A good mechanical design is not just about good ‘look and feel’; it is also about good performance, ease of manufacture, and cost-efficiency. It includes a robust mechanical assembly design that enables smooth production and scalability. Other factors that must be taken into account are ease of maintenance, repair, functional integrity, etc. Even the best designs face mechanical assembly challenges. However, experienced professionals know how to mitigate the challenges that lead to delays, numerous design iterations, or even outright failure.

What are the most common pitfalls in mechanical assembly design, and how do engineers mitigate them? Let’s understand…

1: Overlooking Design for Assembly (DFA) : 

DFA focuses on making assembly easy and quick. Hence, simple component geometry, a minimum number of parts, and improved ease of handling are important.

Common assembly design pitfalls if the DFA is ignored.

Ignoring design for assembly during the early design phase leads to higher assembly times, increased costs, increased risk of assembly mistakes, etc. This affects profitability and slows down the time to market. 

How to avoid the pitfalls?

• Design parts that are easy to assemble
• Minimise part count and use self-locating features.
• Standardise fasteners and components.
• Conduct DFA reviews at the concept stage, not the post-prototype stage.

2: Ignoring Tolerance Stack-Up

Manufacture and assembly need precise measurements; however, some variations are allowed/tolerated depending on the application and its requirements. But when many components fit together, tiny variations in individual parts accumulate and may cause performance issues. Hence, tolerance stack-up analysis should not be overlooked.

What happens if tolerance stack-up analysis is ignored? 

Parts either fit very tightly or very loosely and start rattling; such tolerance stack-up issues result in costly redesign, unplanned downtimes, and assembly failures.

How to avoid the pitfalls?

• Use tolerance analysis tools (e.g., worst-case or statistical methods).
• Collaborate with manufacturing teams to define realistic tolerances.
• Apply GD&T (Geometric Dimensioning and Tolerancing) standards effectively.
• Prioritise critical interfaces and verify them through simulations or prototypes.

3: Inadequate Fastener Selection 

The type, size, material, and placement of the fasteners may vastly impact the integrity of the assembly. Fastener selection mistakes include the wrong choice of material, inappropriate torque requirements, or mismatched threads and heads. 

Result?

Choosing the wrong fasteners can cause loosening, performance issues, vibrations and rattling, and structural failure. This means slower production, higher costs, and risk of damage during installation.

How to avoid these pitfalls?

• Match fastener material with surrounding parts to avoid galvanic corrosion.
• Consider ease of access during installation and servicing.
• Use thread-locking solutions in case of possible vibration.
• Maintain a database of approved fasteners for different use cases.

4: Poor Material Compatibility 

Material selection is key to a high-performing mechanical design. Choosing a material that is not compatible for either manufacturing, assembly, or its intended application can be a costly mechanical design mistake.

Impact?

An incompatible material may cause thermal heating, cracks, warping, or corrosion, thus compromising the product’s performance, quality, and life.

How to avoid material compatibility issues?

• Consult compatibility charts during material selection.
• Use isolating washers or coatings where dissimilar materials must contact.
• Simulate environmental conditions to ascertain compatibility.
• Choose materials with similar expansion coefficients where possible.

5: Neglecting Maintenance & Serviceability

When products are high ticket, users invest only when they are sure of regular maintenance and quick servicing. This is only possible if components are easy to repair and parts that need replacement are easy to procure. 

What is Impact?

If servicing requires complete disassembly or specialised tools, field technicians may struggle or avoid maintenance altogether. This may lead to premature product failure or warranty claims, loss of reputation, and decline in repeat orders.

How to make maintenance and serviceability easy?

• Design assemblies with removable covers, snap-fits, or quick-release mechanisms
• Use modular design principles to isolate serviceable units.
• Provide clear marking, orientation aids, and documentation
• Include feedback from service teams during the design phase
  

6: Ignoring Real-World Manufacturing Constraints (DFM)

DFM is a strategy where manufacturability is a key consideration right from the early design stage. The focus is on easy, cost-effective manufacturing. Ignoring this means that the designs are too complex, too fine, or too expensive to produce and lack practicality.

Impact?

Manufacturing becomes tedious. There may be significant setbacks, like designs getting sent back to the drawing table, expenses shooting up, unplanned downtimes, and product launch getting delayed.

How to avoid this pitfall?

• Collaborate with manufacturing partners early in the design cycle.
• Stay updated on the capabilities and limitations of fabrication technologies.
• Avoid tight tolerances unless necessary.
• Design with standard tooling sizes and shapes in mind.

Conclusion & Key Takeaways

Effective mechanical assembly design is more than making parts fit together. It requires a strategic balance between functionality, manufacturability, serviceability, and cost. Mechanical design mistakes and pitfalls ranging from DFA oversights to ignoring DFM constraints are not uncommon, but they are avoidable.

Here’s what to remember:

• Start early with DFA and DFM reviews.
• Analyse tolerances before prototyping.
• Choose fasteners and materials wisely.
• Design with the end-user and service team in mind.
• Collaboration across teams, design, manufacturing, and quality assurance must work in sync.

Abiding by this checklist during the conceptual and development stages saves significant time, money, and effort down the road. It results in robust products, smoother production cycles, and ultimately, a better experience for the end user.