Perspective Chapter: Analysis of the Operational Reliability of Forest Equipment

Abstract

In the given chapter called Research of operational reliability of forest machines we deal with the reliability of forest machines in the field of logging, handling and removal of wood by world famous manufacturers. The main content of the work is the definition of the issue of operational reliability of forest logging machines, considering that this is very important for the high-quality and economical functioning of maintenance and diagnostics operations. The first part of the chapter focuses on an overview of current topics and thematic analysis. It describes the basics of operational reliability, maintenance and provide basic information about forest machines and their distribution. The second part deals with the reliability of specific forest machines. It discusses the methods and forms of maintenance, control of technological equipment and evaluation of operational reliability of forest harvesting machines.

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 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].

Figure 1.

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

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”.

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

• production quality, e.g., customer services which can manifest themselves in a form of financial penalization,

• stopping or performance reduction of other equipment or activities,

• total production cost increase e.g., direct maintenance costs (due to energy cost increase, etc.),

• development of secondary damages due to the defect.

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

• technological idle time –planned repairs, object exchange, necessary inspections, current maintenance, care, and equipment checking, etc.,

• idle time due to a negative event development – defect, accident, injury, etc.

The downtimes can also be divided into:

1. planned

   • standstill (work rest),

   • repairs,

   • technological,

2. not planned

   • defects,

   • technological,

   • others.

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

• material costs,

• energy costs,

• damaged parts repair costs,

• costs due to production drop out during the downtime.

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).

Figure 2.

A repair characteristics [1].

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-in-progress 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.

Termin	Definice
safety	a state in which the risk of endangering health, life of persons, the environment or property is limited to an acceptable level
durability	the ability of the object to perform the required function under the given conditions of use and maintenance to a limit state that can be characterized by the end of its useful life, economic or technical unsuitability, or other serious reasons
dependability	collective term for the description of readiness and the factors that influence it, i.e. fault-free, maintainability and maintenance assurance
availability	the ability of an object to be in a state capable of performing a required function under given conditions, at a given point in time or interval, provided that the required external conditions are provided
reliability	the ability of the object to perform the required function in the given conditions and in the given time period
maintainability	the ability of an object under the given conditions of use to remain in a state or to return to a state in which it can perform the required function, if maintenance is carried out in the given conditions and the specified procedures and means are used
maintenance support	the ability of the organization providing maintenance services to provide, according to the requirements in the given conditions (they apply both to the own object and to the conditions of use and maintenance), the means needed for maintenance according to the given maintenance concept

Table 1.

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

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

1. 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:

   • the choice of quantitative measures (numerical, functional, etc.), i.e. indicators and characteristics of failure-free, maintainability, repairability, readiness, or next,

   • determination of their required values based on specifications, requirements, or contracts, market research, etc.

   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:

   • it is determined by which indicators reliability (as well as safety and service life) will be described; this task is referred to as the choice of indicator nomenclature,

   • numerical values are assigned to the chosen nomenclature of indicators - in this phase, target values ​​are determined based on the identified requirements for specified technical parameters and conditions of use,

   • a preliminary analysis of the attainability of the set goals (including economic efficiency) is carried out; In this phase, the product is usually considered as a system and the purpose of the analysis is:

     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.).

2. Analysis of reliability objectives

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

   • fault-free requirements, or lifetime of the product (its elements, systems, critical parts, etc.),

   • requirements for maintainability and provision of maintenance (scope and conditions of provision of preventive maintenance, diagnosability, need for operational diagnostics, repairability, general scope of spare parts needs during operation, etc.).

3. Economic evaluation

   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:

• drawing up a list of cost items that will be used,

• the formulation of mathematical relationships for the expression of cost items based on product characteristics and operating conditions, the essence of the LSC method,

• cost correction (instability of the value of money, valorization, inflation, increase in input prices, investments and depreciation, current annual expenses, etc.),

• collection of input data from the manufacturer (design data, fault-free and maintainability data, price lists and other financial data, etc.) and from the user (data on implementation, installation, costs of ensuring conditions of use, maintenance, supply of spare parts, etc.), possible computer processing.

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

1. In the sphere of design, projection, construction and production, the so-called inherent reliability, internally, genetically given to the product by creative workers.

2. In the sphere of users, where the initial inherent reliability properties are developed and formed (mainly, however, maintained and also reduced by the action of the human factor) in the manner of operational use and operational care of the product.

