Understanding Coupling Failures

Power transmission couplings are used to connect two shafts that turn in the same direction on the same centreline. There are three

principle types of couplings; rigid, flexible and special purpose.

Rigid couplings are used in applications where misalignment is not a factor, where flexibility is not required and where the coupling is not required to absorb load shocks or torque changes.

Rigid couplings connect shafts using bolted flanges, keyed sleeves or ribbed clamps bolted together over the shaft ends with keyways. They are used primarily for vertical drive systems. Lubrication is not required, but larger couplings, or those running at high speeds, may require balancing to reduce vibration.

Flexible couplings also connect two rotating shafts, but are designed to dampen vibration, absorb some shock loading and provide some axial movement or end float of the shafts, as well as compensate for minor misalignment.

There are three fundamental categories of flexible couplings; ‘mechanically flexible’, such as gear and chain couplings, ‘material flexible’, such as disc, spring, diaphragm, elastomeric and bellows, and ‘combination’, such as metallic grid couplings. These provide a combination of mechanical and material flexibility.

Special-purpose couplings include such devices as mechanically flexible U-joints and constant velocity joints used for automobile applications; magnetic couplings, such as magnet-to-magnet; eddy current couplings; and fluid couplings such as liquid, silicone and shot-filled types. Magnetic and fluid couplings provide no contact between the drive and driven elements, offer low maintenance and are capable of absorbing shock loads.

A unique special-purpose type, the Schmidt or offset coupling, is designed to handle large parallel shaft offsets of up to 18 in. and 1,000,000 in.-lb. of torque (see Figure 1).

There are more than 80 styles of flexible couplings used today in approximately 90% of all industrial applications. The most common are briefly described here under their respective categories.

1. Mechanically flexible couplings, such as gear and chain couplings, provide a flexible connection by permitting coupling components to move or slide over each other.

Some minor misalignment is provided by the clearances between gear teeth and chain and sprocket teeth respectively, however shafts should be aligned to less than 0.002 in. to ensure long coupling life.

These coupling types require lubrication using recommended coupling grease or EP (extreme pressure) oil. As might be expected, approximately 75% of gear and chain coupling failures are caused by misalignment or improper or insufficient lubrication. Some newer types of gear and chain couplings contain nylon gear sleeves or nylon chains respectively and these types do not require lubrication (see Figures 2 and 3).

2. Material flexible couplings, as the name implies, provide flexibility by incorporating elements that accommodate a certain amount of bending or flexing. The flexing materials that provide the connection between the coupling drive and driven components include laminated discs, bellows, dia- phragms and elastomeric materials that may include rubber or plastics, such as neoprene and urethane.

Generally speaking, these coupling types require little maintenance other than alignment and their service life is limited by the fatigue limit of the flexing material itself (see Figures 4 and 5).

3. Combination mechanical/ material flexible couplings include the very popular grid coupling, which is a compact unit capable of transmitting high torque at speeds up to 6,000 rpm. The construction of the coupling consists of two flanged hubs, each with specially-designed grooved slots cut axially on the outer edges of the flanges.

The flanges are connected by using a serpentine spring grid that fits the grooved slots. The flexibility of this grid provides torsional resilience, can provide some misalignment and end float, dampens vibration and may reduce peak or shock loads by up to 25%. Gridtype couplings require lubrication using good quality coupling grease in the areas of the grooved slots and serpentine spring (see Figures 6 and 7).

Why couplings fail

Couplings fail for several reasons, but the primary causes are improper selection for the particular application; excessive misalignment; improper, inadequate, or insufficient lubrication; harsh environmental or operating conditions; and excessive speeds or loads.

The application factor: Table 1 provides a general guide for coupling application and a corresponding selection recommendation.

The alignment factor: There is a perception that flexible couplings can accommodate a great deal of parallel offset and/or angular shaft misalignment. This is untrue. Depending upon the coupling type, flexible couplings can only accommodate from 1/4 degree to about 2-1/2 degrees of misalignment; high-speed, high-load drive applications must be aligned to much closer tolerances

(see Figure 9).

Symptoms of misalignment include the following:

• Noise at the coupling

• Powdered rubber particles or leaking lubricant directly below the coupling (depending upon coupling type)

• Process fluid and/or oil leaks at the drive, driven (or both) shafts

• Premature shaft, shaft keyway, or key fatigue or breakage

• Premature or frequent bearing or seal failure at one or both machines

• ‘Soft foot’ condition at foot bolts

• Broken or constant loosening of foot bolts at one or both machines • High operating temperatures at or near the coupling

• High vibration conditions, usually at both machines

• Cracked or broken foundation, particularly at or near the foot bolts

• Continuing or intermittent leaks at pipe joints caused by pipe strain

• High energy consumption

• No compensation for ‘thermal growth’ at either the drive or driven machine during initial alignment procedure

• Settling of the machine or foundation after installation.

Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta. He can be reached at 403-932-7620 or texleug@shaw.ca.Illustrations are courtesy of Maintenance Technology International Inc.