In Sword of the Valiant, the knights of Camelot use a crane to hoist young Sir Gawain onto his horse before he rides off for adventure. In criticisms of the Middle Ages, one common comment of derision is that by the end of the period, armor had become so heavy and elaborate that cranes were needed to hoist knights onto their horses. This criticism usually implies that the nobility had nothing better to do than make their armor look fancy while the peasants died in droves fighting their lords’ wars because they couldn’t afford any armor (another misconception). It is also used to make fun of the stupid knights who wore such armor because if they landed face-down in a puddle, they would be unable to lift themselves and would drown (I always wonder why these people don’t consider that such a knight could just roll over).
Evidence for cranes being used to hoist knights onto horseback is sketchy, if it even exists at all. The context in which cranes may have been used is quite limited. Armor used for jousts in tournaments could be heavier than armor used for battle. The extra metal which would have been unrealistic for war protected knights from injury since tournaments were a sport. Jousting injuries were common—fatalities were not. A knight in jousting armor could have had difficulty mounting on his own, but he could have mounted with assistance. I recall once seeing a drawing of a small movable staircase that served the same function as a crane—assisting a knight to mount in heavy jousting armor—but as I have not found this piece of art again, I can’t vouch for its authenticity.
Armor for war was lighter than jousting armor—the same weight as, if not a bit lighter than, the weight carried by modern-day soldiers going to battle. Though a knight might go into real battle on horseback, he could not count on staying there, so he needed to be able to fight on foot if he was unhorsed or if his horse was killed (horse armor was so heavy as to be impractical). Jousting armor mainly protected the areas of the body at danger from a lance strike, but war armor needed to protect the body from all kinds of attacks. Jousts of peace could persist because the risk of injury was low enough to make the sport worthwhile. A joust on the battlefield was nearly always fatal to the loser.
As before, I have no citations above because this information comes from years of research in medieval literature, reading the footnotes carefully, and sometimes, from the introductions of said books. However, if you need a quick reference, the Metropolitan Museum of Art has a fabulous FAQ on myths about armor.