Grease nipples are permanently installed to machinery via a threaded connection. The grease gun connects to the nipple connection on the grease fitting. A small captive ball inside the grease nipple is pushed back against its retaining spring caused by the pressure of the grease gun, thus allowing the flow of grease to the bearing. The ball essentially acts as a valve for the passage of grease. The ball returns to its closed position when the pressure stops. The ball also acts as a check valve to prevent the leakage of grease and contamination from flowing to the bearing.