A woman is fixing a hay bailer when the implement jumps and her are is pinched in the gears of the machine. She is stuck until their bull happens along. She taunts the animal to get him to charge and jar the implement and free her. I'd guess it was from the early to mid eighties

The River (1984)
Starring Sissy Spacek, Mel Gibson
IMDb review excerpt:
Meanwhile, "back on the farm", Mae has her own problems in a scene that still grips my gut to this day: Fixing a piece of farm equipment hundreds of meters away from the farmhouse, she gets her arm trapped in a cog-and-chain and is unable to free herself, plus she starts bleeding from the wound, really badly. I wasn't expecting such a nail-biting scene in such a seemingly placid film. It was really well done.
The River (1984) - IMDb (http://www.imdb.com/title/tt0088007/)

Unfortunately, I couldn't find anything that mentions how she escapes, which is the way you describe.