Evaluasi Perkerasan Jalan Menggunakan FWD* Bagian 1

Evaluasi Perkerasan Jalan Menggunakan FWD*

Bagian 1

Source: http://atmaja.staff.umy.ac.id/2010/11/20/technical-notecategory-pavement/

Oleh: Ridwan Umbara, ST.♦; Sri Atmaja P. Rosyidi Ph.D♥.; Siegfried Ph.D.♣

*technical note dari penelitian hibah bersaing dan menjadi penelitian payung terhadap tugas akhir mahasiswa Ridwan Umbara (Universitas Muhammadiyah Yogyakarta).
♦alumni, ♥dosen, Universitas Muhammadiyah Yogyakarta , ♣peneliti, Pusat Penelitian dan Pengembangan Jalan dan Jembatan, Bandung

Modulus Elastisitas Bahan Jalan

Salah satu parameter yang digunakan untuk menentukan kekuatan struktur perkerasan jalan adalah nilai modulus elastisitas (E). Berdasarkan hasil-hasil penelitian sebelumnya, nilai modulus elastisitas (E) yang dihasilkan dengan menggunakan program BAKFAA, Kosasih dan Siegfried (2008) mendapatkan nilai E di ruas Jalan Soekarno-Hatta berada pada rentang 90 MPa – 4000 MPa untuk semua lapisan. Begitu juga dengan penelitian yang dilakukan di Jalan Soekarno Hatta, Jalan Lingkar Barat, dan Jalan Padalarang-Purwakarta oleh Muhammad (2008), nilai modulus yang dihasilkan berada pada interval 90 MPa – 4000 MPa. Nilai modulus elastisitas (E) yang berada pada rentang tersebut dikategorikan nilai yang logis (reasonable) dan kondisi baik (good performance), karena telah memenuhi batas nilai CBR tanah dasar yaitu  sebesar 5 % atau 50 MPa (Sukirman, 1992), batas nilai CBR lapisan pondasi atas yaitu  sebesar 50 % atau 500 MPa (Sukirman, 1992), dan nilai modulus elastisitas (E) lapisan beraspal yang berada pada rentang 1500 – 3500 MPa (Sukirman, 1992).

Apa itu Falling Weight Deflectometer (FWD)?

Falling Weight Deflectometer (FWD) merupakan peralatan uji lapangan untuk perkerasan jalan yang telah lama digunakan di berbagai negara. Sekitar 30 tahun yang lalu, alat ini diperkenalkan pertama kali di Perancis untuk mengevaluasi struktur perkerasan jalan (Karadelis, 1999).  Selanjutnya pada tahun 1981, Denmark menggunakan FWD untuk menilai daya dukung, umur manfaat, dan disain overlay pada jaringan jalan (Schmidt, 1989). Berbagai penelitian juga telah berhasil dilakukan untuk mengembangkan penggunaan alat FWD dalam evaluasi struktur perkerasan jalan, diantaranya di Amerika Serikat (Parker dkk, 1994, Garg dan Marsey, 2002, Romanoschi dan Metcalf, 2003, Appea, 2003, Zhaghloul dkk, 2005, Henderson, 2006, Westover dan Guzina, 2009), Denmark (Ulidtz, 1998), Jepang (Dong dkk, 2001), Kanada (Tighe dkk, 2003), China (Ji dkk, 2006), Turki (Goktepe dan Agar, 2006., Terzi, 2005), dan di Indonesia (Kosasih, 2004, Subagio, Cahyanto, Rachman, dan Mardiyah, 2005, Muhammad, 2008).
Menurut Rosyidi dkk (2006), terdapat beberapa keuntungan menggunakan alat FWD untuk sistem manajemen jalan, yaitu:

1. dapat menampilkan kinerja perkerasan secara menyeluruh dengan memberikan nilai modulus setiap lapisan struktur perkerasan jalan,
2. peralatan FWD dioperasikan dengan mudah dan memberikan hasil pengukuran yang tepat serta ketelitian yang tinggi,
3. beban pelat dan ketinggian jatuh yang dapat diukur, dengan demikian intensitas beban yang direpresentasikan sebagai beban kendaraan dapat disesuaikan untuk kondisi di Indonesia (8,16 ton).

