Keywords

1 Introduction

Many manufacturing companies design their work systems according to Lean production systems (LPS), which became an industry standard with the publication of the VDI 2870 [1, 2]. Although LPS have always company-specific characteristics, those follow a basic structure that anchors them in the long term goals in the company and, therefore, differ from conventional, more short-term optimization approaches. LPS are based on the company’s objectives and the necessary processes for the achievement of objectives. According to VDI 2870, LPS are used to establish a comprehensive and integrated design of processes [2]. In practice, LPS are mainly oriented towards the optimization of quality, time and costs [3]. The objectives are then cascaded for individual functional areas. For the implementation of LPS, design principles are selected which include various methods and tools. Avoiding waste is the fundamental design principle for LPS. The aim is to avoid job content for which the customer is not willing to pay [4]. In practice, these are e.g., waiting times or unnecessary movements. Following the elimination of waste, there is new and improved standardized operations and recurring continuous improvement processes. However, it is unclear how these far-reaching changes by LPS affect work system and working conditions of employees [5]. From the trade unions perspective, an increased pressure to perform as well as an intensification and densification of work is reported. However, LPS is also granted the potential to improve the working conditions [6]. Weichel characterizes the changes in working conditions in LPS with the increase in compression of performance, an elimination of hidden breaks and compensation movements as well as high repetitiveness of movements. These conditions also lead to physical complaints of employees [7].

The above examples show that LPS have profound impact on the daily work of employees. Overall, however, significant deficiencies exist in accurately understanding the diverse effects of LPS on working conditions [6]. In order to gain an understanding of the causal relationships, a systematic analysis of the impact of LPS on load factors for employees is required. It has to be determined how changes are made to the load situation of workers through LPS. In this way, systematic stress critical measures and combinations of measures in LPS are uncovered, but also positive aspects of LPS for employees are determined. The aim of the presented paper is a systematic analysis of the impact of LPS on the work stress on employees. For this, a reference model is needed that maps the relationships between the work design by LPS principles and its impact on the working conditions.

2 LPS

A LPS is defined as “an enterprise-specific, methodical system of rules for the continuous orientation of all enterprise processes to the customer in order to achieve the objectives set by the enterprise management.” [1, 2]. In general, it focuses on technical and even more on the organizational design of processes. The top target is elimination of waste in all company processes in order to reach higher efficiencies and, consequently, to be more competitive [2, 3, 8]. Waste in this context are all activities that are not increasing the customer value of the product. Seven different kinds of waste can be distinguished:

  • Over production

  • Waiting time

  • Transportation

  • Over processing

  • Inventory

  • Motion

  • Defects and touch up [2, 8].

Methods and tools are used in LPS to eliminate waste. They are systematically integrated into the LPS-structure consisting of the elements: targets, company processes, principles, methods and tools. The structure of an LPS is shown in Fig. 1.

Fig. 1
figure 1

Structure of production systems, based on [2]

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The following example clarifies the structure of the LPS. The top target of a manufacturing company can be quality improvement. Sub-targets are stable processes in the production. To reach this sub-target, relevant company processes need to be defined, e.g., for turning, milling or grinding. A suitable principle is “zero-defects-production”. The principle combines methods and tools that are used to reduce the number of defects that are passed to the next production step and to ensure a high product and process quality [2]. Especially 5× Why, automation, poka yoke, six sigma are methods of this principle.

As it is mentioned above, LPS are used to design technical and organizational processes. The processes are optimized either to lower costs, save time or improve quality. However, by reaching the strategic goals there are eventually tradeoffs in the production systems. This can happen when the work system of the employee’s is negatively affected [9]. Although technical improvements such as new production methods lead to high savings, it can cause higher status of employee’s illness. In the next section, effects of LPS on the work systems are further described.

3 Effects of LPS on Workers

In addition to the impact on efficiency, a LPS has a major impact on the employees and the working system. According to DIN EN ISO 6385, a working system is defined as a system comprising the interaction of a single user or multiple users with the work equipment in order to fulfill the function of the system in the workspace and the working environment under the conditions imposed by the work tasks [10]. S.6 Due to this, a change in a work system leads subsequently to a change in working hours, salaries, and especially health effects [1]. This is also shown in Fig. 2.

Fig. 2
figure 2

Impacts of LPS on different parts of the working system [1]

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Depending on the working conditions and their design, the employee has to handle a multitude of heavy loads during the working hours. According to DIN EN ISO 6385, the work stress is described as the totality of external conditions and requirements in the working system, affecting the physiological and/or psychological state of a person [10]. As an example, a higher weight exposure of the employee may be the result because of the zero defects principle. As a consequence, this can lead to additional pressure at work for the employee. However, the type of load change for the employee must be known in order to prevent negative consequences for the health.

In the section above, the seven kinds of waste, as part of the aim of LPS, are described. In order to show the effects of LPS on working conditions, each kind of waste and the impacts on a working system is described.

