Design and Analysis of High-Torque, Hydraulic Wind-up Mechanism for
Four-Square Gearbox Test Stand, 08-R8266
Rebecca L. Warden
Inclusive Dates: 11/02/11 – 03/02/12
Background — Testing of extremely high horsepower gearboxes rated up to 40,000 hp with output speeds as low as 118 rpm generates torques up to 4,000,000 ft-lb. Such values are representative of applications relating to submarine propellers, wind turbines and helicopter blades and have long posed significant technical, financial and infrastructure challenges to manufacturers. Four-square-type testing of high horsepower gearboxes uses two identical parallel gearboxes in a back-to-front configuration that connects the inputs and outputs of the gearboxes. In this system, the electric motor is sized to overcome the parasitic losses and desired acceleration rate in the gearboxes. Typically, these losses are only 10 percent of the total circulating power in the system, thus eliminating the need for a motor sized to provide 100 percent of the total power requirement. The function of the electric motor is predominately responsible for controlling the speed at which the system operates. To supply the torque requirement of the system, a torque-inducing mechanism is installed within the system in one of the shafts used to connect the two gearboxes. This mechanism induces the torque required for testing by twisting one part of the system's shaft relative to the other. These mechanisms are currently available for low horsepower and low speed applications, but are not available for high torque or higher speed applications. SwRI was asked to submit a proposal to fabricate a four-square test stand to test an 11,000 hp gearbox used on the U.S. Navy's next-generation hovercraft. To date, no torque-inducing mechanisms have been designed for an application at the torques and speeds required for this application. Smaller torque-inducing mechanisms have not been able to be scaled up in size because of the sealing problems associated with larger components.
Approach — In this project, a design for a 39,000 ft-lb torque-inducing mechanism that can operate at speeds up to 1,800 rpm was developed. The technical challenges associated with a hydraulic windup mechanism capable of meeting these requirements are:
- Maintaining a compact size to prevent rotational imbalances
- High-pressure hydraulic sealing on multiple rotating surfaces
- High-pressure hydraulic sealing from a stationary surface to a rotating surface on a large diameter shaft
Hydraulic rotary actuators use vanes attached to one shaft and opposing vanes attached to a second shaft. Hydraulic fluid is pumped between the two vanes, causing one shaft vane to rotate relative to the second vane, which induces torque between the two shafts. The magnitude of the torque will vary based upon the pressure of the fluid. Both the center vane and the housing will rotate at the same speed. The shafts connected to both the inner vane and the housing vane will be supported by two double taper roller-type bearings. The inner vane and housing vane will sit inside an outer housing. The outer housing will remain stationary and provide a means for the hydraulic fluid to be routed through the device. Two specialty rotary unions are used to route pressurized hydraulic fluid to each of the four chambers via ports in the vanes. These allow for sealing of 3,600 psi fluid and can rotate up to 4,000 rpm.
Accomplishments — The results of this project allowed for the development and demonstration of the technical aspects associated with building high-horsepower wind-up mechanisms, as well as generating costs to fabricate the system. The original project this mechanism was designed for has been delayed by congressional funding mandates, but the information developed during this study has been used for other proposal efforts to clients with similar needs.