My nephew Zander Woofenden is into (among many other things) sheep shearing in a big way.
His rig is driven by a large electric motor, which needed a fairly sophisticated adjustable bracket to mount to his portable fencepost (a chunk of 2x6).
He spent a few hours last week with me, designing and building a suitable contraption. It's difficult to see how it works, but the short version is that a motor sits on a frame that adjusts up and down, the motor drives the long power shaft that runs the shears.