A popular management strategy prioritizes problems according to cost or frequency of occurrence and attacks them from the most occurrences to the least. It’s a good concept because you should always be able to expect a good return for your efforts.
Apparently, it’s not a strategy that’s been used by the trucking industry when it comes to out-of-service citations. Brake problems have been, and continue to be, the most frequent reason that trucks are put out of service at roadside inspections.
Fleet customers claim an out-of-service citation can cost between four and eight hours of lost revenue, according to Haldex engineers. Many fleets have just-in-time delivery contracts, so an out-of-service vehicle can cost a huge amount of money, depending on the load and circumstances. These are clearly costly problems.
And, in most cases, there’s really no excuse for such a situation.
“As simple as it sounds, many out-of-service problems result from things that should be caught by simple visual inspections,” says Craig Frohock, director of North American braking systems at ArvinMeritor.
For example, the Commercial Vehicle Safety Alliance reports that out-of-adjustment brakes – those requiring more than the maximum allowable chamber stroke – still cause more out-of-service citations than anything else. Such a situation is really not acceptable, since automatic slack adjusters have been mandated for many years. There are, however, many vehicles, particularly trailers, that are still equipped with manual slacks.
“Change out any manual slacks to automatics,” says Jim Clark, director for foundation drum brakes at Bendix Spicer. “Just that will solve many issues.”
Automatic slack adjusters
Clark says that government inspectors are finding a growing number of instances in which operators are adjusting automatic slacks that they find are out of adjustment without attempting to determine the cause of the problem. Doing so can cause premature wear of brake system components.
“Read the instructions carefully,” Clark says. “You’re not supposed to manually adjust these during normal service.”
Jim Szudy, engineering manager of vehicle systems for Bendix Commercial Systems, says, “The idea is to troubleshoot an automatic slack adjuster if it’s out of adjustment. Don’t manually adjust it. Troubleshooting guides are in all the service manuals. Manually adjusting an automatic slack is not the thing to do. It’s also a good practice to keep automatic slack adjusters as well as cam shaft bushings properly lubricated because it helps the mechanism move more freely. Lubrication also helps to clean out some of the contamination that might have gotten in.”
Regarding the importance of proper lubrication, ArvinMeritor’s Frohock says, “In many cases in which brake stroke exceeds CVSA requirements, we find a binding clevis pin between the actuator and the slack adjuster.”
Haldex engineers advise, “At each PM interval, the shop should correctly measure pushrod stroke at each wheel position in accordance with CVSA Level 1 inspection guidelines. This involves precise pushrod measurements at a prescribed air pressure to ensure uniform measurement. Measurements should be taken from the face of the air chamber housing to the center of the clevis pin. Refer to the CVSA chamber readjustment chart for particular air chamber size. Attention should be paid to regular-stroke chambers and long-stroke chambers, as strokes will differ. Use the power stroke measurement to aid in diagnosing the foundation brake. If stroke issues are found, simply replacing brake shoes and readjusting brake adjusters are only band-aids. The root cause of the problem will resurface later down the road.”
There are a number of situations that can affect linings and sideline a truck. Linings can become contaminated with oil from leaking seals, cracked or simply worn beyond the minimum acceptable thickness.
Rust jacking can be mentioned here, but, according to Meritor engineers, this is not really a serious problem.
“The problem tends to be focused in geographical locations or with particular customers,” Frohock says.
MeritorWabco’s Johnson reports in a published paper that rust jacking occurs when rust forms on the brake shoe table under the lining, causing the lining to lift and crack. In addition, it can cause the shoe to deteriorate. A shoe and lining assembly could be rust jacked if:
• there is at least 1/8-in. of usable brake lining remaining above the rivet heads (3/8-in. total lining remaining);
• the lining is lifted off the shoe, resulting in a gap between the shoe table and the bottom of the lining block; and
• the lining block is cracked along the edge.
There are many conditions that are often incorrectly interpreted as rust jacking:
• cracks on the braking surface of the lining block;
• edge de-lamination without obvious lifting of the block from the brake shoe table; and
• lining cracks that occur when the lining is worn down to the rivet heads.
ArvinMeritor inspected nearly 50,000 lined brake shoe cores for potential rust jacking at its reman shoe center. In addition, one of ArvinMeritor’s friction suppliers conducted its own independent inspection of more than 20,000 lined brake shoes cores. Collectively, the incidence of potential rust jacking was found in approximately one percent of shoes inspected.
Tone ring corrosion
Corrosive road chemicals are taking their toll on more brake system components than linings, according to Frohock.
