Torsional Loading Apparatus

Jericho Moll

Summary:

The torsional loading apparatus was created to be used in the Mechanics of Materials Laboratory as an experiment measuring the strain on a pipe caused by torsional loading in the elastic region. The requirements associated with this design were the following: accurate strain measurement, no permanent deformation, short measurement time, inexpensive, low weight loading, transportable apparatus and no small loose parts.

The initial stages of this design were to come up with several conceptual designs which satisfied the initial design requirements. Several conceptual designs were created and drawn to scale, aiding in the evaluation of each concept. Each concept consisted of a pipe that was loaded in torsion by either a loading arm or a disc with a pulley system to apply a load on the pipe. There were two major methods for measuring the deflection and strain in the pipe. These methods were the use of a strain gage, and measuring the angle of rotation of the end of the pipe manually using a protractor. After the best concept was chosen, the design was brought to Mr. Dave Daugherty in the machine shop to change details that would make the machining process less time consuming. When the final, modified design was decided on, the design was drawn in Pro-E. The Pro-E representation of the project can be seen in Figure 1.

The final design consisted of the following: base plate, steel pipe, dowel, steady rest, loading arm, weights, set screws and fixed end block, and strain rosette. The base plate was used to secure all of the other parts together. The steel pipe was the sample that was loaded and the subsequent strain was measured using the strain rosette. The dowel was fitted in a grove to create a key which kept the steady rest square to the pipe. The steady rest was used to support the loaded end of the pipe so the pipe was loaded only in torsion, not in bending. The load was applied at some fixed distance from the pipe using the loading arm. The weights were hung from the loading arm to create the desired torque on the pipe. Set screws were tightened against the pipe to create a fixed end that would not rotate, and to attach the loading arm to the pipe. The fixed end block provided a place to anchor the pipe to resist the rotation caused by the torque on the pipe. The strain gage was applied on the pipe in the form of a 45 o rectangular rosette. This was used to directly measure the strain in the pipe.

The machining portion of this design was done with the help of Dave Daugherty. The entire apparatus was machined out of an aluminum alloy to reduce the weight of the device.

The strain rosette and strain gage accessories were ordered from Vishay Micro-Measurements. The application engineers at this company were instrumental in choosing the correct gage as well as the appropriate adhesive for this application. The gage was applied directly to the steel pipe.

The results of this project were that the strain in the torsionally loaded pipe could be measured directly using the 45 o strain gage rosette. The calculated shear modulus was 11090 ksi. The literature value is 11300 ksi. This corresponds to a 2 percent difference between the two values. This difference is likely caused by the galvanization that is coating the pipe, which may somewhat affect the material properties.