For decades external gear pumps have demonstrated their usefulness as rugged and economical workhorses. Their primary features – pressures up to 280 bar, extremely high efficiency and low price – have been taken for granted. Noise became synonomous with pumps, but quieter pumps are now available, making noise level a purchasing criterion. Eliminating noise at the source can lower the noise level throughout the hydraulic system. In 1999 Bosch Rexroth introduced its Silence pump, in response to demands for quieter gear pumps. Bosch Rexroth has focused their attention on producing a quieter generation of external gear pumps, satisfying the following requirements:

  • drastic reductions in noise and pulsation

  • displacement volumes from 12 to 28 cm3 per revolution

  • cost-effective
  • identical mounting templates
  • technical specifications as close as possible to the Silence pumps already on the market


The major factors characterizing a gear pump are its intrinsic noise, flutter in the pressure and the pitch of the sound generated. In this development project we aimed to achieve improvements through three central approaches: two-flank contact, helical gearing and eliminating the trapped oil cavity.

Two-flank contact

The gears in conventional external gear pumps make contact during rotation – forming a seal – only at the leading flanks. The previous Silence pumps exhibit very close tolerances for shaft spacing and the tooth profiles. The result is zero-backlash between the two engaging flanks. A further benefit is that the rear flank is also involved in sealing and contributes to moving the fluid. Flow is significantly more uniform and pulsation is reduced by about 75%. Less vibration and noise are induced in the hydraulic system as a whole.

Helical gearing

In an initial step we expanded the principle of two-flank contact by adopting helical gearing, a design commonly found in transmission construction. This has considerable impact on the pump’s intrinsic noise since, due to the angular design of the teeth, transferring the contact line from one pair of teeth to the next no longer occurs across the entire width of the gear at one time. This smoothes fluctuations in the forces being transferred.

These events are distributed both spatially and across time, making for quieter running in the gear set.

Eliminating the trapped oil cavity

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 – between 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-pressure side. We used the calculation tools available in our development setting to eliminate most of these vibration events, but there was always a bit left over. So, we asked ourselves: “Why not develop a pump whose design principle eliminates the trapped oil cavity?”

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 contact at the flanks of the teeth, the top and bottom. Thus, there is no abrupt transfer of contact pressure from one flank to the next. Instead there is always just a single contact line and it moves continuously along a closed-loop engagement pattern. 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. Using calculations and computer-aided hydraulic simulation models, we merged the individual approaches mentioned.

The design used for Silence Plus contains three elements that together bring about significant noise reduction: the zero-backlash drive concept, the helical toothing and the non-involute tooth profile.

Options for using the Silence Plus

Noise reduction is particularly noticeable when an electric motor is used to drive the hydraulic pump. Here hydraulic noise dominates and is similar in pitch to that of an electric motor. Profiting in particular from noise reduction are those who use industrial machine tools and mobile conveyor technology. And all applications involving mobile power packs, such as tail lifts on supply trucks, are significantly quieter.

Since the intrinsic noise generated by the Silence Plus units 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.

Pumps powered by electric motors, with virtually no hydraulic noise, are predestined for use not only in manufacturing plants and warehouses, but in supermarkets and beverage shops. Domestic applications such as passenger lifts, parking lifts and log splitters profit from the development. Trash presses behind supermarkets and hotels can be operated all day and it is acceptable for forklifts to load and unload even at night.

In the case of diesel-powered drives it is the internal combustion engine that makes the most noise, but the pump too, produces a considerable amount of sound. This sound is in a higher frequency range so that it can be heard above the blend of lower-frequency sounds. The pitch of the intrinsic noise of the Silence Plus, owing to the smaller number of teeth, is 35% lower. The human ear senses this as a less intrusive, far more tolerable “hum”. Those previously bothered 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 from the pump’s intrinsic noise can be of some relevance for diesel-powered machinery.

The blower drive on agricultural seeders, for example, represents a noise load for the operator and one that can be mitigated with the use of the Silence Plus.

Less noise, lower costs

Since the problem is attacked at the source, manufacturers can dispense with secondary measures for acoustic decoupling, insulation and encapsulation. Fewer components are required and assembly effort is lessened. These design simplifications result in cost reductions for the system as a whole.

The multitude of potential applications illustrates the extent to which hydraulic noise has crept into everyday life. New technology for external gear pumps points toward a future with (almost) silent hydraulic systems.