The viscous coupling has a hub and housing with a disc space between them. One set of discs is attached to the housing and one set is rotationally attached to the hub. The second set of discs can move axially and is provided with radial slots (42) bounded by lipped profiles. The lip thickness is largest near the hub and tapers towards the edge of the discs. The lip profiles ensure that the discs first make contact near the hub and with increasing pressure difference between the hub and the shaft the discs are pressed further together with a progressive contact along the tapering lipped areas. The discs rotate freely at small differences in the speeds of the shaft and the housing and are progressively pressed together as the relative speeds increase.
