Manajemen Operasi
Pemeliharaan
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Definisi
MAINTENANCE: semua aktivitas yang berkaitan dengan menjaga sistem dan peralatan agar selalu dalam keadaan siap pakai.
RELIABILITY: Probabilitas suatu bagian/suku cadang dari mesin/produk dapat berjalan dengan baik untuk jangka waktu tertentu dalam kondisi yang ditentukan.
Tujuan Menjaga kemampuan sistem dalam keadaan siap pakai dan mengendalikan pembiayaannya.
Mengapa Pemeliharaan Penting.. Karena berpengaruh langsung pada: 1. Kapasitas produksi 2. Biaya produksi 3. Kualitas produk dan jasa 4. Keselamatan pelanggan dan tenaga kerja 5. Kepuasan pelanggan
Posisi Maintenance dlm Perusahaan..
Di bawah Dept. Operasi atau, Langsung di bawah Direksi. Level manajer maintenance ditentukan oleh “The importance of maintenance for the organization”. Departemen maintenance biasanya dibagi 2: ◦ Bangunan (& Pekarangan) ◦ Peralatan
4 Taktik Reliability Tactics 1. Improving individual components 2. Providing redundancy Maintenance Tactics 1. PREVENTIVE MAINTENANCE Implementing or improving preventive maintenance 2. BREAKDOWN MAINTENANCE Increasing repair capabilities or speed
Umur Mesin
Umur Ekonomis ◦ Eksternal ◦ Internal
Umur Teknis
*Maintenance membuat umur ekonomis mendekati umur teknis.
Preventive Maintenance
Keeping machinery and equipment running.
Designing technical and human system that will keep the productive process working within tolerance.
A. Langkah Pencegahan Feel Inspection Tighten Clean Adjust Lubrication
- Rasakan - Periksa - Kencangkan - Bersihkan - Setel - Lumasi
… B. Overhaul (Bongkar) C. Repair (Perbaiki) D. Maintenance Scheduling.
Breakdown Maintenance Perbaikan. Dilakukan setelah mesin rusak.
Jenis Maintenance Menurut Biaya Most necessary maintenance Trouble Free Maintenance
Faktor Penentu Kualitatif Jenis Maintenance Vital atau tidaknya alat tsb Daya tahan alat tsb Struktur kelengkapan pabrik (layout)
◦ Line layout Trouble free ◦ Process layout Tdk harus trouble free
Repair, Replacement & Overhaul
Keputusan2 untuk membongkar dan mengganti. Dapat dikaitkan dengan biaya modal dan operasional (termasuk biaya mantenance) dari peralatan. ◦ Biaya operasional naik untuk sementara. ◦ Adanya kenaikan biaya sedikit demi sedikit, hingga akhirnya sampai pada penggantian yg dapat dibenarkan. ◦ Adanya beda biaya modal dan biaya reparasi mesin baru dengan mesin lama dapat mempengaruhi keputusan dilakukannya penggantian, karena rancangan mesin baru punya kemajuan yg disebabkan adanya mekansime/otomatisasi yang bisa meringankan biaya tenga kerja dan biaya2 maintenance.
Biaya Operasional
Repair
Overhaul
Replacement
Best Practice Walt Disney Company intolerant of failure or breakdown
United Parcel Service Dedicated drivers who operate the same truck every day Dedicated mechanics who maintain the same group of vehicle.
Improving Individual Components
System reliability (Rs) is a product of individual reliability.
Rs = R1 * R2 * R3 * ... Ex: 0.90
0.80
Rs = 0.90 x 0.80 x 0.99 = 0.713 or 71.3%
0.99
Evaluating Maintenance
Reliability ◦ Probability that an item will function for a given time
Mean time between failures (MTBF) ◦ Average time between failures of a repairable item
Failure rate ◦ Reciprocal of MTBF
Formula Number of failures Failure Rate(%) = FR(%) =
* 100% Number of units tested
Number of failures Failure Per Operating Hour(n) = FR(N) = Operating time
Mean Time Between Failure = MTBF
=
1 FR(N)
Example
Twenty air-conditioning systems designed for use by astronauts in NASA space shuttles were operated for 1,000 hours at NASA’s Huntsville, Alabama test facility. Two of the systems failed during the test – one after 200 hours and the other after 600 hours. Compute a. The percentage of failures, b. Number of failures per operating hour and c. MTBF
Answer a) b) c)
FR (%) = 10% FR (N) = 0.000106 failure/unit-hr MTBF = 9,434 hour
Providing Redundancy “Back Up” Redudancy is provided to ensure that if one component fails, the system has recourse to another.
