I. DASAR TEORI
Perancangan elemen elemen mesin pada dasarnya meliputi prosedur yang luas, perhitungan-perhitungan yang kompleks, dan berbagai keputusan perencanaan. Data harus diperoleh dari diagram dan tabel yang banyak sekali. Disamping itu, perencanaan merupakan pekerjaan iteratif, yang mengharuskan perancang melakukan percobaan dengan mengambil beberapa pilihan untuk sembarang elemen tertentu, yang akan mengakibatkan terjadinya pengulangan perhitungan perancangan dengan data yang baru atau keputusan-keputusan perancangan yang baru. Hal ini sungguh sunguh dialami kususnya untuk peralatan-peralatn mekanis yang tersusun dari berbagai komponen mengingat keterkaitan antar komponennya harus di pertimbangkan. Perubahan-perubahan pada satu komponen kerap kali membutuhkan perubahan-perubahan pada elemen terkaitnya. Perancangan mekanis merupakan proses perancangan dan atau pemilihan komponen-komponen mekanis dan menggabungkan keduanya secara bersama-sama untuk mencapai fungsi yang diharapkan. Tentu saja elemen-elemen mesin tersebut harus sesuai, harus selaras dan harus secara aman dan efisien. Perancangan harus mempertimbangkan bukan hanya untuk kerja elemen yang di rancang pada suatu waktu tertentu, tetapi juga memikirkan elemen-elemen yang akan di padukan. Elemen-elemen mesin sangat sering dibuat dari salah satu logam atau logam paduan seperti baja alumunium besi cor, seng, titanuim, atau perunggu. Bagian ini menjelaskan sifat-sifat yang penting dari bahan yang dapat mempengaruhi prancangan mekanis. Sifat-sifat kekuatan, elastisitas, dan keuletan untuk logam, plastik dan jenis bahan lainnya biasanya ditentukan dari uji tarik (tensile tes) dimana sebuah sampel bahan, yang biasanya berbentuk bundar, atau batang datar, dijepit diantara penjepit dan ditarik perlahan hingga putus. Besarnya gaya pada batang dan perubahan panjang (regangan) dipantau dan dicatat terus menerus selama pengujuain tersebut. Karena tegangan pada batang sama dengan gaya yang bekerja pada batang dibagi dengan luas, maka tegangan sebanding dengan gaya yang bekerja pada batang. Berikut ini beberapa sifat kekuatan, elastisitas, dan keuletan logam: 1
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1.
Kekuatan tarik, Su
2.
Tegangan luluh,
3.
Batas proporsional
4.
Batas elastis
5.
Modulus elastisitas tarik, E
6.
dll
Tujuan akhir dari perancangan ,mekanis adalah untuk enghasilkan produk yang bermanfaat yang memnuhi memnu hi kebutuhan konsumen dan pembuatannya cukup cuku p aman, efisien, andal, ekonomis, dan praktis. II.