Perhitungan Balik (Backcalculation)

Struktur perkerasan akan mengalami lendutan pada saat menerima beban roda kendaraan (Kosasih, 2004). Secara teoritis, Kosasih (2004) menjelaskan bahwa besarnya lendutan struktur perkerasan dapat dihitung dari data komposisi dan tebal lapisan perkerasan, karakteristik bahan perkerasan (modulus elastisitas dan konstanta poisson), dan konfigurasi beban roda kendaraan. Di lain pihak, lendutan struktur perkerasan juga dapat diukur di lapangan, yaitu dengan menggunakan alat ukur Falling Weight Deflectometer. Lendutan dalam alat FWD dihasilkan dari pelat beban yang dijatuhkan dari ketinggian tertentu ke atas permukaan perkerasan jalan dan direkam oleh sejumlah sensor (tujuh hingga sembilan buah sensor) yang terpasang pada batang pengukur. Backcalculation telah dikembangkan untuk menghitung balik modulus perkerasan berdasarkan data lendutan dengan mempersamakan cekung lendutan teoritis terhadap cekung lendutan survai FWD.
Menurut Irwin (2002) terdapat tiga faktor penting dalam teknik perkerasan jalan yang menyebabkan perhitungan balik berkembang cukup pesat saat ini, yaitu:

1. pada kenyataannya, lapisan perkerasan yang kuat memiliki lendutan yang kecil, sebaliknya lapisan perkerasan yang lemah memiliki nilai lendutan yang besar oleh karena itu kualitas struktur perkerasan jalan ditentukan oleh besarnya lendutan yang terjadi (konsep ini berkembang pada periode 1935-1960).
2. pengembangan teori mekanistik berkaitan erat dengan ketersediaan data-data struktur perkerasan yang penting yaitu tegangan, regangan dan lendutan pada setiap lapisan (konsep ini berkembang pada periode 1940-1970).
3. adanya kebutuhan akan sistem instrumentasi untuk mengukur lendutan perkerasan jalan yang memiliki mobilisasi tinggi (portable), memiliki akurasi yang baik dan mudah dioperasikan (berkembang pada periode 1955-1980).

Pada dasarnya, proses perhitungan balik masih memiliki beberapa kekurangan, yaitu tidak dipertimbangkannya gradasi modulus tanah dasar dalam arah vertikal akibat perbedaan kadar air, gradasi modulus perkerasan (lapisan beraspal) dalam arah vertikal akibat variasi temperatur dan ketergantungan modulus lapisan agregat dan tanah dasar pada tegangan yang terjadi. Meskipun demikian, di sisi lain ada juga sejumlah potensi manfaat dari modulus perkerasan yang dihasilkan, seperti untuk disain dan analisis disain lapisan tambahan, untuk koreksi variasi pengaruh lingkungan terhadap kekuatan struktur perkerasan atau untuk kontrol kualitas hasil pekerjaan konstruksi (Kosasih, 2004).

Konsep Perhitungan Balik

Konsep perhitungan balik (backcalculation) pertama kali diusulkan oleh Westergaard pada tahun 1925. Prinsip perhitungan ini menunjukkan bahwa modulus perkerasan jalan dapat dihitung dengan mempersamakan cekung lendutan teoritis dengan hasil survai. Besarnya lendutan perkerasan yang dihasilkan dapat dihitung dari data komposisi dan tebal lapisan perkerasan (modulus elastisitas dan rasio poisson), pengaruh lingkungan dan konfigurasi beban roda.
Secara umum, ada dua pendekatan yang dapat digunakan dalam proses perhitungan balik yaitu pendekatan basis data (database) dan pendekatan iteratif (Kosasih, 2004). Pendekatan basis data (database) dilakukan dengan membandingkan cekung lendutan survai terhadap cekung lendutan teoritis yang telah tersimpan dalam basis data untuk rentang data modulus perkerasan dan modulus tanah dasar sesuai dengan variasi struktur perkerasan yang telah ditetapkan terlebih dahulu. Pendekatan ini pada dasarnya dapat dioperasikan dengan sangat efisien. Namun, pendekatan ini tidak selalu siap untuk mengakomodasi variasi struktur perkerasan yang mungkin terjadi di lapangan (Kosasih, 2004). Pendekatan iteratif dilakukan untuk menghitung modulus perkerasan secara iteratif sesuai dengan struktur perkerasan yang ada di lapangan sampai kriteria konvergensi tercapai (Kosasih, 2004). Prinsip dasar pendekatan ini adalah mempersamakan data lendutan hasil survai dengan nilai lendutan teoritis guna mencari modulus elastisitas yang sesuai.
Dalam proses perhitungan balik diperlukan asumsi awal struktur perkerasan sebagai struktur dengan dua lapisan (two layer system), tiga lapisan (three layer system) atau menggunakan empat lapisan (four layer system). Pilihan penggunaan satu dari ketiga sistem tersebut sangat berpengaruh terhadap hasil analisis nantinya. Faktor lingkungan, seperti suhu perkerasan dan jumlah musim per tahun, masing-masing perlu diperhitungkan dalam perhitungan modulus lapisan perkerasan dan lapisan tanah dasar (subgrade).

