A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating bowl-shaped part called a brake drum.
The term drum brake usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or "band" wrapping around the outside of a drum.
The modern automobile drum brake was first used in a car made by Maybach in , although the principle was only later patented in by Louis Renault. He used woven asbestos lining for the drum brake lining, as no alternative material dissipated heat more effectively, though Maybach had used a less sophisticated drum brake. In the first drum brakes, levers and rods or cables operated the shoes mechanically. From the mid-s, oil pressure in a small wheel cylinder and pistons (as in the picture) operated the brakes, though some vehicles continued with purely mechanical systems for decades. Some designs have two wheel cylinders.
As the shoes in drum brakes wear, brakes required regular manual adjustment until the introduction of self-adjusting drum brakes in the s. Drum brakes are also prone to brake fade with repeated use.[1]
Jaguar Cars fielded three cars equipped with disc brakes at Le Mans in , where they won, in large part due to their superior braking over drum-equipped rivals.[2] This spelled the beginning of the end for drum brakes in passenger cars. From the s to the s, disc brakes gradually replaced drum brakes on the front wheels of cars (which receive the majority of braking force). Now practically all cars use disc brakes on the front wheels, and many use disc brakes on all four wheels.
In the United States, the Jeep CJ-5 (manufactured by AM General) was the final automobile (produced for the United States Postal Service) to use front drum brakes when it was phased out in . However, drum brakes are still often used on the rear wheels, and for parking brakes. Some vehicles utilize a "drum-in-hat" parking brake, where the brake shoes are arranged inside the center portion (hat) of a disc brake rotor, which acts as the drum.[3]
Early brake shoes contained asbestos. When working on brake systems of older cars, care must be taken not to inhale any dust present in the brake assembly. After the United States Federal Government began to regulate asbestos production, brake manufacturers had to switch to non-asbestos linings. Owners initially complained of poor braking with the replacements, but brake technology eventually advanced to compensate. A majority of daily-driven older vehicles have been fitted with asbestos-free linings. Many other countries have also prohibited the use of asbestos in brakes.
Drum brake components include the backing plate, brake drum, shoe, wheel cylinder, and various springs and pins.
The backing plate provides a base for the other components. The back plate also increases the rigidity of whole set-up, supports the housing, and protects it from foreign materials like dust and other road debris. It absorbs the torque from the braking action, and that is why back plate is also called the "Torque Plate". Since all braking operations exert pressure on the backing plate, it must be strong and wear-resistant. Levers for emergency or parking brakes, and automatic brake-shoe adjuster were also added in recent years.
The brake drum is generally made of a special type of cast iron that is heat-conductive and wear-resistant. It rotates with the wheel and axle. When a driver applies the brakes, the lining pushes radially against the inner surface of the drum, and the ensuing friction slows or stops rotation of the wheel and axle, and thus the vehicle. This friction generates substantial heat.
One wheel cylinder operates the brake on each wheel. Two pistons operate the shoes, one at each end of the wheel cylinder. The leading shoe (closest to the front of the vehicle) is known as the primary shoe. The trailing shoe is known as the secondary shoe. Hydraulic pressure from the master cylinder acts on the piston cup, pushing the pistons toward the shoes, forcing them against the drum. When the driver releases the brakes, the brake shoe springs restore the shoes to their original (disengaged) position. The parts of the wheel cylinder are shown to the right.
Brake shoes are typically made of two pieces of steel welded together. The friction material is either riveted to the lining table or attached with adhesive. The crescent-shaped piece is called the Web and contains holes and slots in different shapes for return springs, hold-down hardware, parking brake linkage and self-adjusting components. All the application force of the wheel cylinder is applied through the web to the lining table and brake lining. The edge of the lining table generally has three V-shaped notches or tabs on each side called nibs. The nibs rest against the support pads of the backing plate to which the shoes are installed. Each brake assembly has two shoes, a primary and secondary. The primary shoe is located toward the front of the vehicle and has the lining positioned differently from the secondary shoe. Quite often, the two shoes are interchangeable, so close inspection for any variation is important.
Linings must be resistant to heat and wear and have a high friction coefficient unaffected by fluctuations in temperature and humidity. Materials that make up the brake shoe lining include, friction modifiers (which can include graphite and cashew nut shells), powdered metal such as lead, zinc, brass, aluminium and other metals that resist heat fade, binders, curing agents and fillers such as rubber chips to reduce brake noise.