PERANCANGAN PISTON, BATANG PISTON DAN POROS ENGKOL
Dalam rangka perancangan piston, batang piston dan poros engkol dibutuhkan data- data awal agar dalam perancangan dapat diketahui secara pasti tujuannya. Data – Data – data awal yang dibutuhkan yaitu : Ne = Daya kuda (BHP) = 13,5 HP Cm = Kecepatan rata-rata piston (7 – (7 – 22) 22) ≈ 10 m/s z
= Stroke cycle ratio ≈ 2 untuk motor 4 tak
Pe = Tekanan efektif rata-rata = 8,78 kg/cm2 n
= putaran mesin 8000 rpm
III. PISTON
Piston pada mesin juga dikenal dengan istilah torak adalah suatu bagian dari motor yang berbentuk silinder yang bergarak lurus (translasi (translasi)) didalam silinder, gerak lurus tersebut untuk menghisap, memanfaatkan bahan bakar dan udara dan mendorong keluar sisa gas sisa pembakaran, serta memindahkan tenaga desakan dari hasil pembakaran kebentuk mekanis pada badan torak terdapat alur tempat cincin torak yang berfungsi merapatkan silinder dengan badan torak, sehingga proses pembakaran dan kompresi yang terjadi didalam ruang bakar tidak bocor serta mencegah minyak pelumas
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1. Perancangan Piston
Piston akan menerima tekanan dan temperatur dari proses pembakaran, maka torak harus dibuat dari bahan dari bahan yang mempunyai sifat-sifat, ringan kuat, kokoh, tahan aus dan tahan terhadap temperatur yang tinggi, untuk memenuhi syarat seperti diatas bahan torak dapat dipakai allumunium cooper alloy, alloy,
2. Komposisi Kimia a) Ni = 2,0 % Direncanakan 2,0% b) Mg = 1,5% Direncanakan Direncanakan 1,5% c) Cu = 4,0%Direncanakan 4,0% d) Si = 0,7%Direncanakan 0,7%Direncanakan 0,6% e) Zn = 0,3%Direncanakan 0,3%Direncanakan 0,3% f) Fe = 0,8%Direncanakan 0,8%Direncanakan 0,7% g) Al = Sisanya yaitu sekitar 90,9%
3. Sifat Mekanis
a) Kekuatan tarik
σu
= 30 kg/mm2
b) Kekuatan luluh
σ y
= 26 kg/mm2
c) Kekerasan
BHN
= 130 kg/mm2
4. Perhitungan Dimensi Piston
Dimensi dan nama – nama – nama nama bagian piston yang digunakan pada mesin bensin empat langkah ditunjukan oleh Gambar 1.1
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Gambar 1.1 Konstruksi dimensi piston (Kovakh, 1979 : 438 )
Keterangan Gambar : H = Tinggi piston D = Diameter piston H =Tinggi puncak piston ke ring atas hcr = Tebal piston Crown h1 = Jarak antara lubang ring piston H1 = Jarak antara sumbu pen dengan bawah piston H2 = Tiggi piston Skrit bb = Jarak antara lubang pen Lpp = Panjang pen piston dex = Diameter luar pen piston din = Diameter dalam pen piston
Perhitungan dimensi piston meliputi perhitungan – perhitungan – perhitungan perhitungan sebagai berikut : 1. Volume ruang bakar (Vc (Vc)) (Petrovsky, 1971 :26) : Vc
=
= = 25 cm − −
3
2. Tinggi Piston (H pis) (kovakh, 1979 : 439) Di Di
√ , .. . , . = √ , , =
= 7,66 cm = 76,6 mm
V mesin = 0,785 . Di2 .L 160
= 0,785 . 7,662 . L
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H
= (0,9 – (0,9 – 1,3) 1,3) Di
= (diasumsikan 1,1)
= 1,1 . 76,6 mm = 84,26 mm 3. Tinggi dari puncak piston sampai alur ring teratas (Kovakh, 1979 : 439) : h
= ( 0,06 – 0,06 – 0,09) 0,09) Di
= (diasumsikanl 0,09)