Program Perhitungan Balik

Menurut Irwin (2002), proses perhitungan balik telah dikembangkan secara intensif dan berkelanjutan di Eropa dan Amerika Serikat. Beberapa kontribusi penting dalam menentukan modulus elastisitas perkerasan jalan yang disumbangkan oleh para peneliti pada periode awal pengembangan proses perhitungan balik ini, bisa dilihat melalui beberapa metode yang dikembangkan oleh peneliti-peneliti terdahulu seperti solusi nomografis untuk pemodelan struktur sistem dua lapisan yang dipublikasikan oleh Swift (1973) dan Scrivner dkk (1973) serta dua makalah penting yang memuat penyelesaian analisis nilai modulus elastisitas menggunakan komputer (computer-based solution) yang dipublikasikan oleh Irwin (1977) dan Ulidtz (1977).
Pada dasarnya, perhitungan balik dapat dilakukan secara manual dengan salah satu program komputer yang menggunakan teori lapisan elastis. Akan tetapi, proses ini tidak praktis dan memakan waktu yang sangat lama. Oleh karena itu, banyak peneliti mengembangkan beberapa program komputer untuk melakukan proses perhitungan balik, diantaranya adalah:

• ELMOD (Dynatest)
• MICHBACK (Michigan University)
• BAKFAA (Federal Aviation Administration, AS)
• EVERCALC (Washington State DOT)
• MODCOMP (Cornell University)
• MODULUS (Texas A&M University)
• PADAL (University of Nottingham)
• WESDEF (U.S. Army, Waterways Experiment Station)

Sumber: http://labtransportumy.wordpress.com/2010/11/20/falling-weight-deflectometer-a-technical-note-1/

Portable Falling Weight Deflectometer

Light Falling Weight Deflectometer

 „TERRATEST 3000“

The Light Falling Weight Deflectometer in accordance with the german technical regulations TP BF-StB section B 8.3 can be used to obtain the dynamic deformation modulus (Evd value) for soils and stone pavements in road sites. This high-tech instrument has such features that guarantee a fast, practical and, thanks to the GPS system, precise investigation of the measuring data in road and pipe construction:

	integrated GPS system with Google®-Maps interface to determine quickly the position of the test points (patent pending)
	modern designed splashproof electronic box with a wide transparent window for the use of the device also during bad weather conditions
	practical and back-lit graphic display to view the curves during the tests
	clear control panel
	internal memory up to 2000 tests
	automatic plausibility check of the result
	audio signal
	external splashproof buttons
	first-rate plug connection thanks to the use of high quality plugs
	solid screwed safety grip with spirit-level
	ergonomic handle on the drop weight and loading plate with handy transport handles for a better carry
	user-friendly and practical software

Function

„TERRATEST 3000“ service is very user-friendly, as already during the construction and development phase we focused on the fact that wrong tests would automatically be excluded thanks to an internal plausibility check. Because of this, it´s guaranteed that also inexperienced people can immediately use the device and carry out precise measurements right after a quick training given by our qualified technicians.

The test execution described below shows how simple and fast it is to take measurements that can be immediately printed through the integrated mini printer and saved on the chipcard (automatic safe in the internal memory) in order to take afterwords prints also from your PC.

The practical and back-lit graphic display guides you in a very easy and clear way through the different applications and allows you to work also in bad visibility conditions.
„TERRATEST 3000“ service is very user-friendly, as already during the construction and development phase we focused on the fact that wrong tests would automatically be excluded thanks to an internal plausibility check. Because of this, it´s guaranteed that also inexperienced people can immediately use the device and carry out precise measurements right after a quick training given by our qualified technicians.

