The
cable pulleys (also referred to as a sheave or pulley wheel) can
commonly be found on the garage door extension spring setup. The
extension springs are on either side of the door and stretch back over
the horizontal tracks. The door cables attach to the bottom of the
door, go up over the stationary pulley and back over the pulley with
clevis attached to the extension spring.
Over time the pulleys
can split or break causing the door cable to get caught in the pulley.
This will cause the garage door to not open or only open part way. Check
over all four of the pulleys. If you find one bad pulley it's
recommended to replace all four of the cable pulleys since they are most
likely the same age.