For the amount of attention it receives, a truck’s suspension is all too often the “forgotten component.” That way of thinking, however, can have dire consequences. Ignore the suspension, and the result could be a failure that not only means downtime, but also can have serious safety implications.
Fortunately, when suspension failure occurs, there are often telltale signs that reveal the cause, providing valuable information that lets you educate customers about how to improve maintenance and operational practices.
TIGHT IS RIGHT
According to Brad Bowers, president of U. S. Spring Service in Emeryville, Calif., leaf springs commonly fail because of inadequate U-bolt tightness. “There should be absolutely no flexing at the center of a spring,” he says. “The center of a spring leaf is its weakest point, since there’s a hole there. And, if allowed to flex, it’s the point that will flex the farthest, and the spring’s going to fail.” The broken spring leaves in Figure 1 are the result of a loose U-bolt.
Loose U-bolts also allow play between the axle and spring, allowing the two to hammer away at each other to the point of failure. They also can allow the axle to move fore and aft, damaging torque rod bushings.
The upshot: Always torque U-bolts to the suspension manufacturer’s specifications, and observe proper torquing procedures.
FIGHT FATIGUE
Steel springs that don’t fall victim to loose U-bolts eventually can fail from fatigue, a cracking process that is accelerated by overloading or severe service. Fatigue cracks are found between the U-bolt and either end of the spring pack, and gradually progress until the spring fails.
They usually start as a surface imperfection on a spring, which can be anything from a nick caused by a loose tire chain, to corrosion, weld spatter and fretting, which is simply abrasion from spring leaves rubbing against each other. These imperfections are known as stress raisers – areas where stress becomes concentrated. Once started, a crack progresses until the cross section at the site of the crack no longer can sustain the load.
Figure 2 shows typical fatigue breakage. But since it occurs gradually, it gives the astute inspector an opportunity to flag a problem before total failure occurs.
A not-so-obvious contributor to spring fatigue is poor brake balance. One or more underadjusted brakes will cause the remaining, properly adjusted brakes to do more than their intended share of the stopping work. The springs at those axle ends get “wound up” more than they would if all brakes were contributing equal brake torque (see Figure 3).
This situation is compounded when the wheel suspended by a wound-up spring hits a bump. There is little or no reserve capacity available to accommodate the additional loading.
If you see more than one leaf broken in the same area, suspect a fatigue failure. That’s because, when one leaf fails, the remaining leaves become overloaded. Plus, stress is concentrated over the edge of the broken leaf. So, to prevent spring damage like that shown in Figure 4, look for signs of fatigue early and often.
Also, be advised that, if the main leaf in a spring assembly is broken, the vehicle should not be driven until the leaf or assembly is replaced. There are three important reasons for this:
- First, unless the second leaf has end wrappers, the only remaining supporting members are shorter leaves, tied together by spring clips. A broken spring clip in this condition could mean total suspension collapse at that corner of the vehicle;
- Second, depending on the type of suspension, the location of the axle may have been altered;
- Third, if a main leaf is broken (or if more than one-quarter of the leaves in a spring assembly are broken), the vehicle will be placed out of service if caught at a roadside safety inspection.
SUDDEN IMPACT
Impacts with curbs and other obstructions can play havoc with suspension components, especially trailer alignment bushings and slider assemblies. To tell the difference between intentional bushing movement from an alignment procedure and impact movement, look at the crescent formed by the bushing washer and its previous location. If it has a nice, round edge, with no evidence of scuffing, that’s an alignment adjustment, advises Tim Fulkerson, technical service specialist for Dana (see Figure 5).
However, if the assembly looks like the one shown in Figure 6, you’re looking at the result of an impact with a curb, large pothole or something similar, which loosened things up and allowed unintentional movement. Note the scrubbed paint and several “landing marks,” which indicate multiple movement events. Left unchecked, damage will be progressive. “Eventually, the bolt will eat its way into the hanger,” says Fulkerson.
Extreme slider damage also can be caused by significant impacts. The slider in Figure 7 is the victim of a wheel impact with something high enough that the tires couldn’t roll over it, such as a parking lot light base or concrete obstruction, Fulkerson says. The energy of a moving trailer being stopped instantaneously had to go somewhere, and the result is a mangled slider.
However, tires don’t need to hit an obstruction for slider damage to occur. Driving off without the slider pins properly engaged, then making even a moderate brake application, can result in a slider that looks like the one in Figure 8. This can easily be avoided through a visual, pre-trip, pin-engagement check.
JOIN THE AIR GUARD
Air suspensions have their own unique failure modes, and two of the most common – air bag abrasion and leaking (Figure 9) – often are misdiagnosed as road damage or a defective product. However, according to Ralph Norton, assistant communications manager of trailer suspension systems for Hendrickson, these problems often develop from operating the suspension with inadequate or no air in the bags and/or at an incorrect ride height.
“Operating equipment under such conditions can produce internal pinching of the air bag between its top plate and internal bumper (Figure 10),” says Norton. He stresses that the height control valve should rotate freely at both ends, and that any adjustment should be made by moving and re-bolting the valve linkage, not by bending or twisting it. Aside from ensuring that ride height is set to the manufacturer’s specifications, air-ride suspensions need to properly air up before the truck goes down the road, he adds.
Another potential area of concern regarding air suspensions is interference with another suspension component, which usually shows up as heavy, localized abrasion (Figure 11). This can be caused by worn bushings, which allow suspension components to travel out of their intended track. Misapplication, as in too large a spring for the suspension, also may be the culprit.
Finally, don’t discount the importance of shock absorbers in air suspension systems. If shocks are broken or worn out, air bags take the full brunt of impacts, which can result in the bellows separating from the bead plate (Figure 12). Also, shocks limit the extension of air springs and, without them, bellows can be overextended, resulting in similar damage.
In short, it boils down to conscientious inspection and maintenance. Small problems have a way of becoming bigger ones, so it pays to take a look, and learn to recognize early failure signs. Pay attention to details, and the big problems won’t stand a chance.
