Motion System Design
Soft on couplings

Soft on couplings

Flexible shaft couplings are a common component in rotating power trains. But choosing the best coupling is far from simple; there are dozens of configurations and grades at your disposal. A little background on the subject can help bring it all together.

In our imperfect world, shafting between machines doesn't always line up, and the machines themselves behave unpredictably. Therefore, flexible shaft couplings are called upon to save the day, bridging these misalignments and allowing the machinery to shift a bit.

Among the most pliable and resilient of flexible couplings are the elastomer varieties. They can accommodate more misalignment and shifting than all-metal couplings. Plus, they readily soak up torsional shock and vibration. The tradeoff: higher flexibility transmits torque less efficiently.

Elastomeric couplings consist of rubber or plastic elements. No lubrication is needed, so these devices are essentially maintenancefree, except for periodic inspections of the element. They are therefore attractive for connections at hidden or inconvenient locations.

Materials used in the coupling elements may be sensitive to some chemicals and environments. Take time to recognize adverse conditions and consider whether a particular material will react negatively with the surroundings of an operation.

As for the power transmission requirements of your system, two common methods of selection are the HP/100 rpm method and the torque method. The HP/100 rpm method is probably the most popular. As might be expected, it relies on the manufacturer ratings of a coupling's horsepower capacity at 100 revolutions per minute. To translate your equipment needs into a HP/100 rpm rating, multiply the consumed horsepower by 100 and by the service factor, and divide by the shaft rpm. The service factor is a correctional factor provided by the manufacturer. It is intended to compensate for peak torque, cyclic torque, and reversal of torque. Bear in mind that using too high a service factor brings a less flexible, less shock absorbent coupling.

The torque method, which some engineers and designers prefer, measures a coupling's ability to transfer torque from the drive shaft to the driven shaft. Coupling manufacturers generally publish torque ratings as well as HP/100 rpm ratings. To match a coupling with your operation by the torque rating, multiply the consumed horsepower by 63,025 and by the service factor, and divide by the shaft rpm.

Twists and turns

Most elastomers can absorb small quantities of energy during each torque pulse. The energy is then released as heat, in a form of hysteresis. This behavior makes the elastomeric element cushion shock loads and dampen vibration. Such couplings are well suited for applications like steel mill tables and primary crushers, where high torsional shock is frequent.

For applications involving vibratory torques, such as reciprocating pumps, reciprocating compressors, and hammer mills, damping from the coupling is a definite factor. A torsional analysis of the system can help define such effects. For a proper analysis, several concepts should be understood.

Reaction Load
Naturally, a coupling resists being forced into a misalignment, and imparts stress on the shaft and bearings. The extent of this reaction in the coupling depends on its construction.

Internally reinforced elastomeric couplings contain layers of polyester cords that transmit torque and limit the wind-up; this allows the base elastomer to be of a soft composition.

When such an element misaligns in the angular, axial, or parallel direction, the polyester cords and soft elastomer flex easily to accommodate the misalignment, and produce a relatively small reaction.

Some unreinforced couplings use a stiff elastomer to obtain comparable torsional rigidity. But these less flexible elements aren't as forgiving to misalignment, and higher reaction loads can develop.

Torsional Wind-Up
Coupling wind-up is the twist due to torque. The amount of wind-up (in degrees) is found from dividing the operating torque by the torsional stiffness.

Torsional Stiffness
Elastomeric coupling manufacturers publish torsional stiffness data to help the designer select the right service factor. Torsional stiffness is measured in torque per degree of rotation (such as lb-in./degree).

Couplings are generally classified as either torsionally rigid or soft. Torsionally rigid couplings have a metallic flex element that transmits torque and accommodates misalignment. They are usually power dense, which refers to the coupling's ability to transmit a large torque in a relatively small or light package. Disc, gear, and chain style couplings are examples of torsionally rigid couplings.

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Torsionally soft couplings often use an elastomeric element to transmit torque and accommodate misalignment. Soft couplings reduce torque pulses by winding-up and storing the pulse energy within the elastic element. The soft coupling releases the energy back to the system more slowly and gently than a rigid coupling. Increased shock absorption leads to smoother, less detrimental power train operation.

Subjection to high temperatures and high wind-up angles over time can cause creep; the elastomer will undergo progressively greater twist under the same load. Reinforcing cords in the elastomer can diminish this effect. The cords lock up during winding, limiting excess deformation and over-winding.

Elastomeric couplings tend to fail from age distress, overload, or thermal failure. Cracks or tears on the element's surface are signs of age distress. These visually obvious characteristics give clear warning of poor condition and impending failure, so scheduled replacement can occur.

Shock loads or overload failures cause a different crack or tear pattern. The cracks or tears appear as jagged separations of the rubber. Tears in an elastomeric coupling loaded in shear will generally be 45° relative to the shaft centerline, since the layers of polyester are positioned at about that same angle. Couplings that fail in this way may have been undersized, or, if properly sized, they probably protected the balance of the drive from a jam load.

Thermal failure results from heat generation caused by extreme rapid flexing of the rubber. Reinforcing cords in the elastomeric element can hold up under substantial torsional shock. Sometimes, the shock amplitude and/or frequency exceeds what the base elastomer can absorb and dissipate. In severe cases, thermal damage is easily recognized. The rubber has melted, and is shiny, black, and sticky. If the coupling is reinforced, the cord ends are exposed and frayed. Lesser cases of thermal breakdown can defy identification, easily being mistaken for overload or age distress.

A choice checklist

When choosing a coupling, the system or application needs to be fully defined. Here are some checkpoints to consider:
• Type of driving and driven equipment.
• Load and speed (hp and rpm).
• Frequency of starts, shock loads, reversing loads, and overloads.
• Amount of torque fluctuation.
• Torque peaks.
• Ambient conditions, like operating temperatures and exposure to corrosive elements.
• Expected operational misalignments.
• Special or extra-duty requirements for a particular application.
• Shaft diameters and keyseats of driving and driven shafts.
• Nature of shaft attachment (tapered bushings, clearance fits, interference fits, and tapered fits).
• Torsional vibration.
• Appropriate service factor for application.

With these parameters understood, a suitable coupling can be matched to the operation

Trent Kyzer is an Engineer with Dodge PT Components for Rockwell Automation, Greenville, SC

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