This is a specialty part used by the vacuum pump on a Viper 8000 A/C Service Station. The plastic coupling on the Viper 5 CFM vacuum pump is prone to breaking after a long lifetime of use or if the pump becomes hydro-locked with oil. To make matters worse; finding replacement parts for these pumps is near impossible with an alternative of spending $700 on a new vacuum pump. This part slips onto a pair of gears (one on the output shaft of the electric motor and the other on the input shaft of the vacuum pump).This coupling locks the two together allowing the motor to drive the pump. These pumps are very expensive and parts are near impossible to find.