Machine design draws on a relatively small, standardised set of reusable mechanical components, machine elements, that show up again and again across an enormous variety of otherwise completely different machines. Gears, bearings, springs, fasteners and shafts each perform one fairly narrow, well-understood mechanical function, transmitting rotation, reducing friction, storing energy, holding parts together, and a huge share of mechanical engineering design work consists of selecting and combining the right elements from this shared toolkit rather than inventing an entirely new solution for every new machine.
Each machine element solves one narrow, well-characterised mechanical problem
A gear transmits rotational motion and torque between shafts, often changing speed or direction in the process; a bearing lets two parts move relative to each other, typically rotating, while minimising friction and wear between them; a spring stores and releases mechanical energy through elastic deformation; a fastener holds separate components together, ideally in a way that can be undone and reassembled if needed. Because each of these elements is designed around one relatively narrow, thoroughly studied mechanical function, engineers can draw on decades of accumulated design knowledge and established formulas for each element type, rather than working out its behaviour from scratch for every new application.
Standardisation lets engineers combine familiar parts instead of reinventing them
Because machine elements like gears, bearings and fasteners are produced to widely shared, standardised specifications, an engineer designing a new machine can select an off-the-shelf element with well-documented, reliable performance characteristics, rather than having to design and manufacture a custom component for a role countless earlier machines have already solved. This standardisation is exactly what lets machine design proceed as a process of thoughtfully selecting and combining known, well-characterised elements to meet a new machine's specific requirements, rather than starting completely from first principles with every new mechanical design.
What we're still unsure about
The basic function of core machine elements, and their central, well-documented role in mechanical design practice, are well established, foundational mechanical engineering, confirmed across an enormous body of applied engineering work. What's more genuinely an ongoing area of applied research is developing machine elements capable of reliably operating under increasingly extreme conditions, very high speeds, temperatures or loads, that push beyond what existing standardised designs and materials can comfortably handle — engineers continue actively developing new element designs and materials for these more demanding applications, without every extreme operating scenario yet having a fully mature, off-the-shelf standardised solution available.
This sits inside Machine Design & Mechanical Elements, one of eight topics in Mechanical Engineering, one of four domains in Engineering, one of seventeen subjects the app can quiz you on.