The rainbird company and sprinkler professionals recommend that when a rainbird 5000 rotating sprinkler head stops rotating that it has reached the end of its expected and designed lifetime and that it should be replaced. These sprinklers are not designed to be repaired when and if the gears become stripped.
I had three rainbird 5000 rotating sprinklers that had stopped rotating. Instead of replacing the heads, I fixed 2/3 by tapping/light striking on the head with a rubber mallot while the sprinkler was in operation. 2/3 sprinklers rotate perfectly again. The third ratates intermittently.