Funny thing: When you ask maintenance and engineering experts about air-ride suspension systems, the last thing they want to talk about is air. To them, the keys to maintaining modern air-ride suspensions are: shock absorbers, fasteners and bushings.
Oh, they get to the air system eventually, but the overriding message is that the suspension is only as good as the smallest component.
It’s fitting, then, to begin this look at air-ride suspension maintenance with a look at bushings and fasteners.
LOOKS GOOD, RIDES BAD
“The suspension is only as good as the torque rods, and the torque rods are only as good as the bushings,” says ATRO Engineering Systems founder George Sturmon. “The technicians don’t understand the forces and the mechanisms that those bushings are under, and that’s why (bushings) wear out relatively soon.”
Because of this lack of understanding, Sturmon feels that fleet technicians aren’t inspecting bushings properly. “They’ll grab ahold of the (radius) rod and try to move it, and you can’t move it by hand, but it is nevertheless shot,” he explains. “If you can move it by hand, it’s been shot for a long time.”
The problem as Sturmon sees it is that the radius rods themselves only operate—only see loads—whenever the truck is braking or accelerating, or perhaps going over rough road, creating a back-and-forth movement.
“They don’t have constant loads on them,” he says. “So, to check them, you should take a rod or a bar, and put it in there, and see how much you can move it with that. But that’s not done; the technicians will maybe look at them, and on a rubber bushing they might see some checking or cracks, but if they can’t move it they think, ‘Well, it’s good.’ Well, actually, it’s bad.
“Ride quality is affected by bad torque rod bushings,” he explains. “The back and forth movement; the driver can feel that. If he says, ‘This thing is moving back and forth,’ the first thing the technician should look at is torque rods. And then they should put them through a very rigorous inspection: Make sure you’ve got enough force on that bushing to move it. There’s not anybody I know who can grab ahold of a torque rod—they’re supposed to handle up to 30,000 lbs. back and forth—and move it by hand. If you can move that bushing—say you look at it and the material between the two sleeves is a quarter of an inch—if this thing moves upward of an eighth of an inch, it’s bad. If they move that much, they need to be replaced.”
John Morgan, product manager for ArvinMeritor Commercial Vehicle Systems, offers a few guidelines for replacing those bushings.
“When installing a new bushing, follow the manufacturer’s guidelines for lubrication,” he says. “Oil-based lubricants can damage natural rubber bushings.”
“Also, be sure to use the fasteners specified by the equipment manufacturer, and always torque to the indicated level,” Morgan adds. “Be sure the threads are free of contaminants, including paint overspray and dirt, and follow the manufacturer’s recommendations regarding lubricant. Failure to do so can give false torque readings, or yield improper clamp loads, which can cause the connection to fail and result in equipment damage.”
RUBBER VS. ELASTOMER
“Back when I started with this business, when the suspension manufacturer sold you a suspension, the torque rods normally came with it,” says ATRO’s Sturmon. “Now a lot of OEMs are getting their own torque rods made, and they take the rod, they put a rubber cylinder on it, they put a pin in it and they shove it in a hole—not realizing that these things have to be positioned at zero on ride height, so that they’re not stressed at ride height.
“They’re very hard to install, and the guy has to force them into position,” he says. “Well, if you get them in a little bit crooked, particularly on a rubber bushing, you’re immediately degrading the material: Rubber under constant stress degrades.
“Now, with most elastomeric bushings, they’re free-rotating, and you don’t have to worry about that particular problem, because they’ll zero out at ride height, regardless of what the height is,” Sturmon explains. “Because they’ll turn automatically to whatever position, and as the vehicle moves around, the rotation leaves zero torsional stress in the bushing. And the same is true for metal bushings. But, if you don’t grease the metal bushings, you’ve got a heck of a problem. But the elastomeric bushings, elastomeric urethanes, polyethylenes, are self-lubricating.”
WHEN TO INSPECT
“If you know that those bushings are on a new truck, or they’ve been changed within a year, it’s true that you can inspect them annually,” Sturmon says. “But, if you’re not familiar with the truck, you don’t know the last time they were inspected. And in too many fleets, the documentation for checking torque rod bushings is… I don’t want to say nonexistent, but it’s little or none.
“These things are extremely important because the torque rods hold the suspension in place,” he says. “The suspension is really a system, and like a component of any system, when that component wears out, all of the forces that it’s supposed to protect the system from automatically go to other places. So if the bushing isn’t doing its job, the brackets start to take a beating. Or the shocks start taking a beating. The shocks are in there at a specific location, and at a specific angle, and the rods are supposed to hold that, because if you start putting side forces on a shock, it’s going to go quick.”
“FAILURES WILL OCCUR”
“In virtually all circumstances, if fasteners come loose, failures will occur,” says Greg Richardson, vice president of engineering, Reyco Granning. “Pivot connections and u-bolts are some of the most critical areas. And, if a bushed joint comes loose, it can add increased loading to the bushing which will result in premature failure.
