This invention relates to viscous shear dampers.
Dampers of this type consist of an annular seismic mass or inner member enclosed in a ring-shaped casing. The seismic mass is separated from the casing by peripheral and lateral gaps or clearances which are filled with a viscous fluid. The casing is fixed e.g. on the free end of the crankshaft of an internal combustion engine the vibrations of which have to be damped. Upon rotation of the engine shaft and, thus, of the casing, the seismic mass is acted upon only by the viscous torque transmitted by the fluid filling. The significance of such "untuned" dampers resides in that they lower the value of the natural frequency of the engine system while reducing the vibration amplitudes. Viscous shear dampers are discussed in details e.g. in "A Handbook on Torsional Vibration" compiled by E. J. Nestorides, Cambridge at the University Press, 1958, hereby incorporated by reference.
Ordinarily, the viscous fluid is a silicone fluid or oil the lubrication effect of which is re-relatively small. Therefore, it is a tendency in the art to restrict a direct contact of the casing and the inner member as far as possible. To avoid rubbing and pitting, phosphor-bronze rubbing pads have been fitted on the outer peripheral surface of the seismic mass. A simpler solution consists in providing the seismic mass with a central location in the form of a non-ferrous (e.g. phosphor bronze) bush, fitted over the inner wall of the casing. In view of the possibility of axial vibrations such bush may also be designed with thrust surfaces for end location. Another known expedient is to use a bush pressed into the seismic mass and slightly clear of the casing. Further alternative arrangements include the provision of lateral locating rings of bronze inserted on the seismic mass. It is generally considered that a fairly massive section of metal is needed immediately inside the periphery of the casing to give good support to the bearing in cases where the seismic mass runs on its inner bore. It is usual for the bronze locating bush to be a tight press fit onto the inner surface of the casing in order that expansion due to heat may not tend to free the bush.
The above described arrangements permit to obtain fully satisfactory results and are extensively used. Moreover, they have proved satisfactory in operation over long periods. Nevertheless, correct fitting of pads has been found an expensive and laborious procedure. Broadly, the same applies to bushes and lateral locating rings.