The test execution described below shows how simple and fast it is to take measurements that can be immediately printed through the integrated mini printer and saved on the chipcard (automatic safe in the internal memory) in order to take afterwords prints also from your PC.

The practical and back-lit graphic display guides you in a very easy and clear way through the different applications and allows you to work also in bad visibility conditions.

Make sure the plate perfectly adheres to the ground. If necessary use the plate or other auxiliary tools to flatten the ground. At the start, „TERRATEST 3000“ checks the sensor connection, the battery charge, the chipcard and the GPS system and displays their status. By pressing again „START“ you automatically enter into the measuring menu and you will be asked to take the 3 preconsolidation tests and then the 3 measurements After each test, the display shows the displacement and the curves. After the third test it displays also the Evd value. The control panel is well structured and easy to use. Thanks to the back-lit display it´s easy to follow the instruc­tions. After the test, it displays the 3 displacement curves with the Evd value. sumber: http://www.terratest3000.com/english/function.html

Insert the cable and turn the electronic box on. Confirm the status query with the button „START“.

Follow the instruction: take first 3 preconsolidation tests and then 3 measurements.

Both the measuring and Evd data will be displayed. The tests are saved and can be printed later on.

Falling Weight Deflectometer

A falling weight deflectometer, FWD, is a testing device used by civil engineers to evaluate the physical properties of pavement. This could include (but is not limited to) highways, local roads, airport runways and railway tracks. The machine is usually contained within a trailer that can be either towed to a location by another vehicle or, when used on railway tracks, placed on a hand trolley and pushed to the location.
The FWD is designed to impart a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. The load is produced by dropping a large weight, and transmitted to the pavement through a circular load plate - typically 300mm diameter. A load cell mounted on top of the load plate measures the load imparted to the pavement surface. Deflection sensors (most FWDs use geophones, force-balance seismometers are also used) mounted radially from the center of the load plate measure the deformation of the pavement in response to the load. Some typical offsets are 0mm, 200mm, 300mm, 450mm, 600mm, 900mm, 1200mm 1500mm. The deflections measured at these geophones are termed D0, D200, D300 etc.
FWD data is most often used to calculate stiffness-related parameters of a pavement structure. The process of calculating the elastic moduli of individual layers in a multi-layer system (e.g. asphalt concrete on top of a base course on top of the subgrade) based on surface deflections is known as "backcalculation", as there is no closed-form solution. Instead, initial moduli are assumed, surface deflections calculated, and then the moduli are adjusted in an iterative fashion to converge on the measured deflections. This process is computationally intensive although quick on modern computers. It can give quite misleading results and requires an experienced analyst.
Instead, many analysts use simplified methods to calculate related parameters that are empirical in nature. The most common is maximum deflection under the centre of the load plate (D0) which is related to empirical measures such as the Benkelman Beam deflection (after minor adjustment for differences in the two devices). Historically some used the radius of curvature (D0-D200) but this is out of favour now because it is clear that the steel loading plate of 300mm diameter affects the shape of the deflection bowl between the centre (D0) and the D200 sensor at 200mm. However this means that a lot of useful information about the shape of the deflected bowl is wasted. Horak and Emery have published indices that use this information: BLI=D0-D300 and gives an indication of the basecourse performance, MLI = D300-D600 and gives an indication of the subbase performance, and LLI=D600-D900 and gives an indication of subgrade performance. These and other similar indices are known as shape factors. The FWD data can also be very useful in helping the engineer divide the length of the pavement into homogeneous sections.
FWD data can also be used to calculate the degree of load transfer between adjacent concrete slabs, and to detect voids under slabs.
A Light Weight Deflectometer (LWD) is a portable falling weight deflectometer. It is used primarily to test insitu base and subgrade moduli during construction.
A Heavy Weight Deflectometer (HWD) is a falling weight deflectometer that uses higher loads, used primarily for testing airport pavements.
A Rolling Weight Deflectometer (RWD) is a deflectometer that can gather data at a much higher speed (as high as 55 mph) than the FWD. It is a specially designed tractor-trailer with laser measuring devices mounted on a beam under the trailer. Another advantage of the RWD over the FWD is that it can gather continuous deflection data as opposed to discrete deflection data collected by the FWD.
The test materials are described in ASTM D 4694, and the test method is defined in ASTM D 4695.

sumber : http://en.wikipedia.org/wiki/Falling_weight_deflectometer

FAMILY GATHERING

 

 

 

APKJ

VIBRATION CONTROL AND MEASUREMENT

Transmissibility

Assuming that the forcing function is harmonic in nature, we shall consider two cases of vibration transmission - one in which force is transmitted to the supporting structure, and one in which the motion of the supporting structure is transmitted to the machine.

