Gear motor with fluid pressure groove and recess to facilitate starting

A rotary positive-displacement fluid-pressure motor which includes a casing, having an inlet port and an outlet port, at least two intermeshing rotors of toothed or lobed form housed for rotation in the casing, and means for conducting pressure fluid derived from the inlet port as far as a predetermined position in a face adjacent, and engaged by, a side of one of the rotors. A recess is so disposed in that face intermediate the predetermined position and the outlet port that, for facilitating starting of the motor when fluid under high pressure is directed into the inlet port, some of that fluid can gain access to the recess by way of said means and whichever intertooth or interlobe space of that rotor is so positioned as to place the means and the recess in communication.

This invention relates to fluid-pressure operable gear motors, having means 
to facilitate starting. 
Hitherto, in order to reduce frictional resistance, during starting of such 
motors, which otherwise occurs due to certain teeth or lobes of the 
intermeshing rotors thereof being urged by the pressure subsisting in the 
motor high pressure inlet port with substantial force against portions, 
opposite that port, of the respective adjacent cylindrical walls of the 
motor casing, means have been provided whereby fluid under high pressure 
is conducted to those portions to oppose that substantial force. 
In certain constructions such means have comprised what are termed 
"pressure field control grooves" which extend part-way around the 
peripheral edge of end plate means adjacent the respective rotor from the 
motor inlet port. It has been found in practice that if the extent of each 
such groove in the direction away from the inlet port is such that its end 
remote from the inlet port is at a position where the greatest benefit for 
opposing the said substantial force is attained, an excessive amount of 
flow is taken from the inlet port, with the result that the volumetric 
efficiency of the motor is seriously impaired. If, on the other hand, in 
the interests of maintaining an acceptable volumetric efficiency the 
groove is shorter, then the furthest position at which fluid pressure is 
applied in opposition to the said substantial force is not far enough from 
the inlet port to achieve a really adequate reduction in frictional 
resistance at the tips of the teeth or lobes of the respective rotor and 
thus it has been found that the motor, when under extremes of applied 
external loading upon the output shaft thereof, often stalls when an 
attempt to start the motor is made. 
The invention as claimed is intended to provide a remedy. It solves the 
problem of how to design a rotary positive-displacement fluid-pressure 
motor in which fluid under high pressure is conducted to such a position 
as adequately to oppose said substantial force thus significantly to 
reduce frictional resistance at the tips of the teeth or lobes and yet to 
avoid undesirable loss in volumetric efficiency of the motor. 
According to this invention a rotary positive-displacement fluid-pressure 
motor includes a casing, having an inlet port and an outlet port, at least 
two intermeshing rotors of toothed or lobed form housed for rotation in 
said casing, means for conducting pressure fluid derived from said inlet 
port as far as a predetermined position in a face disposed adjacent, and 
engaged by, a side of one of said rotors, and a recess so disposed in said 
face intermediate said predetermined position and said outlet port that, 
for facilitating starting of the motor when fluid under high pressure is 
directed into the inlet port, some of that fluid can gain access to said 
recess by way of said means and whichever intertooth or interlobe space of 
that rotor is so positioned as to place said means and said recess in 
communication. 
The advantage offered by the invention is that the fluid under pressure, 
which so gains access to said recess, at least momentarily provides a 
force in such a direction onto said rotor as, adequately, will oppose the 
substantial force applied directly onto said rotor from said inlet port, 
thereby reducing frictional resistance otherwise set up between the casing 
and those teeth or lobes of the rotor which are positioned in the vicinity 
of said recess, and thus avoiding stalling of the motor when an attempt to 
start it is made.

FIGS. 1 to 4 show a rotary positive-displacement fluid-pressure motor 1 of 
the gear type intended to be driven by fluid, for example hydraulic 
liquid, under high pressure derived from a suitable source (not shown). In 
its basic design the motor includes a casing 2, having an inlet port 3 and 
an outlet port 4, two intermeshing rotors 5, 6 of toothed form housed for 
rotation in the casing, and means 7 for conducting pressure-fluid derived 
from the port 3 as far as a predetermined position 8 in a face 9 disposed 
adjacent, and engaged by, a side 10 of one of said rotors. 
