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Comparison of Elastomeric and Coil Spring Torsional Couplings

( 03/27/2019 ) Written by: Tim Kubat

We previously talked about how important it is to insist on flexible couplings when you purchase heavy equipment. Continuous vibration and intermittent shocks can cause numerous difficulties – from gear tooth fatigue to broken driveshafts—and flexible couplings can prevent these problems by reducing the vibration transmitted to downstream components.

So, it’s simply good business to know what options are available and how they fit into your equipment plans.

There are two technologies in the marketplace that can provide the desired low torsional stiffness required to isolate drivetrain components from engine-induced vibrations – elastomers loaded in shear and steel coil springs loaded in compression.  There are advantages to each technology.

Elastomers are polymers with elastic or flexible properties. Typical uses are for seals, gaskets, and molded parts. Elastomeric couplings use in a molded geometry that torsionally deflects to reduce the magnitude of transmitted forces, mitigate shock events, decrease vibrations, and accommodate small misalignments between components.

Coil springs are made from spring steel wire that is helically coiled. When you put a load on a compression coil spring, making it shorter, it pushes back against the load and tries to get back to its original length.  Their primary use is to maintain a force between two surfaces. 

Here are examples of what we mean by elastomeric and spring coil couplings.

 

How Elastomeric and Coil Spring Couplings Compare to Each Other

Comparison of Advantages - Elastomeric and Coil Spring Torsional Couplings

 

Accommodate axial, parallel and angular misalignment.  Elastomeric couplings can provide misalignment capability between the input and output attachment features; moving in multiple directions and planes. Metal coil springs do not have this capability and can only move in one direction. LORD LCD couplings can accommodate the following:

  • Angular misalignment up to 1.5 degrees
  • Parallel misalignment up to 0.015 inch
  • Axial misalignment up to 0.063 inch

 

Stiffness that does not vary with temperature or service life.

Elastomeric couplings are typically manufactured from natural rubber, and the stiffness is influenced by the temperature of the engine compartment. The stiffness increases in cold conditions and decreases at elevated temperatures. As the coupling acquires service hours at temperatures above 200 F, the stiffness will increase due to the aging process. 

Steel coil springs are not affected by temperature or service time.

 

Simple construction with no moving or sliding parts.

Elastomeric couplings are made by vulcanizing the rubber to the inner member, then installing that part into the outer member. There are no moving or sliding pieces, as only the rubber flexes. 

Coil spring couplings typically have multiple pieces that move in relation to one another, often with bearings to minimize wear, and fasteners to connect the various pieces.  

 

Inherent damping without additional components and large deflection capability to protect drivetrain components during a shift point. High torque impulses can occur during some shift points.  A coupling design that absorbs this energy can protect driveline components from damage. Elastomer materials inherently have damping which aid in the absorption of energy during the high torque impulses that occur during some shift points.  In addition, elastomeric couplings have a deflection capability of over 25 degrees.

Coil springs have no damping and the coils can bottom out on each other or go solid during a shift point event.  Fluid damping can be added, but this adds complexity. Sliding friction can be added but this becomes yet another surface that can wear. 

 

Capable of multi-stage stiffness characteristics. The simplest form of a coil spring coupling provides a single linear torsional stiffness, which is the same as an elastomeric coupling. 

However, numerous coil spring packs, or coil springs nested inside other coil springs can result in a part having several stages of increasing torsional stiffness as each coil engages.

 

Smaller space envelope. Coil spring couplings have a shorter axial length than an elastomeric coupling – taking up less space. The axial length is longer to meet the stress and strain design limits in the elastomer section.  

 

Provides high frequency noise isolation.

High frequency noise produced by drivetrain components can be transmitted through steel components in contact with one another.  This is the case with coil spring couplings. High frequency noise between the engine and downstream components is isolated by the elastomeric coupling. 

 

This comparison should help you know what to look for when you purchase heavy equipment and how to weigh the pros and cons for your specific plans.

If your heavy equipment does not include flexible couplings, there are metal-to-metal contact surfaces experiencing repeated relative surface motion induced by vibration, which causes fretting.  Fretting downgrades the surface layer producing increased roughness and pitting, which reduces the fatigue strength of the component, and eventually results in damage.  This translates into increased maintenance time and costs.

Learn more about couplings here.

 

ABOUT THE AUTHOR MORE BY THIS AUTHOR
Tim Kubat

Tim Kubat is a Principal Engineer in the LORD Industrial Equipment Team, focusing on drivetrains and other applications for agricultural and construction vehicles. He holds three patents for elastomeric designs used in the off-highway market.

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