Modular design

Modular design is a way of structuring a product in which it is divided into relatively self-contained functional units, called modules, with precisely defined and stable interfaces. Modules can be combined into different variants, developed, manufactured and tested separately, and replaced during repair or upgrade without touching the rest of the product. The same idea is also described as modular product architecture or the building-block principle.

The interface is the core of modular design. It consists of the mechanical connection (mounting points, mating surfaces, hole patterns and their tolerances), electrical and data connectors, fluid and air supply connections, and also the installation envelope the module is allowed to occupy. As long as the interface stays stable, the modules on either side of it can be changed independently.

The opposite is an integral design, in which one component performs several functions and the parts are tightly interdependent. An integral solution tends to be lighter and smaller and, for a single variant in high volume, often cheaper. A modular solution wins when many variants are produced, when individual parts of the product evolve at different speeds, or when service and upgrades over the product's life matter.

In practice, modularity often relies on a product platform: common modules are used in all variants (for example the frame, the control unit or a drive in two power ratings) and are supplemented by customer-specific modules. In CAD systems, variants are handled with configurations, parametric models and a skeleton model that drives the assembly. In PDM or ERP systems they are managed with a variant bill of materials that contains all possible items together with the rules for selecting them (the so-called 150% BOM).

Modular design pays off for product lines with many variants (different sizes, power ratings, equipment levels), for machines and lines configured to order, for long-life equipment where parts are expected to be replaced or upgraded, and wherever fast service by swapping a complete module is needed. It also helps split development among several teams or suppliers, who then only have to agree on the interfaces.

Conversely, for a single-purpose product in one variant and high volume, where weight, size or unit cost is decisive (a handheld consumer product, for example), an integral design is usually the better choice. The decision rests on the number of variants, the changes expected over the product's life and the cost of the interfaces.

The most common mistake is poorly defined interfaces. If an interface changes without engineering change control, the modules stop being interchangeable and parts that do not fit show up in service. Define each interface with a drawing or specification including tolerances, give it a revision number, and keep a record of which module versions are compatible with each other.

Modularity also has a cost: fasteners, flanges, connectors and spare room in the installation envelope add weight and cost, and a common module designed for the most demanding variant is oversized for the base variant. So do not introduce modules where variants do not actually arise, and draw module boundaries according to functions, suppliers and service needs, not according to how the CAD model happens to be structured at the moment.