 |
TECHNOLOGY 24 |
drive&control |
Quiet, please!
Special tooth geometry reduces noise in external gear pumps. The SILENCE PLUS innovation offers numerous advantages in practice and can reduce system costs for customers.
by Christian Böhmcker, Marc Lätzel and Dietmar Schwuchow, Bosch Rexroth AG
|
For decades now external gear pumps have demonstrated their usefulness as rugged and economical workhorses. Their pri- mary features – pressures up to 280 bar, extremely high effi- ciency and low price – have come to be taken for granted. Their noise also became something of a given. But quieter pumps are |
provements through three central approaches: two-flank con- tact, helical gearing and eliminating the trapped oil cavity. Two-flank contact The gears in conventional external gear pumps make contact |
now available and the noise level has become an additional pur- during rotation – and thus form a seal – only at the leading flanks.
chasing criterion. Eliminating noise at the source can lower the The previous SILENCE pumps exhibit very close tolerances for
noise level throughout the hydraulic system. As early as 1999 shaft spacing and the tooth profiles. The result is zero play be-
Rexroth introduced its SILENCE pump, responding to demands tween the two engaging flanks. A further benefit is that the rear
for quieter gear pumps. This is a subject to which we, as the mar- flank is also involved in sealing and contributes to moving the
ket leader, have continued to devote attention. The result is fluid. Flow is significantly more uniform and pulsation is re-
something of a revolution: a quieter generation of external gear duced by about 75 percent. Less vibration and noise are in-
pumps, satisfying the following requirements: duced in the hydraulic system as a whole.
· Drastic reductions in noise and pulsation
· Displacement volumes of from 12 to 28 cm³ per revolution Helical gearing
· Cost-favorable In a first step we expanded the principle of two-flank contact by
· Identical mounting templates adopting helical gearing, a design commonly found in transmis-
· Technical specifications as close as possible to the SILENCE sion construction. This has considerable impact on the pump’s
pumps already on the market intrinsic noise since, due to the angular design of the teeth,
The major factors characterizing a gear pump are its intrinsic transferring the contact line from one pair of teeth to the next
noise, flutter in the pressure and the pitch of the sound gener- no longer occurs across the entire width of the gear at one time.
ated. In this development project we wanted to achieve im- This smoothes fluctuations in the forces being transferred.
01 | 2011 TECHNOLOGY
25
These events are distributed both spatially and across time,
making for quieter running in the gear set.
Eliminating the trapped oil cavity
Combining the two principles described was not enough to builda generation of external gear pumps where noise is drasticallyreduced. The development breakthrough was made when ob- serving yet another principle. In external gear pumps using conventional toothing there is continuous alternation between one and two points of contact. Achieving uniform flow makes it necessary to maintain contact – for a certain period of time – be- tween the previous pair of teeth while the next pair is engaging. But in the absence of other engineering measures closed, fluid- filled spaces, the “trapped oil cavities”, will form between these
|
lines of action on both the intake and the discharge sides. The changing volume of this space leads to rapid and severe rises in pressure. These, in turn, can induce vibration. Flow noises can also be generated when this cavity opens toward the low-pres- sure side. We used the sophisticated calculation tools available in our development setting to eliminate most of these vibration events. But there was always a bit left over. And thus we asked ourselves: “Why not develop a pump whose design principle eliminates the trapped oil cavity?” |
The new SILENCE PLUS external gear pump reduces noise by 15 dB(A) on average. |
SILENCE PLUS: The new generation of external gear pumps
We realized this by designing a pump incorporating helical gearing with no play. Its non-involute tooth profile eliminates the trapped oil cavities. The surfaces of the two gears are in
Time t
|
contact not only at the flanks of the teeth but also at the top and bottom. Thus there is no abrupt transfer of contact pressure |
0 |
from one flank to the next. Instead there is always just a single
Standard SILENCE PLUS
contact line and it moves continuously along a closed-loop
|
