What is this article about?
In the world of complex technical projects in mechanical engineering, a smooth start is the key to success. But how do you successfully translate a client’s often vague ideas and expectations into a clearly understandable, feasible project, a functioning machine? This is precisely where two crucial tools meet: the requirements specification and the specifications. What at first glance appears to be dry documentation, upon closer inspection reveals itself to be the foundation of every successful product development. They are far more than just bureaucratic obligations – they structure communication, minimize risks, and create a common understanding between client and contractor. This article delves deeply into the role, structure, and significance of these two documents, clarifying the difference between specifications and requirements specifications in mechanical engineering. It also shows why specifications and specifications are indispensable for every professional mechanical engineering project.
The crucial difference between functional specifications and requirements in mechanical engineering: perspective and level of detail
In mechanical engineering it is
clear separation between specifications and requirements specifications
crucial for successful project implementation. The difference between specifications and requirements specifications will be discussed in detail in the following sections.
Comparison of the core aspects
Both documents fulfill different functions, are created by different actors and differ significantly in perspective, level of detail and timing in the project process.
| Kriterium | Lastenheft | Pflichtenheft |
|---|---|---|
| Wer erstellt es? | Auftraggeber | Auftragnehmer |
| Was ist der Fokus? | Was soll erreicht werden? | Wie wird es erreicht? |
| Detaillierungsgrad | Grobe, aber vollständige Anforderungsliste | Sehr detaillierte technische und organisatorische Planung |
| Zeitpunkt im Projektzyklus | Frühphase – Grundlage der Ausschreibung | Nach Auftragserteilung – Beginn der Umsetzungsplanung |
| Zentrale Fragen | Was ist die Aufgabenstellung? Welche Ziele und Rahmenbedingungen sind zu erfüllen? | Wie werden die Anforderungen konkret technisch umgesetzt? |
The specifications document describes the requirements from the customer’s perspective. It does not contain suggested solutions, but rather defines the functions, performance, and framework conditions that the machine or system must fulfill. It serves, among other things, for tendering, comparing bids, and as a reference for acceptance.
The specifications, on the other hand, are prepared by the contractor and describe in detail how these requirements will be implemented technically, functionally, and organizationally. This document details design principles, control concepts, schedules, test methods, and interfaces. It forms the basis for design, development, and production and is an integral part of the supplier’s project management.
In practice, a
clear demarcation of both documents
essential. It ensures that no misunderstandings arise and that both parties – customer and supplier – work together on a coordinated, transparent basis.
Difference between functional specifications and requirements specifications in mechanical engineering
The requirements in mechanical engineering: The client's perspective
In the requirements , the future client formulates his ideas and the requirements that the machine should meet.
Content focus of a typical specification
The content focuses vary and always depend on the client in mechanical engineering. Nevertheless, there are a few general points that are generally important. This also clarifies the difference between functional specifications and requirements specifications in mechanical engineering.
Clarify and specify the task
Before planning begins, the task should be clarified as comprehensively and completely as possible so that additions and corrections during the processing are limited to the most necessary.
Demands and wishes
For the requirements list/specifications, the objectives and conditions must be clearly defined through requirements in the form of demands and wishes, because otherwise undesirable developments are hardly avoidable.
A distinction is therefore made between
- Requirements that must be met under all circumstances. Without these requirements, the solution is completely unacceptable. Minimum requirements must be defined if necessary.
- Requests should be considered where possible, possibly with the concession that a limited additional effort is permissible. In this case, it is recommended that the requests be considered after
- higher,
- medium and
- to be classified as of little importance.
This differentiation and labeling is also necessary for later assessment, because during selection the fulfillment of the requirements is asked, whereas during evaluation only variants that already fulfill the requirements are considered.
Quantity and quality
Without specifying a specific solution, the demands and wishes must be identified with quantitative and qualitative aspects. Only then can sufficient information be obtained. A distinction is made between:
- Quantity: all information about number, number of pieces, batch size and quantity, often also per unit of time such as power, throughput, volume flow, definition of ppm, etc.