= 0,09 . 76,6 mm = 6,894 mm 4. Tebal puncak piston (Kovakh, 1979 : 439) 0,07 – 0,07 – 0,08 0,08 = hcr
,
= (diasumsikan 0,07)
= 0,08 x 76,6 mm = 6,894 mm
5. Tinggi alur ring piston (Kovakh, 1979 : 439) hi
= (0,03 – (0,03 – 0,05) 0,05) Di
= (diasumsikan 0,04)
= 0,04 x 76,6 mm = 3,064 mm 6. Tinggi piston skrit (Kovakh, 1979 : 439) H2
= 0,68 – 0,68 – 0,74) 0,74) H
= (diasumsikan 0,70)
= 0,71 x 84,26 = 59,82 mm 7. Jarak dari dasar piston hingga sumbu piston pen (Kovakh, 1979 : 439) H1
= (0,41 – (0,41 – 0,61) 0,61) H
= (diasumsikan 0,51)
= 0,51 84,26 mm = 42,97 mm 8. Diameter luar pen (Kovakh, 1979 : 439) dex
= (0,24 – (0,24 – 0,28) 0,28) Di
= (diasumsikan 0,26)
= 0,26 x 76,6 mm = 19,91 mm 9. Jarak antara tengah – tengah – tengah tengah piston pen (Kovakh, 1979 : 439)
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5. Tinjauan Kekuatan Piston
Tinjauan kekuatan dan perhitungan pada bagian piston bagian piston skirt , menggunakan persamaan-persamaan dibawah ini. Tinjauan kekuatan dan perhitungan pada bagian piston bagian piston skirt , menggunakan persamaan-persamaan dibawah ini. 1. Tekanan piston maksimal terhadap dinding liner (Petrovsky, 1962 : 368) Nmax = 0,08 . Pz = 0,08 x 18,5 kg/cm2 = 1,48 kg/cm2 Dengan : Pz = Tekanan akhir pembakaran = 18,2 kg/cm2 2. Tekanan samping spesifikasi maksimal pada permukaan piston (Petrovsky, 1962 : 368) qn
= =
Dengan : qn = 3 – 3 – 3,5 3,5 kg/cm2
, , ,
= 0,032 kg/cm2 Piston skrit Piston skrit dinyatakan AMAN karena tekanan samping yang terjadi pada piston skrit adalah 0,037 kg/cm2 dan masih berada dibawah tekanan samping ijin pada piston skrit piston skrit q n = 3 – 3,5 3,5 kg/cm2. Selanjutnya pada piston crown dianggap distribusi beban merata dari tekanan gas sisa pembakaran (Pz). Ilustrasi pembebanan pada piston corwn ditunjukkan oleh gambar 1.2
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1. Gaya tekan luasan ½ lingkaan piston crown ( Petrovsky, 1962:368)
2.
F = 2 = 8 3, 1 4 7, 6 6 Maka ∶ F = 18 18,5,5 8 =426,058 kg⁄cm Momen bending yang terjadi dengan asumsi = (,1962:369) ,1962:369): M = P Maka ∶ M= , 18,5 8,5 = 34 346,6,454 454 kg. kg.cm cm
3. Momen tahanan lentur pada piston crown ( crown ( Petrovsky Petrovsky,, 1962:370) : W=
.δ
= ℎ = 0,612 0,612 , , Maka : W = 478 = 0,478 = = ,,= 724,803 ⁄
Dengan :
Harga batas tegangan bending untuk material paduan alumunium adalah
= 500 – 900 kg/, maka hasil perhitungan tegangan bending yaitu 835,51 kg/cm memenuhi syarat. IV.
PENA PISTON
Bahan yang akan digunakan pena piston direnvanakan bahan baja paduan ( alloy
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b) Kekuatan luluh
σs
= 2800 Kg/cm2
c) Pertambahan panjang
σ b
= 19 %,
d) Kekuatan impact Wimp
= 5 kgm/cm2
Ilustrasi pembebanan pada pena piston dan dimensi pena piston ditunjukan oleh gambar 1.3
Gambar 1.3 Ilustrasi pembebanan dan dimensi pena piston (Petrovsky,1962:372) 3. Perhitungan Pena Piston ( Kovakh, 1979 : 459 ) dex
= Diameter luar pen = 19,91 mm
din
= Diameter dalam pen
din
= dex . rd
Maka Din
= dex . rd = 1,991 x 0,791 = 1,574 cm
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Dengan∶ Px
= gaya tekan maksimum =
.
