**1. Introduction**

In recent years, forestry in Slovakia has seen a very rapid development trend in all its aspects. With increasing demands on the quality of care in forestry, emphasis is constantly placed on the quality and reliability of forest machinery and technological equipment. At present, forestry is based on the widespread use of forest machinery and equipment.

High requirements for care and processing are closely related to the requirements for the quality and reliability of forest machinery and technological equipment. These are closely related to the care of the facilities. The care of equipment used in forestry consists of daily operation, treatment and supervision of their operation, activities aimed at putting new equipment into operation, to eliminate faults and defects, to improve the technical condition, to technical modernization, to store, conservation temporarily decommissioned forest machinery and technological equipment and for the disposal of decommissioned and finally also the replacement of decommissioned forest machinery and technological equipment with new ones.

The above description of the life cycle of forest machinery shows that the operational reliability and care of forest machinery play an important role in forestry. The current and future degree of mechanization and automation of production in forestry, as well as the tendency to increase the efficiency of forest technology, establish as one of the primary tasks to ensure its operational reliability.

Costs related to the operational reliability of forest equipment can be affected by both the manufacturer and the user of the machine or equipment; the manufacturer with the correct conception and construction of the machine or the equipment and the user by placing the machine or an equipment in such conditions for which it is intended and by quality of maintenance, operation, and repair. Tasks with ensuring the operational reliability of forest stands are complicated by some specific factors e.g., assortment of machines, number of users, differences in the complexity of construction of different machines, seasonality in the use of forest machines, high demands on the qualification of operators, dispersion of technology over a large area, lack of storage space, operating and weather conditions, etc.

The main content of the chapter is to define the purpose of operational reliability of forest harvesting machines, given that this is very important for the quality and economical operation of maintenance and diagnostic operations.

## **2. Forest machines**

There are two interrelated equivalent production processes in forestry, namely: the biological process (organic forest production such as establishment, education, and protection of forest stands) and the logging process [1].

There are many companies in Slovakia which primary activity is logging, concentrating, transporting, storing, and processing wood and wood. The role of Slovak forestry is to ensure the development of the forests to fulfill ecological, social, and economic functions. Regarding the optimization of economic results, it is important to focus on the transport of timber for export and removal by motor means of transport [2]. At present, machinery and equipment are used mainly to facilitate human labour in almost all sectors of the economy [3]. High acquisition costs for securing machines and equipment compensate for the resulting effect of the activity and that is labour productivity and the final profit. To achieve the goal, which currently in the forestry sector is also the care of machinery and equipment used for reforestation and logging, maintenance of machinery and equipment used in this process [4].

For effective provision of the tree felling and transportation process, it is necessary to ensure maintenance, repairs and modernization of machinery, equipment and vehicle fleet of vehicle operators, provided that the equipment meets the strictest safety and environmental criteria [5]. This is aided by the ubiquitous Fourth Industrial Revolution.

#### **2.1 Timber harvest and transportation process**

The tree felling and transportation process has a long history. Slovakia, as a mountainous country in the past as well as in the present, was an ideal place for forestry to flourish. As a conscious queen, Maria Theresa sought to provide education to her subjects to ensure the well-being of her country and the protection of the environment through the care of the forest. Welfare through the extraction of mineral wealth and environmental protection through the care of the forest and our mountains [6]. For this reason, she founded the Mining and Forestry Academy in Banská Štiavnica which

#### *Perspective Chapter: Analysis of the Operational Reliability of Forest Equipment DOI: http://dx.doi.org/10.5772/intechopen.107402*

began to educate experts in the field of mining and forestry. Graduates of this academy contributed their knowledge about the expansion of forestry in Slovakia [7]. Therefore, one of the sectors that has achieved an unprecedented boom is logging and wood concentration.

Logging is one of the activities performed in the care of the forest. It is one of the basic production processes in forestry. Logging consists of activities such as shading trees, their processing, sorting, approaching, concentrating, removal and shipping of individual wood assortments [8].

