Cross Shaft information
A semitrailer is not always parked on even ground, and therefore the loads carried by the two legs of a landing gear are not always equal, and the two legs of the landing gear are not always extending in perfect parallel alignment.
Also, when a loaded trailer is raised on its landing gear, the leg on the far side of the crank lags behind the leg on the near side because of the torsion deflection in the cross shaft together with uneven parking surfaces contribute to cause an unbalance in the loads carried by the two legs of a landing legs and a deflection in the frame supporting the landing gear to the trailer.
It is believed that when the legs of a landing gear do not extend in a perfect parallel alignment, an axial tension or a compression stress is generated in the cross shaft, causing gear friction inside the telescoping mechanisms of the legs.
In these cases, a larger than normal torque is required to raise or to lower the telescopic legs of the landing gear.
It is believed that these large torques have been the major cause of failure of cross shafts on landing gears.
Another contributing factor to explain the failure of cross shafts on the landing gears of semitrailers is believe to be directly related to the handling of the trailers.
Generally, a highway trailer belongs to a pool of trailers, and is often hauled by several trucks during a same week. A trailer is normally dropped off by one truck at a depot, for unloading and reloading, and picked up by another truck for delivery to a new destination. A trailer is also often hauled along one segment of a delivery route by one truck and along a next segment by another truck.
The fifth wheels of tractor trucks are not all at the same height, and it is common for a truck operator to try to hitch a trailer that sits too low for the fifth wheel of his/her truck.
When the trailer fails to reach the fifth wheel, the truck moves ahead causing the trailer to fall back on its landing gear. The leg on the crank side of the landing gear is locked in place by the gearing system of the crank. However, the leg on the far side of the crank, that is the passenger-side leg, is held in place by the stiffness of the cross shaft. Again, if the ground is uneven and higher under the passenger-side leg of the landing gear, the jerk applied to this leg is transmitted directly to the cross shaft, often breaking the cross shaft.