In the UK two common grades of brake shoe material used to be available. DON 202 was a high friction material that did not require a brake power servo. The disadvantage was that the lining was prone to fading on steep hills. A harder lining, the famous VG95 was produced but this required a brake servo. The other snag was that the parking brake would often fail the annual MOT test unless the high friction linings were installed just for the test.
When the brakes are applied, brake fluid is forced under pressure from the master cylinder into the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface on the inside of the drum. This rubbing action reduces the rotation of the brake drum, which is coupled to the wheel. Hence the speed of the vehicle is reduced. When the pressure is released, return springs pull the shoes back to their rest position.
As the brake linings wear, the shoes must travel a greater distance to reach the drum. In systems fitted with automatic adjusters, when the distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the rest position of the shoes so that they are closer to the drum. Here, the adjusting lever rocks enough to advance the adjuster gear by one tooth. The adjuster has threads on it, like a bolt, so that it unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes wear a little more, the adjuster can advance again, so it always keeps the shoes close to the drum. Typically the adjusters only operate when the vehicle is going in reverse and the brakes are engaged.
On vehicles without automatic adjusters, it is required to periodically manually adjust the brakes to take up any excess gap between the shoes and the drum.
The parking (or emergency) brake system controls the brakes through a series of steel cables that are connected to either a hand lever or a foot pedal. The idea is that the system is fully mechanical and completely bypasses the hydraulic system so that the vehicle can be brought to a stop even if there is a total brake failure. Here the cable pulls on a lever mounted in the brake and is directly connected to the brake shoes. This has the effect of bypassing the wheel cylinder and controlling the brakes directly.[4]
Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing."[5] The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.
Disc brakes exhibit no self-applying effect because the hydraulic pressure acting on the pads is perpendicular to the direction of rotation of the disc.[5] Disc brake systems usually have servo assistance ("Brake Booster") to lessen the driver's pedal effort, but some disc braked cars (notably race cars) and smaller brakes for motorcycles, etc., do not need to use servos.[5]
Drum brakes are typically described as either leading/trailing (also called "single leading") or twin leading.[5]
Rear drum brakes are typically of a leading/trailing design (for non-servo systems), or primary/secondary (for duo servo systems), the shoes being moved by a single double-acting hydraulic cylinder and hinged at the same point.[5] In this design, one of the brake shoes always experiences the self-applying effect, irrespective of whether the vehicle is moving forwards or backwards.[5] This is particularly useful on the rear brakes, where the parking brake (handbrake or footbrake) must exert enough force to stop the vehicle from traveling backwards and hold it on a slope. Provided the contact area of the brake shoes is large enough, which isn't always the case, the self-applying effect can securely hold a vehicle when the weight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage of using a single hydraulic cylinder on the rear is that the opposite pivot may be made in the form of a double-lobed cam that is rotated by the action of the parking brake system.
Front drum brakes may be of either design in practice, but the twin leading design is more effective.[5] This design uses two actuating cylinders arranged so that both shoes use the self-applying characteristic when the vehicle is moving forwards.[5] The brake shoes pivot at opposite points to each other.[5] This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is traveling in reverse.[5]
The optimum arrangement of twin leading front brakes with leading/trailing brakes on the rear allows more braking force at the front of the vehicle when it is moving forwards, with less at the rear. This helps prevent the rear wheels from locking up, but still provides adequate braking at the rear.[5]
Because aluminum wears more easily than iron, aluminum drums frequently have an iron or steel liner on the inner surface of the drum, bonded or riveted to the aluminum outer shell.
Drum brakes are used in most heavy duty trucks, buses, some medium and light duty trucks, and a few cars, dirt bikes, ATVs, and a few smaller recreational vehicles like electric scooters. Drum brakes are often applied to the rear wheels since most of the stopping force is generated by the front brakes of the vehicle, and therefore, the heat generated in the rear is significantly less. Drum brakes allow simple incorporation of a parking brake.
Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels use disc brakes as the main brakes. Many rear disc braking systems use a parking brake in which the piston in the caliper is actuated by a cam or screw. This compresses the pads against the rotor. However, this type of system becomes much more complicated when the rear disc brakes use fixed, multi-piston calipers. In this situation, a small drum is usually fitted within or as part of the brake disc. This type of brake is also known as a banksia brake.
In hybrid and electric vehicle applications, wear on braking systems is greatly reduced by energy recovering motor–generators (see regenerative braking), so some hybrid vehicles such as the Toyota Prius (prior to the third generation) and Volkswagen ID.3 and ID.4 use drum brakes at the rear wheels.
Disc brakes rely on pliability of caliper seals and slight runout to release pads, leading to drag, fuel mileage loss, and disc scoring. Drum brake return springs give more positive action and, adjusted correctly, often have less drag when released. It is however possible to design special seals that retract the piston on a disc brake.
Drum brakes emit less particulate matter (PM) than disc brakes, as the wear-particles are mostly sealed in. They are not better in this regard than frictionless brakes though.[6][7]
Certain heavier-duty drum brake systems compensate for load when determining wheel cylinder pressure; a feature which is rare when discs are employed (hydropneumatic suspension systems as employed on Citroën vehicles adjust brake pressure depending on load regardless of if drum or discs are used). One such vehicle is the Jeep Comanche. The Comanche can automatically send more pressure to the rear drums depending on the size of the load. Most other brands have used load sensing valves in the hydraulics to the rear axle for decades.
Due to the fact that a drum brake's friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and braking force. Drum brakes retain heat and are more complex than disc brakes, but are often the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brake's self-applying nature, larger friction surface contact area, and long life wear characteristics (% life used / kW of braking power).
To list advantages of drum brakes:
Drum brakes have also been built onto the transmission's driveshaft as parking brakes (e.g., Chryslers through ). This provides the advantage that it is completely independent of the service brakes—but suffers a severe disadvantage in that, when used with a bumper jack (common in that era) on the rear, and without proper wheel blocks, the differential's action can allow the vehicle to roll off the jack.
Land Rover have used a drum brake on the gearbox output shaft for over fifty years. The advantage is that all four wheels can be braked with the parking brake.
Drum brakes, like most other brakes, convert kinetic energy into heat by friction.[5] This heat should dissipate into the surrounding air, but can just as easily transfer to other braking system components. Brake drums must be large to cope with the massive forces involved, and must be able to absorb and dissipate a lot of heat. Heat transfer to air can be aided by incorporating cooling fins onto the drum, or even drilling holes around the drum's circumference. However, excessive heating can occur due to heavy or repeated braking, which can cause the drum to distort, leading to vibration under braking.
The other consequence of overheating is brake fade.[5] This is due to one of several processes, or more usually an accumulation of all of them.
Brake fade is not always due to overheating. Water between the friction surfaces and the drum can act as a lubricant and reduce braking efficiency.[5] The water tends to stay until heated sufficiently to vaporize, at which point braking efficiency returns. All friction braking systems have a maximum theoretical rate of energy conversion. Once that rate is reached, applying greater pedal pressure doesn't change it—in fact, the effects mentioned can substantially reduce it. Ultimately, this is what brake fade is, regardless of the mechanisms of its causes. Disc brakes are not immune to any of these processes, but they deal with heat and water more effectively than drums.
Drum brakes can be grabby if the drum surface gets light rust or if the brake is cold and damp, giving the pad material greater friction. Grabbing can be so severe that the tires skid and continue to skid even when the pedal is released. Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of drum brakes causes application force to go up. If the pad friction and self-amplification are high enough, the brake stays engaged due to self-application, even when the external application force is released.
While disc brake rotors can be machined to clean the friction surface (i.e., 'turning'), the same generally cannot be done with brake drums. Machining the friction surface of a brake drum increases the diameter, which might require oversized shoes to maintain proper contact with the drum. However, since oversized shoes are generally unavailable for most applications, worn or damaged drums generally must be replaced.
It is quite simple to machine brake drums if one has a slow running lathe (one rule of thumb is that cast iron should not be machined faster than fifty feet per minute). Usually it is only necessary to machine away the ridge that forms that makes brake drum removal difficult, especially if the brakes are self-adjusting. In severe cases the ridge can make the brake drum captive, however most drum brake designs provide a way to externally release the self-adjusting mechanism in order to ease drum removal and service.
