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?”