Perfect Computer System No bug, no crash Extraordinarily reliable
Fighter jets Space shuttle Nuclear power plants
Providing Redundancy Probability of first + component working
Probability of second * component working
Probability of needing second component
= P(R)
Ex: reliability of a component is 0.80 and we back it up with another component with reliability 0.80. Reliabilty result = 0.80 + [0.80 x (1-0.80)] = 0.96
F-14
Tomcat F-14
Only 10% of the F-14’s software is used to fly the plane 40% used to automatic testing and verification 50% is redundancy
Other Example R1
R2
0.90
0.80
0.90
0.80
After redundancy P (R) = 0.94
R3
0.99
Types of Maintenance Breakdown
Preventive Routine inspection & servicing Prevents failures Basis for doing Time: Every day Usage: Every 300 pieces Inspection: Control chart deviations
Non-routine inspection & servicing Remedial Basis for doing ◦ Equipment failure
Preventive & Breakdown PREVENTIVE MAINTENANCE Meliputi inspeksi rutin untuk menjaga fasilitas peralatan dalam kondisi baik dan mencegah kerusakan
BREAKDOWN MAINTENANCE Memperbaiki peralatan yang telah rusak
Formula Expected number of breakdowns
= [(number of breakdowns)x(corresponding frequency)]
Expected breakdown cost = (Expected number of breakdowns)x(cost per breakdown)
Preventive Maintenance cost = Expected breakdowns cost if service contract signed + cost of service contract
Contoh Aplikasi Suatu perusahaan beroperasi dengan mengunakan sistem komputer. Dalam waktu 20 bulan terjadi kerusakan pada perangkat komputer yang ada. Biaya perbaikan setiap kali terjadi kerusakan adalah Rp 300 ribu
Jumlah Kerusakan
Jumlah Bulan Terjadinya Kerusakan
0
2
1
8
2
6
3
4
Total
20
Saat ini perusahaan menerima tawaran kontrak preventive maintenance dari pihak ketiga. Meskipun misalnya nanti perusahaan memutuskan untuk menerima penawaran preventive maintenance tersebut, kerusakan akan tetap terjadi dengan perkiraan rata-rata kerusakan satu mesin per bulan. Nilai kontrak yang ditawarkan adalah Rp 150 ribu per bulan.
4-Langkah Pengerjaan 1.
2.
3.
4.
Hitung expected number of breakdowns (tanpa preventive maintenance) Hitung expected breakdown cost per month (tanpa preventive maintenance) Hitung expected breakdown cost per month (dengan preventive maintenance) Bandingkan kedua pilihan yang ada.
Jawaban 1. 2.
3. 4.
Expected number of breakdowns = 1,6 kerusakan per bulan. Expected breakdown cost = Rp 480.000 per bulan Preventive Maintenance cost = Rp 450.000 per bulan Tanpa Preventive Rp 480.000 sedangkan dengan preventive Rp 450.000 Terima tawaran kontrak.
Breakdown Maintenance Reliability and preventive maintenance are seldom perfect. Good Maintenance facility:
◦ ◦ ◦ ◦ ◦ ◦
Well-trained personnel Adequate resources Ability to establish a repair plan and priorities Ability and authority to do material planning Ability to identify the cause of breakdowns Ability to design ways to extend MTBF.
Isu Mengenai Pemeliharaan TPM (Total Productive Maintenance) Combines total quality management with strategic view of maintenance from process and equipment design to preventive maintenance. Reducing variability & Improving Reliability designing machines that are reliable, easy to operate and easy to maintain. Emphasizing total cost of ownership when purchasing machines, so that service and maintenance are included in the cost. Developing preventive maintenance plans that utilize the best practices of operators, maintenance departments and depot services. Training workers to operate and maintain their own machines.
Challenger
28 Januari 1986 73 detik Bocor tangki bahan bakar bagian kanan.
Columbia 1 Februari 2003 Meledak saat akan mendarat