Another disadvantage of drum brakes is their relative complexity. A person must have a general understanding of how drum brakes work and take several simple steps to ensure the brakes are reassembled correctly when doing work on drum brakes. And, as a result of this increased complexity (compared to disc brakes), maintenance of drum brakes is generally more time-consuming. Also, the greater number of parts results in a greater number of failure modes compared to disc brakes. Springs can break from fatigue if not replaced along with worn brake shoes. And the drum and shoes can become damaged from scoring if various components (such as broken springs or self-adjusters) break and become loose inside the drum.
Catastrophic failure of hardware such as springs and adjusters can also cause unintended brake application or even wheel lockup. If springs break, the shoes will be free to fall against the rotating drum, essentially causing the brakes to be applied. Because of the self-energizing qualities of drum brakes, the unrestrained shoes can even potentially cause the brakes to grab to the point of locking up the wheel. Also, broken pieces of springs and other hardware (like adjusters) can become lodged between the shoes and drum, resulting in unintended application of the brakes (and, as stated above, damage to brake components). For these reasons, brake hardware (such as springs and clips) should always be replaced with brake shoes.
Also, drum brakes do not apply immediately when the wheel cylinders are pressurized, because the force of the return springs must be overcome before the shoes start to move towards the drum. This means that the very common hybrid disc/drum systems only brake with the (nearly always front) discs on light pedal pressure unless extra hardware is added. In practice, a metering valve prevents hydraulic pressure from reaching the front calipers until pressure rises enough to overcome the return springs in the drum brakes. If the metering valve were left out, the vehicle would stop only with the front discs unless the driver used enough brake pedal pressure to overcome the return spring pressure on the rear shoes.
When asbestos was common in drum brakes, there was a danger workers repairing or replacing them would breathe asbestos fibers, which can cause mesothelioma.[8] Asbestos fibers would break off or become separated over time and with the high temperatures induced by braking. Wet brushes and aerosol sprays were commonly used to reduce dust. Safety regulators sometimes recommended using vacuum hoses to suck away the dust, or enclosures with interior lighting and space to use tools inside them, but these were rare and cumbersome. Distinctive shoes designed to protect against asbestos were also recommended.[9] There is evidence that auto mechanics had disproportionate levels of mesothelioma.[8]
Those who do maintenance work on brakes can also be exposed to the solvents 1,1,1-trichloroethane and 2-butoxyethanol (a main ingredient in Greasoff No. 19). Exposure to these solvents can cause irritation, including to the eyes and mucous membranes. Exposure to 1-1-1-trichloroethane vapors can cause central nervous system damage, dizziness, incoordination, drowsiness, and increased reaction time.[9]
Before , it was common to re-arc brake shoes to match the arc within brake drums. This practice was controversial however, as it removed friction material from the brakes, reduced the life of the shoes and created hazardous asbestos dust. After , the current design theory was altered, to use shoes for the proper diameter drum, and to simply replace the brake drum when necessary, rather than re-arc the shoes.
The brake drum has found popular use as a percussion instrument. This was likely first implemented in a composition First Construction (in Metal) by American avant-garde musician John Cage. In more recent times the brake drum has become associated with the front ensemble as used in the marching arts.[10]
This section tells you about air brakes. If you want to drive a truck, bus, or pull a trailer with air brakes, you need to read this section. If you want to pull a trailer with air brakes, you also need to read Section 6: Combination Vehicles in this handbook.
Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large and heavy vehicles, but the brakes must be well maintained and used properly.
Air brakes are really 3 different braking systems: service brake, parking brake, and emergency brake. The:
CDL Air Brake Requirements. For CDL purposes, a vehicle’s air brake system must meet the above definition and contain the following, which will be checked during the vehicle inspection test:
If the vehicle you use for your road test does not have these components, your vehicle will not be considered as having an air brake system and you will have a “No Air Brakes” (“L”) restriction on your CDL.
Note A full service brake application must deliver to all brake chambers not less than 90 percent of the air reservoir pressure remaining with the brakes applied (CVC §).
The parts of these systems are discussed in greater detail in the following paragraphs.
There are many parts to an air brake system. You should know about the parts discussed here.
