Catamount

95 3vze 4x4 pick up. The adjustment bolt on the AC compressor Idle Pulley keeps vibrating loose about every three months. I loosened or maybe removed it last winter to remove the drive belt while doing a water pump and timing belt. My Haynes manual doesn't indicate a lock washer, lock nut or set screw nor do I recall any. Seems thread lock and or a lock washer would fix it but want to be sure this isn't an indicator something more serious is wrong? Any advice would be appreciated.