Designing The Estimation of The Need for Spare Parts and Inventory Policy on The D32 CP8 Compressor Machine

Abstract


Introduction
Angelina, Atmaji, and Santosa in their study of RCS presented an analysis of spare parts and inventory of steel tape and brass inserts as key components in research for Rovema packaging machines.Applying the Reliability Centered Spares (RCS) method to calculate the optimal component stock needs using Poisson Proces and Min-Max inventory analysis [1].The results showed that steel tape and brass inserts should be stored in technical warehouses for a certain amount in the next period to minimize downtime due to the unavailability of spare parts [1].
Inventory management policy is very important in a company.Lacking a good inventory control system, the company cannot carry out production smoothly, and it is also difficult to meet customer needs.Inventory is the part of the company's expenses that absorb the most costs.Causes of Excess Inventory Storage costs are high, but there may be a risk of not being met if the stock shortage needs clients.
Spare parts inventory is one of the important aspects that support the overall inventory systems in the manufacturing process.
XYZ company, a manufacturing company engaged in building materials, also faces obstacles related to the supply of spare parts.In the production section, there are several machines used in the production process, one of which is a compressor.But in reality, machine reliability may degrade over time, leading to machine downtime.Based on the compressor engine inspection data owned by XYZ company, the overheating frequency for each compressor engine can be known.
Based on Figure 1, it is known that the highest overheating frequency occurs in the D32 CP 8 compressor engine, which is 87 times.Based on the data above, the D32 CP 8 compressor engine is the engine that has the highest overheating rate from January 2021 -December 2021, which is 87 times.Compressor D32 CP8 is a compressor machine that is crucial for the production process.Compressor D32 CP 8 is a compressor that functions to supply pressure to one of the main production machines so that the main production machine can work according to its standards.If the pressure required on the main production machine cannot be met, the production process will automatically stop.
The results of an interview with the Mechanical Maintenance Manager, several factors cause the high frequency of overheating in the D32 CP8 Compressor.Such factors can be deciphered using the fishbone diagram in figure 2.
Based on Figure 2, it can be seen that several factors are suspected to be the cause of the damage suffered by the D32 CP 8 Compressor.From the point of the man factor, the operator is poorly trained in handling the D32 CP 8 Compressor both in maintaining and conducting inspections, the available SOPs are not run by the operator.From the point on the machine factor, the D32 CP 8 Compressor engine experiences breakdown maintenance because the engine burns, this is because the compressor engine often overheats.From the point of the method factor, the inspection method is not following company standards.From this point on the material, the factor is that the components are not up to standard and the material that the components are easy to wear out.Problem Identification is the next stage after the results are obtained in field studies.Problem identification is a very important step in this study because it will be the beginning of the problem-solving system.After the problem identification stage is carried out, the determination of the solution and the objectives of the final project will be carried out in the data collection stage that will be used in the research.After the data needed for research has been collected, the next stage is the data processing data.At this stage, the existing data will be calculated according to the theory already studied in the literature study.Analysis of the results of data collection and processing that has been carried out to determine the results of inventory policies that are in accordance with data calculations.Conclusion is the submission of the conclusions in this study as well as the submission of suggestions for the company.

Maintenance
This section explains the meaning of maintenance which is used as a basis in this study.Maintenance is an act of maintenance and repair of an object.In the industrial world, maintenance is meant the maintenance of factory components or machines and updating the performance of components or machines so that the service life of the component or machine is optimal when the engine is damaged and unfit for use [2].Maintenance is divided into two, namely: preventive maintenance and corrective maintenance [3].
Preventive Maintenance is maintenance that is carried out on a scheduled basis.Generally periodically, where many activities such as inspection and repair, replacement, cleaning, lubrication, and adjustment and equalization are carried out, preventive maintenance is machine maintenance activities carried out to prevent damage and find conditions that can cause production facilities or machines to be damaged at the time of production [4].
Corrective maintenance is maintenance that is carried out after an error occurs, it aims to restore the equipment to a state where the equipment can perform the necessary functions directly or suspended, Corrective Maintenance is a remedial action that does not have a routine schedule for repairing equipment or components in ready-made condition [5], [6].Corrective Maintenance usually cannot be planned because this activity is carried out with damaged components and then repaired so that it can run optimally again [3].