            (a) Force excitation

Consider the system shown in Figure 1, where f(t) is the harmonic force acting on the system and fT(t) is the force transmitted to the supporting structure or base. The force transmitted through the spring and damper to the supporting structure is :

(1)

Figure 1 Force Excitation Model

The magnitude of this force as a function of frequency is :

(2)

The oscillation magnitude as a function of frequency is :

(3)

Substituting equation (3) into (2) :

(4)

T is defined as the transmissibility and represents the ratio of the amplitude of the force transmitted to the supporting structure to that of the exciting force.

(b) Motion excitation

The system that illustrates motion excitation is shown in Figure 2. The motion of the dynamic system is represented by the variable x and the harmonic displacement of the supporting base is represented by the variable y. The equation that describes the dynamics of the system is :

(5)

Figure 2 Motion Excitation Model

Then the ratio of the magnitudes of the displacements as a function of frequency, which is the transmissibility, is given by the expression

(6)

Note that the transmissibility expressions for both force and motion excitation are identical. Therefore, it would appear that the engineering principles employed to protect the supporting structure under force excitation are the same as those used to protect the dynamic system from motion excitation.

 

Design Curves

(a) Transmissibility vs. damping ratio

The curve in Figure 3 demonstrates the effectiveness of an isolator to reduce vibration. Figure 3 also indicates a number of important concepts: (i) isolators should be chosen so as not to excite the natural frequencies of the system; (ii) damping is important in the range of resonance whether the dynamic system is operating near resonance or must pass through resonance during start-up; (iii) in the isolation region, the larger the ratio (i.e., the smaller the value of ), the smaller the transmissibility will be.

(b) Isolation efficiency vs. w and

Another graphical method of illustrating the regions of isolation and amplitude as a function of the disturbing frequency and the natural frequency of the system is shown in Figure 4. In using this figure we must note that percent isolation is defined by the expression :

(7)

Figure 3 Design Curves for the Transmissibility vs. the Frequency ratio as a Function of the Damping Ratio z for a Linear Single-Degree-of-Freedom System

The forcing frequency on the ordinate and the percent isolation lines in the graph locate a point, the abscissa of which is the natural frequency of the system necessary to achieve the required isolation. The system parameters may then be selected or adjusted to obtain this desired natural frequency.

(c) Static deflection vs. natural frequency

The static deflection is the deflection of an isolator that occurs due to the dead weight load of the mounted equipment. Since the static deflection is given by the expression , and since the undamped natural frequency of a single-degree-of-freedom system is determined by the equation :

(8)

where is in centimeters.

Figure 4 Design Curves for Isolation Efficiency vs. Frequency

(Damping Ratio, z = 0)

The graphical presentation of this equation is given in Figure 5. Thus, we can determine the natural frequency of a system by measuring the static deflection. This statement is correct provided that the spring is linear and that the isolator material possesses the same type of elasticity under both static and dynamic conditions. As mentioned previously, however, we are assuming a single-degree-of-freedom linear system throughout our analysis, and thus all the design curves presented above are applicable.

Figure 5 Design Curves for the Static Deflection vs. Natural Frequency for a Linear Single-Degree-of-Freedom System

The examples that follow demonstrate the use of these design curves.

EXAMPLE 1

A pump in an industrial plant is mounted rigidly to a massive base plate. The base plate rests on four springs, one at each corner. If the static deflection of each spring is two centimeters, then the natural frequency of the system is given by :

 

Control Techniques

In the control of noise we basically considered three areas: the source, the path, and the receiver. Vibration control may involve one or a combination of the following techniques.

