For Want of a Belt the truck Failed

Bob Deierlein

Hoses Hoses carry fluids to all parts of the engine and must be properly clamped or connected. Touch, sight and smell, as well as a bit of common sense, will help you spot a "bad" hose. For instance, listen for unusual sounds coming from the hydraulic system. When a hydraulic pump lacks sufficient fluid, the backpressure will cause the hydraulic system to emit a shrill whine. Leaking air lines cause a hissing sound.

Unusual smells around a hydraulic system also can be a sign of a problem. When air saturates the hydraulic oil, air bubbles in the system change from a vacuum to a high-pressure condition that generates heat, which singes the oil and gives off a burnt-oil odor.

Puddles under the truck or low reservoirs indicate you may have a leak. Keep an eye out for hoses that show signs of seepage, abrasion or chafing. At roadside inspections, your truck will be put out of service if a brake hose bulges or swells when air pressure is applied. If there is any wear or damage to brake hoses that extend through the outer reinforcement, or if air hoses appear cracked, broken or crimped, your truck also may be put out of commission.

Checking Coolant Hoses However, cooling systems have continual problems with cold water leaks. As cooling system's temperature changes, the stem that the coolant hose is attached to, the clamp and the elastomer in the hose's wall all expand and contract at different rates. Over time, this unsynchronized expansion and contraction reduces compression force on the hose-to-stem seal, which causes cold leaks.

Until recently, the most common method of checking an engine coolant hose was to visually inspect its outside cover for wear or "ballooning" under pressure. But this no longer is considered completely reliable because re-search shows that most hoses fail from the inside out.

Gates Rubber Co., Denver, has identified an electrochemical attack on the tube compound inside the hose as the primary cause of coolant hose failure. Known as electrochemical degradation (ECD), it produces fine cracks or striations in the tube wall. As these fine cracks extend from the inside to the outside of the hose tube, pinholes leak or bursts near one or both ends of the hose.

The best way to check for ECD effects are to ``squeeze test' hoses near the clamps or connectors. Gates suggests you:

1. Make sure the engine is cool.

2. Only use fingers and thumb to check for weakness.

3. Squeeze near the connectors. ECD occurs within two inches from the ends of the hose.

4. Check for a difference in how the middle and the ends of the hose feel. ``Gaps' or ``channels' can be felt along the hose tube's length where ECD has weakened it.

5. Determine whether the hoses are soft or mushy.If so, they likely are under attack by ECD.

6. Replace all 4-year-old hoses because the incidence of failure increases at that point. For vehicles subject to stop-and-go driving, earlier hose replacement is recommended.

The main drive belt types are V-belts or micro-ribbed, and V-rib or serpentine belts.

While V-belts are conventional, micro-ribbed belts rapidly are gaining market share. This is because, according to Gates, micro-ribbed drive belts suffer less wedging, which results in less wear. Additionally, they have better or equal load distribution than V-belt designs, due in part to lower temperatures from reduced bending stress. Less stress reduces the friction between the belt and pulley, and lessens heat buildup. There also is less bearing and shaft load on the components, and automatic belt tensioners can be added.

On the other hand, adding belt tensioners increases costs. With micro-V belts, the pulley alignment is critical, so the system design is more complex. And, as pulley wear increases, foreign material can be trapped in the pulley groove.

As a belt ages or is exposed to extreme temperatures, it loses its flexibility, becomes brittle and shows hairline fissures along the ribs. As these fissures meet, they cause chunks of the rib to break off. Reducing belt temperatures can minimize cracking and chunking.

To get the most from your clutch, you should understand what affects it. A clutch connects and disconnects the engine and transmission by creating friction between the engine flywheel and the clutch pressure plate. During clutch engagement and disengagement, slippage can occur between these surfaces and the clutch discs. To absorb the resulting heat and torque, a friction material is riveted to the clutch facing.

Largely due to the increased drivetrain stress from high-torque/low revolutions per minute (rpm) engines, clutches now use ceramic facings instead of organic discs because they stand up to heat better, eliminate slippage and increase clutch life up to 75 percent. Additionally, ceramics are lighter and spin down faster when the clutch is disengaged, making shifting easier and smoother. Some drivers say ceramic clutches engage more aggressively than organics and have a "different" feel.

Reducing clutch slippage increases clutch life --- it only wears when it is slipping while under load from the pressure plate. This slippage can be intentional --- when the driver normally engages the clutch or uses an inappropriate technique, such as riding the clutch or hill holding. Slippage also can be unintentional --- during operation with insufficient free play at the pedal.

To spec and maintain heavy duty clutches, as well as get maximum clutch life, a task force of The Maintenance Council (TMC) of The American Trucking Associations, Alexandria, Va., has developed a Recommended Practice (RP).

First, the RP suggests you understand what affects your clutch. Such tips include:

1. Powertrain specification(engine, clutch, transmission, axle and tires);

2. Control of powertrain torsional activity;

3. Driver technique(starting gear selection, engagement technique, driving habits); and

4. Clutch maintenance(adjustment, lubrication, protection from rust and contamination).

On the powertrain, the clutch primarily is selected based on rated clutch torque capacity, which must be equal to or greater than the peak torque of the engine. When different sized clutches are available, the larger clutch with more heat capacity will provide longer life. The clutch disc damper specification should be based on the need for torsional resonance vibration control.

When selecting the transmission, the correct startup gear may vary with vehicle weight starting grade and road surface conditions. Remember that selecting a higher startup gear will increase clutch engagement slip time and may significantly reduce clutch life.

Vehicle startability is the most important spec'ing factor affecting clutch life. Component suppliers have a formula for startability, which is used to assess the starting capability of a powertrain specification. This ``startability factor' is calculated based on energy (heat), which must be absorbed by a clutch during vehicle start.

Startability must be determined in the normal startup gear, which is one that can be used to start comfortably without having to fuel the engine.

Clutch engagement torque is important to clutch life. The proper way to start a moving vehicle is to engage the clutch without fueling the engine. Only after the clutch is engaged should the throttle be used. Starting at higher engine rpm will shorten clutch life.

Final axle drive ratio usually is selected to complement transmission and engine choice, and to allow maximum cruising speed with optimized fuel economy. The drive ratio has a significant effect on clutch life and must be used to calculate vehicle startability.

Tire sizes can affect final drive ratios. However, this effect is limited and is not a major factor in clutch life.

Spec'ing Clutches Peak engine torque probably is the most important consideration when spec'ing clutches. If the clutch is not capable of transmitting the peak engine torque, as well as the related torsionals, the clutch or other driveline components will be damaged. Thus, the clutch and drivetrain must be capable of working with the peak engine torque at any time.

If the engine has variable torque, the clutch and drivetrain must be capable of the higher peak torque.

Clutches used to be selected based on peak engine torque and flywheel style. However, the selection process now is more complex due to "system" concerns driven by the interaction of all the drivetrain components.

Clutch damper types are not part of the clutch torque capacity calculation, but they can be critical to performance and life. Soft-rate damped driven disks are strongly recommended and sometimes required by clutch and truck manufacturers.

Increasing plate load due to increasing engine torque or to standardize at a higher plate load will result in an increased bearing load. This, in turn, translates into more pedal effort.

Replacements and Remans Replacement clutches should be the same configuration as the existing clutch, except that a rigid-type damper never should be used as a replacement. Organic facings should not be substituted for cerametallic, as slippage or premature failure can result.

A soft-damped clutch (dampened driven disc with a low spring rate) should not be replaced by a non-soft damped clutch (damped driven disc with a high spring rate). Potentially damaging torsional vibrations can occur.

To get the best performance, be sure that the remanufacturer uses original equipment manufacturer (OEM) grade linings and genuine OEM replacement parts. Also, be sure that the remanufacturer does not add components that were not used or included in the original clutch.

Pay attention to damper style. If the OEM used a soft-damped clutch, follow that design. A typical rebuilt remanufactured dampened disc that uses rubber encased springs have damper stiffness characteristics similar to rigids, and they are not effective replacements. Rigid discs never should be used as a replacement.

Finally, plate load, facings and dampers are critical areas to evaluate when making a remanufactured purchase. The use of original equipment facings and processes in place to control plate loads are positive signs that can guide buying decisions.

OEM remanufactured clutches will have genuine components as well as be subjected to quality and process controls.

Air disc brakes (ADBs) are the most effective braking systems available, yet North American fleet managers often do not spec them. Less than 1 percent of North American Class 5-8 new trucks and buses use ADBs, compared to Europe, where 94 percent of front and 40 percent of rear air disc brakes are ADBs.

This is due in part to the higher initial price, heavier weight, space needs and rotor durability vs. drum brakes, says Pretash Jain of Meritor Heavy Vehicle Systems, Troy, Mich. "There also have been many improvements to foundation drum brakes such as extended maintenance, better automatic adjustment and new lining formulations for better life."

Jain points to several other reasons why the Europeans accepted ADBs earlier. First, Europe has more rigorous breaking regulations than the United States, he says. This creates a demand for increased control, safety standards and restrictions on commercial vehicles, in which ADBs play a critical role. Nearly 100 percent of new hydraulic brake trucks have front disc brakes and 90 percent of rears have disc brakes.

Originally, OEMs held the largest share of the brake market with in-house produced S-Cam brakes. However, with OEMs spec'ing nonexistent today, ADBs are standard because companies are out-sourcing ADBs to replace their proprietary heavier brakes. In Europe, this has made disc brakes comparable to drum brake cost, Jain says.

However, ADBs are not standard in North American trucks. In fact, as an option, it costs about $500 a wheel end. Disc cracking and short pad life, tractor to trailer compatibility, increased lining life of drum brakes, improved drum brake performance and present, and near term market size also make persuading U.S. fleet managers to switch brakes difficult.

Disc brakes provide the best retardation performance, according to Jain. Benefits of this type of brake includes improved braking, fade resistance, reduced speed sensitivity and optimum stability (low sensitivity to friction variation, rapid burnishing/bedding and consistent performance new to worn). No periodic lubrication is required, radial pads lift out for easy replacement and they require only three minutes per axle to change pads, compared to seven minutes for drum brakes, Jain adds.

