Source: {"pile_set_name": "USPTO Backgrounds"}

1. Field of the Invention
The present invention relates to lifting trailers. In particular, the present invention is related to lifting tandem-axle trailers having air ride suspension systems.
2. Description of Related Art
Almost all of the cargo freight transported throughout the United States is carried for at least a portion of its journey by tractor trailers. The trailers used in this type of shipping typically have tandem rear axles. Many of these trailers now utilize air ride suspension systems, in which air bags are used as shock absorbers.
Many years ago, the railroad industry began offering “intermodal” transportation services in which cargo trailers are loaded onto flatbed railcars and transported by rail. The trailers are typically loaded onto the flatbed railcars by overhead cranes having arms that extend downward and terminate with lifting shoes, or hooks. The lifting shoes are configured to connect to the trailer below the lower side edges of the trailer. Usually the arms are positioned so that there is one lifting shoe on each side of the front end of the trailer and one lifting shoe on each side of the rear end of the trailer. This allows the weight of the trailer and its cargo to be distributed across the four arms of the overhead crane.
In a typical intermodal operation, a trailer is positioned near a flatbed railcar. Then, an overhead crane is positioned over the trailer, such that two lifting shoes are positioned on either side of the front end of the trailer and two lifting shoes are position on either side of the rear end of the trailer. The lifting shoes are then secured to the trailer and the trailer is lifted and placed onto the flatbed railcar. Once the trailer is placed onto the flatbed railcar, it is secured into a locking hitch assembly on the railcar so that it does not move.
However, air ride suspension trailers cannot be lifted unless the axle remains in a fixed vertical position. If the axle lowers during lifting, the air bags will stretch and can be damaged. In addition, if the trailer is raised and the air bags are allowed to stretch under the weight of the axles and wheel assemblies, the air bags can be misaligned and damaged when the trailer is lowered again.
Although there have been many developments in the area of intermodal transportation, many shortcomings remain.