Gear train mechanism having reduced leakage

A gear train mechanism includes a housing having side walls that are made of a material exhibiting a modulus of elasticity of at least 140 GPa. A pair of shafts is supported by slide bearings in the housing and exhibit a length of at least of 1.3 times the diameter of the shafts. A first gear of external-tooth type is mounted on one shaft and is defined by an addendum diameter, a top land width and a circular pitch, with the top land width being at least 15% of the circular pitch. A second gear is mounted on the other shaft and is in mesh with the first gear, with both gears disposed between the side walls, wherein the side walls have a wall thickness of at least 75% of the addendum diameter.

BACKGROUND OF THE INVENTION 
The present invention refers to a gear train mechanism for use in e.g. a 
gear pump or gearmotor, and in particular to a gear train mechanism of a 
type including at least one gear of the external-tooth type in mesh with 
at least one further gear between adjoining side walls of a housing, with 
the gears being mounted on shafts which are supported in the side walls by 
means of slide bearings. 
Examples for use of gear trains include external or internal gear pumps or 
external or internal gearmotors. In general, external gear trains exhibit 
two or more gears of the external-tooth type which mesh to provide a 
certain action. Internal gear trains include at least one gear of the 
external-tooth type that is in mesh with at least one gear of the 
internal-tooth type. The constructional principle of rotary positive 
displacement units is also applicable for torque motors. In particular 
gear pumps and gearmotors are configured based on the same principle. 
It is noted at this point that unless a reference is expressly made to gear 
pumps or gearmotors, the use of the term "gear train mechanism" shall 
encompass gear pumps and gearmotors as the structural principles, as 
stated above, are basically the same. 
For energy-saving reasons, the efficiency of the machine is generally of 
great importance. In particular, the volumetric efficiency can be 
influenced by constructional measures. In gear train mechanisms, internal 
and external fluid leakages are encountered at zones that exhibit a gap 
formed between components which move relative to one another, e.g. between 
stationary parts and rotating parts. Fluid leakage is particularly 
experienced between pressure compartment and suction compartment. 
Essentially three different zones of fluid leakages can be differentiated: 
1. at the end face of each gear, 
2. along the gear addendum circle, 
3. at the slide bearing of a shaft. 
Leakage loss increases proportionally with the pressure and is raised to 
the third power of the gap width, i.e. the distance between components 
that move relative to each other, and is inversely proportional to the gap 
length, i.e. the distance being overcome by the fluid leakage. 
Without suitable measures, gear train mechanisms are not suitable for 
higher static pressures in the range above 100 bar because for reasons of 
operational safety and precision, the gap cannot be narrowed randomly. 
Moreover, leakage loss increases in particular along the gear end faces 
due to housing deformations that are caused by rising pressure. 
Leakage loss is conventionally reduced through gap compensation, by keeping 
constant or narrowing the gap between components that move relative to 
each other, e.g. between the end faces of the gears and adjoining housing 
areas, and lead to fluid leakage between the pressure compartment and the 
suction compartment, through pressure-loaded parts in dependence on the 
applied pressure. At least a housing deformation is compensated in this 
manner. 
Gear pumps or gearmotors configured in this manner have the drawback that 
the formed very small gap requires operation with a finely filtered fluid. 
In practice, such preconditions are rarely met. Thus, when using standard 
fluids that contain some impurities, a premature wear of the sealing 
components is a consequence so that the service life of such gear pumps or 
gearmotors is limited and the originally good efficiency gradually 
decreases during course of operation and depending on the degree of 
contamination. 
Moreover, such gear pumps or gearmotors that include gap compensation means 
are complex and cost-intensive. Also, the contact pressure applied on the 
parts for the gap compensation results in frictional forces which 
adversely affect the mechanical efficiency and thus the overall 
efficiency. 
Gap-compensated gear train mechanisms for higher pressures exhibit 
essentially small but short gaps (short gap length) for sealing purposes. 
In non-compensated gear train mechanisms, the omission of compensation 
components results in greater and significantly longer sealing surfaces 
(longer gap length) for a lateral sealing of the gear end faces. 
Especially at higher pressures in the range above 100 bar, the increasing 
gap width encountered with rising pressure due to the bending side walls 
of the housing becomes increasingly disadvantageous. 
Leakage losses are also significant along the gear addendum circle of gear 
pumps or gearmotors. The tooth geometry has to meet a number of 
requirements to attain optimum results, e.g. a great displacement volume 
at given structural volume, involute tooth profile for simplified 
manufacture, length of contact with positive contact ratio, a reduced 
number of teeth for noise reasons, etc. Therefore, gears are 
conventionally formed with relatively pointed teeth, with a top land width 
in the range of only about 5% of the circular pitch. Thus a very short gap 
length is generated between the rotating gear tip and the stationary 
housing part, leading to significant leakage loss along the gear addendum. 
Moreover, conventional gear pumps or gearmotors exhibit external fluid 
leakage at the slide bearings for the shafts. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved gear train 
mechanism for use in gear pumps, gearmotors or the like, obviating the 
afore-stated drawbacks. 
In particular, it is an object of the present invention to provide an 
improved gear train mechanism by which a volumetric efficiency and overall 
efficiency is attained that is comparable to those achieved by 
gap-compensated mechanisms, without requiring such a gap compensation. 
