ABRASIVE-COATED MID-BODY COUPLING FOR IMPROVED TORQUE TRANSMISSION
Publication Date: 2013-Jan-02
The IP.com Prior Art Database
The present invention relates generally to maximizing the efficiency of steam turbines, and more particularly to improving torque transmission in steam turbines with rotor body couplings. The invention described herein proposes to increase the static friction force between mating surfaces of rotor parts at the mid-body couplings by increasing a static friction coefficient rather than the contact force. One of the mating surfaces of rotor parts is coated with an abrasive material, for example Cubic Boron Nitride (CBN or c-BN), or BORAZON®, in accordance with the present invention.
The present invention relates generally to maximizing efficiency of steam turbines, and more particularly to improving torque transmission in steam turbines with rotor body couplings.
Parts of rotors of steam turbines are made up of different materials (e.g., A16Intermediate Pressure (IP) Low-Pressure (LP) rotor) and are typically bolted together at or through mid-body couplings. Torque is transmitted through connecting bolts of the mid-body couplings and subjects mating surfaces of rotors parts bolted together by the connecting bolts to friction.
Rotors of steam turbines are subject to life limitations due to creep relaxation and thermal fatigue due to high temperatures. At high temperatures, the connecting bolts undergo creep relaxation and the compressive force at the mating surfaces of rotor parts diminishes over time. Creep relaxation reduces contact force between the mating surfaces and the static friction force. Thereby amount of transmitted torque that can be endured by the connecting bolts increases. In addition, if the connecting bolts have a higher coefficient of thermal expansion compared to either one or both of the mating rotor parts, bolt force decreases at high temperatures, thereby reducing friction between the mating surfaces of rotors parts.
Conventional solutions ensuring optimal torque transmission over product life of rotors use diverse designs and materials. For example, one conventional solution includes use of creep-resistant bolts. Another solution suggests use of curved couplings and spline disks. However such curved couplings and spline dusks are relatively expensive. Another conventional solution suggests maintenance of small clearances between bolts and bolt holes to minimize potential of circumferential slip during instances of low surface friction.
Yet another conventional solution relies on bolt load to generate contact forces and acceptance of the friction coefficient of bare metal rotor surfaces. However, such conventional solutions relate to friction reduction, friction damping, or friction welding and fail to provide improvement in torque transmission through the mating surfaces of rotor parts.
Further, gas turbine rotors are through-bolted disk assemblies. In gas turbine rotors, the disks are in contact and rely on friction to transmit torque. Several existing techniques to improve torque transmission in gas turbine rotors involve increasing friction coefficients. For example, friction in the gas turbine rotors may be increase by application of both wet and dry grits and plasma coating to the rotor surfaces. Unfortunately, the disks operate at different temperatures and have different thermal growths. In order to simulate the effect of various friction enhancements, a combination of data-matched rotor Finite Element Analysis (FEA) and lab testing of specimens is done. Though the static friction is high, the relative movement be...