Keys and couplings

Keys and couplings - Chapter 11 Keys, Couplings and Seals...

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1 Chapter 11 Keys, Couplings and Seals Material taken for Mott, 2003, Machine Elements in Mechanical Design Keys z A key is a machinery component that provides a torque transmitting link between two power-transmitting elements. z The most common types of keys are the square and rectangular parallel keys. With the square being the most common. The rectangular key is most common in either very small or very large shafts. z The key width is nominally ¼ of the shaft diameter. Parallel Keys z The key seats in the shaft and the hub so that exactly ½ of the key bears on the shaft and the other ½ bears on the hub. z The parallel key is installed on the shaft, before mating the hub. The hub is slide over the key to provide the interface. Mott, 2003, Machine Elements in Mechanical Design
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2 Key Shaft vs. Shaft Diameter Mott, 2003, Machine Elements in Mechanical Design Dimensions for Fabrication z Keys are usually cut with a end-mill or circular-mill cutters and leave square corners which create stress concentrations. z Radiused keyseats and chamfered keys can reduce stress concentrations. Mott, 2003, Machine Elements in Mechanical Design Key Types z Tapered keys are installed after mating the hub and shaft. The taper extends over the length of the hub. z Pin keys reduce stress concentration, but requires a tight fit. Mott, 2003, Machine Elements in Mechanical Design
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3 Woodruff Keys z Used in light loading applications, where it is desirable to easy assembly and dis-assembly. Mott, 2003, Machine Elements in Mechanical Design Selection of Key z The type and size of the key is usually selected after the shaft and hub have been designed. z The length and material specification is determined through by design analysis, while two factors limit their selection: Width of the hub Distance to adjacent stress concentrations. z Key is held in place by; set screws, shoulders, retaining rings, or spacers. z Keys are commonly made of low-carbon, cold-drawn steel; such as AISI 1020CD with 61 ksi, ultimate strength. Stress Analysis z There are two basic failure modes for keys transmitting power. Shear across the shaft/hub interface Compression failure due to bearing stress between key and shaft, or hub.
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Shear Stress Analysis z The torque in the shaft creates a force on the lower left side of the key which in turn creates a reaction force on the upper right side of the key, transmitting the torque into the hub. z This force couple places the key in shear over the surface A = W*L, where F = T / (D/2) Mott, 2003, Machine Elements in Mechanical Design Shear Stress Analysis z Shearing stress z Design stress z Key Length DWL T WL D T F A s 2 ) )( 2 / ( = = = τ N s y d / 5 . 0 = DW T L d 2 = Bearing Stress Analysis z Failure in bearing is due to compressive stress that acts of shaft or hub area: L * (H/2), where failure occurs on the surface with the lowest = σ
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This document was uploaded on 11/21/2011.

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Keys and couplings - Chapter 11 Keys, Couplings and Seals...

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