Blog 2

The team's capstone project will focus on designing, building, and testing an effective test fixture that can run functionally to test mechanical overspeed trip systems installed in gas turbines. Vibrations that occur along the shaft when the mechanical overspeed is tripped will be addressed to achieve low wear on the bearings. The test fixture should be able to support a 250-pound shaft that reaches trip speeds of 8000 RPMS and withstand forces of 8000 pound-force. The shaft experiences a noticeable vibration when the OST is tripped and comes to a full stop. The cause of the vibration is unknown, however, the team has identified two probable causes. One of the key technical analyses that our team will take before executing our final design is measuring the frequency to determine what type of rotor it is. The team will conduct a thorough investigation into whether the current testing rotor that’s been provided is flexible or rigid. This will help better determine the source of the vibrations. A rotor is a rotating component of a machine or mechanical system. Rigid rotors have a high-speed capabilities while experiencing little deformation whereas Flexible rotors have limited-high speed capabilities, being optimized for lower speeds as they have more damping capacity and dissipate more energy which is crucial for vibration control. Diversity Hires will then continue to the next task and a weight and balance testing for the shaft. Another analysis the team will conduct is measuring the balance of the shaft. The first one is the weight and balance of the shaft. As the overspeed trip is triggered, the plunger extrudes out, indicating the system needs to come to a stop as shown in Figure 1. 


Figure 1: Drawing of overpseed trip system 


As the plunger is fully extruded out of the overspeed trip system, which is mounted on the shaft, there is a shift in balance and weight. Having this unbalanced can lead to vibration occurring along the shaft. The team must take calculations of the weight and balance and compare them to the international standard 1940 table. Balancing a shaft to mitigate vibrations involves ensuring its center of mass aligns with its axis of rotation. This is achieved through techniques like static and dynamic balancing, as well as computerized balancing systems as shown in Figure 2. These methods determine the precise location and amount of weight needed to counteract imbalances, reducing vibrations in machinery. Balancing is critical for high-speed applications, optimizing performance, reducing wear and tear, and enhancing safety and energy efficiency. Conducting both of these analyses will assist the team in evaluating the feasibility of utilizing ball element bearings to support the shaft and reduce vibrations during deceleration. 


Figure 2: Balanced shaft 

To enable informed design choices, the team has pinpointed significant constraints to ensure that the project aligns with the essential criteria for achieving a successful solution as detailed in Table 1. 


Major Constraints 

Measured constraints 

Size 

7x7x2 Feet 

Support Capability  

250-pounds

Budget 

$10,000

Bearings Type 

Oil Free 

Table 2: List of constraints 

 One significant constraint involves the selection of bearings that minimize the need for excessive oil or lubricants within the system. Using fluid film bearings can result in significant drag torque and power losses, especially during startup and landing phases before achieving lift-off, potentially causing surface damage. Additionally, when bearings operate well above the rotor-bearing system's critical speed, there's a risk of inducing hydrodynamic instability, leading to a loss of effective damping. The team has been allocated a dedicated space on the manufacturing floor measuring 7x7x2 feet to construct the test fixture, which must support a 250-pound shaft. Having this restricted space poses a challenge to the team as the select components should not take up as much space. All equipment within the fixture must meet weight specifications to withstand shaft-induced stress. The project budget, including sensors, materials, tools, and machinery, is capped at $10,000. The team will communicate with various vendors and explore options that will fit within the budget constraint.  The team will create a comprehensive set of project requirements to further define critical characteristics necessary for a successful solution beyond the mentioned constraints.

Some soft challenges that our team has identified include safety. The team has made safety a top priority in our design as we want the operators to be safe while operating the test fixture. As the test fixture is supporting a shat that reaches a max speed of 8000 RPMS, the risk of parts or components flying off is high. To combat this, the team has decided to implement an enclosure that encapsulates the shaft while it's operating. To allow for the operators and engineers to physically see how the shaft operates through testing, a ballistic glass. Ballistic glass is used on CNC machines and is resistant to heat. Additionally, it will provide safety for the operators to view as they can be protected if there are components or small pieces that do fly. The enclosure will be made out of a matrical that is not only durable but also has a high tensile strength. This will serve as extra protection for the operators.   

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