- Quality: all information on permissible deviations and in particular requirements such as corrosion resistance and other individual aspects, etc.
The requirements should be specified as far as possible by means of figures.
Where this is not possible, descriptions/statements
formulated as clearly as possible
This facilitates subsequent reviews by experts if differences of opinion arise between the contractor and the client during or after the project’s completion.
Particularly important references to influences, intentions or those for implementation can also be included in the list of requirements. The
In this way, the requirements list represents starting positions. Ideally, it is always kept up to date.
Features of the requirements list
Table 2 shows which features should be included in the requirements list/specifications.
| Hauptmerkmal | Beispiele |
|---|---|
| Geometrie | Dimensionen, Bauraum, Raumbedarf, Anordnung, Anschluß, Ausbau und Erweiterung |
| Kinematik | Bewegungsart, Bewegungsrichtung, Geschwindigkeit, Beschleunigung |
| Kräfte | Kraftgröße, Kraftrichtung, Statik, Dynamik, Schwingungen, Resonanzen, Gewicht, Last, Stabilität, Steifigkeit, Verformung |
| Energie | Leistung, Wirkungsgrad, Verlust, Reibung, Ventilation, Druck, Temperatur, Feuchtigkeit, Erwärmung, Kühlung, Speicherung, Arbeitsaufnahme, Energieumformung |
| Stoff | Physikalische und chemische Eigenschaften des Eingangs- und Ausgangsprodukts, Hilfsstoffe, vorgeschriebene Werkstoffe, Materialfluss, Materialtransport |
| Signal | Eingangssignale, Ausgangssignale, Anzeigeart, Betriebsüberwachung, Signalform |
| Sicherheit | Sicherheitstechnik, Schutzsysteme, Maschinenrichtlinie, Maschinenverordnung, Arbeitsschutz |
| Ergonomie | Bedienung, Übersichtlichkeit, Sicherheit des Bedienpersonals, Formgestaltung |
| Fertigung | Einschränkung durch Produktionsstätte, herstellbare Abmessungen, bevorzugtes Fertigungsverfahren, Fertigungsmittel, mögliche Qualität und Toleranzen |
| Kontrolle | Prüfmöglichkeiten, Einhaltung gesetzlicher Vorschriften /Maschinenrichtlinie, Maschinenverordnung usw.) und Regeln (DIN, ISO, AD, usw.) |
| Montage | Zusammenbau, Einbau, Baustellenmontage, Fundamentierung |
| Transport | Begrenzung durch Hebezeuge, Transportwege (Größe und Gewicht) |
| Gebrauch | Lärm, Verschleiß, Einsatzort |
| Instandhaltung | Wartung (Anzahl, Zeitbedarf, …), Instandsetzung, Säuberung |
| Recycling | Entsorgung, Beseitigung, usw. |
| Kosten | Maximale Kosten, Werkzeugkosten, Ersatzteile, Investition und Armortisierung |
| Termin | Entwicklungszeit, Inbetriebnahmezeitraum, Endabnahme |
When clarifying the task, the necessary functions and existing task-specific conditions related to energy metabolism, mass metabolism, and signal metabolism should first be identified. This is achieved through the following characteristics: geometry, kinematics, forces, energy, mass, and signal.
At the end, the author of the specifications must ask himself whether all requirements have been captured.
Classic questions are:
- Are all requirements specified with quantity and quality specifications?
- What purpose must the solution serve?
- What properties must the solution have?
- What properties must the solution not have?
- When drawing up the specifications, were all requirements and wishes for the machine clearly defined?
- Are the desired requirements clearly classified into high, medium and low importance requirements?
The requirements are then sorted:
The main task and characteristic main data must be in the foreground.
After that, after recognizable
- subsystems,
- functional groups,
- Assemblies
structured.
The requirements are also defined for these subgroups.
