GB2139287A - Gear-type rotary hydraulic machine - Google Patents

Abstract

The machine may be a pump or a motor and comprises a pair of gears 12 within a housing 18, each gear being mounted on respective pairs of shafts 30,32, which in turn are supported by bearings 34,36. The housing has two members 20,22 abutting each other in a transverse plane P-P situated midway between the bearings so that the resultant radial force acting on each member is zero and radial displacement of these members is thereby obviated. <IMAGE>

Description

SPECIFICATION Hydraulic Gear Unit Background of the Invention This invention relates generally to hydraulic gear units, such as gear pumps and motors utilized in high pressure hydraulic systems.

More particularly, the invention relates to the design of external casing members for hydraulic gear units. Such casing members must be effective to hermetically seal the operative components of the gear units, especially when such units are subject to cyclic reversing action. As herein defined, "gear unit" means either a gear pump, a gear motor, or a hydraulic unit capable of operating as either.

Prior art casing designs have been limited in their accommodation of extremely high pressure loads over extended periods of time.

In such applications, the casings have required greater thicknesses for rigidity and strength, and have thus had relatively massive designs. As a result, while some applications have required certain dimensional reductions relating to, for example, a shortened axial profile of a housing, the latter requirements have had to be compromised or made subordinate to strength requirements. Where such dimensional requirements have not been avoidable, the load ratings of affected designs have been substantially reduced.

Moreover, notwithstanding compromises between strength and size parameters, many prior art designs have produced an inherent net radial loading at the mating surface interface of such casing members, wherein mated casing members have tended to shift laterally relative to one another. In some applications, the net loading has been so great that the shifting has caused gear plates within the hydraulic units to slide back and forth resulting in losses in unit efficiency. Even in gear unit applications subjected to forces less severe, creep or relaxation in the clamping forces between mating surfaces has also, although not as quickly, resulted in losses in gear unit efficiency.

What is needed is a split casing design which is not subject to compromises between dimensional constraints and strength requirements, nor to net radial loading. The need for such a design will become even greater as hydraulic system pressures become higher with improvements in technology.

Summary of the Invention This invention provides a hydraulic gear unit having a split casing design not subject to loss of radial strength upon reduction of thickness or other structurally related dimensions. Moreover, the casing members do not have an inherent tendency for relative movement between their mating surfaces as hydraulic pressures become higher. As a result, gear plates within the units are not subject to the losses in efficiency experienced in hydraulic gear units of the prior art.

In a preferred form, the subject invention is embodied in a gear unit which includes a pair of external casing members, each having opposed mating surfaces which lie on a radially oriented plane, transversely to a pair of gear shafts in the unit. The two mating surfaces have a resultant net radial separating force of zero between them. Moreover, the radially extending mating surfaces are positioned intermediate the bearings on either side of a pair of mated gears. The mating surfaces are symmetrical with respect to the gears, and are parallel to and equidistant from each of the radial surfaces of pressure plates juxtaposed against either side of the gears.

Brief Description of the Drawings Fig. 1 is a cross sectional view of a preferred embodiment of a hydraulic gear unit constructed in accordance with the present invention; Fig. 2 is a schematic diagram of the gear unit of Fig. 1, including a graph depicting a hydraulic pressure generated force field; and Figures 3, 4, and S are cross sectional views of prior art hydraulic gear units.

Detailed Description of Preferred Embodiments Referring to Fig. 1, a gear unit 10 includes a pair of gears 12, each of which is fixedly supported on a shaft 14 for rotation therewith. In the embodiment shown and described, the unit 10 operates as a pump. For convenience, only one gear and associated shaft system is shown. The gears 1 2 are in constant mesh with one another, and operate to generate hydraulic pressure, and to thus transfer hydraulic fluid from a low pressure port 26 to a high pressure port 28. A keyway 1 6 on one of the shafts 14 provides a coupling means for rotation of the gear 1 2 within the unit 10 by an external source. The hidden mated gear 12, is positioned on a stub shaft 14 immediately behind and in line with the visible shaft and gear assembly.

