Yanal basınçların keskilerin kesme performansına olan etkilerin araştırılması

Abstract

Yeraltında kesici uçlar ile kazı yapan makinalarda, makinanın performansını tayin eden çeşitli faktörler vardır. Bu tez çalışmasında keskiler üzerinde yanal basınçların, keskiye gelen kuvvetler açısından, kesici kafa dizaynında gözönüne alınması gereken s/d oram açısından ve birim hacımdaki kayacı kesmek için gerekli enerji açısından nasıl etkili olduğu üzerinde durulmuştur. Genel olarak arazi gerilmeleri tanıtılarak bunların sınıflaması yapıldıktan sonra arazi gerilmelerini hesaplamanın yöntemlerinden bahsedilmiştir. Ayrıca araştırmacıların in-sıtu ölçmelerden yatay gerilmelerin derinliğe ve düşey gerilmelere göre nasıl değiştiğine dair elde ettikleri sonuçlara yer verilmiştir. Laboratuvarda gerçekleştirilecek deneylerde kullanılması uygun görülen makinalar ve bilgisayar programlan ayrıntılı olarak tanıtılmıştır. Deneylerin yapılmış olduğu pilot kazı seti Uluslararası Kaya Mekaniği Cemiyetinin kabul etmiş olduğu standartlara uygundur. Deneylerde alçıdan yapılmış 15*15*30 cm boyutlarında bloklar kullanıldığından bölüm 5'te bunların nasıl hazırlandıkları ve özellikleri açıklanmıştır. 12.5 mm genişliğinde kama uçlu tungsten karbit keski kullanılarak yapılan 87 deneyin sonuçlarının ortalaması bir tablo şeklinde verilmiştir. Burada verilen yanal basınçlar piston tarafından uygulanan yanal basıncın, piston yüzey alanına ve alçı numunesinin pistonla temasta olan yüzey alanına bölünmesiyle bulunan gerçek değerleridir. Planyaya monte edilmiş kama uçlu keski alçı numunesi üzerinde bir gidiş- dönüşü tamamlıyarak kesme işini gerçekleştirmektedir. Buradan dinamometre ile okunan kuvvetler, ölçülen uzunluk ve pasa hacimleri yanal basınçlara göre kıyaslanarak bilgisayarda değerlendirilmektedir. Deney sonuçlarından faydalanarak çizilen grafiklerde araştırmanın sonuçlan irdelenmiş ve yanal basınçların, keskilerin kesme performansım nasıl etkiledikleri üzerinde durulmuştur.

