Drum brake

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Brake drum is a brake that uses friction caused by a set of shoes or pads pressing outward against a rotating cylindrical part called a brake drum.

The term drum brake usually means the brake on which the shoe presses on the inner surface of the drum. When the shoe presses on the outside of the drum, it is usually called a brake buckle. Where drums are sandwiched between two shoes, similar to conventional disc brakes, sometimes called brake pinch drums, although such brakes are relatively rare. The related type is called a brake band using a flexible belt or "band" wrapping around the outside of the drum.

Video Drum brake

Histori

Modern car drum brakes were first used in cars made by Maybach in 1900, although the principle was then patented in 1902 by Louis Renault. He uses an asbestos webbing for the drum brake lining, since no heat is removed like the asbestos layer, though Maybach has used less sophisticated drum brakes. In the first drum brake, the lever and rod or cable operate mechanically. From the mid-1930s, the oil pressure in the cylinders and small wheel pistons (as in the picture) operated the brakes, although some vehicles remained with pure mechanical systems for decades. Some designs have two wheel cylinders.

When the shoe in the drum brake is worn, the brakes require manual adjustment on a regular basis until the introduction of drum brakes adjusts in the 1950s. Drums are prone to brake fades with repeated use.

In 1953, the Jaguar downgraded three disc-brake cars at Le Mans, where they won, largely because of their superior braking of rivals equipped with drums. It's spelled early from brake drum crossover to disc brakes in passenger cars. From the 1960s to the 1980s, disc brakes gradually replaced the drum brakes on the front wheels of the car. Now almost 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 (produced by AM General) was the last car (produced for the United States Postal Service) to use the front drum brake when it was discontinued in 1984. However, drum brakes are still frequently used. for hand brakes, as it has proven very difficult to design a disc brake that is suitable for holding parked cars. In addition, it is very easy to insert drum brakes inside disc brakes so that one unit works both as a service brake and a handbrake.

The early brake shoes contain asbestos. When working on an older car brake system, it should be careful not to inhale the dust present in the brake assemblies. The Federal Government of the United States began to organize the production of asbestos, and brake producers had to switch to non-asbestos layers. The owner initially complained of poor braking with replacement; However, the technology ultimately advanced to compensate. The majority of older vehicles driven each day are equipped with a layer of free asbestos. Many other countries also limit the use of asbestos in the brakes.

Maps Drum brake

Components

The drum brake components include buffer plates, drum brakes, shoes, wheel cylinders, and various springs and pins.

Backing plate

The back plate provides a base for the other components. The rear plates also improve the overall stiffness of the set-up, support the housing, and protect it from foreign materials such as dust and other road debris. It absorbs the torque from the braking action, and that's why the back plate is also called "Torque Plate". Since all braking operations put pressure on the back plate, it must be strong and wear resistant. Levers for emergency brakes or parking, and automatic brake-shoe adjusters have also been added in recent years.

Brake drum

Brake drums are generally made of a special type of cast iron that is conductive to heat and wear-resistant. Spin with wheels and axle. When the driver slams on the brake, the layer pushes radially to the inner surface of the drum, and subsequent friction slows or stops the rotation of the wheel and the shaft, and thus the vehicle. This friction produces substantial heat.

Wheel cylinder

One cylinder wheel operates the brakes on each wheel. Two pistons operate the shoe, one on each end of the wheel cylinder. The main shoe (closest to the front of the vehicle) is known as the main shoe. Trailing shoe is known as secondary shoes. The hydraulic pressure of the master cylinder works on the piston cup, pushing the piston toward the shoe, forcing it against the drum. When the driver releases the brake, the brake shoe spring returns the shoe to its original position (detached). The parts of the wheel cylinder are shown to the right.

Brake Shoe

Brake shoes are usually made of two parts of steel welded together. This friction material is nailed to a coating table or attached with adhesive. The crescent-shaped part is called the Web and contains holes and slots in different forms for back springs, retaining hardware, parking brake connections and adjustment components. All styles of cylinder wheel applications are applied over the web to the layer and brake pads. Layer table edges generally have three "V" -shaped shapes or tabs on each side called nibs. The remainder of the nibs is attached to the rear mounting pads where the shoe is mounted. Each brake assembly has two shoes, primary and secondary. The main shoe is located on the front of the vehicle and has a layer that is positioned differently than the secondary shoe. Quite often, both shoes can be exchanged, so a careful examination of each variation is important.

The coating must be resistant to heat and wear and has a high coefficient of friction not affected by temperature and moisture fluctuations. Materials that make up brake shoes include, friction modifiers (which can include graphite and cashew nut shells), metal powders such as lead, zinc, brass, aluminum and other metals that retain fading heat, binders, preservatives and fillers such as rubber chips to reduce noise brake.

