Walk Behind Lawn Mowers

I bought a AWD mower (Model #: HU800AWD) about 4 years ago. I have a 3/4 acre zoysia lot and mow in normal mowing cycles throughout the mowing season. My problem is that the blade has broken off the pulley 3 times since I bought it. The last time was a few days ago. I've replaced the blade adaptor twice and the screw and washers each time. This is terribly dangerous obviously but I've had difficulty getting response from Husqvarna about if this is a normal problem or if my mower is defective. I'd like some answers and a resolution.