3.1 Over Production

Avoiding over production may be associated with smaller lot sizes, which results in frequent changes of the product to be manufactured by the worker and, consequently, in a higher diversity of work tasks. This frequent change does not explicitly lead to a positive or negative influence on the work load of employees. So on the one hand side, a more diversified work leads to the prevention of monotony, which is a positive change in an employee’s work load. On the other hand, the workers have to adjust frequently to changes in work tasks, which can cause a higher occupational stress for the worker.

3.2 Waiting Time

The second kind of waste is waiting time. For the purpose of the continuous improvement process, it is important to eliminate times during which the worker is doing no productive work tasks. Those waiting times usually contain so-called hidden breaks during which the worker has the opportunity to rest. Eliminating the waiting times by using a continuous improvement process can cause work intensification for employees. Therefore, the elimination of waiting times is associated with an increasing occupational stress on the employees.

3.3 Transportation

Avoiding transportation can be equivalent to a reduced variation in work tasks and can result in a reduction of compensation movements. Compensation movements lead to a change in the work stress situation of an employee. From an ergonomic standpoint, changing work stresses are necessary for the prevention of musculoskeletal disorders and have, therefore, a positive influence on the workers’ health.

3.4 Over Processing

The fourth kind of waste over processing has neither a positive nor a negative effect on the occupational stress of an employee. Eliminating an ergonomically questionable processing step will have positive effects for the employee’s health. However, does this processing step contain compensating movements, this elimination leads to an intensification of work.

3.5 Inventory

A reduction of stocks, which is the fifth kind of waste, is equivalent to a reduction of buffers for employees. Buffers relieve the employees from certain work tasks to the extent that they reduce the pressure to perform. Conversely, the work stress is increased and the worker has to perform his tasks quickly and correctly in case these buffers are not available. This can lead to a work intensification for employees and increase the level of occupational stresses at work.

3.6 Motion

Motion as the sixth kind of waste describes unnecessary movements by the worker which do not add value to a product. Those unnecessary movements may include compensating movements of the workers like walking. In case inventories and buffers are reduced or eliminated, unnecessary movements are omitted as well. With this example is shown that with the use of LPS elements can affect the work stress of employees in either ways.

3.7 Defects and Touch Up

The seventh kind of waste is called defects and touch up. Those are usually outside the normal, often clocking bonded labor. Therefore, it can also be made by workers who can no longer work stroke bound. For other employees, it may represent a change in work flow and thus relieve the congestion. An elimination of rework can therefore carry monotonous working conditions that pose a burden.

3.8 Using the Principles of LPS

As to overcome these kinds of waste, the eight principles of LPS are used which were introduced in the previous section. These examples illustrate that a systematic analysis of the relationships between the work design by LPS and the employee’s work stress is necessary in order to prevent the employee from overwork.

The explanations show that a production system can have a strong impact on the staff and profoundly change a working system. Rising demands on the employees through a continuous improvement process can lead to an uncontrollable work stress level which may cause in the worst case an inability to work [11]. The stress can be both physically and mentally. In addition to individual health consequences for the employee, the absenteeism also has a negative impact on the employer. In the short term, organizational measures must be taken, such as a substitute. In case of a prolonged illness of the employee, the tasks needs to be taken over by another employee and, later, reintegration of the affected employee must be planned [12].

Therefore, the first step is to identify which occupational stresses exist in a company in order to be able to determine potential relationships between principles of LPS and work stress.

4 Analysis and Results

As shown in the previous section, LPS principles have an impact on an employee’s work stress. Hence, a systematic analysis of occupational stresses is crucial in order to determine the relationships.

The options for categorizing work stresses are as varied as the work stresses themselves. Thus, for the analysis three validated, widely-used models for the systematic analysis are used. Those are described in the following.

The first model from Rohmert et al. determines the connection between work strains and work stresses and is shown in Fig. 3 on the left side [13]. There, work strain and work stresses are described as a mechanistic system. The system consists of a scale, a weight, a spring and a display. On the one side of the scale, the weight is attached which symbolizes the work stress. On the other side of this scale, a spring is attached which stands for the personal performance requirements of the worker. To show the amplitude of the scale, a display is attached on top of the scale which represents the level of work strain.

Fig. 3
figure 3

Models describing work stresses

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The second model is called action-oriented stress model and was introduced by Schönpflug. According to Schönpflug, work stresses can be grouped into the following six dimensions:

  • origin,

  • quality,

  • influence possibility on work stresses

  • probability of occurrence

  • temporal sequence

  • nature of their impact on stakeholders [14].

The last model was introduced by McGrath and divides work into three different work stress segments which are tangible-technical, social and personal. In each segment can work stresses be generated. The tangible-technical segment contains outer environmental effects such as heat, noise or even ergonomic work strains. The social segments describe work strains which are caused by social conflicts or mobbing. Lastly, work stresses can be created by personal circumstances such as family conflicts or character features such as fear for tasks, failure or criticism. In addition, those segments can overlap which creates four additional cross-segments [15].

For the systematic analysis, in the first place it is necessary to compare the different models. The basis for the comparison is the evaluation of work stresses in LPS which was outlined in the previous section. The results are shown in the following Table 1.

