The need to reduce production costs has determined the evolution over time of different systems and maintenance concepts:

  1. CORRECTIVE MAINTENANCE (or after failure maintenance) in which the equipments operates until their accidental shutdown due to wear and tear or due to defects. The repairs usually means replacing the damaged subassembly or even the entire machine. This situation is the most unfortunate and obviously involves high maintenance costs.



  1. PLANNED MAINTENANCE in which the machines are stopped in a planned way, depending on the number of accumulated operating hours, for performing technical overhauls, current repairs and overhauls. In this system it does not matter the degree of wear installed, but the number of accumulated operating hours. Thus, it is possible that some of the still functional components and subassemblies will be replaced. The planned maintenance costs will be lower in this situation than in the situation presented above.





  1. PREVENTIVE and PREDICTIVE MAINTENANCE in which the machines operate in safe conditions until the installation of a certain level of wear, or the appearance of a defect. In this system, the machines will be stopped at an early date a few weeks before, and the repair will be done only where needed.

The preventive and predictive maintenance system allows the early detection, location and identification of the defect or the worn part, as well as the calculation of the safe operating time of the machine.

In this way it is possible to plan the shutdown, prepare the intervention team, order the necessary spare parts and minimize the parking time for repairs so that, clearly, the maintenance costs will be minimal.

In order to place ourselves in this “happy” situation, we must keep in mind that the technical state of operation of a machine can be assessed based on the “symptoms” it manifests during operation: vibrations, noises, increasing bearing temperature, temperature variation and pressure in the cooling circuit, etc. Of all these, the vibration level and the noise level are the main criteria (main parameters) for evaluating the technical condition of a machine. Preventive maintenance involves the periodic measurement of vibrations and noises on the bearings and the comparison of the measured values ​​with those prescribed by the machine manufacturer or with the standards in force.

When it is found that the vibration level has reached the “still permissible” area, the maintenance team is warned to be ready for intervention at any time.

When the vibration level has exceeded the maximum allowable value, it is necessary to stop the machine, dismount it to determine the causes and then repair the defects.




PREDICTIVE MAINTENANCE is a superior qualitative leap in a modern maintenance system, regardless of the industry or the specifics of production, because it provides the necessary information for:

 – early detection of defects;

– their location;

 – fault diagnosis;

– calculation of the safe operating time of the machine.

All this is possible by frequency analysis of vibrations (FFT analysis).

In reality, all the machines are vibrating. All we need ia that the vibration level to be within allowable limits. The study of vibrations showed that each defect has its own characteristic frequency. During operation, all vibration sources transmit their energy through bearings, to the frame and foundation.

The equipments under supervision must be measured to see if normally operates, at intervals specified in the supplier documentation or standards. The records of the measured values ​​and the number of operating hours for the machines are important for fault diagnosys. In fig. 1 represents the vibration level graph for L111 bearing, where in ordinate is the amplitude of the vibration, and in the abscissa is the number of hours or days of operation of the machine.



When it is observed that the amplitude of the vibrations begins to have an increasing tendency, the measurements will be performed at shorter time intervals. After a number of measurements, when certainty is obtained, the trend is increasing (72), signal acquisitions are made for: frequency analysis of vibrations in order to locate the defect and to determine what is defective (fig. 2) and the calculation of the safe operating time (fig. 1) from the moment Ti (initial time) until the moment when the vibration level will reach the maximum allowable limit. An essential condition in the success of the preventive and predictive maintenance system is good collaboration and communication between staff supervision and maintenance and repair personnel.