Load Cases & Assumptions for Mechanical Engineers

📐 What a Load Case Actually Is

A load case is a design scenario that the design can either succeed in or fail in. A load case is more than just a single number. A load case encompasses all the conditions that the design will experience, i.e. all the forces and moments that will act on it, the constraints that it may experience and the operating conditions.

Load cases are used for design and verification purposes. What are the possible real world scenarios that a part will be subjected to? What happens to the part when it is subjected to these scenarios?

⚡ Why Assumptions Matter More Than Fancy Calculations

An exactly converged FEA model is absolutely worthless if your load assumptions are incorrect. The most severe failures in FEA occur because of wrong assumptions, not incorrect mathematics. The hard engineering is making proper assumptions before constructing the FEA model, not simply converging the solution.

Good assumptions are documented, conservative when unsure, and reviewed by an expert in the field. Bad assumptions are left implicit, overly optimistic, and unsuspected.

🔧 Start With Real Use Not Ideal Use

This is a great engineering rule I recently heard: Design to "design load", but first ask: How will someone abuse this? Too many failures occur in the field when someone designs to perfect usage and ignores real world usage, misuse, wear, etc.

Talk to operators and maintenance personnel. Watch how the real world is exploited to generate realistic load cases. Don't let the engineering model be swayed by overly optimistic sales propaganda or unrealistic test assumptions.

⚖️ Identify Main Types of Loads

Mechanical loads can differ in size, direction, frequency and duration. The effect of the load upon the part can also vary. Loads used in design analysis typically differ in some way and the choice of a loading approach determines the design solution that results. Understanding load differences is critical in part design.

• • Static loads: Constant magnitude and direction—self-weight, pressure vessels, dead loads
• • Dynamic/cyclic loads: Repeated loading—fatigue is the concern (rotating machinery, vehicles, bridges).
• • Impact loads: High cycle loading with stress concentrations and dynamic load increase (e.g. drops, impacts, emergency braking events).
• • Thermal loads: Expansion, contraction, thermal gradients creating stress
• • Environmental loads: Wind, snow, seismic, corrosion over time
• • Combined loads: Multiple loads are acting at the same time; This includes the credible worst case scenario, not the simple mathematical sum of all loads.

🔗 Don't Forget Boundary Conditions

Boundary conditions are how the part is held or constrained. They affect the failure mode as much as the loads do. Boundary conditions can be as important as the loads, and if the boundary conditions are incorrect, the stress results are fiction.

It's a common assumption that supports to a real system are perfectly rigid. Rarely is the stiffness (the measure of compliance) of the real support unknown, but if this is the case then one should try different boundary conditions and observe the change in the result.

🛤️ Follow the Load Path

Drawing the load path helps to trace the route that force takes as it enters a structure and follows it down through the building to the foundation walls, the footprint, or other structural boundaries. Until you have traced the load path, you can easily miss an essential structural component.

• Look for locations where components are joined together (bolts, welds, Pins, Shafts etc).
• Trace through structural members (beams, plates, shafts, shells)
• Follow to supports and constraints (foundations, mounts, ground)
• Check for load sharing and redundant paths

Design failures occur in those instances where a load follows a geometric path (i.e. shape). This occurs in those instances where such a path changes direction or transitions from one material to another. Many failures are due to stress concentrations.

📊 Build a Load Case Set

One load case is not enough. You need a set that covers different combinations of operating conditions. Each load case should represent a distinct scenario with different failure modes or stress distributions.

Much complex documentation is needed for only minor components. Don't put too much work into a simple part. Move on to things with greater risk.

⚠️ Worst Case Must Be Credible

Avoid creating an unrealistic worst case by combining all the worst features together. The worst credible case is the most severe service event that can occur, not a mathematical sum of all independent maximums.

Examine: Is the combination physically possible? Has the combination been observed? Under what operating conditions would the combination exist.

📝 When You Don't Have Good Load Data Yet

You won't start with perfect data. You will have to make some assumptions (record them in your analysis notebook!) and they can be refined as you learn more.

• Use same reference for sizing: Use similar systems for sizing, look at similar designs, industry standards, published data.
• Always select a conservative first estimate (in terms of an appropriate load factor) and then reduce this number as uncertainty is reduced (i.e. as more information becomes available).
• Make every assumption explicit: Document the assumption, justify it, highlight uncertainty.
• Run sensitivity analysis: Change uncertain parameters by ±20% and check that this doesn't compromise the design.

Conservative assumptions in code are always better than optimistic guesses for which no evidence exists that they will work. If you don't know, say so.

🚫 Common Assumption Failures

Most load case errors follow a few common patterns. Watch out for these common pitfalls!

Have someone external to your project review your load assumptions. There is great value in having another set of "fresh" eyes identify potential blind spots.

✓ What Good Load Case Definition Looks Like

A good load case definition is one that is specific enough so that somebody else can actually create the same analysis based on what you wrote for the load cases.

• ✓ Loads are quantified: Magnitude, direction, location, duration with units.
• ✓ Explicit definition of boundary conditions (e.g. support types, restraints, interfaces).
• ✓ Environmental conditions stated: Temperature, humidity, corrosive environment
• ✓ Load Point: The load source is traceable. This can be to reference standards, test data, analysis, industry practice or vendor data.
• ✓ Assumptions Baked In (ABI): Assumptions are documented – so that others can later catch the assumptions that went wrong.

📋 Quick Checklist Before You Analyze

• Is coverage provided for standard use, edge cases, misuse and transients?
• Do my boundary conditions match reality, or am I just assuming fixed supports because it is easy?
• Have all different loads types been taken into consideration such as Static Loads, Cyclic Loads, Impact Loads, Thermal Effects, Environmental Effects?
• Are assumptions made clear and areas of high uncertainty highlighted?
• Have you talked to the operators and maintenance personnel as well as the engineers designing the work flow?