the gear pump. The bearing members and pressure plates are thus kept firmly in contact with the gears, and leakage is prevented.

The input and output shafts 1 and 2, are supported by journal bearings, located between themselves and the bearing members. Fig. 1 shows a standard gear pump with bearings around the whole length of the shaft except where the gearwheel is located. Referring to Fig. 2, a gear pump according to the invention includes on each shaft a journal bearing 13 between the first gearwheel and the end housing, and a journal bearing 14 between the two gearwheels. In this illustrated embodiment the second bearing is longer than the first, but this is not necessary, and the two bearings may be of the same length. The shafts between the second gearwheels and the end sealing plate 9 are not mounted in such bearings, and the second gearwheels are supported in a cantilevered fashion by the bearing 14.

Claims (9)

1. A gear pump comprising: a first pair of intermeshing spur gears on first and second shafts respectively, and a second pair of intermeshing spur gears on the same shafts, the gears of at least the second pair being axially slidable relative to their shafts and the gears being aligned such that in use the pressure pulsations produced by the first and second pairs of gears are mutually out of phase; a first end bearing member axially movable relative to the outer side faces of the gears of the first pair; an intermediate bearing member plate axially movable relative to the inwardly directed side faces of the gears of the first and second pairs; and a second end pressure plate axially movable relative to the outer side faces of the gears of the second pair; wherein first journal bearings are provided between the first end bearing member and the portions of the first and second shafts passing therethrough; second journal bearings are provided between the intermediate bearing member and the portions of the first and second shafts passing therethrough; but no journal bearings are provided between the second end pressure plate and the first and second shafts.

2. A gear pump according to claim 1 wherein the second journal bearings are of an axial extent longer than that of the first journal bearings.

3. A gear pump according to claims 1 or 2 wherein the gearwheels of the first pair are axially fixed relative to their respective shafts, while the gearwheels of the second pair are allowed some axial movement on their respective shafts.

4. A gear pump according to any preceding claim wherein the teeth of the gear wheels of the first pair are out of phase with the teeth of the gear wheels of the second pair.

5. A gear pump according to claim 4 wherein the teeth of the gear wheels of the first pair are out of phase with the teeth of the gear wheels of the second pair by one half tooth pitch.

6. A gear pump according to claim 4 or claim 5 wherein the first gearwheel of the second pair is splined to its shaft to allow some axial movement, and the second gear of the second pair is free to move on its shaft.

7. A gear pump according to any claims 1 to 3 wherein the teeth of the gear wheels of the first pair are in phase with the teeth of the gearwheels of the second pair.

8. A gear pump according to claim 7 wherein the second gearwheel of the second pair is splined to its shaft to allow some axial movement, and the first gear of the second pair is free to move on its shaft.

9. A gear pump substantially as described herein with reference to the description and drawings.