The travel nut on LA500DC and LA500DCS gate operators may break when excessive force is applied to the gate such as being struck from a vehicle.
The travel nut can be identified as the white plastic nut that attaches to the internal lead screw of the arm.
The symptom of this problem is that the trolley assembly slides back and forth along the guide slots without mechanical restraint.
LiftMaster has created Trolley Service Kit K75-39337 with all the necessary parts and provides instructions on how to replace a damaged travel nut
within an LA500 arm
Upgrade option when buying a truck door aka Positive Stop Assembly
This is installed onto the panel when the door is opened it is engaged and will keep the door open above the header so there is no hang down of the door.
The door will stay open above the header to eliminated the door being damaged.