In the UK, two commonly used brake shoe materials. DON 202 is a high friction material that does not require a brake power servo. The disadvantage is that the layer tends to fade in the steep hills (counting kilowatts scattered by a ton of cars down 15% at constant 60 mph) The tougher layer, the famous VG95 is manufactured but this requires a brake servo. Another obstacle is that parking brakes will often fail in the annual MOT tests unless a high friction layer is installed only for testing.

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In operation

Normal braking

When the brakes are applied, the brake fluid is forced under pressure from the master cylinder into the wheel cylinder, which in turn pushes the brake shoe into contact with the engine surface on the inside of the drum. This scrubbing action reduces the rotation of the brake drum, which is attached to the wheels. Therefore the vehicle speed is reduced. When the pressure is released, the spring pulls the shoe back to their resting position.

Automatic auto adjustment

When the brake lining is worn, the shoe must travel a longer distance to reach the drum. When the distance reaches a certain point, the adjustment mechanism automatically reacts by adjusting the resting position of the shoe so that they are closer to the drum. Here, the adjusting lever is sufficient to advance the adjuster gear with one tooth. The regulator has a thread on it, like a bolt, so it does not open slightly while swinging, extending to fill the gap. When brake shoes wear less, the adjuster can advance again, so always keep the shoe close to the drum. Normally the regulator only operates when the vehicle moves backwards and brakes are activated.

Parking/emergency brake

The parking (or emergency) brake system controls the brakes through a series of steel cables connected to the hand levers or foot pedals. The idea is that the system is fully mechanical and completely passes through the hydraulic system so that the vehicle can be eliminated even if there is a total brake failure. Here the cable pulls the lever mounted on the brake and connects directly to the brake shoe. It has the effect of passing through the wheel cylinder and controlling the brakes directly.

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Self-applying characteristics

Drum brakes have a natural "self-applied" character, better known as "self-energizing." Drum rotation can drag one or both shoes onto the friction surface, causing the brakes to bite harder, which increases their holding force together. This increases the stopping power without any additional effort being issued by the driver, but it makes the driver more difficult to modulate the brake sensitivity. It also makes the brakes more sensitive to the brakes to fade, as the brake friction decrease also reduces the amount of brake relief.

Disc brakes do not show the effect of self-application because hydraulic pressure acts on the cushion perpendicular to the direction of the disc rotation. Disc brake systems typically have servo assistance ("Brake Booster") to reduce the driver pedal effort, but some disc brakes (especially racing cars) and smaller brakes for motorcycles, etc. No need to use servo.

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Drum brake design

Drum brakes are usually described as leading/trailing or twin leading.

The rear drum brakes are usually the main design/trailing (for non-servo systems), or the primary/secondary system (for the servo duo), the shoe is driven by a single double action hydraulic cylinder and hinged at the same point. In this design, one of the brake shoes always experiences the effect of self-application, regardless of whether the vehicle is moving forward or backwards. This is particularly useful on the rear brake, where the parking brake (handbrake or foot brake) must exert enough force to stop the vehicle from recoil and hold it on the slope. As long as the contact area of ​​the brake shoe is large enough, which is not always the case, the self-effect effect can safely hold the vehicle when weight is transferred to the rear brake due to a slope of the slope or the reverse direction. motion. A further advantage of using a single hydraulic cylinder behind is that the opposite shaft can be made in the form of a doubled cam that is rotated by the action of the parking brake system.

Front drum brakes can be design in practice, but the main design of twins is more effective. This design uses two regulated cylinders so that both shoes use self-apply characteristics when the vehicle moves forward. Brake shoes rotate at opposite points of each other. This gives maximum possible braking as it moves forward, but is not very effective when the vehicle runs backwards.

Optimal front brake front tweaks with front/rear brakes on the back allow more braking force on the front of the vehicle as it moves forward, with less on the back. This helps prevent the rear wheels locked, but still provides adequate braking on the back.

The brake drums themselves are often made of cast iron, although some vehicles have used aluminum drums, especially for front wheel applications. Aluminum does heat better than cast iron, which increases heat dissipation and reduces fading. Aluminum drums are also lighter than iron drums, which reduce the weight of unsprung. Because aluminum is easier to use than iron, aluminum drums often have a layer of iron or steel on the inner surface of the drum, bonded or glued to the outer shell of aluminum.

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Benefits

Drum brakes are used in most heavy duty trucks, some medium and light duty trucks, and some cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels because most of the stop force is generated by the vehicle's front brakes and therefore the resulting heat behind is significantly less. Brake drum allows simple merging of parking brake.