The main objective of this research is to study the effect of the horizontal stresses on the cutting performance of drag tools. About ninety cutting experiments were carried out on plaster blocks in order to understand the basic mechanism of rock cutting mechanics in mine mechanization laboratory of mining faculty of I.T.Ü. In this report, firstly field stresses were defined since it is believed that they play an important role on any tunneling project. The field stresses are classified as primary, secondary and tertiary. These stresses are the result of several different natural, geological, and artificial phenomena. The easiest to predict and the most universal are those caused by gravity. It is vertical (directed towardthe center of the earth ) and is one of the three principal stresses exceptin situations where tectonic processes are active, or where the topography is varying so as to rotate the stress field. Theoretical formulas of stress calculations were given. According to this formulas, the basic factor of the field stress is weight and thickness of the overburden. In flat- lying sedimentary rocks, the vertical stress is taken to be the weight of the individual overburden beds. Some scientists have measured the horizontal stresses in underground openings with the stress tensors. Figure 3.5, a compilation of such measurements, shows that the vertical stress compenent is usually quite close to that predicted from the weight of overburden. The average horizontal stress in hard rocks, however, is much greater than gravitational. At shallow depths the ratio of horizontal to vertical stress is much greater and increases rapidly toward the the ground surface. IX The density of granitic rocks rich in quartz and feldspar is about 2650 kg/m3, and of basic and ultrabasic rocks about 3300 kg/m3. This results in a vertical stress gradient in the range of 26.0- 32.4 kPa/ m, depending on the density of the rock formations at the site. According to investigators horizontal stresses can not be measured with only depth, and horizontal stress/vertical stress ratio is not constant, especially in hard rocks. Horizontal components can not be calculated; they should be measured for a particular site, or estimated from the trends of nearby measurements. Tunnel or gallery cross section is important for the stress distribution around the openings. The calculation formulas of stress around circular openings were explained by KIRCH. Figure 3.9 shows the tangential stress distributions on the boundary of the opening at various confining pressures. Because of symmetry, only a quarter of the circular opening is shown. M is the ratio of the horizontal to vertical applied stress. Thus, by definition, M=0 represents a uniaxial stress, M=l is a hydrostatic stress field. It is seen that as the confining pressure increases, the tangential stress decreases at the horizontal axis but the tensile tangential stress at the vertical axis becomes compressive when M=l/3. Compressive tangential stress is the same all around the boundary with Gq/Sv =2 for M=l, where Sv is the average applied stress. Tangential stresses on circular opening boundary are maximum, and they decrease when go away from boundary opening. Maximum cutting stress on a point of circular opening is equal to fifty per cent of tangantial stress. Most of the mining galleries have rectangular or square cross sections. Stress distribution around this type of galleries is different from circular opening. If there is only vertical stress, tensile strength is on critical situation according to tensile breaking down. When horizontal stress increases, tensile strength which is the center of cross section decreases. In this type of cross sections the stress is especially distributed at corners. The following experimental set up was used during this investigation ; - A shaping machine equipped with a chisel pick having a rake angle of -5°, and width of 12.5 mm. 18 kW SIEMENS electrical motor works the shaping machine helping belt-drum system. It has a tresle for cutting 30*40*50 cm sample blocks. - A piezoelectric dinamometer to record the tool forces in three direction. These are cutting, normal and side forces. Quartz cristalls in dinamometer detect the electric charge when these forces effect the dinamometer. The transducer has a measuring capacity of one ton. - The Charge Amplifier Type 5001 is a mains-operated DC amplifier of very high input impedance with capacity negative feedback, intended to convert the electric charge from a piezoelectric transducer into a proportional voltage on the low impedance amplifier output It is limited with -10 and +10 volts. -Two computers to evaluate the cutting results. The first one is IBM PC-XT having analog digital card. A software written by Dr. Sina Yazacı using Turbo Pascal 3.01 programme for the PC-26 A/D card. This programme enables to read thirty thousand samples in five seconds.The second computer is 486 DX-66 that graphical conclusions are taken with the macro programme in excel 4.0. To prepare sample is important for laboratory tests. A great attention was paid to prepare standard plaster samples for the cutting tests. The samples were prepared using 60 % water and 40 % plaster in weight. Mix ratio is as following, Water density (dw) =1 g/cm3 Plaster density (dp) =2.7 g/cm3 Block Volume =w*h*l= 1 5* 1 5*30=6750 cm3 w=Width h=Height l=Lenghth Vt= Vw +VP 6750=mw/ dw + mp / dp 6750=0.6 mp + mp/ 2.7 6750=(1.62mp + mp)/2.7 mp =6956.12 g. mw =4175.25 g. Samples were cast in steel blocks having dimensions of 15*15*30 cm. They were waited 15 days in steel blocks. Strength of the samples were in average 81.4 kg/cm2. First a plaster block is put on the shaping machine and it is trimmed to have a smooth surface. Cutting depth is constant at 5 mm. During the cutting experiments, cutting width was 12.5 mm and cutting angle was -5°. After finishing of constants and variables control, sample is ready to cutting experiment. Shaping machine works a times on sample and it is stopped. So, one of the cutting experiment finishes. Firstly, cutting experiments were done under 0 kg/cm2 horizontal stress. Secondly, cutting experiments were done under 5.56, 11.12, 16.68, 22.24 kg/cm2 horizontal stress with the help of a hydraulic piston. The piston pressure were kept at 25, 50, 75, 100 kg/cm2 and horizontal stresses were calculated using piston diameter and the lateral surface of the sample. XI Calculation of the horizontal stress is shown on following formulas: Piston pressure = 25 kg/cm2 Piston semi-diameter = 4.5 cm Piston area = 4.52 * % =63.62 cm2 Sample's lateral area 286 cm2 Piston pressure * Piston area =25 kg/cm2 *63.62 cm2= 1590.4 kg Lateral Pressure =1590.4 kg /286 cm2 =5.56 kg/cm2 Different cutting spacing with 0, 5, 10, 20 and 35 mm were used and each test was replicated three times. The cutting spacing is defined as the distance between cutting tool and previous cutting groove. Rock volume per unit length of cut is calculated with the measurement of cutting length and plaster volume coming from the cutting. The specific energy is calculated by dividing the mean cutting force by yield. Yield is defined as the volume of rock per unit distance cut. Average cutting forces, normal forces, side forces, specific energy, plaster volume per unit length, and cutting spacing/depth ratios were tabulated in the text. Results of the research can be given as follows - Horizontal stresses change the optimum spacing/ depth ratio (s/d). When horizontal stresses are 0 and 5.56 kg/cm2 optimum s/d is 1. When horizontal stress is increased up to 11.12 kg/cm2 that equal to 1/8 of sample's compressive strength, the optimum s/d ratio is 2. -When cutting spacing/depth ratio is 0, cutting forces are small. Because, one of the lateral surface of the pick is free and there is not friction. -Horizontal stresses force the plaster sample to be compact until pores become minimum. So, the specific energy increases with horizontal stress reaching up to 16.68 kg/cm2. After mis point, sample can not become more compact and looses its strength, specific energy drops after this stress value. -Horizontal stresses effect cutting and normal forces. It is shown that cutting and normal forces increase with spacing varying from 0 to 35 mm. -Horizontal stresses has very important role on the yield that is plaster volume per unit length. If cutting space is higher than 10 mm, yield increases while horizontal stress is going up.Experiments have shown that horizontal stresses directly helped to cut -Maximum cutting force value is 1.8 times higher than minimum value for 0 kg/cm2 horizontal stress. However this value increases up to 3 times for horizontal stress of 16.68 kg/cm2. XII -ıFor normal forces, maximum value is 2.6 times higher than minimum value for 0 kg/cm2 horizontal stress. However this value increases up to about 4 times for horizontal stress of 16.68 kg/cm2.We can say that the horizontal stresses much more effect the normal forces than cutting forces.