It's inevitable that dump bodies, snowplows, and other severe-service vehicles will enter your facility in need of body or PTO repair. While they are there, are your service writers and technicians checking walking beam suspensions for center bushing wear?
Some of your customers may have misconceptions about walking beam suspensions and may not be inspecting for center bushing wear during the regularly scheduled maintenance check. Some of this confusion is caused by the misinterpretation that a low- or no-maintenance item, such as a center bushing, doesn't wear out. But the lateral movement combined with the vertical stresses encountered while the unit is in service, plus other ingredients such as age, weathering, and other natural factors, do cause center bushings to fail.
When operating a unit with worn center bushings, the deterioration of the bushing can accelerate if not corrected in a short period of time. It does, explains used construc- tion and severe duty buyer Jeff Liljedhahl of RDO Truck Center, Ft Worth, Texas.
"Operators and applications that don't put highway or distance trips on the trucks, or don't have experienced drivers, can be the worst offenders of all," says Liljedhahl. "Sometimes we see trucks come in with tires that are almost rubbing against the frame rails."
This can be a sure sign of bushing problems. That's because the bogie of a walking beam suspension pivots at the two center bushings. These center bushings work in a bipolar plane. On any turn, one is pulled ahead of the centerline of the tandem while the other is pushed away from the centerline of the tandem.
Normal pivot movement will allow for a three-inch movement off of the bogie's centerline. However, when the bogie moves beyond the three inches during a tight turn, that usually indicates the center bushings are worn. It's especially a telltale signal when the unit's axles won't realign after a tight turn. Bushing Replacements Once the need for bushing replacement has been identified, the next question is how to advise customers on the type of bushing needed for their specific type of vehicle use.
Center bushings are offered in a variety of materials. One major walking beam suspension manufacturer offers four bushing styles, each of which should be used for a specific type of truck service. Generally, the main categories of bushing are full rubber, urethane, high-confinement rubber, fully bonded, and bronze.
Rubber bushings are by far the most popular choice for walking beam suspensions. They are effectively a zero-maintenance item after proper installation in the suspension. However, they can be prematurely damaged by corrosive chemicals, natural substances (in some applications), and simple over-taxation by the application that the vehicle was designed for. In the case where a customer has rubber bushings that are prematurely wearing, there are solutions available.
Urethane bushings also provide an option for the customer. Manufacturers of these bushings claim that urethane is less susceptible to chemical and natural solvents. They have found acceptance in areas where high volumes of oil and other petrochemical residue wash onto the suspension. Urethane is a hard bushing substance and has the characteristics of a zero-maintenance product.
One solution is the high-confinement rubber bushing. This is a rubber bushing that is stress loaded into a high-strength steel sleeve. The rubber interior of the bushing has torque-absorbing properties while the steel sleeve is able to absorb a much greater crushing pressure with less sleeve movement when installed in the beam.
Fully bonded bushings and bronze bushings are applicable in severe offroad and extremely heavy-weight carrying applications. These bushings are not designed to promote ride comfort; so service writers should advise installations of these types of bushings only when the customer is encountering severe duty conditions and poor bushing life.
Along with the different types of bushings, reconditioned walking beam sets are available. These sets will have new center bushings pressed in place on a reconditioned beam.
Look for the Problem Ask your service writers if they are looking for these types of symptoms as the truck is moved into the bay for other work. Many customers don't see their trucks operate on a daily basis. They rely on their drivers to advise them that a condition of axle walkout is present. When a driver brings a unit into your facility, does your service writer inquire about the unit's turning characteristics? Looking for the problem and solving it not only is a benefit to your shop's revenue stream. It can save the operator of the equipment countless dollars in further repair and tire cost.