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Main concepts and definitions
Job or Organised Work: it represents the set of tasks (even repeated) made by an operator during his/her shift (even considering multiple cycles of execution).
Task: A specific work activity aimed at obtaining a specific result (e.g., clamping/unclamping a piece, loading/unloading a pallet etc.) that can be repeated after each cycle.
Cycle: A sequence of tasks performed by the upper limbs, which is repeated several times in the same way; in a practical contest, a cycle is considered as the set of tasks that an operator must accomplish to close a working loop, after which he/she will start again.
Cycle Time: The total time assigned to carry out the sequence of tasks that characterises the cycle
Technical Action: An elementary movement involving the upper limbs that represents the simplest operation to be evaluated by the OCRA method; a set of technical actions make a task.
Frequency: Number of technical actions performed per unit of time [min]
Force: Physical effort required by the worker to perform a technical action.
Awkward Postures and Movements: Non-neutral postures and movements of the main joints of the upper limbs adopted to complete a sequence of technical actions characterising a cycle, whom impact in terms of MSDs is functional to the time exposure.
Stereotypy or Repetitiveness :The repetition of the same gesture or series of gestures for most of the work period or shift.
Recovery Period: The time interval within a shift during which the upper limbs are substantially inactive (i.e., the limbs are not performing any technical actions).
Additional Factor of Risk: factor that takes into account the presence of additional risk of both physio-mechanical and organisational nature at the task level.
OCRA Index main formulas
The OCRA Index is obtained by the ratio between the number of actual technical actions (ATA) currently performed in a work shift and the corresponding number of recommended technical actions (RTA):
$OCRA_Index = ATA/RTA$
The number of actual technical actions performed in a shift: ATA**
The definition of the ATA for a single task is a simple procedure divided into two steps:
The computation of the Net Duration of Repetitive tasks (D) in a shift [min/shift]
The computation of the Average Frequency of Action per minute (F) for the task [act./min]
Total Net Duration of Repetitive tasks: D-
The first step in the OCRA methodology always looks at the organisational background of the company: which kind of shifts are distributed over the week? How many shifts per day are there? Which is the break schedule? How significant is the amount of time spent in non-conventional activities like cleaning, maintenance or financial inspections? To derive D, the following formula is applied:
D = Shift Duration - Total break time - Total non repetitive work time - Work time valued as recovery
Where all figures are derived by either organisational structure of the company's shifts or by direct observation of workers' behaviour. Since the literature suggests that the D parameter should be computed for each task considered in the cycle, repeating the same procedure anytime we want to add another job to the set of operations performed in a shift, the amount of required data regarding the scheduling efficiency of a company becomes very high and subjected to uncertainty. (e.g. break times are not always respected precisely, non-repetitive activities can impact on the available time of different sets of tasks, anytime they happen).
For this reason, we assumed for our toolkit a simplification that, despite changing the computation approach for the D parameter of the methodology, doesn't affect significantly the outcome of the analysis, while it reduces significantly the time spent in the data collection phase:
Assumption: The Net duration of a single repetitive task is given by the product between D and the %duration of a task in a working cycle, relative to the Cycle Time:
Dtask = D ⋅ taskdur/CT
Average Frequency of Action per minute/shift for a task: F-
It requires as input the set of tasks performed by the operator within a working cycle, the decomposition of each task in a set of elementary movements (least aggregated data analysed in the methodology), the shift length in minutes, the #pieces manufactured in a shift and the Cycle Time in [sec]. Then, F is computed as:
FF = (#actions\task ⋅ #cycles\shift) ⋅ 60/CT
Assumption: the average number of pieces produced in a shift corresponds to the number of cycles in which all the tasks are performed within a shift. To derive this value, if not already available, is sufficient to estimate the total time (human and machine) required to manufactured a finished product and divide the shift duration for that value:
#pieces manufactured = Shift Duration / Total time to manufactured a finished product
Finally, the ATA is obtained through the following formula, for each task in the cycle:
$ATAtask = F ⋅ Dtask$
The ATA of the entire cycle is given from the sum of all the ATA_tasks in one cycle.
1.2 The number of recommended technical actions performed in a shift: RTA
The computation of the Recommended Number of Technical actions is a more complex procedure involving the computation of the following parameters: DuM, RcM, CF, FoM, PoM, ReM and AdM.
Once D has been computed for the entire cycle of tasks, the table below is addressed to directly computing the value of the Duration Multiplier:
The Recovery Multiplier is empirically estimated once observing the operator's working behaviour and deriving the average maximum time without recovery within a shift; a reference table can then associate this indicator to the RcM:
The action frequency constant is generally set equal to 30 actions/min as a reference value, but to guarantee a good estimated of the RTA, it should be defined roughly considering the ratio between the number of technical action in one cycle and the cycle time.
The computation of the force multiplier requires a considerable degree of accuracy in the data collection phase, since values for the duration, the force load and the body area (right, left or total) involved in the technical action must be properly extrapolated either through operator movements' simulation, general assumptions or direct observation (latter case may not be possible due to privacy issues).
The FoM computation is divided into three steps:
1. Task division into technical actions (T.A.): to ensure the highest degree of accuracy possible, a process supervisor (e.g. an ergonomist, a qualified operator) must decompose all the tasks of a cycle into elementary movements for whom posture and force factors can be estimated without bias.
