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 Rising costs have meant that the cast steel originally used has now been replaced by aluminium.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.