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 non-destructive 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).

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.

Figure 3.

Cumulative failure curve of the product.

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.:

$$R\left(t\right)+F\left(t\right)=1.$$E1

The situation is shown in Figure 4.

Figure 4.

Unreliability (F) and reliability (R) of the product.

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):

$$\lambda \left(t\right)=\frac{\left(\frac{\mathit{dF}}{\mathit{dt}}\right)}{R\left(t\right)}$$E2

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 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.

Figure 5.

Failure rate curve (a “bathtub curve”).

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].

Figure 6.

Six failure rate vs. operating age curves.

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:

$$\stackrel{-}{\lambda }=\frac{\mathit{\text{total}}\mathit{\text{number}}\mathit{\text{of}}\mathit{\text{failures}}}{\left(\begin{array}{c}\mathit{\text{number}}\mathit{\text{of}}\mathit{\text{identical}}\mathit{\text{items}}\mathit{\text{tested}}\end{array}\right)\times \left(\begin{array}{c}\mathit{\text{test}}\mathit{\text{duration}}\mathit{\text{in}}\mathit{\text{hours}}\end{array}\right)}$$E3

3. Maintenance systems

The maintenance kinds include activities by which the maintenance is conducted. The maintenance kinds are as follows:

1. Running maintenance consists of

   • cleaning,

   • lubrication,

   • set up.

2. Inspection activities are:

   • revision,

   • diagnostics,

   • prophylaxis.

3. Repair activities are defined as:

   • small repair,

   • medium repair,

   • general overhaul.

4. Renovation is known as:

   • modernization,

   • reconstruction,

   • module replacement (exchange),

   • upgrading,

   • machine replacement.

Running maintenance means a regular care of objects. It has a prevention character. For this activity, the terms care-upkeeping are used.

It is a running, regular machines, and equipment maintenance. By this maintenance a premature wear and defects can be prevented. The running maintenance includes cleaning, set up of machines and equipment, tightening of loosened connections, refilling of recording (writing) parts of measuring and recording apparatuses. But it is the measurement which has the greatest significance in the whole maintenance process. To the maintenance also small repairs belong, which are conducted during the maintenance. The difference between maintenance and repairs lies also in the fact which workers are conducting them. Whereas specialists conduct the repairs, especially operatives who are also appointed with other duties conduct the maintenance. The operatives of machines and equipment conduct the running maintenance.

Inspection activity in maintenance presents a revision and control activity regarding the condition, maintenance and repair of machines and equipment within the framework of maintenance. From the point of view of a professional, the inspection concerns all kinds and ways of maintenance. It is appropriate to create professional inspection departments (e.g., machinery, electrical, construction, etc.) and to record the results of these controls and revisions into inspection books.

The monitoring of defects belongs to the important duties of an inspection. The inspection is dependent on the maintenance organization. It is not uniform, and the organizational incorporation is not uniform.

The inspection work content results from its basic objective.

Diagnostics presents the detecting of the object condition, e.g., its monitoring and registration of magnitude values which define the condition and development of a subject [14].

By introducing the subject maintenance on the base of the diagnostics of their conditions it is necessary to solve the following tasks:

• subject selection for diagnostics,

• selection of measurable magnitudes on an object that can identify the defect occurrence,

• selection of means and the diagnostics.

The diagnostics of a subject is conducted on the base of:

• a consecutive defect,

• inaccuracy,

• expenses.

Prophylaxis means a regular control of the condition of electrical equipment, information technology appliances, instruments in medicine, etc.

General Overhaul represents the repair of the whole basic equipment. By the general overhaul, the effects of wear and damage are removed. The aim here is to renew the original.

Performance, technical properties, operational quality, and profitability of the basic equipment. It is of advantage to improve its properties based on the technological progress. This can be achieved through the modernization. The general overhaul must ensure that the basic equipment will be able to work until the next general overhaul. It is convenient to conduct the general overhauls in specialized shops. General overhauls belong to the biggest, most important but also most costly repairs.

Medium repair is a repair performance of a greater extent, in which a larger number of machine or equipment parts is replaced or repaired. The extent of a medium repair is determined on the base of inspection results and findings detected during small repairs. Medium repairs represent a significant activity in the operation. Their extent is large, and they are conducted often.

