Grease Nipples
Grease nipples are small but very important fittings used in automotive and other mechanical systems to enable lubricant to be fed into the system. As the name suggest, the lubricant is usually grease.
Grease nipples contain a sprung ball bearing inside the nipple body. When a grease gun is connected to the grease nipple and pressure applied, the bearing compresses to allow the lubricant to flow into the joint.