(a) Source alteration

In the control of vibration it is important to first check and see if the noise or vibration level can be reduced by altering the source. This may be accomplished by making the source more rigid from a structural standpoint, changing certain parts, balancing, or improving dimensional tolerances. The system mass and stiffness may be adjusted in such a way so that resonant frequencies of the system do not coincide with the forcing frequency. This process is called detuning. Sometimes it is also possible to reduce the number of coupled resonators that exist between the vibration source and the receiver of interest. This technique is called decoupling. Although these techniques can be applied during design or construction, they are perhaps more often used as a correction scheme. However, it is also important to ensure that the application of these schemes does not produce other problems elsewhere.

(b) Isolation

In general, vibration isolators can be broken down into three categories: (i) metal springs, (ii) elastomeric mounts, and (iii) resilient pads. Before examining each of these areas, a few general comments can be made which are pertinent to all categories. We must always remember that we are assuming a single-degree-of-freedom system, and therefore our analysis will not be exact in every case. However, practical systems are normally reduced to this model because it is the only one that we understand thoroughly.

When building or correcting a design, always ensure that the machine under investigation and the element that drives it both rest on a common base. Always design the isolators to protect against the lowest frequency that can be generated by the machine. Design the system so that its natural frequency will be less than one-third of the lowest forcing frequency present. The isolation device should also reduce the transmissibility at every frequency contained in the Fourier spectrum of the forcing function.

(i) Metal springs

Metal springs are widely used in industry for vibration isolation. Their use spans the spectrum from light, delicate instruments to very heavy industrial machinery. The advantages of metal springs are: (a) they are resistant to environmental factors such as temperature, corrosion, solvents, and the like; (b) they do not drift or creep; (c) they permit maximum deflection; and (d) they are good for low-frequency isolation. The disadvantages of springs are (a) they possess almost no damping and hence the transmissibility at resonance can be very high; (b) springs act like a short circuit for high-frequency vibration; and (c) care must be taken to ensure that a rocking motion doe not exist.

Careful engineering design will minimize the effect of some of these disadvantages. For example, the damping lacked by springs can be obtained by placing dampers in parallel with the springs. Rocking motions can be minimized by selecting springs in such a way that each spring used will deflect the same amount. In addition, the use of an inertia block that weighs from one to two times the amount of the supported machinery minimizes rocking lowers the center of gravity of the system, and helps to uniformly distribute the load. High-frequency transmission through springs caused by the low damping ratio can be blocked by using rubber pads in series with the springs. A typical damping ratio for steel springs is 0.005.

The design procedure for selecting springs for vibration isolation is outlined below:

EXAMPLE 2

A machine set operating at 2400 rpm is mounted on an inertia block. The total system weighs 907 N. The weight is essentially evenly distributed. We want to select four steel springs upon which to mount the machine. The isolation required is 90%.

(ii) Elastomeric mounts

Elastomeric mounts consist primarily of natural rubber and synthetic rubber materials such as neoprene. In general, elastomeric mounts are used to isolate small electrical and mechanical devices from relatively high forcing frequencies. They are also useful in the protection of delicate electronic equipment. In a controlled environment, natural rubber is perhaps the best and most economical isolator. Natural rubber contains inherent damping, which is very useful if the machine operates near resonance or passes through resonance during "startup" or "shutdown." Synthetic rubber is more desirable when the environment is somewhat hazardous.

Rubber can be used in either tension, compression or shear; however, it is normally used in compression or shear and rarely used in tension. In compression it possesses the capacity for high-energy storage; however, its useful life is longer when used in shear. Rubber is classified by a durometer number. Rubber employed in isolation mounts normally ranges from 30-durometer rubber, which is soft, to 80-durometer rubber, which is hard. The typical damping ratio for natural rubber and neoprene is z = 0.05.

One word of caution when dealing with rubber: it possesses different characteristics depending upon whether the material is used in strips or bulk, and whether it is used under static or dynamic conditions. The steps for selecting an elastomeric mount are essentially those enumerated in the previous section on metal springs. The following examples will illustrate the procedure.

EXAMPLE 3

A drum weighing 120 N and operating at 3600 rpm induces vibration in adjacent equipment. Four vertical mounting points support the drum. Choose one of the isolators shown in Figure 6 so as to achieve 90°/ vibration isolation.

