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Custom Designed Couplings Abstract

Number of visits: Date:2015-09-28 14:17

Many applications of rotating equipment do not require much thought about the coupling that connects the driver and driven equipment, but there are many applications that are critical and very sophisticated. These usually require custom designed couplings to accommodate the system. The applications that require custom designed couplings are not limited by horsepower or speed considerations, but are dictated by the need for a coupling that will provide dependable operation under specified conditions.

Steel Mill Applications 
One of the toughest coupling applications in the steel mill industry is for hot roughing mills. These mills are primary mills that supply hot rolled slabs, plates, or coils to many other finishing mills. If the mill does go down, productivity in other mills is greatly affected.
The spindles used on these drives are limited by the minimum roll size used and they must handle very high continuous loads. These spindle couplings must also handle high misalignment angles due to the constant change of material thickness and roll sizes. This job is also made difficult by the high misalignment angles (up to 6 degrees) for and during roll change that must be handled by these spindles.
If the motor torque was the major criteria to select and size these spindles, a gear spindle designer could easily design a spindle to survive. However, history has shown that these spindles must be sized to handle impact loads due to T.A.F. (Torque Amplification Factors). These T.A.F.'s are one of the culprits that break up spindles and shorten their lives. Torque amplification factors are multiplication factors on normal torque seen by the components of a train due to the system's inertias, backlash and loading cycle of the total system.
The key to the recent success with these applications is the simple fact that the strongest and biggest spindles possible are used, rather than the smallest and cheapest.
The difficult applications use gear spindles made of high nickel alloy steels with high core hardness and carburized gear teeth. The use of these high strength alloys does not solve all the problems, however, it significantly increases the life of the spindle and has well justified the initial investment.
It must also be acknowledged that these spindles are getting more tender loving care at the mill from the maintenance crews due to the fact that they are paying closer attention to lubrication, maintenance, and repair cycles.
Aircraft Applications
Helicopter drive trains are mounted on very soft structures which twist and turn under loads. This movement requires a flexible coupling that can operate at high speeds and angles (18,000 RPM - 7-1/2° misalignment). Helicopter gear couplings for these applications are made from high strength nickel alloys with nitrided gear teeth to minimize distortion. These couplings are usually seal lubed and incorporate specially designed seals to retain lubricate. A seal lubed or grease packed gear coupling is used in this type of application because special lubricants can be used in these couplings that can handle the high compressive loads experienced.
Turbine Helicopter Drives
Illustrated in Figure 11A is a simplified drawing of the drivetrain of a typical turbine-powered helicopter utilizing three types of high performance gear couplings. These types are listed and described briefly below: 
1.Main Drive - Connects engine to rotor gear box. Fins on sleeves aid heat dissipation and stiffening effect. 1,650 HP at 6,500 RPM with ± 4° angular misalignment per gear mesh.
2.Accessory Drive - Connects turbine engine driven main gear box to accessory gear box. 130 HP at 6,600 RPM with ± 2° angular misalignment per gear mesh.
3.Tail Rotor Drive - 100 HP at 2,800 RPM with ± 4° angular misalignment per gear mesh. A series of these single engagement couplings are used in the tail rotor drive system to compensate for air frame deflection.
The new technology that is now being applied to helicopters using composite materials and super alloys will require greater demands on couplings for these applications. Couplings for these applications will be required to be smaller and more compact, and they will probably use light weight super alloys and/or composite materials in their construction.
Industrial Gas Turbine
Many Industrial gas turbines are derivatives of the Aircraft gas turbine or use many of the principles. These units operate at a high speed (3600 - 15000 RPM). They usually require couplings with large axial travel capabilities to over 1/4 inch. These applications require couplings that are usually custom designed and use aircraft technology in their design and construction.
These couplings are made of high strength alloys and are manufactured and constructed to specification and tolerances employed in making Aircraft components.

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