The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a v-belt. The compressor may be air cooled or cooled by the engine cooling system. It may have its own oil supply or be lubricated by engine oil. If the compressor has its own oil supply, check the oil level before driving.
The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the “cut-out” level (around 125 pounds per-square-inch or “psi”), the governor stops the compressor from pumping air. When the tank pressure falls to the “cut-in” pressure (around 100 psi), the governor allows the compressor to start pumping again.
Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times, even if the compressor stops working.
Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. The water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are 2 types:
Automatic air tanks are available with electric heating devices. These help prevent freezing of the automatic drain in cold weather.
Figure 5.1
Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.
Check the alcohol container and fill up as necessary. (every day during cold weather). Daily air tank drainage is still needed to get rid of water and oil (unless the system has automatic drain valves).
A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic.
You engage the brakes by pushing down the brake pedal (It is also called a foot valve or treadle valve). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work.
Foundation brakes are used at each wheel. The most common type is the S-cam drum brake. The parts of the brake are discussed below.
Brake Drums, Shoes, and Linings. Brake drums are located on each end of the vehicle’s axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working.
S-cam Brakes. When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This turns the S-cam (it is shaped like the letter “S”). The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again. See Figure 5.2.
CamLaster. The CamLaster brake has 2 key design differences over traditional S-cam brakes.
One feature is a completely internal adjustment system which is designed to continually keep the brake in proper adjustment. S-cam brakes, on the other hand, require an external slack adjuster. The second feature is a unique cam design that applies the brake shoe. Unlike a standard drum brake that has either a single or double anchor-pin brake, the CamLaster slides the shoes down an inclined ramp on a cam to evenly contact the brake drum.
Figure 5.2
Wedge Brakes. In this type of brake, the brake chamber push rod pushes a wedge directly between the ends of 2 brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber or 2 brake chambers that push wedges in at both ends of the brake shoes. Wedge type brakes may be self-adjusting or may require manual adjustment.
Disc Brakes. In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a “power screw” is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large c-clamp.
Wedge brakes and disc brakes are less common than S-cam brakes.
All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system (or a single gauge with two needles). Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.
This gauge shows how much air pressure you are applying to the brakes. (This gauge is not on all vehicles.) Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. Brakes that are of adjustment, air leaks, or mechanical problems can also cause the need for increased pressure.
A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on when the air pressure in the tanks falls between 55 and 75 psi (or 1/2 the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer may also come on.
Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops between 55 and 75 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 55 and 75 psi. The manual reset type must be placed in the “out of view” position manually. It will not stay in place until the pressure in the system is above 55 psi.
On large buses, it is common for the low pressure warning devices to signal at 80–85 psi.
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Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you put on the air brakes.
Some vehicles made before have a front brake limiting valve and a control in the cab. The control is usually marked “normal” and “slippery.” When you put the control in the “slippery” position, the limiting valve cuts the “normal” air pressure to the front brakes by half. Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front wheel braking is good under all conditions. Tests have shown front wheel skids from braking are not likely even on ice. Make sure the control is in the “normal” position to have normal stopping power.
Many vehicles have automatic front wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard (60 psi or more application pressure). The driver cannot control these valves.
All trucks, truck tractors, and buses must be equipped with emergency brakes and parking brakes. They must be held on by mechanical force (because air pressure can eventually leak away). Spring brakes are usually used to meet these needs. Powerful springs are held back by air pressure when driving. If the air pressure is removed, the springs put on the brakes. A parking brake control in the cab allows the driver to let the air out of the spring brakes. This lets the springs put the brakes on. A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes.
Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi (typically 20 to 30 psi). Do not wait for the brakes to come on automatically. When the low air pressure warning light, and buzzer first come on, bring the vehicle to a safe stop right away, while you can still control the brakes.
The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the regular brakes nor the emergency/parking brakes will work right.
In newer vehicles with air brakes, you put on the parking brakes using a diamond-shaped, yellow, push-pull control knob. You pull the knob out to put the parking brakes (spring brakes) on, and push it in to release them. On older vehicles, the parking brakes may be controlled by a lever. Use the parking brakes whenever you park.
Caution. Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure. Many brake systems are designed so this will not happen. Not all systems are set up that way, and those that are may not always work. It is much better to develop the habit of not pushing the brake pedal down when the spring brakes are on.
