I'm going to disagree with a lot of people in this thread. A lot of aikido techniques, even when done properly, do inflict pain on the uke. That is not to say that goal of the nage ought to be to inflict pain, but I would think there is something wrong if I spent half-an-hour practicing sankyo and felt no pain at the end of it.
Matthew, I have a teacher, a seventh dan Japanese, who has a sankyo that will put you on the mat every single time, no matter how I resist. But it doesn't hurt, it just breaks your posture and you go down. If we did it for half an hour, the result would still be the same—Raul on the mat, no pain. There is the potential for pain—you know that if you do not move with his movement properly, then it will hurt. But it never actually does; he moves just a tiny sliver ahead of your capacity to follow the waza, so that the sharp edge of the technique is always present, even if it never actually cuts you.
One day, my friends and I hope to have a sankyo that's half as good as his.