These objects and others which will become apparent hereinafter are 
attained in accordance with the present invention by providing side walls 
with a wall thickness of at least 75% of the addendum diameter and made 
with a material exhibiting a modulus of elasticity of at least 140 GPa, 
and by configuring the top land width of the gears at least 15% of the 
gear pitch, and by configuring the slide bearing of a length at least 1.3 
times the diameter of the supported shaft. 
Through configuration of the gear train mechanism in accordance with the 
present invention, the deflection of the side walls is reduced to a degree 
which is small in comparison to the gap width. Preferably, the wall 
thickness is in the order of approximately 80% of the addendum diameter, 
and the material for the side walls has a modulus of elasticity of 150-230 
GPa. The configuration of the side walls according to the present 
invention limits fluid leakage through the gap between the side walls and 
the gear end faces. A suitable material for the side walls is e.g. steel 
or spherulitic graphite iron. The use of aluminum is generally unsuitable 
because of the small modulus of elasticity of about 70 GPa. 
According to another feature of the present invention, the gears exhibit a 
tooth flank profile with a top land width as broad as possible. 
Preferably, the top land width of the teeth amounts to 20% of the pitch. 
Compared to conventional top land widths of only 5-8% of the pitch, the 
gear train mechanism according to the present invention exhibits a 
substantially longer gap length and thus a significantly improved sealing 
action. 
Moreover, in accordance with the present invention, the fluid leakage at 
the slide bearings of the shafts can be reduced by incorporating extra 
long bearing bushes of a length preferably in the order of 1.4 times to 
1.8 times the shaft diameter. This results in significantly longer gap 
length compared to conventional configurations that utilize bearing bushes 
at a length of approximately 0.8 times the shaft diameter. Since the slide 
bearings are formed in the side walls, a lengthening of the shafts permits 
the slide bearings to be simply extended. No complicated constructional 
measures are necessary.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Turning now to the drawing, and in particular to FIG. 1, there is shown a 
cross-sectional view of one embodiment of a gear train mechanism in form 
of an external gear pump, according to the present invention, including a 
housing 2 having side walls 1, 3 in form of plates. The housing 2 supports 
two parallel shafts 6, 7 on which meshing gears 4, 5 of the external-tooth 
type are respectively mounted. In the non-limiting example of FIG. 1, the 
shaft 6 is driven while the shaft 7 is idle and driven via the meshing 
gears 4, 5. Each shaft 6, 7 is supported in the side walls 1, 3 by two 
slide bearings 8, 9; 10, 11. 
Fluid leakage can be experienced wherever a gap is formed between 
components that move relative to each other. Essentially, three zones, 
denoted A, B, C, should be considered. 
Leakage zone A is formed in the contact area between the gear end faces and 
the side walls 1, 3. In accordance with the present invention, the side 
walls 1, 3 have a wall thickness w which is about 80% of the addendum 
diameter d.sub.a so as to exhibit the required stiffness for withstanding 
high static pressures of above 100 bar and thus for preventing an 
excessive deflection. The positive effect of the wall strength can further 
be compounded by suitably selecting the material for the side walls 1, 3. 
Preferably, the side wall 1,3 are made of a material exhibiting a modulus 
of electricity of at least 140 GPa Examples for a suitable material for 
the side walls 1, 3 includes steel or spherulitic graphite iron. These 
constructional features limit a deflection of the side walls 1, 3 to such 
a degree that is small compared to the gap width. 
The measures in accordance with the present invention to limit fluid 
leakage at zone B along the gear addendum will be described in more detail 
furtherbelow, with reference to FIG. 2. 
The leakage zone C is formed along the slide bearings 8, 9; 10, 11 of the 
shafts 6, 7. The leakage loss is inversely proportional to the length I of 
the slide bearings 8, 9; 10, 11. The ratio of the length I relative to the 
shaft diameter d is approximately 1.4 to 1.8, preferably 1.5. This value 
significantly exceeds the conventional ratio which normally amounts to 
0.8. Thus, leakage losses are considerably reduced. As the side walls 1, 3 
are broader compared to conventional configurations, a lengthening of the 
shafts 6, 7 effects a greater length I of the slide bearings 8, 9; 10, 11, 
thereby increasing the gap length that is relevant for leakage losses. No 
complicated constructions are required. 
Turning now to FIG. 2, there is shown a fragmented, sectional view of 
meshing teeth of gears 4, 5. Reference character b denotes the top land 
width, and reference character t denotes the circular pitch. In accordance 
with the present invention, the tooth flank profile is defined by a top 
land width b which is about 20% of the circular pitch t. Conventionally, 
the top land width is about 5 to 8% of the circular pitch. The top land 
width b corresponds to the gap length that is responsible for leakage loss 
at zone B. As again the leakage loss is inversely proportional to the gap 
length, the substantially longer top land width b results in significantly 
reduced leakage loss along the gear addendum. 
FIG. 3 shows a cross sectional view of another embodiment of a gear train 
mechanism in form of an internal gear pump according to the present 
invention, with at least one of the gears 4', 5' (here gear 5') being of 
the internal-gear type. 
While the invention has been illustrated and described as embodied in a 
gear train mechanism, it is not intended to be limited to the details 
shown since various modifications and structural changes may be made 
without departing in any way from the spirit of the present invention. 
What is claimed as new and desired to be protected by Letters Patent is set 
forth in the appended claims: