LEGO® Joints

Mechanics Laboratory - Spring 2024 - Team Designed Experiment

Purpose

In my Mechanics Laboratory course during Spring 2024, my lab group was assigned a project in which we were to design an experiment to perform and then present. We decided to test the strength of various orientations of lap joints using LEGO® Technic lift-arms and pins. We chose to do this experiment because, naturally, as engineering students, we love LEGO®, but also because we were all simultaneously taking Strengths of Materials, giving us the power to actually mathematically calculate the stresses that the joints and the parts that compose them can handle. Determining which joint orientations are the strongest also informs set designers which to use to maximize playability, the cornerstone of LEGO® Technic kits.

Equipment

  • Universal Testing Machine (UTM) - Tensile Tester

  • Calipers

  • Ruler - 1 ft

Materials

  • 36 LEGO® Technic 1x11 lift-arms

  • 51 LEGO® Technic 1L pins

Orientations

  • 2 Overlap - 2 Connectors

  • 3 Overlap - 2 Connectors 

  • 3 Overlap - 3 Connectors 

  • 5 Overlap - 2 Connectors 

  • 5 Overlap - 3 Connectors 

  • 5 Overlap - 5 Connectors

The Experiment

The Procedure

  1. Assemble specimens

    1. Three trials for each specimen, totalling 18 trials

      Adjust and tighten UTM clamp positions

    1. Each clamp should span 4 holes on both sides for each specimen at zero displacement

    2. The distance between the clamps is dependent on the amount of overlap in the specimen

    3. Tighten using ratchet wrench

  2. Zero UTM for accurate measurements

  3. Start tensile testing on monitor

    1. Adjust speed to 0.5 in/min

  4. Repeat three times for each specimen

Results & Analysis

It was predicted that more pins used (and therefore more overlap) would increase the tensile strength of the specimen, which turned out to be true for specimens with the same amount of overlap. However, it was discovered that the general trend in increasing tensile strength is rooted in the ratio of the number of connectors divided by the number of overlapping holes. Specimens with a ratio of 1 had the greatest average tensile strength, with the breaking force increasing as the number of pins increased. It should be noted that the specimens with a ratio of 0.67 had a lower average tensile strength than the specimens with a ratio of 0.60, which can almost certainly be attributed to experimental error. This can be observed in Figure 2.