Application of Design Aspects in Uniaxial Loading Machine Development
Summary
Here we present a protocol to develop a pure uniaxial loading machine. Critical design aspects are employed to ensure accurate and reproducible testing results.
Abstract
In terms of accurate and precise mechanical testing, machines run the continuum. Whereas commercial platforms offer excellent accuracy, they can be cost-prohibitive, often priced in the $100,000 – $200,000 price range. At the other extreme are stand-alone manual devices that often lack repeatability and accuracy (e.g., a manual crank device). However, if a single use is indicated, it is over-engineering to design and machine something overly elaborate. Nonetheless, there are occasions where machines are designed and built in-house to accomplish a motion not attainable with the existing machines in the laboratory. Described in detail here is one such device. It is a loading platform that enables pure uniaxial loading. Standard loading machines typically are biaxial in that linear loading occurs along the axis and rotary loading occurs about the axis. During testing with these machines, a load is applied to one end of the specimen while the other end remains fixed. These systems are not capable of conducting pure axial testing in which tension/compression is applied equally to the specimen ends. The platform developed in this paper enables the equal and opposite loading of specimens. While it can be used for compression, here the focus is on its use in pure tensile loading. The device incorporates commercial load cells and actuators (movers) and, as is the case with machines built in-house, a frame is machined to hold the commercial parts and fixtures for testing.
Introduction
Mechanical testing has an interesting history that can be traced back to hardness testing equipment developed by Stanley Rockwell in the early in the twentieth century. While technology has grown to the extent that standard, documented practices guide everything from the verification of machine performance to the guidelines for carrying out specific tests1,2,3,4. Today, mechanical tests are conducted on everything from building materials such as concrete, steel, and wood to food and textile products5,6,7,8,9. Given that the fields of biomedical engineering and, more specifically, biomechanics utilize mechanical testing, loading machines are commonplace in biomechanics labs.
Loading machines run the range of scale in biomechanics. As an example, larger loading machines can be used to conduct full-body impact studies or determine human femoral mechanical properties, while smaller loading machines can be used to test murine bones or stimulate cells10,11,12,13,14. Two types of loading machines are found in the testing laboratory; those that are purchased commercially and those that are built by the user. Loading machines developed in-house are often favored for their personalization and customization options15.
In testing, a specimen is secured in the machine so that a displacement can be applied, generating a measurable force. If the load is used as the driving feedback, the test is load-controlled; if the displacement is used as the driving feedback, the test is displacement-controlled. Loading machines, in general, are built upon a frame that connects a mover to a fixed support. As such, testing generally involves one end of the specimen being moved while the other end remains fixed.
Shown in Figure 1 is a sketch of a simple loading machine demonstrating its basic components. Fundamental to all loading machines is a base or frame. Whereas the vast majority of commercial brands utilize a fixed base, the drawing depicts a platform that allows planar (XY) movement. The mover, in this case, is the upper arm that holds a load cell and is driven by a stepper motor. Attached to the frame are the fixtures which hold the specimen and dictate the type of test that is run. Shown in the drawing are three-point bend fixtures. The top fixture (the single contact) is mounted to the moving arm; the bottom fixture (the double contact) is mounted to the stationary base. During testing, the motor drives the upper fixture downward to where the center contact engages the specimen. As the contact engages the specimen, the load cell records the increase in resistance or the force placed upon the specimen.
There are occasions where machines are designed and built in-house to accomplish a motion not attainable with the existing machines in the laboratory. Here we describe in detail one such device. It is a loading platform that enables pure uniaxial specimen loading or equal and opposite motion at both ends. The device incorporates commercial load cells and actuators (movers); a frame is machined to hold the commercial parts and loading fixtures for specimen testing. Understanding the basic principles of testing machine construction can aid in the design of one's own machine. We have provided the drawing files we created as a starting point to assist researchers with their own machine development. The video will focus on the assembly of the device and the application of mechanical design principles to ensure alignment and reliable testing.
Protocol
Representative Results
Discussion
The goal of this work was to design and fabricate a cost-effective and reliable uniaxial loader for its use with small-scale specimens such as tissue and fibers. A device was constructed that met the requirements set forth while also being flexible enough in design to allow for new attachments to be fabricated as the user needs grow. For example, the device will allow for the testing of dry and wet specimens in a uniaxial or fixed-end configuration.
Critical steps in the design and fabrication…
Disclosures
The authors have nothing to disclose.
Acknowledgements
This work was supported by the National Institutes Health NIDCR [DE022664].
Materials
| Power supply, 24 V DC 2.5 A out, 100-240 V AC in, plug for North America | Zaber Technologies inc | PS05-24V25 | |
| 6 pin mini din-male to female PS/2 extension cable | Zaber Technologies inc | T-DC06 | |
| Stepper motor controller, 2 phase | Zaber Technologies inc | A-MCA | |
| Linear actuator, NEMA size 11, 30 mm travel, 58 N maximum continuous thrust | Zaber Technologies inc | NA11B30 | |
| Corrosion resistant maintenance-Free Ball Bearing Carriages and Guide Rails | McMaster-Carr | 9184T31 | |
| 6061-t6 Aluminum Stock | McMaster-Carr | NA | |
| Plexiglas Stock | McMaster-Carr | NA | |
| Canister load cell, 4.5N | Honeywell Sensotec | NA | |
| USB to 6 pin mini-din | Universal | NA |
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