Ağır Ticari Taşıtlarda Kullanılan Kampanalı Frenlerde Fren Titreşimleri

Abstract

Fren titreşimleri literatürde ‘’brake vibration‘’ ve ‘’brake judder‘’ olarak tanımlanmakta ve genel olarak ‘’brake judder‘’ terimi daha yaygın kullanılmaktadır. Frenleme esnasında görülen titreşimler fren kaynaklı olabileceği gibi frenlemenin tetiklemesiyle süspansiyon sistemi kaynaklı da olabilir veya frenleme esnasında görülen titreşim süspansiyon sisteminden kaynaklanan titreşim ile çakışabilir. Ağır ticari taşıtlarda da kullanılan kampanalı frenlerde, frenleme esnasında fren titreşimleri oluşabilmektedir. Frenleme esnasında görülen titreşim balataların kampananın veya diskin fren yüzeylerine temas etmesi ile hemen oluşabileceği gibi zamanla da gelişebilmekte ve süspansiyon sistemi vasıtası ile araç kabinine, direksiyona ve sürücüye kadar aktarılabilmektedir. Frenleme esnasında görülebilen fren titreşimleri istenmeyen titreşimlerdir. Titreşim sadece titreşim yaptığı bölgede bulunan parçalara zarar vermekle kalmaz titreşim sürücüye kadar transfer olarak sürücüyü rahatsız eder ve araç geneline zarar verip araç performansını olumsuz yönde etkiler. Frenleme esnasında görülen titreşim problemi ; frenleme esnasında oluşabilecek olan termal ve kimyasal kaynaklı olabileceği gibi mekanik kaynaklı da olabilmektedir. Bu kaynaklar, uyarı yada girdi olarak belirtilebilecek şekilde aşağıdaki gibi sıralamak mümkündür.  Geometrik düzgünsüzlükler  Kampana veya disk yüzeyinde olan eşit olmayan aşınmalar  Balata – kampana veya balata – disk yüzeyinde kalınlığı eşit olmayan sürtünme katmanı  Kampana veya disk yüzeyinde eşit olmayan ısı dağılımları  Balata – kampana veya balata – disk yüzeyi arasında oluşan sürtünme karekteristiği ve seviyesi  Bu uyarıların dışında gelişebileşek dış tahriklerdir Frenleme esnasında görülen fren titreşimi nedenlerinden olan ve geometrik düzgünsüzlükler olarak adlandırılan uyarılardan bazıları da balanssızlık ve salgı değeridir. Bu kapsamda balansızlık değeri bilinen kampanalar ve salgı değerleri bilinen poyra – kampana kompleleri 1. dingile sağ – sol tekere takılmıştır. Ayrıca, araca poyra – kampana kompleleri takılmadan önce fren balatalarının servis gereklilik durumunun olup olmadığı ve sürükleyen – karşılayan papuçların çap ayarları kontrol edilerek Z – cam fren sisteminin fren ayar ve bakımı konusunda gerektirdiği zorunluluklar yerine getirilerek araç teste hazır duruma getirilmiştir. Bu çalışma kapsamında; ağır ticari taşıtlarda kullanılan kampanalı frenlerde fren titreşimlerine neden olabilecek uyarılar irdelenmiştir. Ağır ticari taşıtlarda kullanılan Z-cam fren sistemine sahip kampana frenli taşıt üzerinden üç eksenli ivmeölçerler yardımı ile elde edilen titreşimler dataları, teker dönme devri bazında analiz edilerek titreşim frekansları ve tahrik mertebeleri oluşturulmuş ve açıklanmıştır.

