NRS - System

NRS-System for noise level reduction on hydraulic equipment

Standard bellhousings available on today’s market are generally made of metal. Rising costs have meant that the cast steel originally used has now been replaced by aluminium. To save material, the wall thickness – especially in the case of cast aluminium – has been reduced more and more, with the load level remaining constant.


This reduced wall thickness means that there is only minimal absorption of the noise made by the coupling in the bellhousing. Unlike their cast steel counterparts, the more lightweight aluminium bellhousings have comparatively poor damping characteristics.


The pulsation of hydraulic pumps generates very strong vibrations that are not damped sufficiently by an aluminium bellhousing. Moreover, at rigid bellhousings there is no isolation of mechanical vibration between the electric motor and the hydraulic pump due to the lack of damping characteristics.

Vibration studies have revealed that airborne and structure-borne vibration is at frequencies where human hearing is at its most sensitive. Under certain unfavourable circumstances, the thin-walled aluminium bellhousing amplifies the running noise of the pump even more; in these cases, the bellhousing resonates in sympathy, similar to a real bell, which can then lead to an additional increase in oscillation amplitude.

The NRS-solution

In line with EC Directive “Noise“ (2003/10/EG), the planned retrofitting measures are intended to resolve acute problems caused by poor soundproofing and vibration damping, including problems
that occur in hydraulic apparatus already in use. The best noise level reductions were achieved using the NRS System.

The best noise level reductions were achieved using the NRS System.
The NRS System is:

With the NRS System, it was possible to reduce the total noise emission level of a cast aluminium bellhousing in critical frequency ranges by more than 10 dB. This reduction is clearly audible to the human ear. The graph shows examples of frequency ranges of a standard cast aluminium bellhousing and a version with NRS.

Technical data