broken pulley?

i have a 77 chev small block with power stering and alt, two belt set up both go around harmonic and water pump then to ps or alt.. and i have chrome after market pulleys and im running a electric fan so i just have water pump pulley held on with bolts only no shims or spacer or washers.. and my questoin is the water pump pulley cracked and threw belt. whats making it do that? (it cracked around out side of bolts) Thanks

It sounds to me like the nuts came loose and the pulley started wobbling. You need to use lock washers behind the nuts to secure that pulley or they work their way loose. You may also want to check belt/pulley alignment and your harmonic balancer.

how do you check harmonic?