Ground Generator Mobil Unit 10-12.5 kW
The Generator Set, NATO homologated, is a mobile system for power supply on a designed single-axle trailer, developed and manufactured for tactical-logistic use, suitable to operate in extreme environmental conditions. The system is able to supply 220/380Vca outputs at 50Hz and 24Vcc with characteristics of UPS when it is connected to the distribution net and it can assure the supply continuity, offering 2 independent power sources.
Final Customer: Italy Army
Project: RAMT Analysis
Description: Z Lab produced RAMT documentation, as Reliability Prediction, FMECA, Maintainability Analysis, Availability, Testability Analysis and Spare Parts Analysis for the Ground Generator Mobil Unit 10-12.5 kW.
The RAMS is a long-term characteristic of a system and is obtained by the application of data, concepts, methods, techniques and tools of engineering during the system lifecycle (EN 50126). It is defined as a quality and quantity indicator of the system degree, regarding to the system function and to the availability.RAMS is the acronym of Reliability Availability Maintainability, Safety.
Reliability is the probability that an item can perform a required function under given conditions for a given time interval n(t1 –t2).
Availability is the ability of a product to be in state to perform a required function under given conditions at a given instant of time or over a given time interval assuming that the required external resources are provided.
Maintainability is the probability that a given active maintenance action for an item under given conditions of use can be carried out within a stated time interval when the maintenance is performed under stated conditions and using procedures and resources.
Safety is defined as freedom from unaccettable risk of harm.
Reliability prediction is a method to calculate the constant failure rate during the system life time. The reliability predictions is conducted at various system levels and detail’s degrees. It is based, on a system decomposition as tree that is called WBS (Work Breakdown Structure), in order to identify the major components and assign to each of them a failure rate, in accordance with the standard NPRD-2011 (mechanical parts) and MIL-HDBK-217F Notice 2 or Siemens 29500 (electriconic parts). The basic failure rate of the system is calculated by summing up the failure rates of each component in each category multiplied by their quantity (based on probability theory). This is applied under the assumption that a failure of any component is assumed could lead to a system failure. This model assumes that the component failure rate under reference or operating conditions is constant. The failure rate of the electonic items can be calculated:
- at reference conditions (parts count method);
- at operating conditions (parts stress method).
In the part-count method, the failure rate is calculated by appropriate databases that provide the basic failure rate value relative to the component operating environment. The Part-Stress method required detailed information such as: type of technology, year of manufacture, junction temperature, stress factors, thermal expansion characteristics, number of thermal cycles, thermal amplitude of variation, application of the device, etc.. It is also possible evaluate the mission reliability prediction. This analysis can be done after the FMECA analysis : through the FMECA is possible to analyze the failure modes and the percentage of occurrence of each failure mode. In this way it is possible to identify the critical components of the system.
FMECA analysis is a tool used to examine all possible failures, their consequences and the critical components or functions in the system under analysis. The FMECA purpose is to improve and ensure the reliability of complex systems. It is composed of two separate analyzes: FMEA (Failure Modes and Effects Analysis) and CA (Criticality Analysis). The FMECA Analisys can have a functional approach or structural approach:
- functional approach: It is performed on the functions. This approach focuses on the ways in which the functional objectives are not complied
- structural approach: it is performed on the HW system components. This approach tends to provide more detail about the system failure modes and effects at component level
Furthermore, to provide a qualitative assessment of the potential consequences, the level of criticality of failure modes is assigned, according to their effect on the regularity and / or service "comfort" and safety; Evaluating these results, it is possible to suggest mitigation measures relating to the failure mode under analysis. FMECA analysis allows to identify components failures that could be critical in terms of reliability and / or safety, in relation to a particular mission profile. FMECA is the basis of design choices in order to eliminate critical fault, or at least, to reduce the criticality (through corrective actions). MIL-STD-1629 and FMD-97 are the standards used for FMECA (failure mode description and criticality).
Two Maintainability Analysis types exist: Preventive Maintenance Analysis and Corrective Maintenance Analysis. The main purpose of Preventive Maintenance Analysis is to evaluate the maintenance plan that allows to implement all necessary actions in order to prevent the occurrence of faults, through the planned replacement of components subject to wear, or maintenance tasks to ensure the correct system operation (periodic cleaning, functional test, periodic visual inspection…). Corrective Maintenance Analysis has the primary aim to define the corrective actions necessary to restore the nominal conditions of system operation, through the replacement of LRU (Line Reparable Unit) failed. The maintenance analysis provides information in terms of human resources, time and material (spare parts and equipment required for maintenance), through:
- The evaluation of MTTR (Mean Time To Restore): it is the mean time for the maintenance operation considered;
- Compilation of preventive and corrective maintenance schedules: these schedules support to writing of the technical manual for the maintainers, that describe in detail the maintenance operation;
- Definition of the spare parts type and quantity: it is a spare parts list that have to be in depot, in order to minimize the maintenance downtimes.
The analysis of corrective and preventive maintenance times must be evaluated considering isolation time, localization, accessibility, component replacement, component assembly and functional check in accordance to MIL-HDBK-472.
The availability measures the ability of an entity to be able to perform required function under given conditions at a given time, or during a given period of time, assuming that they are insured by external means if it is necessary. For the calculation of same, will be used the “Availability Technique” which correlates MTBF value (Mean Time Before Failure) and MTTR value (Mean Time To Restore). The index measures the availability resulting from the fact of failure events and the ability to perceive this state of the system, report it to address and resolve it, which takes into account both the time to repair, logistics and timing equipment. In the availability formula is considered the mission MTBF, taking into account any physical and functional redundancy of the system.
Purpose of Testability Analysis is to ensure that the failure on the system (especially in difficult controllability and/or observability areas) are clearly and timely detectable. This allows the immediate failure detection and localization in order to minimize the equipment downtime during maintenance operations. To this end, the detection and localization system faults should be as possible safe, timely and cheap. The process typically used for testability analysis is as below:
- Equipment breakdown down to items potentially exchangeable (LRU)
- Identification of the elementary functions included in each exchangeable item (With associated Failure Rates)
- Failure Analysis of each identified elementary function,
- Failure modes identification (and associated failure rates)
- Identification of the effects at exchangeable item output level and at system level
- Identification of all the available elementary tests and links with the covered effects
- Fault coverage and Fault isolation rate computation in accordance with the MIL-STD 2165A.
Spare Parts Analisys is the drawing up a spare parts list that have be in depot in order to minimize the maintenance downtimes.