The transport of timber is important activity in the logging process. Timber transport consists of two stages. The primary transport of timber and timber is referred to as concentration (timber export). Secondary transport of timber is held on the modified roads (timber removal) [4]. Concentration of wood and wood mass is performed in several ways using different forces: manual, gravitational, animal and partially or complex mechanized concentration of wood [5]. The predominant means of concentration in Slovakia are special wheeled tractors. Other types of tractors that are used are universal and tracked (FOREST ACTIVITIES, online).

The transport of wood and timber also includes the removal of wood. The removal of timber follows on from the concentration of timber (export of timber and timber). It is the transport of wood from the forest stock or from the transport point in the forest to the main handling or shipping stock, or directly to the customer. Motor vehicles and trailers are used for these purposes. When transporting wood, transport sets with wood transport in lengths over 6 m predominate (ACTIVITIES IN THE FOREST, online).

The technologies used to remove timber from a hauling site by public road are very diverse. Road haulage vehicles for motor transport are divided according to the mode of propulsion into motor vehicles (self-propelled) and trailers (they do not have their own engine and are attached to motor vehicles). They depend on the form of transported wood (whole trees or their sections, shortened trunks, medium length cut-outs, short cut-outs, etc.), vehicle design and loading equipment [9].

Functions of forest machines Reliability of an object is the object's property which expresses the measure of capability to fulfill stated objectives of the object. It can be expressed by means of time in which the object fulfills the defined objectives until the time when it does not fulfill these objectives (downtime) [10].

Breakdown is a phenomenon resulting from the transition of an object from its working condition into non-working condition e.g., a phenomenon resulting from ending the working condition of an object to fulfill required functions.

Idle time can be defined as a total sum of times when an equipment is out of operation due to a defect, e.g., from the moment of the stop up to the moment of renewed operation. Downtime presents a longer time than the net time required for the equipment repair (**Figure 1**) [11].

Duration of idle time can vary with equal type of a defect, for various reasons. The causes and their consequences are important for the evaluation of the seriousness of down-times but often the downtimes are recorded which represent only the so called "typical worst cases". In real life situation it is common that the down-time duration in night shift during the weekend, caused by the same defect as during the week, lasts longer.

In case that defects affect the defect, it is important to record the downtime, for two basic reasons, as the time before the defect. Many people mistake the word "repair time" with idle-time duration, which can cause a false assessment of the break consequence only from the point of view of interruption of equipment operation, and the estimate of consequences is limited to only so called "typical worst cases".

#### **Figure 1.**

*Comparing down-time duration with the time needed for repair [1].*

Due to idle time in production further influences occur which affect:


The parameters of idle times characterize the effectiveness of the conducted maintenance. The idle times can be divided into two basic groups:


The downtimes can also be divided into:

I.planned


II.not planned


*Perspective Chapter: Analysis of the Operational Reliability of Forest Equipment DOI: http://dx.doi.org/10.5772/intechopen.107402*

The losses that are caused by the downtime include following costs:


Repair is a set of activities by executing of which the object turns from the state of non-operable to the operable condition (**Figure 2**).

*Downtime* has always affected the productive capability of physical assets by reducing output, increasing operating costs and interfering with customer service. By the 1960's and 1970's, this was already a major concern in the tree felling, manufacturing, and transport sectors. In manufacturing, the effects of downtime are being aggravated by the world-wide move towards just-in-time systems where reduced stocks of work-inprogress mean that quite small breakdowns are now much more likely to stop a whole plant. In recent times, the growth of mechanisation and automation has meant that *reliability* and *availability* have now also become key issues in sectors as diverse as health care, data processing, telecommunications and building management.

Reliability: Definitions Reliability was defined as a general property of an object (e.g. a vehicle), consisting in the ability to perform the required functions while maintaining the values of the set operating indicators within the given limits and in time according to the set technical conditions. It was therefore an objective, general and complex property. However, most experts considered this definition of the basic term reliability to be less appropriate. According to the newly introduced standards ISOP 8402 and, in more detail, IEC 50(191), **reliability** is understood more narrowly as a term for describing readiness and the factors that influence it: failure-free, maintainability and assurance of maintenance.