“Clamped joints frequently become loose due to motion between mating parts as a result of unforeseen impacts or overloading of the connection,” Richardson goes on. “Once motion has occurred between mating parts, brinelling of materials may occur which allows the parts to ‘seat closer together,’ thus reducing the clamp load of the fastener. Periodic checks of the torque will generally reduce or eliminate a potential failure.”
Although Richardson feels that fleets generally do a thorough job during inspection and maintenance of bushings and fasteners, he suggests the following tips:
- It is critical that the fleets torque the fasteners to the OEM specifications, and not assume a generic torque for a specific bolt size. An over-torqued connection can be just as detrimental as an under-torqued connection.
- U-bolts are to be torqued incrementally in a crisscross manner to assure distributed clamp load over the connection.
- Some connections require torques that can go as high as 800 ft. lbs. This can cause problems at the fleets since many do not have the equipment to measure such levels of torque, and are forced to guess.
- When bushings are to be replaced, care must be taken not to rip or damage the bushing. Generally the OEM will provide or recommend a lubricant for installation as well as a tool to install the bushing.
- Proper orientation and positioning of the bushing greatly affects its performance and durability.
- Proper torque at the bushing connection is critical to its life. Over-torque may crush the core of the bushing.
NUTS AND BOLTS
Bill Nash, manager, technical services, Hendrickson Truck Systems Group, agrees that proper torque is essential, and often left by the wayside.
“On a new truck, everybody’s concerned with making sure the lug nuts are tight, and it’s probably done as part of the PM cycle, but how many people know that in the maintenance manual that the OEM produces and that the component supplier produces, there’s also a (recommended) maintenance cycle for the fasteners on the suspension?” he asks. “It’s critical that they’re checked, and, if broken, replaced.”
Nash fondly points out that there is a 1928 Hendrickson truck in the lobby of the company’s headquarters, and that truck has an original Hendrickson walking beam suspension with over two dozen grease zerks. “You look at our suspensions today, and we have very few suspensions that even have a grease zerk,” Nash says. “So the concept gets in somebody’s head that it’s maintenance free… but it still needs service.
“Rubber components wear out, and a lot of the components that used to have grease zerks are now rubber, and those joints need to be inspected,” he says. “You can see wear there, or of course with a shock you can grab ahold of it and the rubber mount on it, and you can see whether it’s failed or not. Other places, like torque rods, you can see that the rubber ‘walked out.’ There are other places on the suspension where they’ve used rubber and you can easily tell from a visual inspection whether it needs to be replaced or not.
“I tell you, my guys go out all the time, and every time they go out, depending on what the complaint is, they carry a torque wrench,” Nash says. “A lot of the shops don’t have torque wrenches, and if they do, it’s been dropped so many times it’s not calibrated. And it’s so critical that fasteners on suspension brackets are torqued properly and in sequence. Or they don’t use the right fastener! A lot of times, these suspensions are Hucked, which personally, I prefer. But even the Huckbolts can be checked for torque.
“It’s clamp load that holds that suspension together,” he says.
Like Nash, Bruce Barton, director of engineering, Ridewell Suspensions, encounters horror stories in the field. One experience in particular points out why technicians need to use the right fastener, and use it properly.
“Upon a visit to a customer who was having some difficulties, we found that the nut on the pivot joint had apparently fallen off,” Barton says. “And, instead of replacing it with a like nut, they used a thread chaser. And it wasn’t torqued!
“That was really something to see,” he says. “It just illustrates the importance of making sure that the pivot joint is tight, and that the right hardware is being used.”
In Barton’s view, the pivot joint is the key component to an air-ride suspension. “On most trailer-type suspensions, there is one pivot point, and it’s important that that fastener be re-torqued early in the life of the vehicle,” he says. “Most suspension manufacturers would indicate a re-torque between 5,000 and 10,000 miles; that is often missed, and that is so key to the life of the pivot joint. When the vehicle is new there is some relaxation in that joint, and that first re-torque is critical to the robustness of that pivot joint.
“If that pivot joint is loose, it will more than likely destroy the hanger that is attached to the frame of the trailer,” Barton warns. “It has the potential to destroy and force the replacement of the bushing.”
As with the rest of the system, the manner in which it’s torqued is as important as whether it’s been torqued.
“A lot of times, mechanics will hit it with an impact wrench, as opposed to torquing it to the suspension OE recommendation,” Barton warns.
“The knowledge level of technicians isn’t where it should be,” says Randy Petresh, vice president-technical services for Haldex. “The biggest thing is that (suspension maintenance) is ignored. It’s an awareness thing that needs to be impressed upon the director of maintenance, who has to impress it on his technicians.”
Why does such an important system get ignored? “I think it’s this perception that the suspension systems are very robust, they take a lot of beating, a lot of abuse, and you don’t have to worry about them,” Petresh says.
Are you worried yet? Stay tuned: we’ll continue our air-ride suspension maintenance story next issue with a look at shock absorbers, air springs and ride height adjustment.