Brake vibration is known as ‘’brake judder’’ in litterateur. Generally, ‘’brake judder‘’ is used more common in litterateur. Which brake induced vibrations can be seen during braking also, can be seen from suspension systems due to triggered by braking or vibration during braking can conflicted with the vibration caused by the suspension system, too. Among brake vibration and noise problems, brake judder is a problem that automobile manufacturers and brake suppliers have researched for a long time, also brake judder is a problem costs manufacturers millions of dollars in warranty cost and undesirable trade off. The magnitude of judder responce depends not only brake torque variation but also on the suspension design characteristic. As a luxury feeling and comfort are required expectations of newer passenger cars, improvement of judder performance continues to be an important concern. Braking induced vibrations and noises are undesired effects that are being targeted by NVH (Noise, Vibration and Harshness) engineers as part of improving comfort levels of today’s automobile. These effects are usually categorized according to customer perceptions. An automotive brake provides a means of converting kinetic energy of a moving vehicle into heat. The heat generated is dissipated by conduction through adjacent components, radiation through the atmosphere, and also by chemical processes, resulting in metallurgical changes and wear. The above mentioned processes are responsible for changes of the shape of the original components and their physical characteristics which can result in either variations in normal force or friction coefficient. This will cause variations in the brake force and consequently Brake Torque Variations (BTV), which are a source of excitation. In disc brakes, variations of normal force can also be the cause for movement of the piston if the pads can move. This will induce pressure fluctuation in the hydraulic system, that will also influence the normal forces in other brakes in the car. The hydraulic pressure fluctuation can be felt by the driver through the brake pedal and is referred to as brake pedal pulsation. The level of pedal pulsation can be influenced by the amount of hydraulic damping and possibly by parameters such as line length, brake cylinder diameter etc. High frequency noises (noises in the audible range of frequencies above 1kHz) are usually called squeal noises, however more descriptive names are also used. Low frequency noises (within 200 Hz and 500 Hz) includes moan, howl and groan. Lower frequency effects are in the judder (or roughness) range. Judder effects are related to the automobile speed in contrast with other noises mentioned before. It is usual to relate judder frequencies to vehicle speed, so frequencies at two times the number of wheel rotations per second are called second order. By just mentioning the above relation NVH engineers deduces that judder is a driven responce. That responce requires source (braking action), transmission path (vehicle structure and linkages), and finally the perception causing element (steering wheel vibration, vehicle body vibration or cabin noise). There has been reference to two main types of judder in braking industry, cold (or rotor thickness variation) judder and thermal (rotor to pad friction coefficient variation) judder. Thickness variation judder ; this type of judder has typically the lowest frequencies. A main assumption of the mechanism is that the variation value of the thickness of the rotor or out of cylindricity of the drum drives the friction surface according to the rotor speed. The order of excitation is then related directly to that of the topography of the rotor. The vibration motion at the friction surface is then transmitted through the brake structure to the knuckle and the later is transmitted through the vehicle chassis and kinematic links as well as the brake hydraulic lines throughout the vehicle. The perception of the driver is most pronounced by the steering wheel vibration, braking pedal pulsation, and even the vehicle floor vibration. It can be deduced from the proposed model for this driven responce that both the input impedance at the knuckle and the brake sides are factors of how much of the surface variation is transmitted to the knuckle, and based on the transmissibility of vibration between the knuckle and the steering wheel, the level of perception of judder is determined. Thermal judder ; this type of judder is occurs at higher frequencies than the thickness variation judder. It occurs at higher orders of wheel rotation (6…12 have been typically noticed). Typical brake design and machining operations induce thickness variations that are primarily second and third order. These low orders are never observed in thermal judder. At the higher frequencies of thermal judder audible perception is also a factor. The mechanism suggested for this type of judder is believed to be the variation of the friction coefficient around the rotor. This variation is caused by chemical changes occuring to the friction couple when intense heat is generated at the friction interface. Visual observation of hot or blue spots around the rotor is common. Even, after cooling one can notice this effect by the color of the rotor surface. The order of vibration is related directly to the number of spots. If we stil use the simple model of transmission of judder vibration, one can see that a variation of the torque applied at the brake components is transmitted through the braket o the knuckle then through the vehicle structure to the point of observation. One should expect less or no pedal pulsation in this type of brake judder. Brake vibrations can occur during braking on drum brakes which is used for heavy commercial vehicles. Vibration occurring during braking, will be able to occur immediately while brake linings start to contact with the surfaces of the brake drum or disc or also develop over time. Thus, vibration is to be transferred by means of suspension system to vehicle cab, steering wheel and to driver. Brake vibration is a unwanted vibration which can be seen during braking. Vibration not only damage the parts or components which only occured region also transmitted vibration will disturb the driver and damage the all vehicle and affect the vehicle performance adversely Vibration problem which occurring during braking ; that may occur during braking, as well as the mechanical origin can be either thermal or chemical origin. These origins can be specified as an input and listed as below.  Geometrical irregularities  Uneven wear on drum and disk surface  Uneven friction film on drum and disk surface  Uneven heating on drum and disk surface  Friction characteristic and level between drum – lining or disk – lining  External forces, like road..etc The causes of brake vibration during braking is which so-called geometrical irregularities. Unbalance and run-out are geometrical irregularities and they can be accepted as an input to the system. In this context, which was known hub – drum assembly total run – outs and drum unbalance values were assembled to 1. axle right – left wheel of the vehicle. Also, before assembling the these parts, brake linings were checked in respect of seviceability and leading – trailing lined shoes diameter adjusting. Shortly, vehicle was prepared according to Z – cam brake system maintenance manuel. In this project context ; brake vibration causes or input signals were examined that could cause brake vibration on drum brakes which used for heavy commercial vehicles. Brake vibration datas collected as downhill speed up at neutral gear and speed down by braking at neutral gear with the aid of three-axis accelerometers on drum brakes which has Z – cam brake systems for heavy commercial vehicles. These datas have been analyzed and explained in respect of wheel rotation speed versus vibration frequencies and orders of excitation.