Drum brakes are also sometimes installed as a parking brake (and emergency) even when the rear wheels use disc brakes as the main brake. Many rear disc braking systems use a parking brake where the piston in the caliper is driven by a cam or screw. It suppresses the bearing 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, small drums are usually installed inside or as part of the brake discs. This type of brake is also known as brake bankia.

In hybrid vehicle applications, wear and tear on the braking system is greatly reduced by energy-restoring motor-generator (see regenerative braking), so some hybrid vehicles such as GMC Yukon Hybrid and Toyota Prius (except the third generation) use drum brakes on the rear wheels.

The disc brakes depend on the elasticity of the caliper seal and a little runout to release the pads, leading to drag, fuel mileage loss, and disc valuation. Brake drum back springs provide more positive action and, adjusted correctly, often have fewer obstacles when released. It is however possible to design a special seal that pulls the piston on the disc brake.

The drum brake emits less particulate (PM) than the disc brakes, since the worn particles are mostly sealed. They are no better at this than brakes without friction.

Heavy heavy duty drum brake system balances the load when determining wheel cylinder pressure; a feature that is rare when a disc is used (The hydropneumatic suspension system as used on the Citroë vehicle nà © n adjusts the brake pressure depending on the load regardless of whether the drum or disk is used). One such vehicle is the Jeep Comanche. The Comanche can automatically send more pressure to the rear drum depending on the size of the load. Most other brands have used valve load sensors in the hydraulics to the rear axles for decades.

Due to the fact that the drum brake friction contact area is on the circumference of the brake, the drum brake can provide more braking power than the same diameter disc brakes. The increase of the brake drum brush contact area on the drum allows the drum brake to last longer than the disc brake pads used in the brake system with the same dimensions and braking strength. The drum brakes retain heat and are more complex than disc brakes but are often the more economical and powerful type of brake for use in rear brake applications due to low heat brakes from the rear brakes, self-applied drum brake properties, greater friction surface contact areas and age use long characteristics (% life used/kW of braking power).

To list the advantages of brake drum:

• is cheaper to generate
• slightly lower maintenance frequency due to better corrosion resistance than disks.
• Internal self-energizing effects require less input power (such as hydraulic pressure).
• The wheel cylinder is somewhat simpler to be reconditioned than the calipers.
• small weight savings, especially from much smaller hydraulic cylinders vs. calipers.

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As parking brake/emergency brake

The drum brakes have also been built into the driveshaft transmission as a parking brake (eg, Chryslers until 1962). It gives the advantage that it is completely independent of the service brakes - but suffers severe losses in that, when used with the bumper jack (common in that era) behind, and without proper wheel blocks, differential action can allow the vehicle to roll the jacks.

Land Rover has been using drum brakes on the output shaft of the gearbox for more than fifty years. The advantage is that all four wheels can be braking with parking brake.

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Disadvantages

The drum brakes, like most other brakes, convert kinetic energy into heat with friction. This heat must disappear into the surrounding air, but can easily transfer to other braking system components. Brake drums must be large to overcome the large forces involved, and must be able to absorb and throw away a lot of heat. Heat transfer to air can be helped by inserting cooling fins into drums. However, excessive warming may occur due to heavy or recurrent braking, which may cause distorted drums, causing vibrations under braking.

Another consequence of overheating is the brake fade. This is because one of several or more processes is usually an accumulation of all.

1. When the drum is heated by hard braking, the drum diameter increases slightly due to thermal expansion, so the shoe must move further and the driver must press the brake pedal further.
2. The properties of the friction material may change if heated, resulting in less friction. This can be a much bigger problem with drum brakes than disc brakes, because shoes are in drums and are not exposed to ambient air. The loss of friction is usually only temporary and the material regains its efficiency when cooled, but if the surface is too hot to the point where glazed is reduced then the braking efficiency is more permanent. The surface glazing can be lost with the use of further brakes, but it takes time.
3. Excessive drum brake heating can cause the brake fluid to evaporate, which reduces the hydraulic pressure applied to the brake shoe. Therefore, the brakes provide less deceleration for the amount of pressure applied to the pedal. The effect is exacerbated by poor maintenance. Brake fluid that is old and has absorbed moisture has a lower boiling point, so the brakes fade faster occurs.

Fade brakes are not always due to overheating. Water between friction surfaces and drums can act as a lubricant and reduce braking efficiency. Water tends to stay hot enough to evaporate, at which point braking efficiency returns. All friction braking systems have maximum theoretical energy conversion rates. Once the figure is reached, applying a larger pedal pressure will not change it - in fact, the effects mentioned can substantially reduce. Ultimately, this is what fades the brakes, regardless of the underlying mechanism. Disc brakes are not immune to any of these processes, but they handle heat and water more effectively than drums.

