GB2312476A

GB2312476A - Pressure balance control in gear pumps

Info

Publication number: GB2312476A
Application number: GB9608582A
Authority: GB
Inventors: Roderick Howell Spurry; James Edward Wood; John Hudson
Original assignee: Sauer Sundstrand Ltd
Current assignee: Sauer Danfoss Swindon Ltd
Priority date: 1996-04-24
Filing date: 1996-04-24
Publication date: 1997-10-29
Anticipated expiration: 2016-04-24
Also published as: GB9608582D0; GB2312476B

Abstract

In a gear pump first and second toothed gears (4,6,Fig 1) are supported in a housing (8) between floating bearings (10) and (12) and fluid is pumped from an inlet 22 to an outlet 24, the gear pump 2 including means associated with at least one gear for placing the outlet 24 in fluid communication with a plurality of spaces between the gear teeth, at least one such space being on the inlet side of the gears. This controls and stabilizes the location of the seal point 28. The means may be a chamfer 35 on a bearing 12' or a relief pocket (38, Fig 6A) in pump housing.

Description

TITLE Pressure balance control in gear pumps DESCRIPTION Inventive Field The invention relates to external gear pumps and more particularly to a method of reducing and stabilizing the loading on the journal bearings which support the gears.

Background of the Invention In an external gear pump, liquid, usually oil, is carried in gaps between gear teeth from a low pressure inlet to a high pressure outlet. It is known that around the circumference of a gear, there is a fairly abrupt point at which the pressure rises from inlet pressure to outlet pressure, rather than there being a smooth "cushioned" pressure increase. This abrupt point is known as the "seal point". The position of the seal point is not constant; when the speed of the pump is increased, it moves towards the high pressure outlet side, and when the pressure in the pump increases, the seal point moves towards the low pressure inlet side. Further, when the gears are forced by hydraulic pressure to track into the body of the pump in an arc which commences adjacent the inlet (see Fig. 4) the seal point will be at an arc distance (from the track commencement) where such "cut in" arc is always less than 180 degrees and is on the low pressure side of the pump. Therefore, during normal operation of the pump, the seal point has a tendency to move around quite substantially.

In a gear pump having "figure 8" shaped floating bearings, the journal bushings which support the gears are an integral part of the floating bearings. Loads on the bearings are caused by a number of factors but the dominant one is the difference in hydraulic pressure between the input and output sides of the pump.

This pressure difference exerts a force on the gears, and this force is transmitted via the shafts to the bearings.

The net force on a gear in a direction towards the inlet side depends on the variation of pressure around its circumference. This depends on the position of the seal point. As can be seen in Figure 3, when the seal point is near the inlet (at point a), only a small amount of the gear's circumference is at low pressure. The high pressure on the inlet side (area A) partially balances out the high pressure on the outlet side (area C) . The net force on the gear and therefore the bearing is relatively low.

When the seal point is nearer the outlet side (at position c), there is only a very small high pressure area (area B) on the inlet side to balance out that on the outlet side and the net force on the gear and on the bearings is relatively high.

The loaded area, or output projected area, of the gear is equal to the width of the gear multiplied by the loaded length (x or y in figure 3). This quantity gives an indication of the force on the bearings.

Thus, when the seal point moves around during operation of the pump, the net load on the bearing varies, and the resultant movement can cause wear and possible leakage.

The high loads on the bearings which occur when the seal point moves significantly away from the inlet are also undesirable.

The Invention The invention provides a gear pump including a pair of toothed gears for pumping fluid in spaces between the gear teeth from an inlet to an outlet, including means associated with at least one gear for placing the outlet in fluid communication with a plurality of the spaces between the gear teeth, at least one such space being on the inlet side of the gear.

By the inlet side of the gear, it is meant that if the gear is divided down the middle, i.e. along line A-A in Figure 2, the inlet side is everything to the left of this line as viewed in the Figure.

By allowing fluid communication of the outlet with a plurality of spaces between gear teeth, at least one of the spaces being on the inlet side, the seal point can be stabilized at a desired location on the inlet side of the pump. This is because the seal point will always be at the end of the path of fluid communication. Here the pressure will always change from inlet to outlet pressure.

Preferably the path for fluid communication from the outlet extends around to a point quite near the inlet. In this case the seal point is held near the inlet, and the forces on the bearings are kept constant and relatively low.

The fluid communication path may be a chamfer cut into the face of the bearing adjacent the bearing, the chamfer extending from the outlet to a point on the inlet side of the gear. Oil from the high pressure outlet fills the chamfer, and thus the seal point is fixed at the end of the chamfer.

Alternatively, a pocket or pockets may be milled into the housing of the pump, again to allow fluid communication between the outlet and a plurality of spaces between gear teeth.

Preferably, means are provided to ensure that axial balance is maintained. This avoids excessive variations in bearing face loading. These means may be a peripheral chamfer on the rear face of the bearing which can fill quickly with oil at pressure and ensure quick response to any changes in the pressure generated by the load on the pump.