Maintainability
Maintainability is the opportunity for a damaged system or component to be restored to full working condition within a predetermined time and with certain maintenance procedures.Maintainability also relates to the duration of maintenance or how long it takes to complete a maintenance action (ease and speed).The characteristics of the maintenance capability are usually determined by the design of the equipment, which then establishes the maintenance procedure of the machine and determines the length of the repair time.

Reliability Centered Spares
Reliability Centered Spares is an approach to determining the level of spare parts inventory based on through-life costing and equipment needs and machine maintenance operations in support of inventory.Reliability Centered Spares (RCS) is one of the parts management methods by considering aspects such as the maintenance needs of the machine needed, what happens if the tribe spare parts are not available, anticipate the need for spare parts, the p-ISSN: 2087-1627, e-ISSN: 2685-9858 number of spare parts stock storage needed, and what machine maintenance requirements should be carried out [7].

System Breakdown Structure
System Breakdown Structure is the first step in identifying the components of a system by conducting a more detailed analysis of the system [8].The main purpose of this system breakdown structure is to analyze all components contained in the system based on functions so that it can help to determine critical components [9].

Management Parts
Spare parts are components that must be available to maintain the ability of machine equipment to always be in good condition and can operate properly.In maintenance engineering, the cost of parts is a large expense, and spare parts are very important for the availability of equipment, spare parts are a significant resource in equipment maintenance [10].
Parts Classification is a grouping activity on parts that is useful for deciding which management parts strategy to apply to these components.Spare parts are divided into 2 types, namely repairable components, and non-repairable components, therefore spare parts management is needed (spare parts management) and controlling the availability of spare parts [11].
Components are considered non-repairable when they are difficult to repair and the cost to repair is much more expensive than buying new components.In the calculation of the number of non-repairable parts component needs, the number of failures that occur is equal to the number of parts needed.The number of parts component needs is the minimum value of (n) with the following calculation: When reconditioning the machine and involving the replacement of components it is necessary to hold the spare parts components, so the calculation is: Information: P: Level of trust

Risk Matrix
The Risk Matrix is a mechanism to clarify process risks that are usually identified through one or more objectives.The following are the advantages of using a risk matrix: 1. Very functional When used in field practice workers can avoid all kinds of risks that occur.2. Make a standard tool for establishing the relationship between probability and consequence of risk.3. Disables the acceptance of unacceptable risks and allows for operational decision-making.
Characteristics of the risk matrix, namely: 1. Simple and easy to understand 2. Have a clear orientation to its application 3. Have a clear sense of the consequences that have a range of values in each existing consequence 4. Provides very clear guidance on what actions are needed to deal with an unbearable level of risk.

Mean Time to Failure (MTTF)
Mean Time to Failure (MTTF) is the average time between the failure of a machine or system when it starts operating until it experiences a failure [12]- [14].When the rate of damage to components/systems depends on their service life, the statistical distribution used is the Normal and Weibull distributions. 1

. Normal Distribution
The parameters used in the normal distribution are the mean (μ) and the standard deviation (σ).The reliability functions on this distribution are:

Exponential Distribution
The parameter used on the Exponential distribution is lambda (λ).Has a constant value against time.The average time between damages to this distribution is:

Weibull distribution
The parameters used in the Weibull distribution are the service life parameter (α), the form parameter (β), and the location parameter (γ).The average time between damages in this distribution is: With:

Min-Max Stock
Min-Max stock is a method for determining the minimum amount of stock and maximum stock of spare parts [15].The purpose of implementing min-max stock is so that the warehouse part knows the inventory of spare parts needed to meet the production capacity and maximum stock of spare parts in the warehouse [16].The following is how to calculate the min-max stock method.ReOrder Point is the point or limit of inventory to be reordered [17].The calculation of ReOrder Point is determined by the length of lead time, average usage, and safety stock [18].Here is the equation in the ReOrder Point calculation.

