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Couplings — A User’s Point of View

Number of visits: Date:2015-09-28 10:59
Why Use a Flexible Coupling?
The coupling serves to connect the shaft between the driver and driven equipment in such a manner that it will transmit the required torque without imposing undue stresses on the shafts and bearings due to inherent misalignment and axial displacement. This can, in some cases, be accomplished without using a flexible coupling, i.e. with a solid shaft. However, it is rare to see this simple but superior technique used, particularly where the driver and driven equipment are produced by different manufacturers. Greater care must be taken to insure proper alignment with the solid shaft coupling, or bearing damage and shaft breakage may result. Quill shafts are also a means of obtaining flexibility without the used of flexible coupling. It should first be determined that a flexible coupling is required before using one. It seems that the petrochemical industry has, without due consideration of solid couplings, committed itself to the almost total use of flexible couplings, thus introducing unnecessary complexity.
Coupling Selection
After all the data required for selection has been acquired (such as maximum rated power, maximum rated speed, maximum torque, the geometry that the coupling must fit into, the thermal growths of each machine), the next task is to determine what type coupling is best for the application. Overall choices are a solid shaft, gear type coupling, or a diaphragm type. Until about ten years ago, the gear type coupling was the standard choice. The advent of the diaphragm coupling gave the industry new features and advantages that solve many of the chronic problems that were characteristic of the gear type coupling.
Advantages of Flexible Diaphragm Over Gear Type Couplings
1.No lubrication required, thus eliminating the need for regreasing or special oil filtration to reduce sludging.
2.Will tolerate greater parallel and angular misalignment without the distress wear that results from large misalignment in a gear type coupling.
3.There are no rubbing or sliding parts to wear. When applied within the fatigue limitations of the materials, the diaphragm type coupling will give much longer life.
4.There are a lot fewer dynamic balance problems since there are no pilot/tooth clearances that will allow the spool to crank and create imbalance.
5.There is much less angular movement, which is predictable since it does not depend on an indeterminable coefficient of sliding friction. Seven to ten times less angular moment is produced with a diaphragm coupling.
6.It lasts as well at maximum rated misalignment and axial distortion and load as at zero misalignment. In the gear type, the wear rate depends on the degree of misalignment, load, speed, and adequacy of lubrication.
7.Zero backlash: Backlash in a gear type coupling can amplify tooth loading from torsional vibrations that may be present.
8.Longer life in radial and thrust bearings due to predictable and lower moment and thrust loads.
9.High and low temperature capability is not limited to the lubricant’s temperature limitations.
10.Coupling hub to shaft fit fretting wear is practically eliminated, due to lower angular movement.
11.Hub, sleeve, and spool cracking is common in gear type couplings due to large angular moments produced.

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