INTERMODAL TRAILER HITCH AND TRACTOR CONTROL DESIGN
At one point in the WABTEC Rrampcar development program, a manufacturer of Terminal spotting tractors demurred from quoting on a tractor for use with pull-up type stanchions because where they had tractors in this service they had experienced damage and warranty claims that they attributed to faulty operation of the hitches. Railmotive analyzed the problem and found that the manufacturer was basically correct, but that the problem could be solved by either programmed maintenance of the stanchions or a modification of their design along with refinement of the tractor controls.
The operation of these stanchions is outlined on the accompanying exhibit titled STANCHION OPERATION. Fig.1, which shows a railcar with a semi trailer, tied down and ready for transit while another is being loaded. The figure illustrates why a collapsible stanchion (often called a “hitch”) is necessary for sequential loading where the deck of the car must be kept clear except when trailers are actually in place and tied down. This method frequently called “Circus loading” doesn’t require specially reinforced liftable trailers nor high capacity lifting cranes, thus if properly employed, can produce time, capital and labor savings in loading/unloading where highway trailers are to be transported by rail.
The operation of the stanchion when loading (where the damage was happening) is shown in Figs 2, 3, & 4. Fig 2 shows the first step where a hook that is part of the tractor is dropped into a receiving opening in the Stanchion’s vertical Strut, which is lying flat on the Deck of the train.
` In Fig 3 the hook is shown engaging the vertical strut and, as a result of forward movement of the tractor, pulling it up off the deck by rotating it about a pin connection at the bottom of the vertical strut which acts as a hinge. Note that as the Stanchion is erected, a second, diagonal, strut initially folded like a jackknife, is straightening out and will, when the hitch is fully erect, become a simple link bracing the vertical strut in its erect position.
This fully erect position is shown in Fig. 4. Note that when the diagonal strut is pulled straight, all forward motion of the Vertical strut, hook, and Tractor is constrained and must stop. By design the unfolding of the diagonal must occur at or above a certain minimum speed in order to travel beyond a straight line under the influence of inertia and operate a spring catch which will lock the diagonal strut in its straight position, making a rigid connection for the trailer.
Unfortunately the speed necessary to bring this about rises with increasing friction, and requires drivers to “hit it harder” when latch-up fails, in order to assure the desired solid connection. This was the cause of the failures and a simple design was provided by Railmotive Systems which would drive the Diagonal strut to its locking position no mater how low the speed of the Tractor, eliminating the “need for speed” and permitting the use of speed governing on the tractor if desired. The system was patented and patents assigned to WABTEC.