Results and Discussion
Compressors are designed to work automatically 24 hours a day, 7 days a week to produce compressed air used to drive production machines that require the availability of compressed air continuously to ensure the quality of the machine [19].The most commonly used type of compressor is a screw compressor.Screw compressors are high air flow compressors compared to other types of air compressors and can operate non-stop 24 hours a day [20].In this study, the compressor used as an object was presented in table 1. Main Bearing 2.
Cylindrical Bearing Oil Cooler

System Breakdown Structure
System Breakdown Structure is the first step in identifying the components of a system by conducting a more detailed analysis of the system.Table 2 shows the system breakdown structure of the D32 CP8 Compressor engine.

Machine Downtime
Downtime is defined as the time a system component cannot be used (not in good condition), thus making system functions not run.Another definition of downtime is the accumulation of time that occurs in machines/equipment when they cannot operate due to other activities that must be carried out.One of the causes of downtime is the disruption of the production process.Table 4 presents compressor engine downtime data for the period 2019-2021.

Risk Matrix
Critical components are determined based on the results of risk matrix calculations.the following is a table of likelihood and severity risk matrix scores based on questionnaires filled out directly by employees.Risk Matrix is applied at the beginning of data processing to determine the critical components of the D32 CP 8 compressor machine.The filling of the Risk Matrix is carried out directly by the person in charge of the company's compressor.Table 4. Shows the risk matrix for the D32 CP8 Compressor.There are three critical components of the D32 CP8 Compressor, namely the Screw Motor, Refrigerant Air Cooler, and Cylindrical Bearing Oil Cooler.

Critical Component Classification
Critical components are clarified to be non-repairable or repairable.If critical components belong to the category of non-repairable components, then if they are damaged, they must be replaced with new components.If critical components include repairable components, then if they experience damage, repairs can be made.This classification is carried out because the calculation of the number of components needs a different calculation method.Table 5 shows the classification of each predetermined critical component.

Determination of Time to Failure Distribution
The Distribution Test was performed to determine the distribution representing time to failure (TTF) using the Anderson Darling (AD) test.The test is carried out on three distributions namely the Normal distribution, the Exponential distribution, and the Weibull distribution.The AD value will determine that the distribution represents the spread of data by looking at its AD value.The smallest AD value will determine the distribution that represents the spread of data.The P-Value value is used as a parameter for the acceptance or rejection of a hypothesis provided that H0 is rejected on the P-Value < α.This data uses a significant level (α) of 0.05 (95%).In determining the distribution for each critical component, it is assisted by using Minitab software.Table 6 shows the results of the distribution test using Minitab software [21].
Based on Table 6, can be seen the selected distribution for each spare part.Screw motor is normally distributed, Refrigerant Air Cooler is normally distributed, cylindrical bearing Oil Cooler is normally distributed

Determination of Time to Failure Parameters
Parameterization on the distribution of critical component TTF data using AvSim+ 9.0 software.The obtained parameters will be used to calculate the Mean Time to Failure (MTTF) value.Table 7 describes the parameters obtained from the distribution of TTF data using AvSim+ 9.0 software.

