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Flexible Couplings

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In a perfect world, multiple components could be produced in a single piece, or coupled and installed in perfect alignment. However, in the real world, separate components must be brought together and connected onsite. Couplings are required to transmit rotational forces (torque) between two lengths of shaft and despite the most rigorous attempts, alignment is almost never perfect.

To maximize the life of components such as bearings and shafts, flexibility must be built in to absorb the residual misalignment that remains after all possible adjustments are made. Proper lubrication of metal-to-metal type couplings is vital to their performance, however, rubber element types are maintenance free – each serving a different purpose with their unique advantages.

Rubber element type couplings such as Pin and Bush, Spider and Rubber-In-Compression are classified as maintenance free as they do not require periodic lubricating. These couplings (although at a disadvantage to metal-to-metal couplings with regards to torque versus size capability), offer far superior flexibility and are very forgiving with misalignment. The rubber elements have the additional advantage of vibration damping and shock absorbing, making these couplings perfect fit options around mills and crushers, pumps and fans etc.

Unless specifically noted by the manufacturer, metal-to-metal couplings such as Gear, Grid Spring and Chain types are generally grease lubricated. Coupling components are protected primarily by an oil film, which bleeds from the grease and seeps into the loading zone, lubricating the two metal contact surfaces. Greases made with high-viscosity base oils, anti-scuff and metal-wetting agents are recommended to overcome the boundary (mixed film) conditions that often exist in flexible couplings. High oils viscosity also slows the leakage rates.

Centrifugal forces in flexible couplings can be extreme, becoming greater as distance from the rotational axis is increased. Even moderately sized couplings can generate forces thousands of times greater than gravity (referred to as Gs). Grease manufacturers place high priority on formulations that resist premature separation of oil and thickener due to the high G forces.

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