In accordance with the invention the motor is provided with a recess 11 so 
disposed in the face 9 intermediate the predetermined position 8 and the 
outlet port 4 that, for facilitating starting of the motor when fluid 
under high pressure is directed into the inlet port 3, some of that fluid 
can gain access to the recess 11 by way of the means 7 and whichever 
intertooth space 12 of the rotor is so positioned as to place the means 7 
and the recess 11 in communication. 
As shown, the means 7 comprises a groove which opens directly to port 3 and 
which so extends across the face 9 to the predetermined position 8 as to 
be in direct communication with those intertooth spaces 12 which at any 
instant are passing that groove. 
The face 9 is formed upon end plate means 14 suitably located in the 
casing. The end plate means is provided with a pressure-balancing 
arrangement (not shown) associated with that face 15 thereof remote from 
the rotors. This arrangement, of which there are many known examples, is 
intended to avoid inadvertent tilting of the end plate means and the onset 
of consequent premature wear of the sides of the rotors and of the 
adjacent faces of the end plate means otherwise experienced, during 
operation, in motors not so provided with a pressure-balancing 
arrangement. In order to achieve pressure-balancing the face 15 is so 
divided by suitable sealing means as to define areas thereof which are 
subjectable to low fluid pressures and areas thereof which are subjectable 
to high fluid pressures, the relative sizes of these areas and their 
disposition being so predetermined that the pressures acting thereupon in 
operation of the motor do so in opposition to the pressures which subsist 
in the working spaces of the motor and which are effective upon the face 9 
of the end plate means adjacent the rotors. The pressure-balancing 
arrangement is intended also to urge the end plate means into adequate 
sealing engagement with the rotors without the generation of undue 
friction between them. In this embodiment the end plate means 14 is common 
to the two rotors 5, 6, and a further common end plate means 16 is 
provided, being disposed on the other side of the rotors and in engagement 
therewith. This further end plate means is also provided with its own 
pressure-balancing arrangement (not shown) associated with that face 17 
thereof remote from the rotors. 
Also in this embodiment a groove 7 and a recess 11 are provided in the face 
9 in the manner shown adjacent the sides 10 of both of the rotors 5, 6 and 
likewise a groove 7 and a recess 11 are provided in the face 18 of the 
further end plate means 16 adjacent the other sides 19 of both of the 
rotors. 
Each of the grooves 7 is in direct communication with the inlet port 3, in 
the manner shown in FIGS. 2, 3 and 4, and is produced by milling away an 
appropriate arcuate length of the respective peripheral edge of its end 
plate means. Each of the recesses 11 is likewise produced by milling away 
a respective portion of the same edge. 
The rotors 5, 6 are each provided with shafts 20, 21; 22, 23 extending from 
both side thereof, these shafts being respectively supported in bushes 24, 
25; 26, 27 suitably retained in the casing 2. The shaft 23 extends to the 
exterior of the casing as shown, thus forming the output shaft of the 
motor. 
Both of the end plate means 14, 16 are each apertured to receive two of the 
shafts 21, 23; 20, 22 respectively, and each end plate means is generally 
of figure-of-eight shape, being housed in overlapping bores 28 and 29 
provided in the casing. 
In this embodiment the two recesses 11 adjacent the gear 6 are each one 
half a tooth pitch closer to the ends 8 of their respective grooves 7 than 
the two recesses 11 adjacent the gear 5, as shown in FIGS. 2, 3 and 4. 