Once this has been done, it is advisable to review feedback, objections and additions after some time and incorporate them into the list of requirements in the specifications.
Only when the goal
sufficiently clarified
and the client’s representatives are of the opinion that the product is clearly defined, potential suppliers can be enquired with the drawn up specifications.
Very important aspects for the requirements list in the specification are:
Things that are taken for granted are not included.
Rather, it is important to ensure that
formulated as precisely as possible
becomes.
Project framework conditions in the mechanical engineering specifications
The project framework forms the organizational backbone of a mechanical engineering project. They define binding specifications regarding time, content, responsibilities, and quality requirements. A precise description of these framework conditions in the specifications is essential to
to create clear expectations, minimize risks and achieve project goals on time and economically.
Schedule and milestones
A structured schedule ensures that all project phases – from planning to commissioning – are carried out in a realistic and coordinated manner. The plan includes
Key dates and milestones
which serve as checkpoints for project progress. Typical milestones in mechanical engineering are:
- Concept approval,
- Design release,
- Start of production,
- Start of assembly,
- Start of commissioning,
- Acceptance by the client.
Compliance with these deadlines is regularly reviewed. Deviations from these deadlines must be communicated promptly and accompanied by appropriate countermeasures.
Scope of delivery and responsibilities
The scope of delivery describes all services and products to be provided by the contractor. These include, among others:
- Design and layout of mechanical and electrical components,
- Manufacturing and procurement of assemblies,
- Final assembly and commissioning,
- Preparation of technical documentation,
- Conducting training courses.
The client is responsible for providing production-specific and site-specific information as well as the infrastructure (e.g. media connections, hall layout, safety requirements).
Responsibilities must be clearly defined to avoid interface problems. Each task must have a responsible and decision-making authority.
Acceptance criteria
The project will only be accepted if the defined acceptance criteria are fully met. These include both functional and qualitative requirements and are
objectively measurable.
Important criteria are:
- Compliance with technical specifications and performance requirements,
- Carrying out and passing all functional and safety tests,
- Completeness of documentation in accordance with legal requirements (e.g. Machinery Directive 2006/42/EC and in future Machinery Regulation 2023/1230, etc.) and customer specifications,
- successful completion of defined acceptance tests.
Fulfilling these criteria is
Prerequisite for project acceptance.
The specifications in mechanical engineering: The contractor’s response
Definition and purpose of the specifications
In mechanical engineering, the specifications provide the
binding implementation specification of the contractor
It describes in detail how the requirements formulated in the specification are to be implemented technically and organizationally. The specification primarily defines the objectives, functions, and framework conditions from the client’s perspective.
The focus of the specifications is the
Description of the technical solution approach.
It answers the question of which
- means,
- Methods and
- Technologies
the requirements are met. This includes, among other things, design concepts, control strategies, components used, software architectures, security solutions, and interface concepts.
Typical contents of a functional specification are:
- System overview and technical architecture,
- Description of the functioning of assemblies and subsystems,
- Selection and justification of technical components,
- Circuit diagrams, process descriptions, cycle times and cycle diagrams,
- Security compliance and
- Compliance with standards,
- Test concepts and tests.
The purpose of the specifications is to ensure detailed planning, transparency, and traceability of the implementation. It forms the basis for internal design and development, but also serves the client to check whether his
Requirements are correctly understood and adequately solved.
In addition, the specifications are an essential reference document for the subsequent acceptance of the project:
Only what is defined in the specifications is considered to be binding.
Therefore, it is usually approved by both parties and may be contractually relevant in the event of deviations.
Content focus of a typical specification
A functional specification in mechanical engineering represents the technical and organizational elaboration of the requirements formulated in the specifications. It serves as a detailed planning and implementation basis for design, development, and production. The functional specification is divided into several central content areas.
Technical specifications
The most important part of the functional specifications covers the technical details of the solution. These include:
- Design drawings that describe the mechanical design of the machine, including all relevant dimensions, materials and tolerances.