Determination of the Value of Mean Time to
The determination of the value of MTTF is carried out based on the type of distribution of each critical component with a formula corresponding to the selected distribution.Table 8 shows the calculation of MTTF of critical components.
The calculation of the number of spare parts needs is carried out to meet the needs of spare parts in the next 1 year using the Poisson Process method.The calculation of the number of parts is carried out based on the type of repair of components.Screw Motor Components, Refrigerant Air Coolers, and Cylindrical Bearing Oil Coolers are components that are included as non-repairable components.Then in the event of damage, the component must be replaced.Some of the variables used to calculate the number of non-repairable parts needed are the Mean Time to Failure (MTTF) value, the number of components, the level of confidence, the number of machines used, the period of use, and the operating time.The level of trust in this study is 95% obtained based on referrals from the  9 shows the results of the calculation of the

Inventory Policy Determination
The determination of inventory policy is carried out to determine the minimum and maximum inventory quantities of spare parts so that they are always available and there is no buildup in the warehouse.Inventory policy determination is carried out using the Min-Max Stock method, and ReOrder Point.Min-Max Stock is used to determining the minimum and maximum inventory quantities.

Conclusion
From the calculation process that has been carried out, it can be concluded that based on calculations using the Risk Matrix, there are three critical components of the D32 CP8

Figure 3 .
Figure 3. Flowchart Research Field Studies are the initial stage in the implementation of this research.Literature and literature studies are conducted to help the author understand the concepts and methods used to solve the problems contained in field.Problem Identification is the next stage after the results are obtained in field studies.Problem identification is a very important step in this study because it will be the beginning of the problem-solving system.After the problem identification stage is carried out, the determination of the solution and the objectives of the final project will be carried out in the data collection stage that will be used in the value (the amount of damage that occurs at a time) MTTF : Mean Time Between Failure A : Number of installed components N : Number of machines M : Machine operational time Q : Usage period Usage-T)  C (6) Min Stock = (TC)+S Max Stock = 2(TC) p-ISSN: 2087-1627, e-ISSN: 2685-9858 Description: Q: Average usage per given period (Units) C: Lead time spare parts (Month) S: Safety stock (Unit)

Table 3 .
Data Compressor

Table 2 .
Risk Matrix

Table 5 .
Classification of Critical Components

Table 7 .
Mean Time to Failure

Table 8 .
Spare Parts Needs

Table 9 .
Usage Data and Leadtime Spare Parts

Table 11 .
Spare Parts Inventory Policy

Table 9 ,
it can be known that to meet 95% of the availability needs of critical components in the next 1 year, the company needs 8 Screw Motor components, 8 Refrigerant Air Cooler components, and 8 Cylindrical Bearing Oil Cooler components.
Table 10 shows data on the use of critical components and lead time for 2019-2022.Based on Table 11, it can be seen the minimum number of stocks, maximum stocks, and ReOrder Points of each component.The minimum stock on the Screw Motor is 3 components, a refrigerant air cooler is 4 components, Cylindrical Bearing Oil Cooler is 2 components.The maximum stock on the Screw Motor is 8 components, a refrigerant air cooler is 7 components, Cylindrical Bearing Oil Cooler is 6 components.The ReOrder Point on the Screw Motor is 4 components, the Refrigerant Air Cooler is 3 components, and the Cylindrical Bearing Oil Cooler is 2 components.
Compressor Machine, namely the Screw Motor, Refrigerant Air Cooler, and Cylindrical Bearing Oil Cooler.Based on the results of the calculation of the number of spare parts needed for the next 1 year using the Reliability Centered Spares method, namely the Poisson Process, 8 Screw Motor components were obtained.components, Refrigerant Air Cooler as many as 8 components, Cylindrical Bearing Oil Cooler as many as 8 components.Based on the results of the inventory policy calculation using the Min-Max Stock method, and ReOrder Point, the minimum value of the component stock, maximum component stock, and ReOrder Point for each component are obtained critically.Min-max stock for Screw Motor components is 3 components and 8 components, Refrigerant Air Cooler is 4 components and 7 components, and Cylindrical Bearing Oil Cooler is 2 components and 6 Components.The ReOrder point for each component is 4 Components Screw Motor, 3 Components Refrigerant Air Cooler, 2 Components Cylindrical Bearing Oil Cooler.