Small repair includes all remaining repair performances, the extent of which is smaller in comparison with medium repairs and general overhauls. Small repairs present a large number of repairs. A part of small repairs can be done during preventive inspections by workers who conduct an inspection.

Renovation of an Object.

An intensive technical development requires that during repairs (especially during general overhauls) not only the physical but also the moral wear of equipment be removed. The total dismantling of a machine is to be used for the removal of its technical obsolescence. In this way a modernization is conducted. It includes various technical improvements of a machine, by which a higher performance can be achieved, easier handling (operation), higher precision, longer life, higher safety at work, mechanization of operational manipulation, etc [13].

The modernization as a sustainable process acting against the moral wear, becomes an organic component of a repair. In association with the modernization the term reconstruction is to be specified. A modernization can represent such an improvement, which changes its original character. A reconstruction is such a performance, by which the original character of the reconstructed object is (permanently) changed (e.g., when general purpose machines are adopted to a single purpose, special machines).

Current trend of machines and equipment development, new trends in IT- equipment maintenance, when the manufacture of a module in a large series production is cheaper than the repair of a faulty one and it results in module exchange or when the downtime due to the repair, and the loss due to this downtime, and repair costs are comparable with the value of a new module.

In IT, where the replacement of an original is enlarged by a new one which ensures the original function or extends it, is called upgrading. In case that any of this kind of activity does not put the object into an operable condition, or if the costs for its maintenance are inadequate, the exchange of the machine takes place (Table 2).

Maintenance is set of activities to secure the determined operational state of an object by technical conditions, as well as to define and consider its actual state.
	Care Examination Checking	Inspection	Repairs
Definition	Set of activities for maintaining the determined operational state of an object by technical conditions	Set of activities for determination and consideration of the actual state of a technical object	Set of activities for the restoring of a given operational state of a technical object by technical conditions
Example	Cleaning; Lubrication; Setting	Measurement; Testing; Diagnostics	Replacement of object parts; Adjustment; Adaptation of an object part

Table 2.

The characteristics of maintenance kinds [15].

4. The objective of maintenance and repair function

The basic function of the maintenance and repair activity is to ensure work capability of HIM and NIM in production. The significance of this function increases simultaneously with the increase of production technical level, and specially with the increase of automatic machines, lines, and integrated production processes.

The sum of work, the task of which is to secure the capability of machine and equipment operation, and their total effective running, is called maintenance-repair activity, sometimes only a maintenance. In a narrower sense, under this term we understand cleaning, lubrication.

and other regular protection of basic means prior to repair e.g., current maintenance. In a broader sense, under the term maintenance we understand any care of basic means, by which their working capability is ensured. Here belongs current maintenance of machines during operation, as well as all kinds of planned and non-planned repairs, by which the consequences of wear are removed.

In real life situation it is difficult to draw the exact border line between maintenance and repairs because these activities often overlap. During current maintenance small repairs are done, and vice versa, during repairs, sometimes current maintenance must be conducted.

Principally the removal of wear consequences is called repair. From the economic point of view, it is execution of additional work on the fixed assets, by which their wear is to be removed. The inner differentiation of maintenance-repair work is being derived from the level of functioning of the fixed assets (basic means). In trouble-free functioning the supervisory-inspection activity will be enough. It must avoid undesired conditions. If the functioning worsens, it is necessary to conduct repairs. According to their extent there are various degrees of maintenance/repair activities:

Current maintenance consists of cleaning, lubrication, inspection, and other pre-repair activity.

Small repair consists of such work as repair and replacement of smaller parts, which are subject to wear, replacement of sealing of taps, valves, cleaning and exchange of oil and cooling systems, oil cups, etc.

Medium repair is characterized by the fact, that during its execution the opening and inspection of all boxes and closed mechanisms is done, larger parts and aggregates are being repaired and replaced, the whole lubrication system is being flushed. According to circumstances machine slide ways are being repaired and the whole set-up of a machine is conducted, including precision checking. The medium repairs are conducted on the site, where the machines are installed. Only exceptionally they are conducted in repair shops, specially determined for repairs. In any case, after the medium repair a control protocol is written down, where there is the record of handing over the machine and the record on precision control results.

