A coupling is a mechanical component that is used to firmly connect the driving shaft and driven shaft in different mechanisms together, rotate together, and transmit motion and torque. It is also sometimes used to connect shafts and other parts (e.g. gears, pulleys, etc.). It usually consists of two parts, which are connected by a key or clamping fit, respectively, and fastened at the two shaft ends. Couplings can compensate for deviations (including axial, radial, angular or combined offset) between two shafts due to inaccurate manufacturing and installation, deformation or thermal expansion during operation, as well as shock and vibration absorption.
Advantages of Universal Joint Universal joints have always been recognized for their inherent advantage of large misalignment capabilities upward of 15° compared to a maximum of approximately 6° for typical gear-type spindles. In recent years, research and improvements in design and manufacture have contributed to substantial increases in the torque capability of universal joints. Achievement of power transmission densities that allow universal joints to meet rolling mill diameter limitations has increased their competitiveness with gear spindles. Today, U-joints are generally available with power densities equal to or greater than geared spindles, especially when operating misalignment angles exceed 1½°. The advantages of universal joints include: • Consistent operating characteristics. • Reduced backlash and runout. • Reduced maintenance. • Elimination of possible mill chatter. • Lower operating costs. Operating characteristics - U-joints utilize a cross and bearings instead of engaging gear teeth for the flexing component. Gear spindle teeth are constantly sliding and wearing during their operational life, which can have significant effects on the life and capacity of the gear spindle. U-joint bearings roll and have minimal and relatively predictable wear during their operating life. The torque capacity of gear spindles decreases as mis- alignment increases because fewer teeth are in full contact to share the load. U-joint torque capacity is not significantly influenced by the misalignment angle, and reduction in bearing life is statistically predictable regardless of operating angle.Backlash and runout - Gear spindles, by design, must have 0.020 to 0.100 in. of backlash in order to flex-this backlash increases as the spindle wears.U-joints typically have less than 0.003 in. of backlash, which does not increase until after complete bearing failure.Gear spindles typically have 0.020 to 0.060-in. runout as a result of heat treatment distortion (which will increase due to wear). U-joints are assembled with virtually no radial runout.Maintenance - Gear spindles should be lubricated weekly for optimum life. U-joints typically require only monthly Relubrication.Gear spindles typically expel small amounts of contaminated grease as a result of wear.Since bearing seals are not subjected to eccentric loading regardless of operating angle, U-joint seals have long service life and expel comparatively little grease. Mill chatter - The use of gear-type spindles can, under certain conditions, provide a vibration forcing function that causes mill chatter and resultant product quality defects.3,4 Operating costs - Life expectancy for U-joints is 3 to 6 times that of gear spindles (less change-out frequency). Initial cost of U-joints is generally 1.5 to 1.8 times the cost of gear spindles. Additional cost reductions include: maintenance and repair; lubricant quantity; and clean-up.
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