Several science fiction books picture bigger Terran planets with higher gravity. I'm curious to know what would be the maximum gravity mammals similar to the one on earth could survive. Only bones types and structures that exist in our animal world are allowed, no science fiction or magic (no adamantium bones)

A 10G planet seems tough. Elephants can weight up to 5000kg, but if they were smaller, their bones wouldn't be so tough. Therefore if gravity was 10G, I'm not sure whether a 10 times smaller elephant could still exist. Their bones would be too thin and crack under their own weight.

On the other hand, a 2G planet seems ok. Living there would be like constantly carrying a backpack of your own weight. Some very strong humans could probably survive there. We'd have lots of spinal and circulatory problems but eventually our muscles would grow stronger and we would barely cope with such a world. 

What do you think would be the limit?