Modulating Control Valves. In some vehicles a control handle on the dash board may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.
Dual Parking Control Valves. When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency. One of the valves is a push-pull type and is used to put on the spring brakes for parking. The other valve is spring loaded in the “out” position. When you push the control in, air from the separate air tank releases the spring brakes so you can move. When you release the button, the spring brakes come on again. There is only enough air in the separate tank to do this a few times. Therefore, plan carefully when moving. Otherwise, you may be stopped in a dangerous location when the separate air supply runs out. See Figure 5.3.
Figure 5.3
Truck tractors with air brakes built on or after March 1, , and other air brakes vehicles (trucks, buses, trailers, and converter dollies) built on or after March 1, , are required to be equipped with anti-lock brakes. Many commercial vehicles built before these dates have been voluntarily equipped with ABS. Check the certification label for the date of manufacture to determine if your vehicle is equipped with ABS. ABS is a computerized system that keeps your wheels from locking up during hard brake applications.
On newer vehicles, the malfunction lamp comes on at start-up for a bulb check, and then goes out quickly. On older systems, the lamp could stay on until you are driving over 5 mph.
Test Your Knowledge
These questions may be on your test. If you cannot answer them all, reread Subsection 5.1.
Figure 5.4
Most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has 2 separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle (and possibly one rear axle). Both systems supply air to the trailer (if there is one). The first system is called the “primary” system. The other is called the “secondary” system. See Figure 5.4.
Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has 2 needles in one gauge). Pay attention to the low air pressure warning light and buzzer. The warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 55 psi.
The warning light and buzzer should come on before the air pressure drops below 55 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop, and have the air brakes system fixed.
This device allows air to flow in one direction only. All air tanks on air-brake vehicles must have a check valve located between the air compressor and the first reservoir (CVC §). The check valve keeps air from going out if the air compressor develops a leak.
You should use the basic 7-step inspection procedure described in Section 2 to inspect your vehicle. There is more to inspect on a vehicle with air brakes than one without them. These components are discussed below, in the order that they fit into the 7-step method.
Check the air compressor drive belt (if the compressor is belt-driven). If the air compressor is belt-driven, check the condition and tightness of the belt. It should be in good condition.
Check slack adjusters on S-cam brakes. Park on level ground and chock the wheels to prevent the vehicle from moving. Release the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment. Adjust it or have it adjusted. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspections. Be safe. Check the slack adjusters.
All vehicles built since have automatic slack adjusters. Even though automatic slack adjusters adjust themselves during full brake applications, they must be checked.
Automatic adjusters should not have to be manually adjusted except when performing maintenance on the brakes and during installation of the slack adjusters. In a vehicle equipped with automatic adjusters, when the pushrod stroke exceeds the legal brake adjustment limit, it is an indication that a mechanical problem exists in the adjuster itself, a problem exists with the related foundation brake components, or the adjuster was improperly installed.
The manual adjustment of an automatic adjuster to bring a brake pushrod stroke within legal limits is generally masking a mechanical problem and is not fixing it. Further, routine adjustment of most automatic adjusters will likely result in premature wear of the adjuster itself. It is recommended that when brakes equipped with automatic adjusters are found to be out of adjustment, the driver takes the vehicle to a repair facility as soon as possible to have the problem corrected. The manual adjustment of automatic slack adjusters is dangerous because it may give the driver a false sense of security regarding the effectiveness of the braking system.
The manual adjustment of an automatic adjuster should only be used as a temporary measure to correct the adjustment in an emergency situation. It is likely the brake will soon be back out of adjustment since this procedure usually does not fix the underlying adjustment problem.
Note Automatic slack adjusters are made by different manufacturers and do not all operate the same. Therefore, the specific manufacturer’s service manual should be consulted prior to troubleshooting a brake adjustment problem.
Brake drums (or discs) must not have cracks longer than 1/2 the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease and must not be worn dangerously thin (less than 1/4 inch). Mechanical parts must be in place, not broken, or missing. Check the air hoses connected to the brake chambers to make sure they are not cut or worn due to rubbing.