General overhaul (GO) presents the biggest repair performance in maintenance. In this kind of repair, the whole machine undergoes the repair, not just its parts. For this reason, it is being released from its base, it is disassembled to its individual parts and repaired in a special workshop. The extent and quality of GO must ensure that the machine will obtain the original technical properties and its planned life will be obtained. The machine or other fixed asset undergoes a test according to standards after the general overhaul and is handed over to operation by means of a protocol.

The intensive technical development requires still more that in repairs (specially in general overhauls) not only physical, but also moral wear is removed. The complete disassembly of a machine must be also used for eliminating its technical obsolescence. In such a way modernization takes place. It includes various technical improvements of the machine by which its higher performance, easier handling (operation), higher precision, longer life, improved safety at work, mechanization of operation manipulation, etc. can be achieved. In such a way the modernization, as a continuous process acting against the moral wear, becomes an organic part of repairs.

In association with the modernization the term reconstruction must be specified. Modernization can represent such a machine improvement which does not change its original character. Reconstruction, on the other side, presents such a performance, by which the original character of the reconstructed machine has been changed permanently (e.g., when multi-purpose machines are adapted to single-purpose, special machines) (Tables 3 and 4).

Maintenance activities	Maintenance kinds	Stages
Cleaning; Lubrication; Set up	Current maintenance	I.
Testing Diagnostics Prophylaxis	Inspection	II.
Small repair; Medium repair; General overhaul	Repairs	III.
Modernization; Reconstruction; Module replacement(exchange); Upgrading; Machine replacement(exchange)	Renovation	IV.

Table 3.

The objective of maintenance and repair function [16].

Assessment criteria	Central form	Decentral form	Integrated form	Combined form
Operation know-how	satisfactory	good	very good	satisfactory up to very good
Roads, ways	long	negligible	negligible	negligible to long
Decision acts	many	negligible	negligible	negligible to many
Capacity balancing	very good	good	insufficient	good to very good
Rationalization possibility	very good	negligible	almost none	negligible to good
Investment for machine deployment	average	high	very high	average to high
Quality	very good	good	satisfactory	good
Cooperation within the company	very good	negligible	none	negligible to very good
Organizational demand	high	average	negligible	average
Qualification of workers	average	average	very high	average to high
Production kind sequence	none	negligible	very strong	negligible to strong
Experience exchange	very good	satisfactory	negligible	satisfactory to very good
Number of workers required	negligible	high	average	negligible to average
Worker’s motivation	negligible	good	high	good to negligible

Table 4.

Organizational maintenance forms [17].

Decentralized organization form results from the fact that workers are assigned on the base of their qualification and work-legal relations to individual lower organizational units, e.g., plant. With consequent implementation of this form of maintenance the incorporation of maintenance operation into the structure of each production unit is assumed. This unit works independently and is linked directly to the organizational structure of the production unit. The workers have been specialized in the equipment of the given operation. This fact is of advantage specially both at work within current maintenance, as well as with equipment inspection.

Integrated organizational form – is based on the supposition that service workers (service men) conduct, beside the maintenance activities, also current operational work. For this work universal specialists with broad knowledge and skills are required.

5. Research into the operational reliability of forest machines

This methodology of operational reliability research is intended for monitoring the actual condition of the machine in operating conditions and evaluation of selected indicators of operational reliability of selected forest harvesting machines.

There are several methods of processing information on reliability indicators, but some of them are very complex and difficult to apply under normal operating conditions [18]. The information processing method listed below is simpler but accurate enough which in turn facilitates its implementation in real business conditions.

The methodological procedure of mathematical processing of empirical information on the reliability indicators of forest machines is as follows:

1. Create a Machine Card for each device separately

   The machine card contains:

   • CV of the machine, i.e. its current use,

   • empirical values of machine reliability indicators arranged in an arithmetic series.

2. Using mathematical statistics and software, calculate individual characteristics for failure times (standard deviation S, coefficient of variation V, arithmetic mean, left confidence interval limit, right confidence interval limit, median, minimum, maximum, lower quartile, upper quartile, variation range, quartile range, variance, standard deviation, standard error, asymmetry, point).

3. Construction of histogram and curve of cumulative relative number of failure rate indicator (resp. time between failures).

4. Selection of theoretical division and determination of its parameters.

5. Determination of fault intensity λ (t) on the basis of theoretical distribution.

6. Determination of the mean time to TS failure.

Combined organizational form is composed of a combination of central, de-central and integrated organizational form of maintenance activities, while the principles of management are changing within hierarchical levels [19]. This form is most convenient for companies in the metallurgy industry, with a broad production program and with a large number of workers with various structure of qualification.

