Goal: The torsion testing device had a primary goal of providing accurate torsion testing devices through a budget friendly design, thus design decisions were made to optimize performance while reducing cost.
Design
The device is designed to hold rods made from different materials with a maximum diameter of 1/8”. The rod must have a 90 degree bend at both ends pointing in the same direction. The length of the bent parts can range from 2 cm to 5 cm. The part of the rod which undergoes torsion has a length of 19.5 cm. The device performs the torsion test by firmly holding the rod in place at one end while holding and twisting the other end using a pulley attached to a weight. The device is able to made almost entirely out of acrylic in order to be cheap and easy to manufacture, with nuts and bolts required to fasten the parts together.
New pieces were required upon redesign considerations. The most notable and important feature added was the 90 degree aluminum plate that has shoulder bolts inserted that support the weight of the pulley disks. This redesign was the result of bending caused by the weight of the pulley being supported solely by the test piece. By supporting the weight of the pulley with the shoulder bolt and 90 degree aluminum, no bending will exist and torsion will be the only factor that causes deformation.
Base Design
The base of the device needed to be stable and allow the end of the rod to be firmly held in place while also allowing the length of the rod to rotate freely. The end of the rod is securely held in place by the top plate (transparent) and the base plate (red). The small rectangle and L-piece (both orange) ensure the rod is unable to shift along the plate in either direction.
There are also two long pieces (yellow) that run parallel to the length of the rod. These pieces act as rails to ensure that the rod only undergoes torsion and not deflection. These pieces do not firmly clamp the rod however, as this would introduce friction and not allow the rod to freely rotate.
In order for the clamping mechanism to be capable of holding rods of varying diameters, slots were made in the pieces that sit on top of the base plate so that the space in which the end of the rod fits is adjustable.
The base plate is supported by four legs that are secured by nuts both above and below (only pictured on left side). This allows the plate to be raised and lowered as required for the assembly process. It will also be clamped down to the table using a C clamp during the actual test to ensure stability.
Pulley Design
The pulley is made up of three acrylic discs that are 1/8” thick and held together by up to 4 screws (can be replaced by pins). The acrylic disc in the middle (blue) has a smaller diameter than the outer discs (orange and purple), creating a space for string to be wound up. The middle disc has a slot that can fit rods with diameters as large as 5mm. The rear disc has a hole near the edge so that string can be tied to it. The first disc (orange) also has protractor markings etched into it. This allows students to determine the amount of angular deflection more easily.
One of the challenges for this design was to come up with a way for the pulley to be able to securely hold rods of varying diameters. The solution for this is an L-shaped piece (pink) with slots in it. The slots in the middle disc and front disc (orange and blue) have a width of 5mm. In order to firmly hold smaller specimens, the L-shaped piece has diagonally positioned slots which allow it to push the rod into the top corner of the slots in the front disc and middle disc. This allows smaller diameter rods to stay as close to the center of the discs as possible. Also, the corner of the slots in the front and middle discs is offset from the center of the discs by 2mm in the x and y directions. The reason for this is to ensure that the center of the disc is as close as possible to the center axis of the rod. This allows the pulley to spin as concentrically as possible.
90 Degree Aluminum Redesign
The 90 degree aluminum plate (green) hangs over the edge of the table and sits as low on the feet support as possible. All of its holes are threaded, allowing the shoulder bolts (black) to screw in. The shoulder bolt covers (white) support the weight of the pully disks while still allowing it to rotate about the shoulder bolt.
The SolidWorks model of this device can be found on GrabCAD.
Manufacturing
Acrylic Prototype - Lasercutting
Since this device is made of acrylic pieces, the manufacturing process of the device consisted mostly of lasercutting. All of the pieces were made from lasercutting 3 mm thick sheets of acrylic. When cutting parts that need to mate together, it is important to consider how lasercutting affects the measurements of the piece. The laser is able to cut pieces by melting the material along the specified lines, which causes a certain thickness of the material to be lost. This means that holes made by the lasercutter will be slightly larger than what was specified in the drawing, and extrusions will be slightly smaller. If a hole is dimensioned to have an exact fit with a screw (the diameter of the hole is equal to the OD of the screw), the fit will be noticeably loose. This was important to consider specifically for the holes on the base plate and disks that needed to be tapped, as they were cut smaller than the final design to allow for the tapping. The protractor design was also etched into the front disc using the lasercutter.
(more on lasercutting can be found here)
Aluminum Prototype
One of the goals for the Winter 2022 term was to build a functional prototype out of aluminum. Using aluminum as the material allows for more durability for the device and will ensure that they will be reusable/break less (from dropping, cracking, etc.). The aluminum used was 1/8” thick and was waterjet cut. Some of the holes (mainly on the base plate and the front disk) were also tapped for the appropriate screw size. The aluminum prototype was not successful due to the lack of support for the weight of the pulley, and thus redesigns were warranted and only a new acrylic prototype was made.
Preparing the Specimen
The rod which was used to make the specimen was cut into several pieces measuring 23-29 cm long using a hacksaw. Each of these pieces will be made into a specimen that can be used for the torsion test
The ends of the rod were bent 90 degrees using a metal brake. The bent ends of the rod should be equal in length, and the length of the middle (the part of the rod which will be under torsion) should be 19 cm. (Verify with every prototype build)
A list of all materials used can be found in the following document: https://uofwaterloo.sharepoint.com/:w:/r/sites/tm-eng-engineeringideasclinic/Shared Documents/Materials Design Activities/Torsion Testing Device/Winter 2022 Design Iterations.docx?d=w5534f0c92d07439195817db7b1fe711b&csf=1&web=1&e=wuA3hh
Next Steps
Design an aluminum prototype that is capable of completing a torsion test until the test material fails.
Incorporate a more accurate method of measuring the angle of deflection. Improve on the current method by introducing a needle or marker to measure the angle from and by adding more accuracy to the protractor engraving.
Digitalize the reading of the angle of deflection using a rotary encoder.
Look into making the device more compliant with ASTM A938 torsion testing standards.