Kelly bushings are mechanical devices used in the drilling industry to connect the rotary table to the drill pipe during drilling operations. They provide a means to rotate the drill string and transfer torque from the rotary table to the drill pipe. While there are different variations and designs of kelly bushings, the main types include: Square Drive Kelly Bushing: This type of kelly bushing has a square-shaped opening that matches the cross-sectional shape of the kelly. It is the most common type used in drilling operations. The square drive kelly bushing typically has four pins that engage with corresponding holes in the kelly, providing a secure connection. Hex Drive Kelly Bushing: Hex drive kelly bushings have a hexagonal-shaped opening that matches the shape of the kelly. Similar to the square drive bushing, it typically has six pins that engage with the kelly. Hex drive bushings are less common than square drive bushings but are used in some drilling applications. Master Bushing: The master bushing is a specialized type of kelly bushing that is typically larger in size and designed to accommodate various sizes of square or hexagonal kellys. It provides versatility by allowing different sizes of kellys to be used with the same drilling rig. The master bushing has multiple sets of pins or inserts that can be changed to match the size and shape of the kelly being used. Split Bushing: Split bushings are used when a kelly or drill pipe needs to be replaced without removing the entire drill string from the wellbore. They have a split design that allows them to be opened and closed around the kelly or drill pipe, providing a temporary connection. This type of bushing is useful in situations where a damaged kelly or drill pipe needs to be replaced quickly. These are the main types of kelly bushings commonly used in the drilling industry. The specific type used depends on the drilling rig and the requirements of the drilling operation.