Here we present a protocol to develop a pure uniaxial loading machine. Critical design aspects are employed to ensure accurate and reproducible testing results.
The information in this video is aimed at explaining some of the more important aspects of developing mechanical loading platforms. The information is intended for researchers who may be interested in developing their own platforms, but unfamiliar with some of the basic principles involved. This video is intended to demonstrate the two key features involved in developing a reliable testing platform, specifically, reproducible assembly and disassembly, and accurate alignment.
Demonstrating the assembly of the platform will be Mr.Robby Thoerner. Robby Thoerner recently received his undergraduate degree at the University of Akron, from our department of biomedical engineering in the biomechanics track. The goal is to assemble the testing device.
Here is an example of an assembled device which has two actuators, load cells, and a frame to support them. Gather the parts to construct the device. Use two programmable actuators, each with 30 millimeters travel, 60 millimeters when working together.
Daisy chain the actuators to sync them for equal extension and retraction. Next, prepare two load cells suitable for confined spaces. To support the actuators, prepare one carriage for each of them along with a rail.
Create the base from aluminum stock. Cut a piece to the appropriate size and use a mill to machine the plate accordingly to specifications. Machine two side plates according to specifications.
The two should mirror one another, and each should have a slot and drilled holes to accommodate the rail. Drill and tap the bottom faces of the side plates. Mount the side plates in the track on the base.
Be sure the mount points for the rail are near each other. Fasten the side plates to the base plate from underneath. Next, fasten the rail to the track using its clearance holes and the drilled holes in each side plate.
When done, the rail connects the two side plates and carriages slide onto the rail. Move on to work with the mounts for the actuators. Use L-shaped stock to machine a rear mount attachment and a bar to attach to the bottom of the mount.
This creates a key for alignment in the track. Screw the bar into the bottom of the mount. At this point, get the selected actuator for the experiment.
Attach the rear mount to the body of the actuator using the hole pattern in the actuator. The series of holes in the side plate allow the mount position to be adjusted. Slot the base of the mount to the frame to attach the rear actuator.
Use two screws to secure it. Duplicate the steps to mount an actuator on each of the side plates. Get the parts for supporting the front of the actuator.
One part is an L-shaped piece with a hole for a tapered connector. On its side is a track to accommodate a plate. The plate for this track also has a track to accommodate fixtures and ensure alignment.
Machine an aluminum cylindrical part to connect the load cell and actuator. Machine an aluminum tapered cylindrical part to connect the load cell to the fixture and carriage. Pass the cylinder into the hole of the front actuator mount and use a set screw to anchor the cylinder end.
Now, work with the fixtures for the front mounts. For vertical adjustments, machine a central vertical slot in the fixture holder. Attach the actuator front mount to the rectangular side plate, with the vertically aligned holes in its center.
Here is the completed assembly for the front mount on one side. Duplicate the steps to mount both the left and the right actuators of the device. These are representative data sets from load testing knotted 25.4 millimeter 2-0 sutures until failure.
The gray dash curve represents results from the device described in this protocol, using a loading rate of 0.61 millimeters per second. The black curve represents results from a fixed-end device using a double loading rate, or 1.22 millimeters per second. In all tests, failure occurred at the knot, and measurements showed no statistical difference between the devices.
In designing a loading machine, it is always important to design keeping alignment in mind. Focusing on alignment will make sure that your results are reproducible and accurate and that you're testing your specimens appropriately. In this video, we've tried to demonstrate some of the more important aspects of designing loading machines.
If you understand some of these basic principles, you can apply them in developing platforms for you own testing needs.
