Rigid couplings connect rotating members such as shafts and transmit torque and motion. They permit neither axial nor radial relative motion between the shafts of the driver and driven unit. When the two shafts are connected solidly and properly, they operate as a single shaft. A rigid coupling is primarily used for vertical applications like vertical pump. Most rigid couplings are made of aluminum, steel, or stainless steel. Shrink discs are rigid couplings with two external rings that tighten together axially and exert inward force on a third ring via tapered surfaces. This closes the inner ring onto the shafts and provides a secure connection. Rigid couplings with keyways include keys that can be used to stop the rotation of the shaft couplings. The couplings require accurate machining to avoid a potentially catastrophic misalignment.