An injection system for use in injecting a liquid into a shaft, the system comprising: (A) a liquid supply assembly (10) comprising: a liquid supply inlet (34) for supplying the liquid to the assembly at ambient pressure, an electric pump (68) to pressurize and discharge the liquid in the assembly at a first pressure above ambient pressure through a liquid supply outlet (36), a pressure switch (72) connected to the supply outlet and in electrical communication with the pump to selectively drive the pump when the pressure of the liquid supply outlet falls below said first pressure； (B) an injector gun (40) comprising: an injector inlet (41) in fluid communication with the liquid supply assembly to receive the liquid from the liquid supply assembly at said first pressure； an injector outlet (45) to discharge the liquid from the injector assembly； an activator assembly (56) that is in fluid communication with the injector inlet and the injector outlet and can be moved from a first position to a second position； (C) a liquid dosing assembly (46) to provide a metered dose of the liquid to the shaft comprising a first chamber (50) to be alternately in fluid communication with the injection inlet and the injection outlet； and a second chamber (48) cooperatively associated with the first chamber that is adapted to contain a gas under a second pressure above the ambient pressure that deflects the first chamber so that the volumes of the first and second chamber change in proportion inverse one with respect to the other where the first chamber is to alternately receive the measured dose of the liquid from the injector inlet to said first pressure when the activator is in the first position, and provide the measured dose of the liquid to the outlet to said second pressure when the activator is in said second position； and wherein the pressure switch is configured to drive the pump when the first pressure is greater than the second pressure and / or corresponds to a state where
