8.2.1 Example 1
The example considered here are big gearboxes with a power output of about 3.6 to 5.4 MW. We analyzed two basic conditions for failure in risk evaluation of parts of gearboxes:
– Current operation with all suitable parameters – Operation with improper maintenance
8.2.1.1 Description of the Mill Train Set
The observed gearboxes (six gearboxes with constant speed and three with variable speed) are working in the hot mill train set and they transmit power from 3.6 to 5.4 MW. Gearboxes are one- or two-stage types with helical gears and with a synchronous motor (Figure 8.1). According to the last analysis, it is just the gearbox that is the part with the highest number of failures.
8-2 Fundamentals and Assessment Tools for Occupational Ergonomics
8.2.1.2 Method of Solution
To evaluate the technical risk of the above-mentioned gearboxes, one needs to determine the probability Pand the weight factor or consequenceD.
There are three information sources available for the determination ofP: statistics, expert system, individual estimate of an expert-worker.
We chose the way of an expert system, which is suitable for our purposes (Figure 8.2). Description of this flexible observation system is too complicated.
The software is based on the fuzzy groups and this method enables one to perform condition-based maintenance — CBM. It is a well-known fact that the CBM is the best maintenance method at the present time.
On the other hand, the consequenceDis most frequently expressed in terms of money. Only the primary consequences are taken into consideration here, that is, price of spare parts and price of work. But the secondary consequences (losses due to lost time) are often much higher. There is a list of gearbox parts shown in Table 8.1. Two states of the gearboxes were simulated by means of the expert system:
1. Current operation with optimal parameters 2. Operation with insufficient maintenance
The result of the simulation is the final comparison of risk values for individual parts of the gearbox (Figure 8.3).
8.2.2 Example 2
8.2.2.1 Risk Evaluation in the Pipe Mill Hall
In the next part the example considered is the pipe mill hall. It is a big technological complex from the technical risk point of view. Products of this pipe mill are pipes with diameters from 500 to 1420 mm, FIGURE 8.1 Gearboxes are one- or two-stage types with helical gears and with a synchronous motor.
Statistics
Expert system Non-planned
monitoring Planned monitoring
Gearbox monitoring R = P . D
Statistical monitoring
Financial costs Gearbox conditions
Real task
FIGURE 8.2 The complicated flexible observation system.
Application of Risk Theory in Man – Machine – Environmnet System 8-3
with thickness from 5.6 to 12.5 mm and with length from 9 to 12 m. The arrangement of pipe mill hallis shown in Figure 8.4.
8.2.2.2 Risk Level Determination
To obtain the values of technical risk levels in this technological complex we need to know the values of probability Pand consequence D. Number of failures or the frequency of failures equals to the probability of risk state occurrence. The kind of accident equals to the range of health-injury, that is, equals to the consequence or to the weight factor. We can see all the important parts of pipe mill hall in the next table. For each part, the frequency of failures and the consequence of accident are determined. Combination of these two parameters is the risk level. Operation is with three levels of failure frequencies:low, middleandhigh.Three kinds of consequences are taken into consideration:
accident light, accident difficult and deadly accident. Hence three risk levels are also obtained: low, middle and high (Table 8.2). It is possible to determine from this table the so-called weak points in the whole technological complex. From the analysis of maintenance costs and the technical risk it is evident that the most important subject of interest is the ultrasonic welding 1. Therefore, it is very useful to perform the technical risk analysis for all components of this machine for ultrasonic welding 1 (Figure 8.5).
The result is shown in the graph in Figure 8.5. In this graph the risk level becomes equal to the point evaluation: 1 – 5 is unacceptable risk; 6 – 9 is undesirable risk; 10 – 17 is risk acceptable with control and 18 – 20 is risk acceptable without control.
It is necessary to take into consideration that during the process of risk determination two aspects were combined together: technical factor — frequency of failures and human factor — kind of accident.
TABLE 8.1
Part of Gearbox Sign
Pinion bearing Rlp
Pinion Rp
Bearing and pinion Rlþp
Countershaft bearing Rlpr
Countershaft gearing Rpr
Countershaft shaft Rhpr
Whole block Rcb
Big gear Rlvk
Output gear Rvk
Bearing and output gear Rlvkþvk
0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016
Rlp Rp Rl+p Rpr Rlvk Rvk Rlvk+vk
Parts of Gearbox
Risk Values
optimal parameters unsufficient maintenance
Rhpr Rcb Rlpr
FIGURE 8.3 Comparison of risk values.
8-4 Fundamentals and Assessment Tools for Occupational Ergonomics
Therefore, we obtained the technical – human combined risk values or levels. These levels are only the primary and preliminary information for the designer. But this information is very important for all the successive steps of detailed technical risk analysis.