The respective gear and shaft assemblies are encased within a split housing 18, which consists of separate casing members 20 and 22. The members 20 and 22 are hermetically mated together at radially extending mating surfaces 24 positioned circumferentially about the gear 12 and defining a radially extending, plane P-P (for reference). As apparent, the plane P-P extends transversely to the shafts 1 4. Although the shaft 14 is typically a single member which extends through a bore in the gear 12, separate shaft portions 30 and 32 positioned on either side of the gear 1 2 are herein defined.

As depicted, the first shaft portion 30 represents the upper portion while the second shaft portion 32 is defined by the lower portion of the shaft 14. The first shaft portion 30 is supported within the upper casing member 20 by a first shaft sleeve bearing 34, while the second shaft portion 32 is supported in the lower casing member 22 by a second shaft sleeve bearing 36. Per conventional hydraulic gear unit practice, first and second pressure plates 42 and 44 are juxtaposed against first and second sides 38 and 40, respectively, of the gear 1 2. The latter pressure plates insure the volumetric efficiency of the gear unit, as will be appreciated by those skilled in the art.

Referring now to Fig. 2, a schematic diagram of the gear unit 1 0 is shown in conjunction with a force field as generated by the high pressure side of the unit, which herein acts as a pump as earlier noted. It will be appreciated that the difference in pressures between the high and low pressure sides of the unit will be effective to create a resultant or net force differential between respective sides. The magnitude of such a force will depend on parameters of each individual gear and shaft system, and more specifically will be a function of gear width W, gear diameter D, and the hydraulic pressure differential between the high and low pressure sides of the unit.

It will also be apparent to those skilled in the art that such pressure differentials in hydraulic gear units vary axially from one end of one to the opposite end of the other of the axially spaced bearings 34 and 36. As such, there will be a resultant force vector "F" acting centrally on the gear 12, which will be carried by inverse reactions equal to "1 /2 F" over the latter bearings. The result is an axially extending force field depicted by the graph shown in Fig. 2, wherein "L" equals the length of each bearing, and "X" represents the abscissa extending parallel to the shaft axis from the extreme left end of the bearing 34.Thus, if "A" represents the effective area over which the pressure differential "P" is exerted, "A" will equal "W times D", and at any given localized point along X, the value of the force exerted will equal "RAP", where "R" is a dimensionless constant.

It will also be apparent to those skilled in the art that the axial location 25 (shown as a dotted line in Fig. 2) of the mating plane along the abscissa X will determine the specific value of the sliding force realized in a particular casing arrangement. Thus, in accordance with the graph, maximum forces are reached whenever the mating plane P-P is positioned at the inboard ends of the bearings, i.e. at the sides 38 and 40 of the gear 12; or referring to Figure 1, at the line of contact between the pressure plates 42 and 44 and the gear 1 2. Minimal, or zero, forces will be realized whenever the mating plane is positioned at either of the outboard ends of the bearings 34 and 36, and at the central or intermediate point of the gear 1 2. Referring now to Figs. 3, 4, and 5, several examples of prior art mating plane locations are represented for comparison purposes. For example, in Fig. 3, the mating plane P'-P' is positioned interjacent the ends of the bearing 34'. Referring momentarily to Fig. 2, it will be seen that the particular location of the plane P'-P' presents a near maximum net mating surface sliding force value, particularly to the extent that the plane is positioned through the inboard end of the bearing 34'.Thus, there is substantial potential for relative movement between the mating surfaces 24' of the casing members 20' and 22' of Figure 3.

In Fig. 4, a three-piece housing is employed having three separate casing members 20', 21', and 22'. This particular system results in a pair of mating planes P'-P' and thus presents a double exposure to the sliding force phenomena. Moreover, it will be noted by reference to the graph of Fig. 2 that the double exposure is further aggravated by the fact that both planes P'-P' are radially positioned through inboard portions of the bearings 34' and 36'.

