Design Philosophy - 2 - Design of Machine Elements - Mechanical Engineering

Factors to be considered in machine design

Design of a machine element or an entire machine requires careful consideration of many interrelated factors. Some of these considerations are common-sense, whereas others need technical judgement and calculation. The principal factors a designer must examine are listed below and are then explained in detail.

Principal factors

• Choice of device or mechanism
• Material
• Loads and resulting stresses
• Size, shape, space requirements and weight
• Manufacturing method and assembly
• Mode of operation
• Reliability and safety
• Inspectibility
• Maintenance, cost and aesthetics

Choice of device or mechanism

Deciding what device or mechanism will perform the required function is the first and often the most important step. A given function can sometimes be achieved by different mechanisms; the designer must select the most effective solution in terms of performance, cost, space and manufacturability. A preliminary layout or rough sketch of the assembly helps to visualise the relative positions of parts and to identify interference, accessibility and space constraints.

Material

The choice of material strongly influences strength, stiffness, durability, manufacturing method and cost. An incorrect material selection can cause premature failure, excessive weight or unnecessary expense. Selection should be based on required mechanical and physical properties (such as tensile strength, ductility, hardness, toughness, corrosion resistance, thermal properties), suitability for fabrication (machining, casting, forging, welding, forming) and economic considerations. Where appropriate, standard materials (for example, common engineering steels, aluminium alloys, cast irons, brass, bronze, polymers or composites) are chosen to take advantage of known properties and availability.

Loads on the elements

External loads produce internal stresses in machine elements; accurate estimation of these loads is essential because design dimensions are derived from the stresses. Loads acting on a component may arise from a number of causes:

• Energy transmission by a machine member (for example torque in a shaft).
• Dead weight of the structure or component.
• Inertia forces due to acceleration or deceleration of parts.
• Thermal effects caused by temperature variations or thermal gradients.
• Frictional forces at interfaces or bearings.

Loads can also be classified by their temporal characteristics:

• Static load - does not change appreciably in magnitude or direction and usually increases gradually to a steady value.
• Dynamic load - varies with time. Examples include loads that change in magnitude (for example varying traffic loads on a bridge) and loads that change in direction (for example alternating forces on a piston rod in a double-acting cylinder). Vibration and shock loading are special forms of dynamic loading and must be treated accordingly.

The nature of these loads is often illustrated by diagrams that show variation with time or position; such information is necessary when performing stress analysis or fatigue analysis.

Size, shape, space requirements and weight

Preliminary calculations suggest approximate dimensions for components. When standard, catalogue parts are available, designers commonly select the next larger standard size to ensure compatibility and interchangeability. The shape of parts is governed by function, manufacturing method and spatial constraints in the assembly. A scale layout drawing of the assembly helps to confirm clearances, access for maintenance and ergonomic requirements. The weight of a product is a critical design parameter in many applications: aircraft and spacecraft require very light structures; ships and marine equipment often favour materials with a high strength-to-weight ratio; portable tools and devices must be light to be user-friendly.

Manufacture

Design must be compatible with available manufacturing processes and production facilities so that parts can be produced economically and to the required tolerances and surface finish. Considerations include:

• Choice of manufacturing process (casting, forging, machining, forming, welding, additive manufacturing) suitable for the geometry and material.
• Specification of tolerances and surface finish to meet functional requirements without unnecessary manufacturing cost.
• Design for assembly and disassembly, including location and access for fastening, joining and inspection.
• Standardisation and interchangeability of components where possible to reduce inventory and simplify maintenance.

How will it operate

The designer must ensure that the finished machine or product can be operated easily and reliably. Human factors and the sequence of operations are important: controls, starts and stops should be straightforward and safe for users. Complex sequences should be simplified so that the operator does not require excessive force or training to perform normal tasks. Examples where ergonomic operation has been refined include scooters and cars, whose controls and starting procedures are designed for simplicity and safety. User-friendliness often improves through iterative design and testing.

Reliability and safety

Reliability is the probability that a component or machine will perform its intended function without failure for a specified period under stated conditions. By convention, reliability is expressed as a number between 0 and 1:

• 0 ≤ R < 1

To achieve reliable performance, designers must examine details such as expected loading, wear, lubrication, environmental effects, stress concentrations, fatigue and possible misuse. Use of a factor of safety (a conservative multiplier applied to loads or a reduction applied to allowable stresses) is a common practice, but using a factor of safety alone does not guarantee overall reliability. A systems approach, including redundancy where appropriate, protective devices, proper material selection and quality control in manufacture, is necessary to ensure dependable operation.

Safety requires that machines are designed to minimise the risk of injury or damage during normal operation and foreseeable misuse. Safety measures include guards, interlocks, safe operating speeds, controlled release of energy, and compliance with statutory regulations. Manufacturers are legally and ethically responsible for producing machines that are safe for their intended use.

Inspectibility, maintenance, cost and aesthetics

Inspectibility and maintenance are closely linked: design should allow straightforward inspection, maintenance and repair. Provision for lubrication, easy replacement of wear parts, access to fastenings and clearances for tools extends service life and reduces downtime. Regular maintenance schedules and clear maintenance instructions are part of good design practice.

Wear and friction are inevitable where relative motion occurs. Designs should minimise frictional losses and wear by specifying appropriate surface finishes, fits, bearing types and lubrication systems. Excessive friction wastes energy; excessive wear shortens component life and increases cost.

Cost depends mainly on material choice, complexity of manufacture, assembly effort and lifecycle expenses (maintenance, energy consumption, spare parts). Designers must balance initial cost against expected service life and operating cost to arrive at the most economical solution.

Aesthetics and ergonomics influence market success and user acceptance. Even when not strictly necessary for function, good aesthetic and ergonomic design can improve usability and perceived quality.

Practical guidelines for the designer

• Understand the problem completely before proposing a solution; list functional requirements and constraints.
• Use standard parts where possible to save cost and simplify maintenance.
• Perform load and stress calculations with appropriate safety factors and consider fatigue where loads are cyclic.
• Choose materials for the combined needs of strength, toughness, weight and manufacturability.
• Design for manufacturability and assembly to reduce production cost and improve quality.
• Include provisions for lubrication, inspection and replacement of wear parts.
• Ensure safety and compliance with relevant standards and regulations.

In summary, successful machine design requires a balanced assessment of function, loads, materials, manufacturing, operation, reliability and cost. Considering these factors early in the design process and iterating-using sketches, layout drawings and simple prototypes-helps to produce a safe, reliable and economical machine that meets its intended purpose.