How to Hook Up Tractor Trailer Doubles

by Devorah Fox

A "double" is a popular tractor-trailer combination consisting of a tractor tractor/semi-trailer and another trailer. The first and second trailers are coupled with a converter, or dolly -- a fifth wheel on an axle. Coupling, or hooking up, is not a difficult procedure, but for safety's sake you must do it right. Except for the use of the dolly, coupling doubles is essentially the same as coupling a tractor and one semitrailer.

Inspect the dolly thoroughly for damaged or missing parts and grease dolly fifth wheel. Open the air tank petcock -- or depress the air release valve. With a helper, put the dolly in front of the second trailer, Trailer 2 --or use the tractor/semitrailer combination to pick up, drive and position the dolly near the front of Trailer 2. Place the dolly so its fifth wheel touches the front of Trailer 2 and its fifth wheel jaws are in line with Trailer 2's king pin.

Back the tractor-semitrailer combination into position in front of the dolly. Lock the tractor-only brake if you have one, put the trailer air supply valve in the emergency position and get out of the cab. Lift the tongue of the dolly by hand. Hook the dolly eye hook onto the Pintle hook of Trailer 1, the trailer hooked to the tractor. Close the Pintle hook safety latch and hook the safety chain from the dolly onto Trailer 1's O-ring. Raise the dolly landing gear and connect the dolly air lines and electrical cord to the back of Trailer 1. Close the dolly's petcock drain valve and Trailer 2's shut-off valves. The dolly fifth wheel jaws should be open.

Make sure Trailer 2's height is slightly lower than the dolly fifth wheel. If Trailer 2 is equipped with spring brakes, the parking brakes will be "On." Return to the tractor and back the dolly under Trailer 2. If Trailer 2 does not have spring brakes, connect the air lines and the electrical cable from the dolly to Trailer 2. Open the shut off valves on the back of Trailer 1.

Make sure nothing and no one is behind Trailer 2 or between or under any of the units. Release the tractor-only brake and slowly back the dolly under Trailer 2 until you hear and feel the fifth wheel jaws lock around the king pin. Use the landing gear crank to raise Trailer 2's landing gear just slightly. Return to the tractor, shift into low and gently try to move the tractor forward.

Get out of the cab and visually check that there is no space between the trailer upper coupler plate and fifth wheel. Secure the fifth wheel locking lever. Crank up the trailer landing gear and secure the landing gear handle.

Tips

  • check Work on level ground.
  • check Check for enough clearance between the rear of the tractor frame and the landing gear. When the tractor turns sharply, it must not hit the landing gear.
  • check Check that there is enough clearance between the top of the tractor tires and the nose of the trailer.

Warnings

  • close Don't try to back under the trailer at an angle.
  • close Never drive with the legs only part way up. They may catch on railroad tracks and objects in the road.

References

About the Author

Devorah Fox has been writing professionally since 1977. She is president of Mike Byrnes & Associates, Inc., publishers of the "Bumper to Bumper" commercial motor vehicle books and authors of the "Easy CDL" iPhone app. She also writes for the “Island Moon” newspaper. Fox has Bachelor of Arts degrees in anthropology and linguistics from State University of New York, Binghamton.

Photo Credits

  • photo_camera semi trailer image by max blain from Fotolia.com