When a fluid is supplied at constant pressure to the inlet port of a gear 
motor, the resulting torque output varies cyclically during rotation of 
the gears in the manner shown graphically in FIG. 5 which plots torque 
against rotation. If, when it is required to start a gear motor not 
embodying the present invention the gears happen to be meshing at the 
condition which results in, or approaches, the trough indicated at 30, it 
may prove very difficult to start the motor and in some cases starting may 
be impossible regardless of the magnitude of the fluid pressure applied at 
the inlet port. In the case of motors having pressure-balanced end plate 
means, but again not embodying the present invention, this problem can be 
aggravated by variations in the fluid-pressure loading upon the end plate 
means because that loading is reacted partly mechanically by the gears and 
partly by the pressure of fluid between, and under, the gear teeth. Due to 
the fluid pressure present in the inlet port, on attempting to start, 
certain of the gear teeth seal at their tips against those portions of the 
wall of the motor casing 2 opposite to the inlet port. Thus the build-up 
of the desired pressure field over the face of the end plate means 
adjacent the gears is impaired and a high mechanical friction load occurs 
due to the imbalance across the end plate means. With the meshing gears, 
just prior to attempting motor starting, at the position of minimum cyclic 
torque, the motor starting torque will therefore be low, or even zero. The 
dotted line 31 on FIG. 5 indicates the typical sawtooth friction torque 
curve resulting from the loading of the end plate means. The zone of 
intersection of this line just above the trough 30 of the cyclic curve 32 
indicates that such a poor starting torque condition occurs only over a 
small angle of rotation and shows that the starting torque is zero when 
the friction torque is greater than the hydraulic torque depicted by the 
curve 32. 
The provision of the recesses 11 in accordance with this invention is 
intended to overcome such starting difficulty, these recesses ensuring 
that such a condition of poor starting torque over that small angle of 
rotation is avoided. 
FIG. 3 shows the tooth 33 of the gear 5 in a position beyond the 
predetermined position 8 at the end of each groove 7 adjacent that gear 
and the tooth 34 in a position passing over that groove so that the space 
12 between the teeth 33 and 34 is sealed from each recess 11 adjacent that 
gear. Since, however, the teeth of the gear 6 are one half a tooth pitch 
out of phase, in rotation, with respect to the teeth of the gear 5 and 
since each recess 11 adjacent the gear 6 is disposed one half a tooth 
pitch closer to its respective associated groove 7, then, simultaneously 
at the condition shown in FIG. 3, the tooth 35 of the gear 6 is in a 
position beyond each recess 11 adjacent that gear and the tooth 36 is in a 
position passing over that groove 7 so that the space 12 between the teeth 
35 and 36 places each groove 7 adjacent the gear 6 in communication with 
its associated recess 11. 
This meshing condition of the gears 5 and 6 corresponds to the peak torque 
condition, or substantially so, indicated at 37 in FIG. 5, and thus if the 
gears are in this meshing condition when fluid, for example hydraulic 
liquid, under pressure is directed to the inlet port, there is no 
difficulty in the starting-up of the motor. 
If, however, when it is required to start the motor it so happens that the 
meshing condition of the gears 5 and 6 is one half a tooth pitch 
out-of-phase from the condition shown in FIG. 3, that is at the condition 
shown in FIG. 4, the motor is at the minimum torque condition indicated at 
the trough 30 of the curve 32 in FIG. 5 and but for the provision of the 
recesses 11 starting up of the motor would prove to be extremely 
difficult. In the manner shown in FIG. 4 the tooth 33 of the gear 5 and 
likewise the tooth 36 of the gear 6 are in registry at their tips with the 
respective recesses 11 while the tooth 34 (the tooth following tooth 33) 
of the gear 5 and likewise the tooth 37 (the tooth following tooth 36) of 
the gear 6 are still adjacent the respective grooves 7. Hence, the space 
12 between the teeth 33 and 34 places the associated grooves 7 in 
communication with the respective recesses 11 adjacent the gear 5, while 
the space 12 between the teeth 36 and 37 places the associated grooves 7 
in communication with the respective recesses 11 adjacent the gear 6. 
