Apparatus and method for an improved driveline coupler having a reversible saddle thereon which allows it to be configured to work with different sizes and shapes of shafts. The reversible saddle is configured on one side to work with one size of driveline shaft and configured on the opposite side to work with a different size driveline shaft so that, in the field, an operator can modify the driveline coupler from use with a first size of drive shaft to a second size of drive shaft easily and quickly by turning the saddle upside down.

Apparatus and method for an improved driveline coupler having a reversible saddle thereon which allows it to be configured to work with different sizes and shapes of shafts. The reversible saddle is configured on one side to work with one size of driveline shaft and configured on the opposite side to work with a different size driveline shaft so that, in the field, an operator can modify the driveline coupler from use with a first size of drive shaft to a second size of drive shaft easily and quickly by turning the saddle upside down.

A linkage system for transmitting torque includes a central element, an input arm and an output arm. The central element has four sides and a top and bottom. The input arm and an output arm each have a head portion for connecting with the central element and a body portion. The head portion of each of the input and output arms defines a bearing surface having a first radius. The input and output arms engage the central element such that the bearing surfaces of the heads are offset by 90 degrees.

A double knuckle coupling suitable for transmitting torque applied to a first input shaft to a second shaft wherein the coupling accommodates angular changes between the shafts. Additionally, the disclosure describes an improved mud motor transmission incorporating the coupling.