engagement pattern in the form of the figure 8. The axial forces impinging on the gears as a consequence of the helical toothing are absorbed by bearing elements. Hydrostatic compensation grooves help to absorb the additional forces – without wear. We first built design prototypes proving the validity of the concept. Then we assembled a team drawn from the testing, engineering, product management, manufacturing, planning, purchasing, quality assurance and controlling departments. Supported by extensive calculations and computer-aided proaches mentioned above. hydraulic simulation models, we merged the individual ap- The design used for SILENCE PLUS thus contains three essen- tial elements that together bring about significant noise reduc- tion: the zero-play drive concept, the helical toothing and the non-involute tooth profile. What practical advantages do these engineering features offer the user in regard to inherent noise, pressure fluctuations and the pitch of the sound generated? |
The 75 percent decrease in flutter also reduces vibration. The pump also emits a more pleasant sound, at a frequency which is 35 percent lower. |
|||
|
Options for using the SILENCE PLUS Noise reduction is particularly noticeable when an electric mo- tor is used to drive the hydraulic pump. Here hydraulic noise |
This novel profile gives continuous, zero-play contact between the teeth, without cavities that trap oil. |
|||
|
TECHNOLOGY
26 |
drive&control |
dominates and is similar in pitch to that of an electric motor. The blower drive on agricultural seeders, for example, repre-
Profiting in particular from noise reduction are those who use sents a noise load for the operator and one that can be mitigated
industrial machine tools and mobile conveyor technology. And with the use of the SILENCE PLUS.
all applications involving mobile power packs, such as tail lifts
on supply trucks, are significantly quieter. Less noise, lower costs
Since the intrinsic noise generated by the SILENCE PLUS units Since the problem is attacked at the source, manufacturers can
|
is almost inaudible at low pressures, the pump can readily be recommended for use in all kinds of lubrication, filtering and cooling circuits. The noise reduction also unfolds its full effect when lubricating transmissions in vehicles. |
dispense with secondary measures for acoustic decoupling, in- sulation and encapsulation. Fewer components are required and assembly effort is lessened. These design simplifications result in cost reductions for the system as a whole. |
Pumps powered by electric motors, with virtually no hydraulic The multitude of potential applications illustrates the extent to
noise, are predestined for use not only in manufacturing plants which hydraulic noise has crept into everyday life. Today we
and warehouses but also in supermarkets and beverage shops. may have to accept some of that as an “immutable fact” – but we
Domestic applications such as passenger lifts, parking lifts and won’t have to put up with it tomorrow. New technology for ex-
log splitters profit from the development. Trash presses behind ternal gear pumps points toward a future with (almost) silent hy-
supermarkets and hotels can be operated all day and it is ac- draulic systems.
ceptable for forklifts to load and unload even at night.
In the case of diesel-powered drives it is the internal combus-
tion engine itself that makes the most noise. At first glance,
attenuating hydraulic noise would seem to make little sense.
But the pump, too, produces a considerable amount of sound.
And this sound is in a higher frequency range so that it can be
Authors: heard above the blend of lower-frequency sounds. The pitch of
Bosch Rexroth AG, Schwieberdingen, Germany
|
the intrinsic noise of the SILENCE PLUS, owing to the smaller number of teeth, is 35 percent lower. The human ear senses this as a significantly less intrusive, far more tolerable “hum”. Neighbors previously annoyed by noise from construction sites or garbage trucks in the early morning hours profit from such advances. The sanitation crew and the driver also appreciate the far lower noise level. Direct airborne sound resulting from the pump’s intrinsic noise can be of some relevance for diesel-powered machinery. |
www.boschrexroth.com Dipl.-Ing. MBA Christian Böhmcker, Product Management |
Dr. rer. nat Dr.-Ing.
Marc Lätzel, Dietmar Schwuchow,
Development Project Manager
280 bar Acoustic pressure at 1 m 80
50 bar 70
60
50
40
|
30 20 |
Rotation speed |
The significantly lessened noise level at the SILENCE PLUS in comparison to a conventional external gear pump. |
0 500 1000 1500 2000 2500 3500 3000
Standard 16 cm3 / r.p.m. SILENCE PLUS 16 cm3 / r.p.m.
sp_professional_article_quiet