“Recently, we’ve seen corrosion-related issues with tone rings, which can lead to out-of-service problems because of the impact they have on the ABS system,” he says. “This is most often seen in medium-duty trucks equipped with hydraulic disc brakes.”
Such problems generate costs beyond simply replacing the affected rings.
“The costs associated with such failures multiply because fleets don’t want to put used pads back on a wheel with a new rotor,” says David Moore, vice president of OEM and technical sales at Performance Friction.
While heavy-duty disc brakes struggle to make headway into the over-the-road market, they have found success in niche areas of the Class 8 market and in growing areas of the medium-duty marketplace. While disc brake suppliers report good results from fleets that have moved to disc brakes, rotor roughness continues to be bothersome in some applications.
Arnold E. Anderson, a consultant to the Link Engineering Co., says that disc brake roughness is a rigid body vibration, mostly at wheel rotation frequency, that is caused by brake torque variations. It is readily detected by the driver, since brake roughness often feeds back through the brake pedal in the form of a tactile pulsation and may produce a similar feedback through the steering wheel.
Many vibrations can occur at wheel rotation frequency that are not brake roughness. For example, tire and wheel unbalance can cause vibrations that may be sensed at the steering wheel. However, such vibrations do not require application of the brakes and tend to occur only at specific narrow speed ranges. Poor suspension alignment, bent wheels and some road surface irregularities can produce vibrations that are similar to brake roughness. Sometimes, these may be more pronounced when the brakes are applied. It is important, therefore, to be careful in diagnosing brake roughness on a vehicle.
“Experienced test drivers often choose a smooth road, then use specific vehicle speeds and brake usage sequences to search for brake roughness,” Anderson says.
He indicates that even some new vehicles may exhibit a brake roughness, often including a pulsing brake pedal feel, especially during a light brake application. These symptoms may disappear after a few brake applications. If so, they probably resulted from contamination or local rusting of the rotor surface. If the problem worsens with use, a systematic diagnostic procedure is justified.
Rotors from problem vehicles should be measured for thickness variation (TV) and lateral runout. At a minimum, this should be measured at the rotor mid-plane and also near the OD and ID. Vehicles vary in their sensitivity to rotor dimensional characteristics. Sensitivity studies should be performed using production brake linings for the vehicle. The brake linings used to evaluate brake roughness should be fully burnished.
Anderson says that roughness can worsen in high-mileage, low-brake-use applications. This, he says, is explained by abrasive particles at the brake lining surface that can be the first to contact the rotor. Under normal brake pressures, and when the brakes are heated, most abrasive particles are embedded into the brake lining matrix. This limits their abrasive action. However, when driving at highway speeds with the brakes released and cool, a brake lining may gently and locally rub the rotor. The abrasive particles may rise above the surface and dominate the contacts at such times. Eventually, this local contact of the rotor by the brake lining (especially by abrasive particles at the lining surface) locally wears the rotor. This local rotor wear provides a rotor thickness variation that produces uneven braking torques, which may be especially noticeable on gentle brake applications. The resulting periodic brake torque variations, and their associated brake pedal pulses, provide initial brake roughness.
Very localized rotor wear produces most brake roughness. This local wear almost always is produced when the brake is released, and it is commonly worse when brake linings are cool. Under these conditions, a small amount of local brake dragging wears the rotor at the local contact site. With most disc brakes, this wear is confined primarily to the inboard rotor face. Once the rotor has developed significant TV, gentle brake applications provide uneven heating of the rotor. This thermally induced TV increases the initial rotor TV. Now, the brake roughness is more severe. When vehicle speed causes the brake roughness input to excite some suspension or steering component, brake roughness peaks.
While it’s always tempting to save a buck or two, using inexpensive service parts is rarely a wise move. The old adage “Pay me now or pay me later,” usually applies.
“We want to reduce the number of unplanned service incidents,” says Performance Friction’s David Moore. “We want to provide a longer component life so service intervals can be extended,”
While he was talking about the products his company offers fleets, he actually could have been representing the position of all of the suppliers in the industry.
“Every time you get into a brake, there are other associated costs that far exceed the cost of the replaced components themselves,” Moore says. “Typically, the cost of components is only half the total cost. If you can double the life or, more importantly, reach the next scheduled maintenance interval, there can be considerable savings over time. For fleet maintenance, the key is to reach the next maintenance interval. It doesn’t help to get 10 percent more life. You need to reach the next time there is scheduled maintenance.”
ArvinMeritor’s Frohock says, “You can get some very inexpensive drums, but their chemistry might not be adequate to resist cracking or heat checking like an OEM-level part would.”