Figure 6 Typical Load vs. Deflection Curve for an Elastomeric Mount

(iii) Isolation pads

The materials in this particular classification include such things as cork, felt, and fiberglass. In general, these items are easy to use and install. They are purchased in sheets and cut to fit the particular application, and can be stacked to produce varying degrees of isolation. Cork, for example, can be obtained in squares (like floor tile) 1 to 2.5 cm in thickness or in slabs up to 15 cm thick for large deflection applications. Cork is very resistive to corrosion and solvents and is relatively insensitive to a wide range of temperatures. Some of the felt pads are constructed of organic material and hence should not be employed in an industrial environment where solvents are used. Fiberglass pads, on the other hand, are very resistant to industrial solvents. A typical damping ratio for felt and cork is z = 0.05 to 0.06.

EXAMPLE 4

A large machine is mounted on a concrete slab. The lowest expected forcing frequency is 60 Hz. If the isolator will be loaded at 7 N/cm2, choose the proper fiberglass isolator from the manufacturer's data shown in Figure 7 to produce 80% isolation. Assume that the damping ratio of the material is z = 0.05.

Figure 7 Typical Natural Frequency vs. Static Load Curves

for Fiberglass

(iv) Inertia blocks

Isolated concrete inertia blocks play an important part in the control of vibration transmission. Large-inertia forces at low frequencies caused by equipment such as reciprocating compressors may cause motion that is unacceptable for proper machine operation and transmit large forces to the supporting structure. One method of limiting motion is to mount the equipment on an inertia base. This heavy concrete or steel mass limits motion by overcoming the inertia forces generated by the mounted equipment.

Low natural frequency isolation requires a large deflection isolator such as a soft spring. However, the use of soft springs to control vibration can lead to rocking motions which are unacceptable. Hence, an inertia block mounted on the proper isolators can be effectively used to limit the motion and provide the needed isolation.

Inertia blocks are also useful in applications where a system composed of a number of pieces of equipment must be continuously supported. An example of such equipment is a system employing calibrated optics.

Thus, inertia blocks are important because they lower the center of gravity and thus offer an added degree of stability; they increase the mass and thus decrease vibration amplitudes and minimize rocking; they minimize alignment errors because of the inherent stiffness of the base; and they act as a noise barrier between the floor on which they are mounted and the equipment that is mounted on them. One must always keep in mind, however, that to be effective, inertia blocks must be mounted on isolators

Consider the system shown in Figure 8. The equations of motion that describe the systems are :

(9)

Figure 8 Model for the Analysis of Vibration Absorber

The magnitude of the frequency response is obtained from the following equations :

(10)

Now note what happen to the equations above if the forcing frequency w is equal to the natural frequency of the vibration absorber (i.e. ). Under this condition :

(11)

Therefore, the motion of the main mass is ideally zero and the spring force of the absorber is at all times equal and opposite to the applied force, . Hence no force is transmitted to the supporting structure.

 

Vibration Measurements

Measurements should be made to produce the data needed to draw meaningful conclusions from the system under test. These data can be used to minimize or eliminate the vibration and thus the resultant noise. There are also examples where the noise is not the controlling parameter, but rather the quality of the product produced by the system. For example, in process control equipment, excessive vibration can damage the product, limit pro-cessing speeds, or even cause catastrophic machine failure. The basic measurement system used for diagnostic analyses of vibrations consists of the three system components shown in Figure 9.

Figure 9 Basic Vibration Measurement System

(i) Transducers

In general, the transducers employed in vibration analyses convert mechanical energy into electrical energy; that is, they produce an electrical signal which is a function of mechanical vibration. In the following section, both velocity pickups and accelerometers mounted or attached to the vibrating surface will be studied.

(a) Velocity Pickups

The electrical output signal of a velocity pickup is proportional to the velocity of the vibrating mechanism. Since the velocity of a vibrating mechanism is cyclic in nature, the sensitivity of the pickup is expressed in peak milli-volts/cm/s and thus is a measure of the voltage produced at the point of maximum velocity. The devices have very low natural frequencies and are designed to measure vibration frequencies that are greater than the natural frequency of the pickup.

