Why doesn't my washing machine spin during the spin cycle?

My washing machine won't spin during the spin cycle. It seems to run fine for the rest of the washing cycle but when it gets to the spin cycle, the tub just stall and there is a faint smell of burning rubber. In the past I have been able to get it going by spinning the tub by hand, and then the tub would take off but that seems like it no longer works. I have a Whirlpool Estate washing machine Heavy Duty, Super Capacity, 8 Cycle, 4 speed combination, and it is about 4 years old.

Model # SM 8280867