Hydrodynamic coupling

A hydrodynamic coupling comprising coaxial rotary bladed impeller and runner elements defining a toroidal working chamber for working fluid which transmits torque from said impeller element to said runner element, said chamber having a radially outermost region and the magnitude of transmitted torque being dependent on the degree of filling of said chamber with fluid, said torque increasing in response to increasing ratio of RPM of said impeller element to RPM of said runner element whereby the pressure of fluid which circulates in said chamber increases abruptly as a result of abrupt increase of said ratio;

• and a casing element having at least one section rigid with one of said first mentioned elements and surrounding the other of said first mentioned elements, at least one of the elements including said casing element and said other element defining a compartment in communication with said outermost region of said chamber so that the liquid level in said compartment normally equals approximately the liquid level in said chamber but the liquid level in said compartment advances toward the common axis of said impeller and runner elements in response to abrupt rise of fluid pressure in said chamber, said coupling having at least one opening communicating with said compartment radially inwardly of said outermost region to discharge at least some of the fluid whose level advances in said compartment radially inwardly in response to said abrupt rise of fluid pressure in said chamber and to thereby limit the magnitude of said torque on abrupt change in speed of said impeller element, especially an abrupt increase in RPM of said impeller element.