= 18,5 x 0,785 x
7,66 = 852,116 kg/
b = 3,064 cm ,(, ,) Maka : = = 652,72 kg/ L =
5. Tegangan bending yang terjadi
=
( − ) −, , , = ( , ) = 0,4 , = 1631,8 kg/ Maka : = ,
Dengan : W = Momen tahanan =
Tegangan bending yang diijinkan = 1500 – 1500 – 2300 2300 Kg/cm, maka dari hasil perhitungan diatas yaitu 1631,8 kg/cm2 memenuhi syarat dan AMAN 6. Tegangan geser yang terjadi
=
)
Dengan : f : f = luasan = luasan melintang piston pin = ( = 0,785 ( 1,9912 – 1,574 1,5742) = 0,785 . (3,96 – (3,96 – 2,47) 2,47) = 0,785 . 1,49
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kompresi dan ring oli. Bahan yang dipakai untuk piston ring kompresi dan piston ring oli direncanakan dari bahan besi tuang. 1. Komposisi Kimia a) Si = 0,9 – 0,9 – 1,15% 1,15% Direncanakan= 1,13% b) Mn = 0,8 – 0,8 – 1,0% 1,0% Direncanakan = 0,9% c) P = 0,1 – 0,1 – 0,3% 0,3% Direncanakan = 0,2% d) Ni = 0,6 – 0,6 – 1,2% 1,2% Direncanakan = 1,0% e) Cr = 0,3 – 0,3 – 0,5% 0,5% Direncanakan = 1,0% f) V = 0,1 – 0,1 – 0,2% 0,2% Direncanakam = 0,2% g) Mo = 0,1-0,4% Direncanakan = 0,4% h) S< 0.12% Direncanakan = 1,0%, i ) Fe = 94,5% 2. Sifat Mekanis a) Kekuatan tarik σu ≈ σt =1800 kg/cm2 b) Kekuatan bengkok bengkok
σ b
= 4800 kg/cm2
c) Kekuatan tekan σc
= 900 kg/cm2
d) Kekerasan brinell
BHN
= 190 – 190 – 230 230
Ilustrasi dimensi pada ring kompresi dan ring pengontrol oli ditunjukan oleh gambar 1.4
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3. Perhitungan ring kompresi ( Petrovsky, 1962 : 374 )
a. Lebar ring piston b = (0,029 – (0,029 – 0,033) 0,033) . Di
(Diasumsikan0,030) (Diasumsikan0,030)
= 0,030 x 7,66 = 0,229 cm
b. Tebal ring piston h = (0,6 – (0,6 – 1,0). 1,0). b
(Diasumsikan 0,9)
= 0,9 x 0,229 = 0,206 cm
c. Jarak antara ujung ring sebelum masuk kedalam selinder L = (0,10 – (0,10 – 0,18) 0,18) . Di
(Diasumsikan 0,15)
= 0,15 x 7,66 = 1,149 cm
d. Jarak antara ujung ring setelah masuk piston
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= 0,45 – 0,45 – 0,7 0,7 kg/cm2 f.
(Direncanakan : 0,5)
Momen tahanan pada ring kompresi
=
W = b . h2 0,229. 0,206 2
= 0,0025 cm2 g. Tegangan bengkok yang terjadi σ b
, = 1408 kg/cm = ,
σ b =
2
Tegangan yang diijinkan untuk besi – besi – besi besi tuang pada ring kompresi adalah 1000 – 1000 – 1500 1500 Kg/cm2, maka dari hasil perhitungan diatas yaitu 1408 kg/cm2 memenuhi syarat dan AMAN. 4. Perhitungan ring oli (Petrovsky, 1962:372)
a. Lebar ring oli : b = ( 0,029 – 0,029 – 0,033). 0,033). Di
(diasumsikan 0,30)
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= . 0,229 . 0,229
W = . b . h2
2
= 0,0025 cm3
g. Tegangan bengkok yang terjadi σ b
, = 1320 kg/cm = ,
σ b =
2
Tegangan yang diijinkan untuk besi-besi tuang pada ring oli adalah 1000 – 1500 1500 kg/cm2, maka dari hasil perhitungan diatas yaitu 1320 kg/cm2 memenuhi syarat dan AMAN.
VI.
CONNECTI NG R OD ) BATANG PENGGERAK ( CONNECTI
Pada ujung connecting rod dipasang small end bearing atau bush dibuat dari bahan perunggu phospos, pemasangan dilakukan d ilakukan dengan mengepres. Melalui pena piston, connecting rod berfungsi meneruskan gaya – gaya dari piston ke poros engkol, dan sebaliknya. Sedangkan pada connecting rod akan menerima gaya tekan dari pembakaran,
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Bagian – Bagian – bagian bagian yang akan dihitung pada connecting rod ditunjukan oleh Gambar 2.1
Gambar 2.1 Connecting rod
(Petrovsky,1962 : 378 )
3. Connecting rod rod small small end a. Panjang small end bearing akibat beban full, dapat dicari dengan menggunakan rumus ( Khovakh 1979 : 439 ). b b = (0,40) Di
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= 0,01991 cm f. Diameter luar bush
= + (2 x ) + Δ = 1,991 cm + (2 x 0,1672 ) + 0,0199 = 2,14 cm g. Jari – Jari – jari jari luar bush
, = 1,17 cm =
R=
h. Radius luar small end ,dapat dicari dengan menggunakan rumus (Khovakh ,1979 : 467 ) Ro = (1,2 – (1,2 – 1,3 1,3 ) x r = 1,3 x 1,17 = 1,521 cm i.
