Self Locking Gear

Good day Sir,

In the 150 tonne hydraulic crawler crane, to lifting the boom and load, they use pump to drive motor. As motor rotated, it make the worm gear rotate together. and the spur gear rotate too. So the winch rotate and the boom is lifted. I know that the worm gear can be Self Locking as the lead angle of the worm is less than the friction angle. But when I remove the motor out of system,heavy boom make the spur gear rotate, and incrediblly the worm gear rotate too. I mean the worm gear CAN NOT Self Locking. So please help me to solve this probem, I think,after long time in service. The worm gear loss ability of self locking due to wear.SO the coefficient friction between the worm and the gear be changed. Is it possible ? Thanks a lot