The examined forest machines of various manufacturers are necessary for the needs of data collection from operation and for the needs of subsequent analysis and evaluation divided into the following subgroups [20]:

1. Cabin, control units, cabin equipment (on-board computer/control unit, tachograph, outside thermometer)

2. Sensors - electrical (safety) equipment of the vehicle

3. Engine (fuel system with tank, block and lower engine cover, cylinders, pistons, pins, connecting rods, bearings, shaft, flywheel, wiring)

4. Engine and its cooling and lubrication

5. Gearboxes and transmission mechanisms (gearboxes, clutches, shafts, joints, gearboxes, differentials, distributions)

6. Chassis (body, frame, shock absorbers, suspension, wheels, tires, axles, steering, brakes)

7. Hydraulic system of the machine

8. Hydraulic crane with log grab

9. Trailer/semi-trailer

10. Trailer poles, trailer equipment [21].

6. Conclusions

The main reason for paying attention to this area of reliability and maintenance is that for every company, creating the most reliable system possible is a challenge and nowadays a common need. It is therefore necessary to be able to assess the reliability of all machinery and equipment and, in the event of deterioration in the characteristics of the means of transport monitored or stagnation, to be able to take appropriate steps to remedy this situation over time. The dependence of companies and people on technology is growing, and it is therefore necessary to ensure that the failure rate of used machinery and equipment is kept to a minimum or that machinery and equipment have maximum controlled maintenance based on real operating conditions and conditions [22].

Regarding the electronic systems used in forest machinery, we can say that this area of ​​the logging and transport process has already been affected by Industry 4.0. All manufacturers try to implement their own know-how, their own technologies, which they try to address their customers and operators of the vehicles. Most of the mentioned systems of manufacturers are usable only with a certain limitation and therefore the manual collection of operational data is currently used for the evaluation of operational data. As advances in software and hardware for vehicles as such are growing very rapidly, there is a real need for tools to be available to collect and evaluate in-service data, thus increasing the efficiency of data evaluation and the operational reliability of these vehicles. In the future, the use of Industry 4.0 for the collection of operational data using computer technology as well as its subsequent evaluation is interesting. At present, it is possible to collect operational data only partially through the control unit and then their laborious analysis.

Operational reliability prediction grows with the introduction of software and hardware innovations. For this reason, it is clearly necessary to say that Industry 4.0. has also affected this area of ​​industry, which greatly affects the use of these machines and equipment in real operating conditions [23].

The maintenance organization system is an important internal source for increasing the reliability of machinery and equipment. A positive result can be achieved through planning, management, improvement of work organization and data recording up to the management of spare parts [24]. In other words, a maintenance system is a means of maintaining or restoring equipment to good technical condition for the duration of its technical life or for a period which its operator can maintain in view of the amount of costs incurred.

The current state of the study of operational reliability in real conditions still carries a great deal of ambiguity and it can be said that the electronic area (software as well as hardware) has not yet matured, even with the current progress in the field of forest machinery. The high requirements for care and processing are closely related to the requirements for the quality and reliability of forestry machines and technological equipment. And these, in turn, are closely related to the care of the devices in question. 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, eliminating malfunctions and defects, improving technical condition, technical modernization, storage, preservation of temporarily decommissioned forestry machines and technological equipment and for the liquidation of decommissioned forestry machines and technological equipment and finally for the replacement of decommissioned forestry machines and technological equipment with new ones. The current and prospective degree of mechanization and automation of production in forestry, as well as the tendency to increase the performance of forestry technology, establish as one of the primary tasks the provision of its operational reliability. The system of organizing maintenance represents an important internal source of increasing the reliability of machines and equipment. A positive result can be achieved through planning, management, improving the organization of work and recording data up to the management of spare parts. In other words, the maintenance system is a means for equipment to keep them in good technical condition or restore this good technical condition for the duration of their technical life or for the period that their operator is able to maintain considering the amount of costs incurred.

Acknowledgments

This publication is the result of the project implementation - VEGA project No. 1/0364/21 “Research of forest machines working mechanisms regarding the new constructional parameters and working principles”.

Conflict of interest

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