Diameter small end :
= 2 x Ro = 2 1,5 1,521 21
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4. Ketahanan terhadap lengkungan pada beban kritis untuk cast steel, dapat dicari dengan menggunakan rumus ( Petrovsky : 380 )
= (3350 – (3350 – 6,2 6,2 x
) α
a. Total gaya connecting rod , dapat dicari dengan menggunakan rumus (
Petrovsky : 380 ):
=
=
Tegangan kompresi yang diijinkan untuk : Karbon steel = 800 – 800 – 1200 1200 kg / cm2 Alloy steel = 1200 – 1200 – 1800 1800 kg / cm2 b. Cross sectional area pada area pada connecting rod , dapat dicari dengan menggunakan
rumus ( Petrovsky : 380 ).
(,−,) = 1,68 cm = = ρ = √ = √ ,, = 8,11 cm (3350 6,2 x , ) 1,68 = 5615,61 kg 2
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Mmax
)
=(
2
0,0078 x 1,17 x 1,68 x 9,65 = 6,57 kg.cm
7. Modulus penampang terkecil connecting rod W
=
= , , = 2,02 cm
3
8. Bending stress pada connecting rod , dapat dicari dengan menggunakan rumus ( Petrovsky : 381 ): σ b
=
= , = 3,25 kg/cm ,
2
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e. Diameter dalam big end bearing Dinb
= dcp + Δcp = 5,13 + 0,004 = 5,134 cm
f. Diameter bagian luar big end D bixer
VII.
= (1,2 – (1,2 – 1,3) D 1,3) Dbed = 1,3 . 5,53 . 5,53 = 7,18cm
POROS ENGKOL (CRANK SHAFT)
Crank shaft menerima gaya – gaya – gaya gaya dari connecting rod, gaya yang diterima crank shaft yaitu : a. Gaya tekanan gas b. Gaya inersia dari bagian bagian yang bergerak translasi
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2. Sifat mekanik a) Kekuatan tarik (
)
b) Brinel Hardnes ( HB )
= 110 kg / mm2 = 311 – 311 – 373 373
Bagian – Bagian – bagian bagian yang akan dihitung pada crank shaft ditunjukan oleh gambar 4.9
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= (0,500,65) × = 0,60 × 5,13 = 3,07 c. Radius crank shaft
= Dengan : L Maka :
= Panjang langkah piston
= , = 2,17 17
6. Counter weight ( pipi engkol ) Bagian – bagian yang bertranslasi dan berotasi menimbulkan gaya inersia, maka dibutuhkan counter weigth yang bersatu pada crank web. web. Adapun tujuan dibuat counter weight adalah :
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f. Tinggi pipi engkol
= () ) =2,171,9 (5,135,89) = 9,58 58 Kesimpulan Pada proses perencanaan sebuah mesin ataupun komponen dari bagian-bagian mesin (khususnya piston, batang piston, dan poros engkol) perlu beberapa data yang digunakan sebagai acuan dasr untuk menentukan data data untuk perencanaan ditugas ini yaitu :
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Gaya tekan pada poros engkol
= 852,11 kg,
Diameter main jurnal
= 5,89 cm
Diameter crank pin
= 5,13 cm
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DAFTAR PUSTAKA
Kovakh.,M., 1979, ” Motor Vehicle Engines”MIR Engines”MIR Publisher, Moscow Petrovsky., M., 1973, ” Marine Internal Combution Engine”MIR Engine”MIR Publisher, Moscow