Force transmission using 3D printed chain 88

Building the OSM structure using a 3D printer provides an opportunity to print the transmission system in the same process. This process produces a “print to go” structure that could attach the actuators immediately. This fabrication approach reduces the assembly process that was required when using other string types.

The design of the 3D printed transmission system or string depends on the material type and the shape of the string. Three types of 3D printed string were made. The first one is a cylindrical rod made from Tango Plus material, the second one is a square section bar made from combining two materials which are the Tango Plus material and the Vero material, the third one is a cylindrical chain made from Vero material. See Figure 5.5.

Figure 5.5 Three types of 3D printed strings. From up to down, cylindrical rod (Tango Plus material), Tango Plus with Vero segments, and Vero chain.

The first type is made from Tango Plus flx930 material. This material was tested in chapter four and its properties are known. From a tensile test, it seems that the extension of the material is 250- 260% and the yield stress is 0.68 MPa. The breaking load can be calculated by using the stress equation and knowing the cross-section diameter of the rod and the yield stress. The result of the breaking load is (4.45N) and this is a very small load compared with the required load to manipulate the OSM structure which is approximately (20-23N). Furthermore, the high extension value of this material makes it stretch under the load effect and transmit a small amount of force.

The second type shows the same behaviour as the first type when it is tested because the intervals of the Tango Plus material control the bar behaviour. The solid parts that are made from Vero material reduce the extension value of the bar but they do not increase the breaking load. Using the stress equation to calculate the breaking load of the Tango Plus interval, it seems to be (3N). This breaking load is smaller than the breaking load of the first type.

The third type is the chain which is designed to be a cylindrical chain and consists of several links that are connected together. The essential link part is a small rod with 1mm diameter, that is connected with the other rod of another link to form the chain. This design gives high flexibility with very low extension ability. This chain was printed using Vero material. Figure 5.6 shows the shape and dimensions of the chain link, showing the chain printed using Vero material.

Figure 5.6 The design of 3 mm diameter 3D printed chain. (a) Design and dimensions of one element from the chain. (b) final shape after print. Since the 1mm rod that connected every two links in the chain is the weakest part, the breaking load of this rod is assumed to be the breaking load of the chain. The breaking load is calculated analytically using the maximum shearing stress theory (Tresca yield criterion) which predicts that “yielding begins when the maximum shearing stress equals the maximum shearing stress at the yield point in a simple tension test” [74]. The maximum shearing stress is assumed to be equal to the yield stress for the Vero material which is known from its datasheet. Then the load can be calculated by using:

𝝉

=

5-2

Where

τ

F = the load.

By using the yield stress of the Vero material, which equals (50 MPa), the breaking load value is calculated as (39.27 N). This analytical value of breaking load is more than the force that is required to manipulate the 3D printed OSM structure. However, the breaking load was calculated using other methods so as to ensure that the 3D printed chain could be used as a force transmission tool. The second method that is used to evaluate the breaking load is finite element analysis (FEA). SolidWorks software was used to perform the FE analysis with a model of chain link under applied axial load. Figure 5.7 shows the stress distribution results under applied axial loads of 19N and 23N. It seems that the maximum von Mises stress exceeds the yield stress of the Vero material when the load is equal to 23N. A decrease of 41% was noted when comparing this

breaking load to the analytical value. Moreover, this breaking load is nearly equal to the force required to manipulate the OSM structure and that means that the chain could fail during operation.

(a) (b)

Figure 5.7 The finite element stress distribution results under applied axial loads for the 3D printed chain link (a) applied axial load 19N (b) applied axial load 23N.

The third way to determine the breaking load is the empirical method. Six specimens with different lengths (100 mm, 120mm, 150mm and 200 mm, shown in Figure 5.8) were tested on tensile test machinery. The Instron 3369 tensile machine was used for this test. The tensile test machine was calibrated before starting the test and the test velocity was set at 4 mm/min. This movement caused an increase in the pulling load until the specimen broke, See Figure 5.9. The results of the load and extension for the six specimens were plotted as shown in Figure 5.10.

Figure 5.9 (a) Tensile testing machine. (b) The 3D printed chains after testing.

Figure 5.10 Results of tensile testing for the 3D printed chains.

The result of the average breaking load is 4.05N, and the result of average extension at the breaking load for the 100mm length specimens is 4.3mm. This breaking load is small compared with the load required to manipulate the 3D printed OSM structure. The empirical results prove that the 3D printed chain cannot be used as a force transmission tool. Furthermore, these results show that the “print to go” structure cannot be performed because the chain will fail during operation.

To increase the breaking load, two other specimens were printed with the diameter of 4.5mm and 100mm length, see Figure 5.11. The two specimens were tested on tensile test machinery using the same test procedure as the previous specimens. The results are shown in Figure 5.12. The average breaking load is 8.3N which is increased compared with the breaking load of the 3mm diameter chain but it is still below the load required to manipulate the 3D printed OSM structure.

The 3D printed chain gives a high flexibility performance to use as a force transmission tool for 3D printed structure but the breaking load is the issue. Furthermore, there is a big difference between the breaking load calculated analytically and the empirical one. The reason is that the material behaviour depends on the printing process. Therefore, every 3D printed structure showed be tested before using it.

Figure 5.11 The 4.5 mm diameter 3D printed chains compared with the 3 mm diameter 3D printed chain.

Figure 5.12 Tensile testing of the 4.5 mm diameter 3D printed chain.