2. Force Score (F.S.) computation: The force score is obtained by computing the force load, expressed through Borg CR-10 Scale (table below), that accounts for the weight of the object carried out in a task. If multiple T.A. are included in a single task, then the Weighted Force Score is computed by weighting the force load of each T.A. for the %time spent in the force exertion relative to the Cycle Time:
3. FoM computation: once the Weighted F.S. has been defined for a task, a multiplier table can be directly consulted for the extrapolation of the FoM:
The estimation of the risk associated to operators'exposure to awkward postures is the most time-consuming step of the OCRA methodology, due to the high accuracy required and the multiple body areas considered in the analysis. Like the FoM, also the posture multiplier computation is task-specific and considers a weighted value if multiple T.A. are investigated for a single task.
Assumption: For the sake of consistency, in our model we rely on the same tasks' duration considered in the previous step, assuming that there is no distinction between the T.A. execution time and the time spent in an awkward posture (regardless the body area involved) for that T.A.;
According to the method, 4 body areas for the Upper Limbs are assessed in terms of MSD risk: Shoulders, Elbows, Wrists and Hands. As shown in the tables below, for each area of the operator's upper limbs, a set of reference harmful movements and postures is defined; a posture score is then set by matching the type of harmful movement/posture and the %time spent in that posture relative to CT:
Assumption: considering the same body area, at most one dangerous movement can be identified in a T.A. performed by the operator; however, a T.A. can be harmful for multiple body areas. To sum up, the number of awkward postures associated to a T.A. can go from 0 to 4 at most.
The computation of the PoM follows two steps:
1. Posture Scores (P.S.) computation: respecting the task sub-division defined in the previous analysis , each elementary movement is assessed by either simulation or direct observation of the operator's behaviour to identify the presence of possible awkward postures. If multiple T.A. belonging to the same task have the same awkward posture, the cumulative %time respect to the CT, spent in an awkward posture must be considered in the score table. In this way, posture scores are properly computed and grouped at the task level.
2. Total posture score (Total P.S.) computation: Once computing the P.S. for each awkward posture made in a task, the Total P.S. is obtained by summing together the partial scores belonging to the same Posture Area (shoulder, elbow, wrist or hand).
3. PoM computation: for each task, the highest among the Total P.S. is picked considering the four body areas; then, this value is used in the posture risk table to derive the correspondent Posture Multiplier:
The literature review suggests that repetitiveness of movements or postural efforts can become a relevant driver of MSD when at least one out of three of these conditions is satisfied:
a. Identical technical actions or groups of identical technical actions are repeated for almost the entire cycle time (0-50% absent risk; 50%-80% moderate risk; >80% high risk)
b. Static postures are sustained along a great portion of the cycle time (0-50%: absent risk; 50%-80%: moderate risk; >80%: high risk)
c. Operators execute multiple times extremely short cycles of activities featuring actions that involve the upper limbs. (CT>15 sec: low/absent risk; 8<CT<15 sec: medium risk; CT<8 sec: high risk)
Assumption: Being an indicator that is representative for the entire shift evaluation, a single value of the ReM is computed for the entire set of tasks referring to the same data collected about the awkward movements in the PoM computation phase.
Finally, the worst case among the three conditions is taken as a reference to get the correspondent ReM value from the table provided below:
The Additional Risk Multiplier embodies the incidence on the OCRA Index represented by two components:
Presence of physio-mechanical dangerous conditions (PMC) in the working environment: like the presence of vibrating tools or the operator's exposure to harmful temperatures while performing a task. 8 factors are considered in the methodology and the presence of each of them, with the relative %time exposure to this risk, is assessed by observing the operator task's execution.
Organisational risk conditions (ORC) linked to operators working rhythm: it's not strange to believe that the operator's pace may be affected by the work pace of the machine in some processes. An ORC score of 0 is set if the operator is working independently from the machine; a score of 8 if the operator works together with the machine but flexibility in delays is allowed by buffers; a score of 12 is given if the operator works synchronously with the machine. It's obvious to notice that ORC score is a unique value referred to the entire shift.
The AdM is computed in two steps:
PMC Score computation for each task: relying on the table shown below, the PMC score is computed as a function of %duration of a task relative to CT and of the type of PMC:
AdM computation: the AdM is obtained for each task by firstly adding the ORC score to the PMC score of each task and then associating the Total Risk Score to the correspondent AdM by referring to the additional risk table, shown below;
Finally, the RTA is obtained by the following formula, for each task:
$RTAtask = CF ⋅ D ⋅ DoM ⋅ RcM ⋅ FoM ⋅ PoM ⋅ AdM ⋅ ReM$
The overall value of the RTA for the entire cycle is derived by summing all the RTAtask in the cycle.
It's crucial to remember that, since both the RTA and ATA computation is performed at the task level, both indicators are influenced by the "Body Area" that the operator uses to accomplish each task: hence a value of the OCRAright and OCRAleft index must be computed in a precise and coherent way. At this purpose we have formalised the following assumptions:
Assunption 1: An operator can potentially perform each technical action (T.A.) either with the Left or Right part of his/her upper limbs. However, if the T.A. involves the lifting or handling of a heavy object (Hypothesis: weight of the object over 5kg), then he/she must use both upper limbs.
Assumtpion 2: Once an operator has decided to make a T.A. with either the Right/Left/Both areas of his/her upper limbs, he/she must perform all the remaining T.A. needed to make the correspondent task with the same part of his/her body. E.g.: exchanging of objects from Right to Left hand within a single task are avoided because can be time-consuming.
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