PURPOSE: To prevent the fall of a reduction gear and to improve durability and maintainability by a method wherein a rollable ball is arranged to a tip part to bring an inclination preventing member, making contact with the back of the gearing part of a reduction gear during heavy load running to suppress the fall of a reduction gear, into contact with the back of the gearing part of the reduction gear.

CONSTITUTION: Since, during operation of an inclination preventing member 48, a ball 54 is brought into contact with the back 46 of the gearing part of a reduction gear 38 and lightly rolled, a bending load exerted on the inclination preventing member 48 is reduced to a value extremely lower than that of conventional constitution, breakage of the inclination preventing member 48 is reliably prevented from occurring. Further, during operation, the ball 54 is brought into contact with the gearing part back 46 of the reduction gear 38 and lightly rolled, and wear of the ball 54 is extremely low, whereby there is no need to frequently regulate the position of the inclination preventing member 48 to compensate wear and maintainability is remarkably improved.