Long spacer-type flexible couplings are used on cooling tower applications to transmit power from the motor to the gear box and accommodate the misalignment that takes place between these two shafts during operation. These long spacer couplings require unique design and manufacturing considerations to assure proper operation on these applications. The center spacers are usually fabricated of tubular construction to minimize weight on the connected shafts and provide adequate lateral rigidity. Typical materials used for these couplings and spacer assemblies were stainless steel and carbon steel with platings or coatings for corrosion resistance. Materials, such as monel, inconel, and beryllium copper, were applied for special corrosive environments. Nowadays, the material of shaft are being changed to the composite since these have many advantages over metal couplings as follows; A. The Higher Strength and Stiffness properties of fiber-reinforced polymers allow rotational components, such as flywheels and gyroscopes, to function significantly faster than those made of metal alloys; thus more energy per unit weight is stored. Composite components also have benign, or non-catastrophic failure modes. B. The decreased moment of inertia for drive shafts by high modules fiber reinforced composites result in higher critical speeds allowing reduced number of intermediate bearings and supporting structures or higher rotational speeds for faster feed rates. C. The Corrosion Resistance is an inherent property of composites by proper resin selection within multi-functional designs. To achieve similar corrosion resistance with metals, such as carbon steel, a polymeric liner or coating is required with expensive pretreatment and bonding processes of higher alloys must be used. D. The Thermal Expansion of Composite materials can be tailored to comply with a broad range of design requirements to minimize thermal stresses. Aramid and graphite fibers have a negative longitudinal coefficient of thermal expansion that must be recognized and accommodated. E. Low Bending Loads on Bearings, No Fretting Failures of Flexible Element and Fatigue Resistant Flexible Element, Light Weight (approx. 1/8 times to metal shafts), Long Single Shaft (up to 40 feet diameter of fan) Many factors must be considered when evaluating the drive system for coupling selection. The basic torque analysis must be performed to determine the coupling size. Motor nameplate ratings for torque and operating speed are usually used with a service factor applied depending on design margin of the specific coupling applied. The equipment shaft size, distance or span between the motor and gear box, and operating speed must be reviewed for proper coupling/spacer sizing. Tubular spacers are applied with varying tube diameters and shell thickness to provide adequate torque transmitting capacity and lateral stiffness under dynamic loading. Many flexible disc couplings may be applied to operate at approximately 70 to 80 percent of the first lateral critical frequency. These couplings require precision straightening and dynamic balancing of the tubular spacers with special close piloting of the coupling component parts and tight shaft to hub connections. It is imperative that these designs have non-wearing pilots to assure dynamic balance repeatability upon installation and removal cycles. Dynamic stability of long floating shaft flexible couplings is critical. Detrimental characteristics can result if the coupling:    * is not properly designed/selected.    * has inadequate pilots or wearing parts.    * has excessive clearance in bores to shafts.    * is not properly dynamically balanced.    * is improperly installed and/or aligned.    * becomes severely corroded or damaged. Other drive system problems can surface from non-coupling related causes such as long shaft overhangs, radial clearance in shaft bearings, bent shafts, improper structural supports, etc. Coupling spacers require special straightening and dynamic balancing procedures for best performance in this application. Most of these spacers are connecting relatively small diameter shafts because of the low torque requirements. These large diameter, thin wall tube members must be accurately piloted across the coupling with a high degree of assembly repeatability. Most coupling components will shift as a result of pilot clearances and this potential change in unbalance should be considered in the application of the coupling. Couplings using pins, body bolts or rebated fits offer best dynamic balance repeatability. Couplings