Table 1 Comparison of evaluation possibility with introduced models
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The comparison of the different models shows that each model has a different consideration of work stress.

In order to reduce over production (1) smaller lot sizes are attempted. This leads to the alteration of the work system in a way that the work stress is changed. This change in work stress can only be almost determined by the model of Schönpflug since this model distinguishes the work stresses in different dimensions. Therefore, the dimensions can be more precise for example in the description of the number of manufactured products. With this model, each work stress can be determined in its characteristics. The models of Rohmert et al. and McGrath do not distinguish in different dimensions and just group the work stresses. Especially the model of Rohmert et al. summarizes all the work stresses and relates the impact of work strain on a worker to his personal features. Therefore, the work stresses do not need to be determined in depth. The same applies for the model of McGrath. There, the work stresses are categorized in the above-described segments. With this, the work stresses can be identified and do not need to be further determined e.g., in numbers. Thus, both models only describe half of the work stresses.

Waiting times (2) contain times in which the worker has to wait for a process to end or for further instruction. Therefore, he is not able to add a customer value to the product. Sometimes those waiting times are hidden breaks for the worker which is a positive influence on the work stress. Due to this, all models are just hardly able to determine the correct impact on a workers work strain. The model of Rohmert et al. is not able to show a positive effect of work stress since the beginning of the scale is in the green segment. The model of Schönpflug can determine the work stress but is not able to give an evidence of positive effect on the workers’ strain. The same applies to McGrath’ model.

Avoiding Transportation (3) consequently reduces the work task diversification and the ergonomic change of body positions. This has a negative impact on the employee. All models are able to determine and describe the change in work stress on the worker. In the model of Rohmert et al., the work strain changes from green to orange or even red. With the different dimension of Schönpflugs model, the work strain can be quite precisely described. Lastly, McGrath model with its segments can determine which work stress is involved and, therefore, be described.

Over processing (4) means that additional steps or movements are made which do not add to the customer value. However, avoiding over processing can either lead to the reduction of ergonomic critical movements in case of a not ergonomically movement or it increases work load by eliminating compensation movements. This example shows that over processing can have a positive or negative effect on work stresses. So the models of Rohmert et al. and McGrath are just partially useful to determine the work stresses due to the aggregated structure. On the contrary, the dimensions of McGrath allow a more specific evaluation of the work stress.

Inventory (5) describes the establishment of buffers in the production which consequently leads to a reduction of walking distances for the employees which gives the employee more time for further production steps. In case inventories are avoided, the walking distance increases which leads to reduced times. This negative impact can be determined by all models.

Motion (6) is necessary for the worker in order to get the needed materials. However, an optimization towards no motions for the worker is with respect to ergonomics not to be aimed. The same applies for far distances which lead to a high number of motions. Thus, the optimal amount of walking distances has a positive influence whereas too less resp. too much motion has a negative impact on work stresses. Because of this optimal range of motions for a worker, all models are only hardly able to determine the work stress reasonably.

Defects and touch ups (7) are additional work tasks in case the quality level was not reached. Therefore, the worker is not bound to a work cycle which leads to a reduction in work load. Furthermore, it enlarges the work tasks of the worker and has a positive influence on the work stress. The models of Rohmert et al. and McGrath are partly able to determine the work stress due to the difficulty in description of the parameters. Schönpflugs model with its dimension can determine and describe the work stress almost precisely which leads to an adequate evaluation of it.

All in all, the comparison showed that all three models are useful for the first determination of work stresses. However, no model has the ability to define the work stress parameter in the way that leads to the understanding of the relationship between work stress and work strain. Further research needs to be done in the field of work stress models. The first step needs to be a thoroughly literature search in different scientific fields since the introduced models mostly derive from psychological research.

5 Summary

In this paper, a deeper insight of the impact of LPS on a work system was given. For this, it was necessary to introduce the general elements of the LPS. Furthermore, the effects of the LPS on the work system were outlined. It was shown that most design principles of LPS, which serve to overcome the drawbacks of the seven kinds of waste, not only affect the production system but the work system as well. An evaluation of the effects showed that most methods of LPS can lead to a deterioration of the workers performance. As a consequence, the relationship between the change of work systems and work strains need to be further investigated. For this, three different models to determine work stresses were introduced.

The model of Rohmert et al. focuses on the relationship between work strain and work stress. There, the impact of work stress depends on the personal features of a human. The better a person is prepared for the work tasks, the lower does the work strain impact the workers performance. However, the model does not distinguish between different work stresses. Those external factors are just summarized to work stresses. The model of Schönpflug is more precise and distinguishes between different dimensions of work stresses. With this it is possible to evaluate each work stress. McGrath model is similar to Rohmert et al. model which determines segments for the different work stresses. However, it does not give any evidence of dependence to work strains and is, therefore, only partly useful for determining work stresses. Further research need to be done to identify different models which derive from other fields of science. Otherwise, a new model needs to be developed to meet the requirements for an adequate determination of work stresses.