Hydra Dyne: Hydraulic Analysis

Undergraduate Capstone project:

Background:

In my fourth-year undergraduate capstone project, my team and I have worked with a hydraulic equipment supplier named Hydra Dyne. The company has asked us to investigate how torque-induced loading and external pressure forces influence the piston integrity of their hydraulic equipment. They were unsure how the hydraulic fluid pressure affected the piston components in the assembly. The company asked us to model the stress behaviour of the members (coloured pieces, see Figure 1) and analyze the factor of safety of each component under maximum loading.

Bolt Connection:

The members found in Figure 1 slide onto the spud and are tightened in place by a nut. This device is identical to a bolt and nut connection. As the spud (or bolt) is tightened, tension develops between the spud head and the top of the nut. This tension causes the spud to stretch a small amount, like pulling on a spring. And like a stretched spring that tries to return to its relaxed state, the stretched spud attempts to relieve the tension by returning to its original length. The result is a compression, or clamp force, which pulls the spud head and the nut towards each other, clamping the members (or joint) together (see Figure 2). However, applying an external load (fluid pressure) will increase the clamping force acting on the members and decrease the tension in the spud. This relationship is not widely understood because the load distribution between the rod and the members due to the external load is non-linear. If you’re interested to learn more about this topic, visit this website.

Hardware simplification:

To accurately model this problem like a bolted connection, the complex grooves of the joint members were simplified to become flat. The problem is that those complex geometries distribute their mass differently from other components in the assembly. To accurately find the outer diameter of all the members about one common axis, the planar moment of inertia is calculated. The planar moment of inertia provides insight into the cylinder's resistance to bending due to the applied force about a common axis. The final simplified members are cylindrical pieces that are much easier to model and work with (see Figure 3).

Structural & Finite Element Analysis (FEA):

An FEA model requires input values for the forces applied to the system. I calculated the axial load due to the fluid pressure and the torque. Then I found the factor of safety of each component based on its material property. The axial torque and pressure force cannot be assumed to act entirely on the members or the spud. The distribution of the axial pressure force is nonlinear. It depends on the stiffness of the members relative to the spud. Stiffer components will carry the majority of the load, and therefore will experience higher stresses. Therefore, I also calculated the stiffness of the members and the spud by hand and demonstrated how the force distribution was different. The FEA model (Figure 4) was created by applying the calculated axial forces and revealed that our assumptions were correct.

Physical Testing:

To verify our findings, we manufactured and tested the hydraulic components under the same loading conditions. Strain gauges and LabVIEW software were utilized to extract the required data. In comparison, the FEA simulation, hand calculation, and physical testing results are within 5% error. We demonstrated to the client how their hydraulic cylinders performed and confirmed that it is safe to use.