Watching Your Back

Inspection guidelines for trailer air suspensions.

"One of the biggest issues is the dock walk limiting devices," says Dan Carter, project engineer for Great Dane Trailers. "Because of the inherent geometry of a trailing arm suspension, when you raise or lower that suspension when the brakes are locked, the trailer's going to roll away from the loading dock, and it tears up the landing gear. Well, there are a lot of devices that eliminate that dock walk, but do people know how to operate them?"

Carter suggests that trailers with spring suspensions can fall victim to road debris and other assorted impacts that can throw the suspension out of alignment and cause wear. He suggests that maintenance technicians pay close attention to cracks in the springs, as well as visible wear in the springs, the rocker arms, or any other mechanical parts. The same goes for inspection of welds.

Like Barton, Carter believes there is not nearly enough fastener re-torquing going on in fleet maintenance shops. "We say to check it after 3,000 to 5,000 miles in the break-in period, and then the torque should be re-checked every 25,000 miles," he says.

"Those are the things that can really pay off, if they're part of your regular maintenance," he says.

Carter saves his final tip for the upper running gear rail on a sliding suspension, which is often set once and then forgotten. He recommends that fleets slide the suspensions on a regular basis.

"If they order a slide suspension and they don't change their bogey position much, they come to the end of four or five years and they want to sell it, and somebody tries to operate this slide and it's stuck. You can really get into some issues trying to get that broken loose and trying to get it to slide again. So, regular exercise of that suspension goes a long way."

Bruce Barton, director of engineering for Ridewell Corporation, offers this checklist for trailer suspension inspection and maintenance:

Trailing arm air suspension:

  • Understand the suspension—read the service and installation literature that is supplied by the manufacturer.
  • Inspect for obvious wear situations:
  • Air spring abrasions, fatigue cracks in the areas where the flexmember rolls over mating parts
  • Worn pivot bush
  • Excessive play, complaints of axle walk
  • Fatigue cracks in rubber
  • Suspensions often use "wear" or thrust washers adjacent to the pivot bush. Wear can be expected on this washer, but if it is missing or worn through, it should be considered for replacement.
  • Worn shock absorbers—because shocks supply most of the damping, vibration complaints often occur when the shocks are worn.
  • Cracks and corrosion damage to general suspension structure, beams hangers and axle.
  • Perform periodic fastener maintenance—this is the single most commonly seen lack of maintenance
  • Torque to manufacturer's recommendations
  • First re-torque most critical
  • Bushing fastener (pivot bolt/nut): "mission critical" joint
  • U-bolts if part of the suspension
  • Shock absorber
  • Hanger attachment to frame, if bolted
  • Air spring fasteners
  • Leveling valve hardware/linkage
  • Check ride height
  • Proper functioning of suspension requires correct ride height setting and functioning of the leveling valve and linkage.
  • Check air supply
  • Air suspensions rely on an air supply. Ensure that all pneumatic components and plumbing upstream of the air springs function properly.
  • If an air spring is used as an actuator (i.e., as part of a liftable suspension that uses an air spring for lift force), use a pressure regulator to limit pressure to 100 psi. Often, the air spring top and bottom mounting surfaces don't remain parallel when inflated, and under full system pressures premature failure can be expected.
  • Tools
  • Normal tools suffice for most suspensions.
  • Ridewell offers an easily-changed clamped-in pivot bush, while some suspension manufacturers offer special tools to replace the bushing. Service literature should always be consulted in regard to special tool requirements.
  • New technology
  • Weight savings drives material selection in the form of steel, aluminum and plastics. Non traditional suspension materials cost more than steel, so emphasis must be placed on the cost vs. benefit (value proposition).
  • Expect increased durability of the rubber materials, such as improved compounds that better resist aging (ozone). Ridewell will shortly introduce a new composite pivot bush that is lighter and more cost-effective than present designs.
  • Single tires and disc brake applications will drive suspension and slider design changes.

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