Note: Descriptions are shown in the official language in which they were submitted.
111~3989
The invention relates to a gear pump containing driving and
driven gears and a multi-section housing with end plates enclosing the gears
at the sides. Such gear pumps are well-known and find general application~
These pumps are characterized by their pulsating delivery rate,
which is determined by the geometry of the gearing. The relevant literature
indicates the coefficient of delivery rate pulsation as being the degree of
irregularity ~, which is defined as follows:
= Q max - Q min = ~Q
Q mean Qo
where: Q = delivery rate
The delivery rate pulsation brings about a pressure pulsation~
being for instance, the result of a pressure reductîon through a pressure
regulating valve. A distinct disadvantage of this pressure pulsation is the
noise generation not only in the pump and pressure reducing valve but through-
out the hydraulic system. Further disadvantages are the oscillations caused
by the pressure pulsations in specific parts of the system, natural fre-
quencies of oil heads and intermittent seal loadings.
The degree of irregularity in gear pumps cannot be reduced at
will. The selection of favourable gearing parameters can bring about a
reduction in the degree of irregularity to 2% and more, especially with
internally geared pumps. Further Teductions can only be made within very
narrow limits, since economic considerations, such as output capacity, must
be taken into account.
;~
Whereas the delivery rate pulsations of gear pumps are determined
by the geometry of the gearing and the initial pressure, th;e pressure pulsa-
tions resulting from the delivery rate pulsations through a pressure regulat-
ing val~e are proportional to the static pressure in a system free of natural
frequencies. The formula below applies:
` Pw = 2 po ~ -
where: Pw = amplitude of the pressure pulsations
Po = static pressure.
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For instance, with a static pressure of 100 bars and ~ - 2%, the
amplitude of the pressure pulsations is 4 bars, already giving rise to a
considerable noise level.
In contrast the internal leakage in pumps with clearances that
remain constant is proportional to the initial pressure. Leakage pulsations
give rise to pressure pulsations through a pressure reducing valve, increasing
by the square of the static pressure.
Upon initia~ion of a leakage pulsation having the same amplitude at
the selected pressure Pn (e.g. nominal pressure) as the geometric delivery
rate pulsation, being out of phase with it and showing the same wave shape,
the delivery rate pulsation resulting at the delivery end o~ the pump is
zeroed at static pressure Pn with the pressure pulsation disappearing at the
same time. Upon exceeding pressure Pn, the delivery rate pulsation and with
it the pressure pulsation increase again.
A significant reduction in the pressure pulsations may, even though
to a differing degree, be expected over the entire pressure range.
The object of the invention is to design a gear pump to bring about
a reduction in the pressure pulsations caused by the delivery rate pulsations
and in the noise levels.
According to the present invention there is provided a gear pump
comprising? a housing with inwardly facing end plates, meshing, driving and
driven gears mounted in said housing between said end plates for rotation
about respective axes, the gears being formed with peripheral gear teeth,
each tooth including a working surface which engages a working surface of a
; tooth on the other gear along a line of contact that travels, during operation
of~the pump, along a line of action intersecting the contacting driving and
driven flanks, the gears having pitch circles that are mutually tangent at
a pitch po mt, and at least one of the elements comPrising the working
surfaces of the driving gear teeth, the working surfaces of the driven gear
teeth and said end plates being formed with recess means providing a leakage
path permitting leakage past the line of
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B
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contact between each pair of engaging working surfaces, the recess means being
configured such that the size of the leakage path varies as the line of con-
tact moves along the line of action from substantially zero at the beginning
of the line of action to a maximum at the pitch point and then decreasing
continuously to substantially zero at the end of the line of action, so as
to provide a pulsating oil leakage which constitutes a means for compensating
for delivery pulsations in the output of the pump.
The "working surfaces" of the gear teeth are those surfaces that
engage one another during normal operation to transmit force and motion from
the driving gear to the driven gear. The opposite surfaces are "non-working
surfaces" and are not required to engage one another for normal driving of
the set of gears. Where back lash is present, the non-working surfaces will
not engage during normal operation.
Consequently, a pulsating leakage can optimally be produced in a
simple manner and superimposed upon the geometric delivery pulsation, thereby
bringing about either partial or total compensation or even over-compensationO
The geometric delivery pulsation is sinusoidal according to the
; following formula for the involute-tooth system:
V0 = b2m [21 Z~ Z-2) ( ~ 4 ) ],
where: b ~ gear face width
.~ .
m ~ pitch coefficient
Zl ~ number of teeth of the driving gear
. .
Z2 ~ number of teeth of the driven gear
= static pressure factor
:
~ = angle of engagement
` ~ Yl F angle o rotation
The delivery rate attains its maximum at the origin, where the
angle of rotation is ~l - 0; in other words, where the point of engagement
~ .
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of the teeth is located at the pitch point, i.e. along the line containing
the centres of rotation of both gears. The minimum value is present when a
pair of tooth surfaces commence or terminate engagement. The recess or
recesses may then be configured such that the maximum value of compensating
leakage is attained at the origin ~ ~1 = 0), with zero leakage compensation
when the gears commence or terminate engagement. (For reasons of simplicity
in representation, the cover is assumed to be zero.)
According to a preferred embodiment of the invention, the recesses
are located along the lines of steepest gradient on the tooth working sur
faces of at least one of the gears. In another embodiment the tooth working
surfaces of at least one of the gears are provided with chamfers either on
one or both sides.
In the accompanying drawings which illustrate exemplary embodiments
of the present invention:
Figure 1 illustrates a first embodiment of the invention;
Figure 2 is a detail of Figure 1, as a perspective representation;
and
Figure 3 i}lustrates the inwardly facing surface of an end plate
of the housing of a further embodiment of the invention.
As represented in Figure 1, the gear pump comprises a driving gear
`:
'~ 1, a driven gear 2 and a housing, not represented, enclosing both gears.
, ~ .
Driving gear 1 rotates in the direction of arrow A when in operation and has
a tooth 3, which has a groove-like recess 5 along the line of steepest
gradient on the working surface 4O Recess 5 has a varying cross-section
which is~maximum at the pitch circle, and decreases a~ zero at either end,
within the working depth of the toothD
As opposed to the illustrated embodiment in which the tooth working
surfaces of only one gear are provided with recesses, the working surfaces of
both gears 1, 2 may be provided with recesses. According to a further embodi-
; 30 ~ ment, again not represented, the recesses may also be chamfering on the tooth
3~
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1l~3989
surfaces, specifically on one or on both sides of one or of both gears.
By suitable dimensioning of the recesses of variable cross-section
the leakage past the line of contact may be controlled and to a great extent
adapted in amplitude and wave shape to the delivery pulsation. The leakage
is restricted by the open cross-section at the sealing point, i.e. along
the line of contact separating the suction and pressure sides.
In a further embodiment, represented in Figure 3, one of the two
end plates 6 contains a recess 8 along the line of action 7 of the gearing.
The cross-section of the recess can also in this case be varied within wide
limits and adapted to the delivery pulsation, thereby circumventing the
sealing point, i.e. the line of contact separating the suction side 9 from
the pressure side 10 with an oil-leakage recess of varying cross-section.
Needless to say, the volumetric efficiency drops as a result of
the pulsating oil leakage in addition to the oil leakage normally present.
With losses of this efficiency being only negligible, the measures that have
~ been described above may be put into effect solely on those gearings having a
; low degree of irregularity. This applies to optimized inner gearings in
particular. The efficiency decreases by a slight a~ount not greater than
1%, which is of no practical consequence.
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