Thus, fluid under pressure derived from the inlet port 3 is applied to the 
gears as far around the periphery thereof as the tips of the teeth 33 and 
36, effecting pressure-loading on the gears sufficiently far around them 
towards the outlet port 4 as adequately to reduce frictional resistance at 
the tips of the teeth with the casing and also to avoid undue frictional 
resistance between the sides of the rotors and the faces 9, 18. In this 
way the undesirable condition as indicated at the zone 38 in FIG. 5 is 
avoided, thereby enabling the intermeshing gears then to commence their 
rotation with no likelihood of any delay in starting, abruptness in 
starting, or stalling. 
Although in the embodiment above-described with reference to FIGS. 1 to 5 
of the drawings the recesses 11 adjacent the gear 6 are one half a tooth 
pitch closer to their associated grooves 7 than those adjacent the gear 5, 
in the second embodiment of the invention and as shown in FIG. 6 the 
recesses 11 adjacent the gear 6 are spaced at the same distance from the 
associated grooves 7 as those adjacent the gear 5. Whilst this might have 
a slightly adverse effect upon motor start-up, it would still be an 
improvement over motors not so provided with such recesses and have the 
advantage from the production standpoint that "handing" of the end plate 
means in manufacture would be unnecessary. 
In the embodiment above-described with reference to FIGS. 1 to 5 of the 
drawings the motor is uni-directional in its operation in that since the 
grooves 7 open directly from the inlet port 3, the gear 5 can only be 
rotatable in the anti-clockwise direction and the gear 6 in the clockwise 
direction as viewed in FIGS. 2, 3 and 4. However, in the third embodiment 
as shown in FIG. 7 where it is desirable for the motor to be operable in 
either direction of rotation of the output shaft 23, the grooves 7 do not 
so open directly from the inlet port. Instead, each of these grooves is 
much shorter in arcuate length and a further one of said recesses 11 is 
provided on that side of each such shorter groove remote from the 
respective first recess 11, the arcuate spacing between each shorter 
groove 7 and its further recess 11, being the same as the spacing between 
that groove and the respective first recess 11. Thus, whichever of the two 
ports 3, 4 of the motor is chosen to be the inlet port the improved 
start-up facility is afforded for the resultant direction of rotation of 
the motor output shaft. However, since there is no direct communication 
between each said shorter groove 7 and whichever port is chosen to be the 
high pressure inlet port, in this embodiment high pressure fluid reaches 
that groove 7 either by way of gear tooth tip/casing clearances 
established at those teeth moving away from the inlet port, or, 
alternatively, by way of a suitably provided passageway 38 disposed within 
the respective end plate means, which directly connects the inlet port to 
the respective shorter groove. 
Further, although in the embodiment above-described with reference to FIGS. 
1 to 5 of the drawings the motor includes end plate means having 
pressure-balancing arrangements, in alternative embodiments of the 
invention no such end plate means are provided and the rotors of the motor 
bear at their sides directly upon the casing. In this case the said 
grooves and the said recesses are formed in the faces of the casing 
adjacent the rotors. 
Again, although in the embodiment above-described with reference to FIGS. 1 
to 5 of the drawings each of the two end plate means is common to the two 
rotors, in alternative embodiments of the invention each rotor has 
individual end plate means one on each side thereof. In this case the end 
plate means may be of D-shaped cross-section, the two on each side of the 
rotors thus having flats which are in abutment one with the other. 
Further, in other embodiments the end plate means may be so formed as to 
provide the entire bushings for the respective shafts. 
Finally, although in the embodiment above-described with reference to FIGS. 
1 to 5 of the drawings the said grooves and recesses are produced by 
milling away edge portions of each end plate means, in cases where the end 
plate means are cast, for example in aluminium, the grooves may instead be 
formed during the casting process. Also, instead of the grooves 7, as well 
as the recesses 11, being of the cross-sectional shape shown in FIG. 1 
they may be of any other desired shape, or simply formed by chamfering the 
respective edges of the end plate means.