Fig. 5, however, shows a version of a prior art hydraulic gear unit housing 1 8' which improves over the versions of Figs. 3 and 4, but only from the standpoint of net radial loading. Thus, in the embodiment of Fig. 5, the mating plane P'-P' is positioned at the outboard end of the bearing 34', and hence falls at a zero net sliding force point according to the graph of Fig. 2. It will be apparent to those skilled in the art, however, that the mated casing design of Fig. 5 is not as amenable to shorter axial dimensions or profiles as is the version of Fig. 1. For example, larger gear unit sizes will result in an elongated lower casing member 22', and at some point the member 22' will have an objectionably deep bore 50' (Fig. 5).As such, those skilled in the art will realize that resultant load ratings must commensurately and of necessity be lowered. The preferred embodiment of Fig.

1, however, allows for considerably shallower bores 50 in any single member, to the extent that both casing members 20 and 22 may be utilized to share the overall total bore depth required. The result is a considerable potential for a stronger housing 18 even though overall physical sizes or dimensions must become larger to meet operative requirements.

In summary, two major benefits result from the unique location of the mating plane P-P (Fig. 1) of the present invention. Firstly, the net radial loading between the mating surfaces is zero and thus provides for an inherently stronger hydraulic unit. Secondly, with the mating surface plane lying parallel to and equidistant from the operative surfaces of respective pressure plates 42 and 44, relatively smaller casing members may be utilized with out commensurate reductions in load ratings per prior art practices.

The aforedescribed preferred embodiment is believed to represent only one of many variations which fall under the appended claims.

Claims (8)

1. In a hydraulic gear unit comprising a pair of external casing members having radially extending, axially opposed mating surfaces, said gear unit further including a pair of gears in mesh with one another, a pair of shafts respectively supporting each gear medially thereon, and first and second bearings disposed at opposed ends of each of said shafts; an improvement comprising: said mating surfaces lying radially outwardly of said gears in a plane positioned transversely through said shaft and intermediately of said first and second bearings.

2. The gear unit of Claim 1 wherein each gear includes a pair of radially extending, axially spaced sides, said unit further comprising a pair of pressure plates, each having a radially extending surface disposed against each of said sides, wherein said surfaces extend parallel to and equidistantly of said plane.

3. The gear unit of Claim 1 wherein said gears generate a variable force field extending axially between one outboard end of said first bearing to a second outboard end of said second bearing. each localized force along said field being potentially operable to slide one casing member laterally relative to another. wherein said mating surfaces are positioned in a plane containing a localized net radial force value of zero.

4. The gear unit of Claim 3 wherein a maximum localized force along said variable force field comprises a function of the width of the gears, the diameter of the gears, and the hydraulic pressure developed by the rotation of said gears.

5. The gear unit of Claim 4 wherein said first and second bearings are sleeve bearings.

6. A gear unit comprising a pair of external casing members including radially extending, axially opposed mating surfaces, said gear unit further comprising a pair of gears in mesh with one another, a pair of shafts respectively supporting each gear medially thereon, first and second bearings disposed on each of said shafts, said mating surfaces lying radially outwardly of said gears in a plane positioned transversely through said shaft and intermediately of said first and second bearings.

7. A hydraulic gear unit comprising a pair of external casing members including radially extending, axially opposed mating surfaces, said gear unit further comprising a pair of gears in mesh with one another, a pair of shafts respectively supporting each gear medially thereon, first and second bearings disposed on each of said shafts, said mating surfaces lying radially outwardly of said gears in a plane positioned transversely through said shaft and intermediately of said first and second bearings, each of said gears including a pair of radially extending, axially spaced sides, said unit further comprising a pair of pressure plates, each having a radially extending surface disposed against each of said sides of said gears, wherein said surfaces extend parallel to and equidistantly of said plane.

8. A gear unit substantially as described herein with reference to, and as shown in, Figures 1 and 2 of the accompanying drawings.