The following was published in CCJ's Air Brake Book , 11th Edition, sponsored by SilverbackHD . The CCJ Air Brake Manual is a supplemental industry resource thanks to our partnership with SilverbackHD, the Maintenance and Technology Council and the Commercial Vehicle Safety Alliance. You can download the complete book on air brakes here _
A balanced braking system is one in which all of the vehicle's brakes are applied and released approximately simultaneously, with each brake developing the correct stopping power for its respective load.
Looking at this concept, it's not hard to imagine the safety and maintenance issues that unbalanced brakes can cause on a commercial vehicle.
For example, if a tractor brakes more aggressively than the trailer it is towing, uneven brake wear is the most obvious consequence, as brake pads with increased wear wear faster than brakes that do not. Part of the job Worse, this condition could result in a trailer hitting the braking tractor, skidding, or stopping unintentionally if the brakes lock.
The frequency of brake imbalance situations has decreased somewhat thanks to improved braking technology. Today's braking systems consist of countless components that must work correctly and in the right order to ensure safe and effective braking. If only one of these components fails, the vehicle's braking system begins to work unsynchronized.
Brake imbalance is a warning sign that a braking system is not working as it should. Torque imbalance, pressure imbalance, and different tire sizes are common issues that can lead to a brake imbalance problem.
Each of these circumstances can be further complicated by the extreme temperatures at which basic brakes operate. For many technicians, tracking down a brake imbalance issue can be like a forensic investigator trying to uncover a crime scene. Many possibilities must be investigated and ruled out before the culprit is identified.
When considering the causes of brake imbalance, it is important to remember that if all the brakes on a heavily loaded station wagon are cold but poorly adjusted, the stopping distance of a vehicle equipped with drum brakes will increase.
When these poorly adjusted brakes get hot, their drums naturally expand, causing the pads to fade and the master cylinder to move more. In these cases, the vehicle can travel up to 75% more to come to a complete stop. Automatic valve lifters are designed to counteract this problem, but do not always react correctly due to worn parts.
maintaining torque balance
It's common sense: A brake shoe or pad with a thicker layer of friction material will engage sooner and more aggressively than a brake shoe or pad at the other end of the wheel with a thinner layer of material.
"Even pad or pad wear means balanced brakes," said Keith Roth, SilverbackHD's vice president of operations. "Uneven wear means unbalanced brakes."
Other factors can also degrade the braking power of the friction material. These include oil or grease on the lining or shoe, enameled friction material, polished drums or rotors, or linings and linings with mixed friction properties on one or more wheels.
Out-of-spec drums or rotors, improperly adjusted brakes, mismatched brake cylinder sizes, improperly installed automatic slack adjusters, inoperable or improperly adjusted antilock wheel brake sensors, and incorrect axle specific gross vehicle weight rating can also cause brake imbalance. Subjects. .
Differences between coatings are more of a nasty surprise when you least expect it. This is because the friction, discoloration and recovery behavior of coatings at different temperatures can vary greatly.
In the early days of asbestos-free linings, some friction materials swelled excessively when exposed to high temperatures. For this reason, swollen pads often caused well-adjusted brakes to slip after the pedal was released. However, once the fairing had cooled, it could not always be expected to return to its previous dimensions. In extreme cases, this permanent pad growth required clearance removal before the brakes could be released.
Other friction-induced imbalance problems remain. Edge codes, for example, with which the aggressiveness of a coating can be easily identified.
However, edge code markings often wear out as the friction material wears during braking. It's not a huge loss, however, as friction can vary by as much as 40% even within the same edge code.
To maintain a degree of consistency, always specify the same brand and type of liner for tractors and trailers, and use the same liner material. A good guide is TMC's RP 628C Aftermarket Brake Lining Classification, which provides an ever-changing list of different brands and types of pads that meet FMVSS-121 and their tightening torques.
Understand the couple's breakup.
Even if the tractor and trailer are initially well matched, the torque balance can deteriorate over time. Friction material can become contaminated from improperly installed and leaking wheel oil seals or from improper and overly liberal lubrication of the cam assembly on drum brakes.
Leaking oil seals on new equipment require spot checking of other units on the tractor or trailer as assembly line failures could be to blame. Leaking seals on older vehicles and equipment may indicate that higher quality seals need to be specified or technicians retrained in proper lubrication procedures. Another option is to specify "unified" (sealed) hubs on new machines.
Over time, brake drums can become deeply indented or bell-shaped, and brake discs can "bulge out" and even prevent contact with friction material on one or more wheels. Also, never assume that new or replacement drums are automatically ready for use. They may come with errors from the manufacturer, such as B. flared or eccentric, which requires turning on a lathe. In some cases it may be necessary to grind the trimming radius to get a proper fit. Although a slight heat test is acceptable, any drum or rotor with deep cracks should be discarded.
Remember that brake shoe return springs can stretch or even break over time. That's why it's a good idea to replace the springs every time you recoat your brakes, even if they look good. The same rule should be applied to reels that have gone flat.
S-cams can wear to the point where braking torque is also severely affected. Likewise, worn camshafts and their splines and bushings can affect braking performance. Pay special attention to the condition of the bushing as it is responsible for centering the cam and shoe assembly on the drum. Like the springs, the bushings must also be replaced every time the brake pads are replaced.
Over the years, more than one anchor pin has been removed with heat and a hammer. But this can warp the spider, and bent spiders worsen the contact between the liner and the drum. Use only light tapping, or better yet, use specially designed pullers to remove anchor pins when servicing brakes.
Slipping disc brake calipers can seize, leading to accelerated wear of the inner disc brake pad. To combat this problem, make sure the caliper pins and sliding surfaces are properly lubricated for the caliper to function properly. Gum and carbon deposits from oil and water contaminated air can clog valves over time, causing them to slow down or fail. There's an easy way to avoid this disease: make sure the air tanks are emptied regularly and specify an air dryer if you don't already have one.
Upgrading to the wrong size master cylinder or slack adjuster will affect performance and compatibility.
It is also not recommended to mix two brands of automatic slack adjusters on the same axle as they will not function identically and will cause uneven brake wear. And while they are very reliable components, automatic lash adjusters can fail or wear out over time. Lubricate them sufficiently and measure the push rod caster when the brakes are applied. Inspecting the assembly for excessively worn triple clamp and slack adjuster holes, worn clevis pins and general play should also be a standard maintenance procedure.
To ensure proper performance, ABAs must be mounted at the correct angle, which is determined using installation templates that vary by application and brand. However, in the real world, the mounting bracket can be affected by OEM clearance issues. For this reason, the play must be checked before changing the installation position.
Low profile tires can save you money at the fuel pump, but mismatched low profile tires on the tractor and trailer can cause brake compatibility issues. And it's a mistake to fit aftermarket tires without reconfiguring the vehicle's braking system.
A vehicle or tractor with low profile tires that have a radius 18% smaller than the original tires may cause the vehicle to brake excessively. In fact, an 18% reduction in rolling radius can result in an 18% increase in braking power, causing lightly loaded non-ABS trailers to lock up under normal braking.
And because smaller tires spin faster at a given speed on the freeway, the liners will grab the drums at higher rpm and get hotter, especially when braking downhill. If you want to specify low profile tires, consider specifying the next smaller tube on the vehicle, which will reduce torque by about 20%. And switching to a less aggressive setting, or, with technical approval, introducing some sort of pressure modifier into the system, will also help correct an excessive braking problem.
Constant overheating, localized wear due to lack of even contact with friction material, or contact with abrasive material can damage drums and rotors. Always inspect rotors and drums during relining work. All highly polished friction surfaces should be sanded with 80 grit emery cloth and when accompanied by a glaze on shoes or pads should prompt a search for a more suitable friction material.
Foreign abrasive materials can also cause excessive wear at the edges of the trailer cowling's contact area or in areas coincident with the cowling's rivet holes. In this case, remove the bottom dust cover (if installed) to provide an exit for foreign objects. When inspecting a drum for excessive wear, its inside diameter should not exceed original specification by more than 0.12 inch.
When resurfacing drums, the finished inside diameter must not exceed 0.08 inch beyond the original specification and run out must not exceed 0.01 inch. The same applies to brake discs. When checking rotor thickness, they should be no more than 0.12 inches below original specs and come back no more than 0.08 inches below original specs. Lateral runout should not exceed 0.01 inch. In any case, it is important to follow each manufacturer's turning and finishing requirements.
pneumatic imbalance
A tire (or air pressure) imbalance occurs when a truck's air system supplies improper air pressure to the vehicle's brake chambers. This is often caused by incorrectly specified or defective relay valves, but quick exhaust valves can also disturb the air pressure balance. Other common causes of air pressure imbalances include air leaks, air system contamination, a front axle limiter valve and excessive use of the trailer manual control valve. (See SAE J1505 and 1860 for more information.)
Timing imbalance occurs when some brakes are released faster than others. Common causes include oversized control lines (found on pre-1991 trailers) preventing brake application; poor plumbing design or installation; and no use of pressure relief valves, if any.
Most manufacturers say that maintaining proper air balance is critical to improving braking response. Ideal air balance is achieved when the air supplied to each axle does not vary more than 2 psi when 10 to 40 psi is applied. An exception to this rule would be incorrectly coupling an S-cam equipped tractor to a wedge brake trailer. Because wedge brakes have smaller chambers and require more psi than S-cams for the pads to contact the drum, the trailer with wedge brakes would require higher air pressure than the tractor to compensate for the braking.
low pressure imbalance
Les engineers du système de freinage design qu'environne 95% du freinage implica des pressions d'application pour les applications de transport de ligne inférieures à 20 psi, et around 84% du freinage est efectué à des pressures d'application de 15 psi ou fewer.
When Federal Motor Vehicle Safety Standard 121 went into effect in 1975, trailers had to be compatible with a tractor simulator that blew out a large puff of air. To meet timing requirements, trailers required 1⁄2 inch (3⁄8 inch ID) OD control lines instead of 3⁄8 inch (1⁄4 inch ID) OD lines. However, during normal braking, a tractor does not provide enough air to fill a trailer's oversized control line. As a result, trailer braking is delayed and the problem is magnified in multi-trailer combinations.
In some cases, this delayed air supply causes the driver to feel a noticeable push from behind as the trailer pushes the power unit forward. At its extreme, that bump can quickly turn into a full thrust under braking in slippery conditions or through a corner, resulting in a jackknife.
To eliminate the delay, the National Highway Traffic Safety Administration modified the tractor simulator and changed the maximum mooring/release times for trailers built on or after May 3, 1991, and specified air supply times for the non-contact control line at the rear of tractors. Trailers and dollies built on or after May 3, 1991 that should theoretically be rare today.
However, when working with older tractors and trailers, trailer jolt can be eliminated by installing a smaller control line on the trailer and making tractor modifications that speed up manual braking. This allows the trailer brakes to be applied faster during normal braking without reducing stopping distances during panic stops.
high pressure imbalance
Conversely, while a tractor's brakes do most of the work, a station wagon cannot decelerate quickly without heavy pressure from the driver on the brake pedal.
And although ABS will prevent over-braked wheels from locking, it is not a substitute for a properly balanced braking system. Sustained high pressure braking of an ABS equipped tractor is not recommended as a non ABS equipped trailer (or one with a non-functional ABS) may gain enough air to lock up its brakes, potentially driving it out of its lane. Worse still, a tractor with non-working ABS is likely to be damaged by sustained hard braking. For these reasons, the NHTSA advises drivers not to interfere with their normal braking habits when operating ABS-equipped station wagons.
Air disc brakes provide more effective braking on tractors, which could be a problem when paired with trailers still fitted with drum brakes. Trailer drum brakes tend to fade faster when paired with tractor air disc brakes. As a result, the tractor brakes work harder to slow the header, resulting in unbalanced brakes.
The key to solving this high pressure imbalance is the trailer cracking pressure. Cracking Pressure, expressed in psi, is the air required to open a valve when the air brakes are applied. Some tractors rapidly draw air into their own brakes before sending it to the trailer behind them. However, some trailers withstand a tractor's air because they have a relay valve set with a relatively high cracking pressure.
In general, valve character, including opening pressure, has minimal impact on high-pressure braking, and mainly affects low-pressure braking and wear.
Remember that upgrading to aftermarket or remanufactured air valves can destroy pneumatic balance since the opening pressure of relays or quick release valves fitted with aftermarket springs can vary widely. Just because a valve "looks good" or "fits" doesn't mean it's a suitable OE replacement. Even when substituted valves of the same make and model are used, burst pressures and pressure differentials may vary due to differences in orifice size and manufacturing tolerances. (See SAE 1860 for more information.)
oversized brakes
A final but relatively simple problem that causes brake imbalance is a tractor with axles that have been oversized for the loads it is carrying. Some fleets do this to increase the tractor's resale value or to extend axle life by using larger gears and bearings. But the safety considerations far outweigh any increase in payload or durability, as the tractor will over-brake whenever the axle load is well below its rated capacity. The same rule applies to trailers with over-specified axles. This can lead to compatibility, wear and maintenance problems.
One way to fix an over-spec truck or trailer is to switch to less aggressive brake pads. Another option (not always possible) is to connect the cam pushrods to a different hole in the hydraulic valve lifter, which will reduce braking power. However, keep in mind that the spacing between the clearance adjustment holes varies by brand. It is advisable to seek technical advice from your vehicle's original equipment manufacturer or supplier of braking components before making any modifications.
It's also a good idea to conduct an on-site braking torque and balance test before making any fleet-wide changes. A suitable procedure is suggested by Recommended Practice (RP) 613 "Brake System Torque Equalization Test Procedure", which is offered by the Maintenance and Technology Council of the American Truck Associations.
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