The Drawings Figure 1 is a cross-section through an external gear pump; Figure 2 is a schematic view of the gears, showing the pumping of oil from an inlet to an outlet; Figure 3 is a side view of a bearing showing the variation in seal point and its effect on the loaded area of the bearing; Figure 4 shows the position of a "cut -in" arc; Figure 5 shows a bearing according to the invention; Figures 6A to 6C show a gear pump according to an alternative embodiment of the invention; Figures 7A to 7C show a gear pump according to a further alternative embodiment of the invention; and Figure 8 shows a bearing designed so as to maintain axial balance.

Description with reference to the drawings Referring to Figure 1, a gear pump 2 includes first 4 and second 6 gears supported in a housing 8. The gears are supported by floating bearings 10 and 12, which include journal bushings 14,16,18 and 20 through which the shafts of the gears pass. Referring to Figure 2, oil is pumped from a low pressure inlet 22 to a high pressure outlet 24 in the spaces 26 between the gear teeth.

Figure 3 shows how the position of the seal point 28 can vary. The nearest that it can be to the inlet is at point (a) one gear pitch away from the junction 29 of the body and bearing relief cusps. (In this example, the body inlet port protrusion 32 has no effect since it is located inwardly of the junction 29). The further the seal point moves away from the inlet, the greater the output projected area and therefore the net loading on the bearing, as previously described, and shown in Figure 3.

When the seal point is at position (c), the net load on the bearing is much greater than when the seal point is at position (a). This net loading on the bearing therefore alters significantly during the normal working of the pump, and can be quite high when the seal point moves away from the inlet.

Figure 4 shows the situation where the gear has been forced by hydraulic pressure to "track" into the body of the pump in an arc 33 which commences adjacent the pump inlet.

Figure 5 shows a first way in which the position of the seal point can be stabilized, and maintained at a position where the net loading on the bearing is low. Referring to Figure 4, a bearing 12' has a peripheral chamfer 34 on its front face. The front face is the face that contacts a gear of the pump. When pumping takes place, oil from the outlet flows into the space left by the chamfer, so that the whole of this space is at outlet pressure. The point of transition from inlet to outlet pressure is thus fixed at the end 36 of the chamfer 34. This point is relatively near the inlet so the net loading on the bearing is low for the reasons described previously.

Figures 6A to 6C show an alternative way of achieving a similar effect. A relief pocket 38 is milled into the housing. Oil from the high pressure outlet side enters this pocket in just the same way as it enters the chamfer of Figure 4. Thus, the seal point 28 is reliably defined.

Figures 7A to 7C show a further embodiment in which multiple relief pockets milled into the body profile. The finish point 39 of the relief nearest to the pump inlet is in the same location as the end 36 of the chamfer in Figure 4, in order to define a seal point 28.

Preferably provision is made to ensure that axial balance is maintained, to avoid excessive variations in bearing face loading. The pressure balance on the rear face of the bearing may be fed by a peripheral chamfer 40 on the rear face of the bearing 12 to ensure instant response to any changes in pressure generated by the load on the pump (see Fig.8)

Claims

CLAIMS: 1. A gear pump including a pair of toothed gears for pumping fluid in spaces between the gear teeth from an inlet to an outlet, including means associated with at least one gear for placing the outlet in fluid communication with a plurality of the spaces between the gear teeth, at least one such space being on the inlet side of the gear.

2. A gear pump according to claim 1 wherein the means for placing the outlet in fluid communication wiih d plurality of the spaces between the gear teeth includes a chamFer cut into a bearing adjacent the gear 3. A gear pump according to claim 2 wherein the chamfer extends from the outlet to a point on the inlet side of the gear.

4. A gear pump according to claim 1 wherein the means for placing the outlet in fluid communication with a plurality of the spaces between the gear teeth includes a pocket or pockets milled into a housing of the pump adjacent the gear.

5. A gear pump according to any preceding claim including means for maintaining axial balance.

6. A gear pump according to claim 5 wherein the means for maintaining axial balance include a peripheral chamfer on the rear face of bearing adjacent the gears.

Amendments to the claims have been filed as follows 1. A gear pump including a pair of toothed gears for pumping fluid in spaces between the gear teeth from an inlet to an outlet, including means associated with at least one gear for placing the outlet in fluid communication with a plurality of the spaces between the gear teeth, at least one such space being on the inlet side of the gear, to stabilize the seal point at the end of the path of fluid communication, on the inlet side of the gear.
2. A gear pump according to claim 1 wherein the means for placing the outlet in fluid communication with a plurality of the spaces between the gear teeth includes a chamber cut into a bearing adjacent the gear.
3. A gear pump according to claim 2 wherein the chamfer extends from the outlet to a point on the inlet side of the gear.
4. A gear pump according to claim 1 wherein the means for placing the outlet in fluid communication with a plurality of the spaces between the gear teeth includes a pocket or pockets milled into a housing of the pump adjacent the gear.
5. A gear pump according to any preceding claim including means for maintaining axial balance.
6. A gear pump according to claim 5 wherein the means for maintaining axial balance include a peripheral chamfer on the rear face of bearing adjacent the gears.
7. A gear pump substantially as described herein with reference to Figure 5, Figures 6A and 6B, Figures 7A and 7B or Figure 8 of the drawings.