A discussion of the evolution of log loaders couldn't possibly be complete without mention of the inventor of them, Leo Heikkinen, and the line of Prentice loaders that he developed before selling the business to Omark Industries.
In my youth logs were loaded with cables and rigging. It was common for loggers to erect an "A-Frame" (two leaning poles tied together at the top with the legs spread at the bottom, held up by a single guywire behind, and hopefully a loose 'snap guy' on the front to keep the poles from flipping over backwards if something broke), or a single leaning "gin pole" supported by 2 guy wires, and a snap guy.
The logs could be lifted onto the truck with a dozer or a donkey pulling the lifting line. Often 'end hooks' were used. These hooks stuck in the ends of the log thus kept the log level. Loading required usually 3 people. The machine operator would control the lifting cable, while a hooker would set the hooks. The truck driver stood on an always present platform on above the cab of the truck. Each hook had a tag line (a length of rope) on it. The tag lines would be used to help move the log into position on the truck and when the cable was released, then to pull the hook free. The truck driver managed the front hook from above the cab while the hooker worked from the ground.
Later loggers used a crane either with end hooks or two sets of tongs and a spreader bar in the same general fashion. Still later with the advent of mobile machinery, the cable heel boom came along. With the heel boom, a single set of tongs or a grapple was used, and the log was 'steadied' by heeling it under the boom. This implies that the tongs or grapple was set so the log was heavy to the front, and then as it was lifted, the back of the log would come up first hitting the bottom of the 'heel boom', and finally the front end of the log would come up, and the machine could then rotate and place the log on the truck.
The cable grapple was a step beyond tongs, as the grapple did not require a man to set it. The cable grapple was usually managed with 3 cables. Two of the cables would hang down from the end of the boom, the grapple would hang open if it was supported by one line, and closed if its weight was supported by the other line. The third line was not always present, but if used was a 'tag line' that ran out from the crane to the side of the grapple, and a tug on that would rotate the grapple, or if the log was just out of reach you might use it to pull the grapple toward the machine and then release it suddenly so it would swing out beyond the end of the boom. If all this seemed tedious, it was. With small numbers of large logs it wasn't too bad, but as the numbers of logs per load increased it all became maddeningly tedious.
It was perhaps no accident that the hydraulic loader was first developed for the pulpwood market which characteristically involved handling numerous small pieces. Early on they tended to be mounted on trucks, and grew into the logging business from the bottom up, first displacing other methods of handling small logs and gradually moving up the scale until all logs used hydraulic technology.