While the definition of fault-free has not changed much, the term maintainability is understood more generally and corresponds to the original definition of general maintainability under the legislation in force until 1993 in **Table 1**.

**Maintenance** is generally understood as a combination of all management, technical, organizational, control and administrative activities, aimed at maintaining or returning the object to a condition in which it can perform the required function

**Figure 2.** *A repair characteristics [1].*

(generally defined maintenance in this way = preventive maintenance + maintenance after a failure); is performed by the user, operator, supplier, manufacturer, etc.

a. Reliability quantification

Reliability theory examines the patterns of occurrence of failures and methods of predicting them, looks for ways to increase the reliability of products, in the period of their design, projection, construction and production, but also methods of achieving inherent reliability (internally given) and operational reliability during the period of operation and storage. The concept of operational reliability includes the technical side of reliability and the reliability of people - operators, operators, etc. Reliability theory also elaborates methods of checking and testing reliability. The brief definitions of reliability properties given in **Table 1** have only a **qualitative** character - they provide data on the content of each property. **Reliability** quantification involves two steps:


Determining reliability indicators (safety, durability) for a specific type of product depends on its functional complexity, purpose of use, etc. It generally takes place in the following steps:


**Table 1.**

*Overview of basic terms and definitions according to the new legislation [2].*

*Perspective Chapter: Analysis of the Operational Reliability of Forest Equipment DOI: http://dx.doi.org/10.5772/intechopen.107402*

	- 1.divide requirements into components, i.e. system elements (subsystems, modules, identified critical or newly developed parts, etc.),
	- 2.establish requirements for ensuring the quality of supply (i.e. materials, services, software, etc.),
	- 3.evaluate the capabilities and capabilities of the manufacturer and suppliers (technical and personnel level, technological equipment, control equipment, people's qualifications, etc.).

It is implemented in the defining, preparatory phase of the product life cycle from the viewpoints of:


One of the basic aspects of assessing a newly developed or innovated product is the consideration of expected costs and achieved benefits for its required (or assumed) reliability, or lifetime while respecting safety requirements. To optimize reliability costs, two related methods are usually used: the LCC qualitative assessment method "Life Cycle Cost" and the LSC quantitative assessment method "Life Support Cost", which according to IEC recommendations have be part of the technical documentation.

The LCC analysis broadly includes the following steps:


The level of reliability as a whole and the level of individual sub-properties is created, shaped and maintained in two basic spheres:


Operational care of an object (vehicle) is an integrating term that includes all measures and activities carried out in the sphere of users (in the ACR, e.g. in units, departments, etc.) and in the ACR logistics units and departments. Operational care mainly includes preventive care, i.e. system of inclusion and use, maintenance, technical diagnostics, repair system, etc.

Maintenance is an activity carried out for the purpose of maintaining the object in an operable condition for a period determined by the technical conditions; it consists in checking the condition of the object, carrying out preventive interventions and maintenance after a fault. Maintenance includes washing, cleaning, adding fuel, lubricants and operating fluids, lubrication, adjusting, adjusting, checking parameters, troubleshooting, repairs and more.

Technical diagnostics is a field dealing with methods and means of determining the technical condition of objects. Technical diagnostics means non-dismantling and nondestructive diagnostics. Its purpose is to evaluate the technical condition and draw conclusions for further operation, repair, etc.

Repair is a set of activities carried out after a fault in order to return the object to an operational state. Includes disassembly, replacements, adjustments, partial repairs, assembly, etc.

The entire complex system of the mentioned activities, which make up the operational care of the vehicle, can significantly influence the achieved parameters of reliability properties. E.g. the level of maintainability depends not only on the level of construction and manufacturing, but also on the quality of maintenance, the level of people performing maintenance (driver, crew, workshop specialists), workshop and mechanization equipment, diagnostic technology and its adoption, tools, devices, workshop equipment, etc.; the same applies to other reliability properties.

#### *2.1.1 Failure rate and product life characteristics curve*

In practice, reliability is determined by the *number of failures per unit time during the duration under consideration* (called the **failure rate**).