Drum brakes can be achieved if the drum surface gets light rust or if the brakes are cold and humid, giving the material a larger friction pad. Grabbing can be so severe that the tire slips and keeps slipping even when the pedal is released. Grab is the opposite of fading: when friction pad rises, the self help nature of the brakes causes the application's strength to rise. If the pad friction and self-amplification are high enough, the brakes keep moving because of the application itself, even when the external application force is released.

While disc brake rotors can be worked on to clean the friction surfaces (ie, 'rotate'), the same can not generally be done with brake drums. The machining of brake drum friction surfaces increases the diameter, which may require large shoes to maintain contact with the drum. However, since large sized shoes are generally not available for most applications, worn or damaged drums should generally be replaced.

It is simple enough for a brake drum machine if one has a slow running lathe (one of the rule of thumb is that cast iron should not be done faster than fifty feet per minute). Usually only necessary to get rid of the ridge that makes drum brakes difficult. If the abandoned ridge on a big problem can occur later especially if the brakes are adjusting. In a bad case this ridge can make a brake drum captive. Most drum brake designs provide a way to eject self-regulation mechanisms externally for easy removal of drums and services.

Another disadvantage of drum brakes is their relative complexity. One must have a general understanding of how the drum brakes work and take simple steps to ensure the brakes are properly reattached while doing the work on the brake drum. And, as a result of this increased complexity (compared to disc brakes), tromol brake maintenance is generally more time-consuming. Also, more and more parts produce more failure modes compared to disc brakes. The springs can break from exhaustion if not replaced along with worn brake shoes. And drums and shoes can become damaged from scoring if various components (such as broken or adapted springs) break and become loose in the drum.

Fatal hardware failures such as springs and adjuster can also cause unwanted brake applications or even wheel locking. If the spring breaks, the shoe will fall freely against the rotating drum, which essentially causes the brake to be applied. Due to the self-energizing qualities of the drum brakes, the shoes are uncontrolled and even potentially cause the brakes to reach to the point of locking the wheel. In addition, splinters and other hardware (such as adjuster) may be jammed between the shoe and the drum, resulting in unwanted brakes (and, as stated above, damage to the brake components). For this reason, brake hardware (such as springs and clips) should always be replaced with brake shoes.

Also, the drum brakes do not apply immediately when the wheel cylinder is pressed, because the spring back force must be over before the shoe starts moving toward the drum. This means that a very common hybrid disk/drum system only brakes with discs on light pedal pressure unless additional hardware is added. In practice, the measuring valve prevents hydraulic pressure from reaching the front calipers until the pressure rises enough to overcome the spring back in the drum brake. If the measuring valve is abandoned, the vehicle will only stop with the front disc until the spring pressure on the back of the shoe is overcome.

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Security

When asbestos is common in brake drums, there is a danger workers repairing or replacing them will inhale asbestos fibers, which can cause mesothelioma. Asbestos fibers will break apart or become separated over time and with high temperatures caused by braking. Wet brushes and aerosol sprays are usually used to reduce dust. Security regulators are sometimes recommended using a vacuum hose to vacuum, or enclosures with lighting and interior space to use the equipment inside, but this is rare and impractical. Special shoes designed to protect against asbestos are also recommended. There is evidence that car mechanics have disproportionate levels of mesothelioma.

Those taking brake treatment may also be exposed to 1,1,1-trichloroethane and 2-butoxyethanol solvents (the main ingredient in Greasoff No. 19). Exposure to these solvents can cause irritation, including eyes and mucous membranes. Exposure to 1-1-1-trichloroethane vapors can cause damage to the central nervous system, dizziness, incoordination, drowsiness, and increased reaction time.

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Relive

Prior to 1984, it was common to wade through brake shoes to fit the bow in the brake drum. This practice, however, is controversial, as it removes friction material from the brakes and reduces shoe life as well as creating harmful asbestos dust. The current design theory is to use shoes for the right diameter drum, and only replace the brake drum when needed, rather than the curved shoes.

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Use music

Brake drums are used in modern concerts and film music to provide a metal sound that does not sound like a runway. Some are more resonant than others. To produce the clearest sound, percussion hangs the drum with a nylon rope or puts it on top of the foam. Other methods include the installation of brake drums at the snare drum booth. Either way, the percussion is attacking the brake drum with a hammer or various weight rods. Brake drums are also often in steelpan ensembles, where it is called "iron."

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See also

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References

• Patent ES0217565, ES0195467 by Sanglas, on brake discs looks like drum brakes, nice on rainy days

Source of the article : Wikipedia