Velocity pickups can be mounted in a number of ways; for example, they can be stud-mounted or held magnetically to the vibrating surface. However, the mounting technique can vastly affect the pickup's performance. For example, the stud-mounting technique shown in Figure 10(a), in which the pickup is mounted flush with the surface and silicone grease is applied to the contact surfaces, is a good reliable method. The magnetically mounted pick-up, as shown in Figure 10(b), on the other hand, in general has a smaller usable frequency range than the stud-mounted pickup. In addition, it is important to note that the magnetic mount, which has both mass and spring like properties, is located between the velocity pickup and the vibrating surface and thus will affect the measurements. This mounting technique is viable, but caution must be employed when it is used.

Figure 10 Two Transducer Mounting Technique

(a) Stud-Mount Pickup; (b) Magnetically Held Velocity Pickup

The velocity pickup is a useful transducer because it is sensitive and yet rugged enough to withstand extreme industrial environments. In addition, velocity is perhaps the most frequently employed measure of vibration severity. However, the device is relatively large and bulky, is adversely affected by magnetic fields generated by large ac machines or ac current carrying cables, and has somewhat limited amplitude and frequency characteristic.

(b) Accelerometers

The accelerometer generates an output signal that is proportional to the acceleration of the vibrating mechanism. This device is, perhaps, preferred over the velocity pickup, for a number of reasons. For example, accelerometers have good sensitivity characteristics and a wide useful frequency range; they are small in size and light in weight and thus are capable of measuring the vibration at a specific point without, in general, loading the vibrating structure. In addition, the devices can be used easily with electronic integrating networks to obtain a voltage proportional to velocity or displacement. However, the accelerometer mounting, the interconnection cable, and the instrumentation connections are critical factors in measurements employing an accelerometer. The general comments made earlier concerning the mounting of a velocity pickup also apply to accelerometers.

Some additional suggestions for eliminating measurement errors when employing accelerometers for vibration measurements are shown in Figure 11. Note that the accelerometer mounting employs an isolation stud and an isolation washer. This is done so that the measurement system can be grounded at only one point, preferably at the analyzer. An additional ground at the accelerometer will provide a closed (ground) loop which may induce a noise signal that affects the accelerometer output. The sealing compound applied at the cable entry into the accelerometer protects the system from errors caused by moisture.

Figure 11 Mounting Technique for Eliminating Selected Measurement Errors

The cable itself should be glued or strapped to the vibrating mechanism immediately upon leaving the accelerometer, and the other end of the cable, which is connected to the preamplifier, should leave the mechanism under test at a point of minimum vibration. This procedure will eliminate or at least minimize cable noise caused by dynamic bending, compression, or tension in the cable.

(ii) Preamplifiers

The second element in the vibration measurement system is the preamplifier. This device, which may consist of one or more stages, serves two very useful purposes: it amplifies the vibration pickup signal, which is in general very weak, and it acts as an impedance transformer or isolation device between the vibration pickup and the processing and display equipment.

Recall that the manufacturer provides both charge and voltage sensitivities for accelerometers. Likewise, the preamplifier may be designed as a voltage amplifier in which the output voltage is proportional to the input voltage, or a charge amplifier in which the output voltage is proportional to the input charge. The difference between these two types of preamplifiers is important for a number of reasons. For example, changes in cable length (i.e., cable capacitance) between the accelerometer and preamplifier are negligible when a charge amplifier is employed. When a voltage amplifier is used however, the system is very sensitive to changes in cable capacitance. In addition, because the input resistance of a voltage amplifier cannot in general be neglected, the very low frequency response of the system may be affected. Voltage amplifiers, on the other hand, are often less expensive and more reliable because they contain fewer components and thus are easier to construct.

(iii) Processing and display equipment

The instruments used for the processing and display of vibration data are, with minor modifications, the same as those described earlier for noise analyses. The processing equipment is typically some type of spectrum analyzer. The analyzer may range from a very simple device which yields, for example, the rms value of the vibration displacement, to one that yields an essentially instantaneous analysis of the entire vibration frequency spectrum. As discussed earlier, these analyzers, which are perhaps the most valuable tool in a vibration study, are typically either a constant-bandwidth or constant-percentage-bandwidth type of device. They normally come equipped with some form of graphical display, such as a cathode ray tube, which provides detailed frequency data.

  Source : http://personal.cityu.edu.hk
Practical Methods for Vibration Control of Industrial Equipment, can download in here.   
Reference standards for Vibration Monitoring and Analysis, here.