Leslie Kahn, senior product manager for heavy-duty parts at SKF, says, “We’ve seen some counterfeit replacement parts in the marketplace and, of course, have tested them. We’ve found that they do not come anywhere near the performance level of original parts. What you risk is a vehicle that will not be able to hold pressure, which puts added demand on the compressor. You may save some money up front on a parts purchase, but you’ll spend a considerable amount in the future because you may have to replace that ‘inexpensive’ part and also the compressor. It really doesn’t pay to try to save money up front on less-than-OE-level parts because you’re risking an out-of-service vehicle.”
Moore offers another example of the potential problems that can be caused by inadequate service parts. “If there is significant variation in performance in replacement brake pads, you might install seven good pads on a truck and one bad one,” he says. “When that bad pad wears out, you need to do service – at least on that axle. It doesn’t matter if the average life is 30,000 miles, if one pad wears out at 15,000 miles, you’re going to be doing maintenance on that axle.
“Reduction in the amount of variation in components is a critical factor because it’s the weakest link that will cause expense. It’s not the average behavior; it’s when the first part fails that causes the equipment manager to spend money.”
Consider the use of replacement parts designed to help reach the next scheduled maintenance interval.
“ArvinMeritor, as well as other suppliers, offer extended lubrication products that increase the required lube interval for the brake and slack adjuster so that some of the regular maintenance is not required for three to five years,” says MeritorWabco’s Johnson. “We also recommend the use of automatic lubrication systems. Anything that can be done to extend lubrication intervals will save money.”
Jim Szudy from Bendix suggests the use of wider brake drums. “Go from 16.5 x 7 in. to 16.55 x 85/8 in.,” he says. “They cool more rapidly and wear longer.”
Clark says, “We’ve also found that dust shields help brakes live longer – 15 to 20 percent, especially on trailers.
Is it time to address the brake system? Paul Johnson, senior director for compression and braking products for MeritorWabco, might be offering the answer when he says, “Most of the problems are the result of inconsistent maintenance practices or using non-OE-level service parts.”
Air dryers also need to be spec’ed
By Leslie Kahn, Senior product manager for heavy-duty parts, SKF
Desiccant air dryers have become standard components in air brake systems, but these items should be carefully spec’ed to fit the application. A dryer suitable for line-haul applications would not likely live very long in a transit application where the compressor runs most of the time because of the heavy air demands of air actuated components – doors, kneeling mechanisms, windshield wipers.
Moisture is a significant, but not the only, concern for an air system. As the compressor runs in high-duty cycle applications, it becomes hot and can start to pass oil, called “blow by.” The dryer’s role is to remove both the moisture and any blow by before it can enter the air system. If water gets into the desiccant, it can enter the air system. The reserve tanks will accumulate water, and the air supply for braking can be limited. Excessive oil also can cause the dryer’s exhaust valve to leak, causing rapid cycling of the compressor. Drivers should keep an eye on exhaust valves to see if oil is being emitted.
“Prior to the winter freezing temperatures, the air dryer should be serviced to avoid a sideline. The desiccant cartridge, purge valve and turbo valve should be replaced to ensure a trouble-free winter. Also, the heater element should be checked for proper operation. Either use an ohmmeter (a closed circuit indicates the heater is functioning) or cool the lower dryer housing to below 35ºF and feel for warmth. At this service, and whenever the vehicle is in the shop, check to see that the vehicle is holding air pressure. The dryer will blow off when it reaches governor cut-off pressure at around 120 psi. Time how long it takes between blow offs. With the vehicle idling in the shop, the interval shouldn’t be fewer than six minutes because no air is being consumed. Check for the source of the air leak if it is fewer than six minutes. The leak may not be at the air dryer, and soapy water spray at fittings and along air lines will help reveal the leak source.”
SKF manufactures a full range of air dryers, from light-duty units suitable for automobile and light truck applications to heavy-duty units that can be used in applications in which the compressor runs 100 percent of the time.
The case for retarders
By Harry Dozier, Chief engineer, Brake Systems Inc.
The biggest problem with brakes is trying to use them as retarders coming down a hill. They generally don’t have the capacity to do that. If you can use a retarder for downhill speed control and your foundation brakes for stopping, you’ll extend the life of your brakes and increase the safety factor of your vehicle.
Brake fade from hot brakes increases a vehicle’s stopping distance. While that is a major concern for drum brakes, it is still a factor with disc brakes. Using an engine retarder relieves the brakes of the burden of holding the vehicle to a reasonable downhill speed.
Newer engines generate higher back pressure, so they have more efficient engine braking than older models – as much as twice the retarding force.
Brake Systems Inc. manufactures the Blue Ox exhaust brake in a range of models to suit most engine applications.