- Selection of components and materials , with information on manufacturers, types, performance data, etc.
- Control technology and software concepts
- Interface specifications , both mechanical and electrical/digital, to ensure integration into higher-level systems.
Implementation planning
Another key component is the planning of implementation:
- A work breakdown structure divides the overall project into manageable work packages with clear responsibilities.
- The schedule includes milestones, start dates and end dates for each phase, as well as critical paths.
- Resource planning specifies which human, technical and financial resources are required.
Quality management and testing procedures
To ensure the required quality, the specifications define:
- Quality standards (e.g. according to DIN ISO, factory standards or customer specifications).
- Test and inspection procedures , for example for functional, stress and safety tests, which are described in much more detail than in the specifications.
- Acceptance criteria that are documented and verifiably defined for each assembly and function.
- Documentation requirements , e.g. regarding test reports, CE marking or risk analyses.
Service and maintenance
To ensure long-term operation of the machine, the specifications also include statements on:
- Maintenance intervals and maintenance instructions , including recommended tools and activities.
- Spare parts lists with ordering information, storage recommendations and replacement instructions.
- Training concepts for operating personnel, maintenance personnel and repair personnel, including planned training content and training formats.
The importance of the interaction between requirements and functional specifications
The smooth interaction of specifications and requirements is a key factor in mechanical engineering.
essential success factor for the development and implementation of technically complex projects.
Both documents are intertwined and together form the foundation for targeted and reliable cooperation between client and contractor.
The functional specifications document is therefore a direct response to the requirements. Both must be consistent in terms of content and logic.
A common understanding of the project goals is essential.
The contractor must not only formally capture the requirements but also correctly interpret them in a technical and economic context. This includes regular consultations, precise clarifications, and structured documentation.
Central to project success is that responsibilities, areas of authority, and scope of work are clearly defined. This is the only way to avoid misunderstandings, duplicate work, or gaps in the project’s progress. It’s equally important that both documents together form the basis for acceptance, warranty, and any subsequent claims – anything that isn’t clearly agreed upon can lead to conflicts later on.
Typical pitfalls in interaction
In reality, problems often arise. There are various reasons for this:
- Unclear or incomplete wording in the requirements leaves room for interpretation in the functional specifications and carries a high risk of errors.
- Missing or insufficient specifications , e.g. regarding interfaces, tolerances or environmental conditions, make precise implementation difficult.
- Conflicting requirements that lead to conflicting objectives.
- Lack of communication between client and contractor – especially in early project phases – prevents a common understanding of the solution.
- Informal changes , for example through telephone calls or emails without formal documentation, create a lack of transparency and legal uncertainty.
Professional change management , transparent communication, and consistent maintenance of both documents throughout the project are therefore essential. Only when the specifications and requirements specifications are continuously coordinated and updated together as needed can mechanical engineering projects be implemented successfully, efficiently, and without conflict.
Conclusion
The structured interaction of requirements and functional specifications is a key prerequisite for the successful planning and implementation of technical projects in mechanical engineering. While the requirements are defined by objectives and framework from the client’s perspective, the functional specifications represent the contractor’s detailed response as to how these requirements will be technically implemented.
Both documents differ significantly in terms of their perspective, level of detail, and their development phase in the project cycle. A systematically prepared specification creates transparency and comparability during the tendering phase, while the functional specification forms the basis for design, development, assembly, and acceptance.
Particular attention must be paid to the interaction between both documents: Only when requirements are clearly formulated, fully specified, and agreed upon between the parties can misunderstandings, undesirable developments, and subsequent requests be avoided. Typical sources of error such as imprecise wording, contradictory specifications, or informal changes can be significantly reduced through professional communication and effective change management.
Ultimately , a
clear requirements and a technically sound functional specification
are not only documentation tools, but
strategic success factors from project idea to acceptance
and beyond. They promote technical quality, adherence to schedules and costs, and a collaborative partnership between client and contractor.