All air brake system tests in this section are considered important and each can be considered critical parts of the in-cab air brakes tests. The items marked with an asterisk (*) in this section are required for testing purposes during the vehicle inspection portion of the CDL skills test. They may be performed in any order as long as they are performed correctly and effectively. If these items are not demonstrated and the parameters for each test are not verbalized correctly, it is considered an automatic failure of the vehicle inspection portion of the skills test.
Do the following checks instead of the hydraulic brake check shown in Section 2, Step 7: Check Brake System.
To perform this test, the driver must start with the engine running and with the air pressure built to governor cut-out (120–140 psi or another level specified by the manufacturer). The driver identifies when cut-out occurred, shuts off the engine, chocks the wheels if necessary, releases the parking brake (all vehicles) and tractor protection valve (combination vehicle), and fully applies the foot brake. The driver then holds the foot brake for 1 minute after stabilization of the air gauge. The driver checks the air gauge to see that the air pressure drops no more than 3 pounds in one minute (single vehicle) or 4 pounds in 1 minute (combination vehicle) and listens for air leaks. The driver must identify how much air the system lost and verbalize the maximum air loss rate allowed for the representative vehicle being tested.
Note For a Class A combination vehicle, if the power unit is equipped with air brakes and the trailer is equipped with electric/surge brakes, the pressure drop should be no more than 3 psi.
Important The maximum air loss rate for a combination of 2 or more vehicles is 3 psi if the towed vehicles are not equipped with air brakes.
An air loss greater than those listed above, indicates a problem in the braking system and repairs are needed before operating the vehicle. If the air loss is too much, check for air leaks and fix any that are identified.
Note For testing purposes, you must be able to demonstrate this test and verbalize the allowable air loss for your vehicle. For testing purposes, identify if the air loss rate is too much.
To perform this test the vehicle must have enough air pressure so the low-pressure warning signal is off. The engine maybe on or off; however, the key must be in the “on” or “battery charge” position. Next, the driver begins fanning off the air pressure by rapidly applying and releasing the foot brake. Low-air warning devices (buzzer, light, and flag) must activate before air pressure drops below 55 psi or the level specified by the manufacturer. The driver must indicate the approximate pressure when the device gave warning and identify the parameter at which this must occur; no lower than 55 psi. See Figure 5.5.
For testing purposes, identify and verbalize the pressure at which the low air pressure warning signal activates and identify the parameter(s) at which this should occur. On large buses, it is common for low-pressure warning devices to signal at 80–85 psi. If testing in a large bus, identify the parameter(s) mentioned above (55–75 psi) and inform the examiner that your vehicle’s low-pressure warning devices are designed to activate at a higher pressure.
If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single-circuit air system. In dual systems, the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.
Note Farm labor vehicles and Type I school buses must be equipped with both an audible and visible type warning device.
Figure 5.5
To perform this test, the parking brake (all vehicles) and tractor protection valve (combination vehicles) must be released; (engine running or not) as the driver fans off the air pressure. Normally between 20-45 psi (or the level specified by the manufacturer) on a tractor-trailer combination vehicle, the tractor protection valve and parking brake valve should close (pop out). On other combination vehicle types and single vehicle types, the parking brake valve should close (pop out). The driver must identify and verbalize the approximate pressure at which the brake(s) activated.
Note The parking brake valve will not pop out on buses that are equipped with an emergency park brake air reservoir (tank). If your bus is equipped with an emergency park brake air tank, you must perform the spring brake test for triple reservoir vehicles to check the automatic actuation of the spring brakes.
Spring Brake Test for Triple Reservoir Vehicles
If the parking brake valve does not pop out when the air pressure has been reduced to approximately 20 psi, you must demonstrate that the spring brakes have activated. To do this, you must:
The spring brakes should drag and prevent the vehicle from easily moving forward. If the spring brakes do not prevent the vehicle from easily moving forward, your road test will be postponed.
Note This test must only be performed on single vehicles designed with an isolated parking brake reservoir. Do not perform this test on combination vehicles.
Check the Rate of Air Pressure Buildup
To perform this test, the engine must be running at normal operating idle, typically 600–900 rpms. Observe the air gauge to determine if the pressure builds at the proper rate. For dual air systems, the pressure should build from approximately 85 to 100 psi within 45 seconds. For single air systems (in pre- vehicles), the pressure should build from approximately 50 to 90 psi within 3 minutes.