with sliding parts are normally not used on these applications because of the potential offset and resulting unbalance from component wear. The potential offset of coupling components and the dynamic unbalance reaction is easily calculated using mass displacement and force reaction formulas. American Gear Manufacturers Association defines unbalance in Specification 515.2 as mass times displacement from rotating axis of: U = DW, where U = Unbalance (in. lbs.), D = Microinch displacement (in.), W = Weight (lbs.) Any rotating coupling will exert a force on the equipment to which it is connected - this is known as centrifugal force. If the mass of the coupling is evenly distributed about the rotating axis, these forces counteract each other, and the coupling is said to be "imbalance". However, if there is an excess of mass on one side, the centrifugal force on the "heavy" side exceeds that on the "light" side., and the coupling/shaft is pulled in the direction of the heavy side. The magnitude of this force may be calculated from the following equation: F = (W/g) x C x (2 x 3.1416 / 60 x RPM)2 where, W = Unbalance weight (lbs.) g = Gravitational constant (384 in/sec.2) C = Distance of weight from axis (in.) RPM = Speed of rotation Since every action must have an equal and opposite reaction, the force (F) is felt directly at the bearings of the connected machines. For example purposes, we will use the thermal bow as measured at a cooling tower. The spacer tube applied on this application is 8 5/8 inches outside diameter with a wall thickness of 0.188 inches and is approximately 170 inches long. The tubular shafting material is stainless steel. Field measurement showed a 70oF thermal difference from top to bottom or shaded section of tube. The bow was measured in excess of 0.280 inch. See below figure.           The calculated force as a result of the thermal bow is: Weight (W) of center member = 250 lbs. Mass displacement (D) as a result of the 0.280-inch bow = 0.083 inch Unbalance (U) = 0.083 inch x 250 lbs. = 20.8 inch lbs. Total Force (F) = (250 lbs/384 in/sec2) x 0.083 in x (2 x 3.1416 / 60 x 1,200)2 = 850 lbs. total The force at each shaft end will be one-half or 425 lbs. Standard mathematical analysis using an 8-5/8 inch beam x 170 inches long with a thermal gradient difference of 70oF also closely duplicates this field reaction. X = (- gL2/8t) x (T2-T1)= (- 9.6 x 10-6 x 1702 /8 x 8-5/8) x (100 - 170) = 0.281 inch  Where, g = Thermal coefficient for stainless steel (9.6 x 10-6 in./in./oF.)              L = Length of Shaft (inch)             t = Beam thickness (8-5/8 inches)             T1 = Temperature of top section (oF)              T2 = Temperature of bottom of shaded section (oF)  The flexible couplings at each end of the spacer also see angular misalignment as the result of shaft bow from thermal gradient. It may be calculated: q = tan-1 {2 x X/(L/2)} = tan-1[{2 x (- g L2/8t) x (T2-T1)}/ (L/2)] = tan-1{(- g L2/2t) x       (T2-T1)}    = tan-1{(- 9.6 x 10-6 x 1702 /2 x 8-5/8) x (100 - 170)} = 0.0066 radian or 0.379 degree Most flexible couplings have misalignment capabilities within this value. However, when combined with initial setup angular misalignment and axial offset, the coupling may also be damaged. The high unbalance force combined with the resultant angular offset caused detrimental response to the system. Shading the coupling from the sun remedied this problem by stabilizing the thermal gradient.  It is not uncommon for long coupling spacers to react to thermal gradient differences while in static positions. In critical long rotor applications, such as steam turbines and generators, turning gears are used to prevent natural sag from thermal gradient and concentrated weight effects. In the balancing operation on most coupling spacers, thermal stabilization is applied prior to making balancing corrections. It is not uncommon for long shafts to run with higher unbalance/vibration at startup for a short time until temperature gradient around the shaft becomes uniform.  These mass shafts from thermal gradient changes are not a significant factor in most applications but should be considered on long spacer coupling applications where thermal gradients are non- uniform.  In summary, the application and use of long single-span cooling tower couplings provides many advantages. To get the desired results, many up-front considerations should be thoroughly evaluated:    * Normal and peak torque loading    * Operating speeds    * Coupling shaft connections    * Environment    * Shaft misalignments    * Dynamic stability