*Perspective Chapter: Analysis of the Operational Reliability of Forest Equipment DOI: http://dx.doi.org/10.5772/intechopen.107402*

In considering the failure rate of a product, suppose that a large group of items is tested or used until all fail, and that the time of failure is recorded for each item. Plotting the cumulative percent of failures against time results in a curve such as the one shown in **Figure 3**.

It can be seen from **Figure 3** that 34% of items failed within the period from 0 to 500 hours and 86% of items have not survived more than 4500 hours operating time. The latter data can be re-interpreted as follows: when the product is placed in service, then, as time goes on, 66 items of 100 probably continue to meet specification after 500 hours, while 14 items of 100 are expected to survive more than 4500 hours operating time under given operating conditions. Hence, the **cumulative failure curve** can be also used to estimate the reliability of the tested product.

Both` reliability and unreliability vary with time. Reliability *R(t)* decreases with time; an item that has just been tested and shown to meet specification has a reliability of 1 when first placed in service, 1100 hours later this may have decreased to 0.5. Unreliability *F(t)* increases with time; an item that has just been tested and shown to meet specification has an unreliability of 0 when first placed in service, increasing to 0.5 after 1100 hours. Since, at any time *t*, the product has either survived or failed, the sum of reliability and unreliability must be 1, i.e.:

$$R(t) + F(t) = \mathbf{1}.\tag{1}$$

The situation is shown in **Figure 4**.

For example, knowledge of a product's reliability is useful in developing warranties.

The **instantaneous** failure rate (failures per unit time) *λ* at any point in time *t* is defined by eq. (2):

$$
\lambda(t) = \frac{\left(\frac{dF}{dt}\right)}{R(t)}\tag{2}
$$

The second phase of the life characteristics curve describes the normal pattern of random failures during a product's **useful life**. This period usually has a low, relatively constant failure rate caused by uncontrollable factors, such as sudden and unexpected

**Figure 3.** *Cumulative failure curve of the product.*

**Figure 4.** *Unreliability (F) and reliability (R) of the product.*

stresses due to complex interactions in materials or the environment. These factors are usually impossible to predict on an individual basis. However, the collective behavior of such failures can be described statistically.

Finally, as age takes over, the **wear-out period** begins, and the failure rate increases, a common experience with automobile components or other consumer products [12].

It is very easy for a machine manufacturer to declare "our machines are reliable", but behind that statement hides complex verification and reliability analyses. Each analysis contains a detailed knowledge function and all possible available failures of the analyzed equipment, failure rates of individual components, etc.

The curve in **Figure 5** is typical only for some types of simple devices. The course of the period of life-threatening disorders is e.g. often affected by wear and tear.

The bathtub curve theory also describes the course of breakdowns and maintenance from a historical perspective. In general, today's devices are much more complex than they were twenty years ago. It follows that the curves of life indicators change – **Figure 6**. The curves in the figure show the failure rate of various electrical and mechanical elements depending on the time of operation (curve A describes the behavior of about 4% of objects, B - 2%, C - 5%, D - 7%, E - 14% and curve F - about 68%). Although the relative representation of objects with different behavior (curves of type A to F) is not the same in industrial sectors, the failure rate curves of equipment are increasingly approaching curves of type E and F as their complexity increases [13].

Knowing the product life characteristics curve for a particular product helps engineers predict behavior and make decisions accordingly. Though many research institutes and large manufacturers conduct extensive statistical studies to identify distinct patterns of failure over time, gathering enough data about failures to generate as smooth a curve as shown in **Figure 5** is not always possible.

If limited data is available, the **average failure rate** is computed using the following formula:

$$\overline{\lambda} = \frac{\text{total number of failures}}{(number \text{ of identical items tested }) \times (\text{test duration in hours})} \tag{3}$$

*Perspective Chapter: Analysis of the Operational Reliability of Forest Equipment DOI: http://dx.doi.org/10.5772/intechopen.107402*

**Figure 6.** *Six failure rate vs. operating age curves.*