For testing purposes, you must verbalize the parameters of the test and identify if the vehicle met the appropriate standards.
There are 3 tests as follows:
Static Leakage Test
With a basically fully-charged air system (within the effective operating range for the compressor), turn off the engine, release all brakes, and let the system settle (air gauge needle stops moving). Time for 1 minute. The air pressure should not drop more than:
Important The maximum air loss rate for a combination of 2 or more vehicles is 2 psi if the towed vehicles are not equipped with air brakes.
An air loss greater than those listed above, indicate a problem in the braking system and repairs are needed before operating the vehicle.
Fasten your seat belt. Set the parking brake, and gently pull against it in a low gear to test that the parking brake will hold.
Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph), and apply the brakes firmly using the brake pedal. Note any vehicle “pulling” to one side, unusual feel, or delayed stopping action.
This test may show you problems, which you otherwise would not know about until you needed the brakes on the road.
Test Your Knowledge
These questions may be on your test. If you cannot answer them all, reread Subsections 5.2 and 5.3.
Push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the clutch in until the engine rpm is down close to idle. When stopped, select a starting gear.
If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. This is a good response if there is enough distance to stop, and you use the brakes correctly.
You should brake in a way that will keep your vehicle in a straight line and allow you to turn if it becomes necessary. You can use the “controlled braking” or “stab braking” method.
Controlled Braking. With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make a larger steering adjustment or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.
Stab Braking. Apply your brakes all the way. Release the brakes when wheels lock up. As soon as the wheels start rolling, apply the brakes fully again. (It can take up to one second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.)
Stopping distance was described in Section 2.6 under “Speed and Stopping Distance.” With air brakes there is an added delay, “brake lag”. This is the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light/medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (one half second or more) for the air to flow through the lines to the brakes. Thus, the total stopping distance for vehicles with air brake systems is made up of 4 different factors.
Perception Distance + Reaction Distance + Brake Lag Distance + Braking Distance = Total Stopping Distance
The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, at 55 mph for an average driver under good traction and brake conditions, the total stopping distance is over 450 feet. See Figure 5.6.
Figure 5.6
Brakes are designed so that brake shoes or pads rub against the brake drum or discs to slow the vehicle. Braking creates heat, but brakes are designed to take a lot of heat. However, brakes can fade or fail from excessive heat caused by using them too much and not relying on the engine braking effect.
Excessive use of the service brakes results in overheating and leads to brake fade. Brake fade results from excessive heat causing chemical changes in the brake lining, which reduce friction, and cause expansion of the brake drums. As the overheated drums expand, the brake shoes and linings have to move farther to contact the drums, and the force of this contact is reduced. Continued overuse may increase brake fade until the vehicle cannot be slowed down or stopped.
Brake fade is also affected by adjustment. To safely control a vehicle, every brake must do its share of the work. Brakes out of adjustment will stop doing their share before those that are in adjustment. The other brakes can then overheat and fade, and there will not be enough braking available to control the vehicle(s). Brakes can get out of adjustment quickly, especially when they are hot. Therefore, check brake adjustment often.
Remember, the use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the correct low gear, the following is the proper braking technique:
When your speed has increased to your “safe” speed, repeat steps 1 and 2.
If your “safe” speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade.
If the low air pressure warning comes on, stop and safely park your vehicle as soon as possible. There might be an air leak in the system. Controlled braking is possible only while enough air remains in the air tanks. The spring brakes will come on when the air pressure drops into the range of 20 to 45 psi. A heavily loaded vehicle will take a long distance to stop because the spring brakes do not work on all axles. Lightly loaded vehicles or vehicles on slippery roads may skid out of control when the spring brakes come on. It is much safer to stop while there is enough air in the tanks to use the foot brakes.
Any time you park, use the parking brakes, except as noted below. Pull the parking brake control knob out to apply the parking brakes and push it in to release. The control will be a yellow, diamond-shaped knob labeled “parking brakes” on newer vehicles. On older vehicles, it may be a round blue knob or some other shape (including a lever that swings from side to side or up and down).
Never leave your vehicle unattended without applying the parking brakes or chocking the wheels. Your vehicle might roll away and cause injury and damage.
Test Your Knowledge
These questions may be on your test. If you cannot answer them all, reread Subsection 5.4.
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