While he predicts the cost of ADBs to remain higher than the cost for drum brakes in North America, manufacturers are making attempts to overcome the obstacles. The North American ADB market is investigating unique sizes for packaging and torque output reasons, to overcome fleet managers' preferences to improved cam brakes and lower operating costs such as extended service cam brakes. A few manufacturers also are trying to offer cost-effective ADBs that prove the equipment provides shorter stopping distances than conventional tractor drum brakes.

How ADBs Work In an ADB, an advanced eccentric actuation mechanism increases the brakes' efficiency and clamping force distribution. This eccentric actuation increases the efficiency of the brake, which means more input force out in the brake load. The eccentric mechanism fits into a block housing with two pistons attached to a load plate. When the brake is applied, air chamber pressure rotates the actuation mechanism, pushing two pistons and the load plate against the inner shoe, applying more uniform braking pressure to the rotor.

The caliper, in turn, slides on pins as the outer load plate contracts the outer shoe, applying the same uniform braking pressure. This allows the ADB to produce higher torque for increased stopping performance and reduced stopping distances in a lighter weight package.

In addition to better brake stability, ADBs also reduce fade during stops, and provide a better brake feel (modulation), which can be especially helpful by reducing the potential for front-wheel lockup in non-ABS situations. The rotor expands toward the pads as heat builds, eliminating brake fade and maintaining effectiveness by keeping brake pedal pressure constant, Jain explains.

With controlled steering in extreme braking conditions, front disc brakes are less susceptible to torque variation encountered with drum brakes that causes brake pull. As an added component, an intermediate saddle design lets the caliper to float freely and puts most of the load on the saddle, resulting in longer component life, reduced drag and more uniform braking. According to Meritor, ADBs benefits include:

* Integral automatic adjusting mechanism, synchronously acting on the two actuating pistons;

* Flexible air chamber positioning for better packaging capability;

* Larger diameter rotor design for higher cooling rate and improved heat dissipation, which results in longer lining and rotor life;

* Easier maintenance.By reducing lining change-out time, ADBs can contribute to lower operating costs than drum brakes. Additionally, the internal adjustment mechanism eliminates the need for an external automatic slack adjuster;

* Time savings.It allows for quick and easy pad change with minimum dismantling; and

* Optimized pressure distribution between the pad and rotor via a twin piston design.

Finally, maintenance is lower because ADBs require no lubrication and brake pads can be changed in 80 percent less time than S-cam drum brake linings. And because ADBs use the same size brake unit on all axles, fewer parts are required, resulting in less inventory.

Belt chirps, rumbles and squeals can be caused by many factors. Nevertheless, any unusual drive belt noise should be investigated.

Intermittent belt chirps that increase as you rev your engine can be caused by belt vibration, which results from misaligned drive pulleys. A screeching or squealing noise that occurs when pulling away from a stop can indicate lack of tension, so check the belt tension and automatic tensioners.

Tapping or grinding noises commonly are caused by a pebble imbedded in the belt. However, grinding sounds also can result from damaged pulley bearings, which must be replaced, aligned and lubricated to eliminate the noise and further damage. Vibration and noise can develop over time as pulleys and spring tensioners wear out of tolerance, as bearings and brackets loosen, or as belts wear and stretch.

Determine whether a noise is loudest when the engine is cool, at idle speed, or when accelerating or shifting gears. To check this, spray a light mist on the belt. If the noise quiets for a very short time then returns, your belt probably is misaligned. If that doesn't work, remove the belt and reinstall so that it runs in the opposite direction.

For severe misalignment, you may have to use shims to reposition drive components or change the press fit of the pulley or shaft. Pulley alignment and tension must be correct on all V-ribbed applications to

Most drivers know how to properly use a clutch. But sometimes a reminder can be helpful to those that have developed ``sloppy' habits.

* Do not apply the clutch brake while the vehicle is moving.This will shorten its life. A worn out clutch brake also will cause gear clash in the transmission.

* Do not shift the transmission before attaining minimum shift speed. Improper shifting places a severe shock load on the drivetrain and may damage the disc hub assemblies.

* Do not coast downhill with the transmission in gear and the clutch disengaged.This puts a severe shock load on the drivetrain and can damage friction facings and the hub of the disc assemblies.

* Try to minimize the slip time during clutch engagement by depressing and releasing the clutch pedal quickly, but not abruptly. Minimizing the slip time reduces heat and wear on the facings, increases clutch life and decreases the number of times the clutch needs to be adjusted.

* Do not keep or rest a foot on the clutch pedal. Doing so while the vehicle is running will cause the clutch to be partially disengaged, resulting in too much slippage.

* Make sure the clutch pedal has at least 1/4-inch of free travel.If there is less than that or if there is a false free-play (when the pedal movement does not generate release fork movement), slippage can occur.

* Do not use the clutch as a brake, such as holding the vehicle on a hill.This results in faster disc lining wear and causes the clutch to overheat.