How to lubricate flexible couplings

Chris Decker, Americas Products Technical Advisor – Grease, ExxonMobil Fuels & Lubricants

Across industries, flexible couplings protect machinery from stress, wear and costly machine downtime. These components, although usually small in comparison to the equipment they connect, play a very large role in equipment reliability. When properly selected, installed and maintained, they can help to significantly prolong the life of your equipment.

In an ideal world, industrial operators wouldn’t need to rely on flexible couplings as their machinery would operate in perfect alignment at all times. However, in the real world, it is likely your equipment will occasionally slip into slight degrees of misalignment, placing additional stress upon the connected gears and bearing systems.

Here’s where flexible couplings come to your rescue.

The primary functions of flexible couplings are to:

• Connect equipment at the ends of two shafts
• Transmit torque
• Accommodate some degree of shaft misalignment

A secondary function of the flexible coupling is to provide some torsional flexibility and shock load protection to the connected equipment.

Even though the flexible coupling compensates for some misalignment, it should be stressed that maintenance personnel should be aware of the maximum misalignment specification established for a given coupling. This will require the maintenance personnel installing the equipment to check each one, measuring and adjusting the connector appropriately to within these specifications. This ensures that everything will function properly in accordance with the coupling design during operation. Coupling manufacturers will provide these specifications along with the proper coupling recommendations for a given application.

There are three kinds of mechanical flexible couplings that require lubrication:

• Gear couplings
• Grid couplings 
• Roller chain couplings

In such applications, operators will need a viscous lubricant to protect these small parts supporting very heavy loads. Grease is usually the lubricant of choice for flexible couplings due to the ease of application and the ability to keep it in the assembly.

Properly formulated coupling grease stays exactly where it’s needed, making it ideal for applications that cannot be frequently relubricated. During operation, the thickener of the grease will hold the lubricating oil in reserve until external forces –such as movement, vibration and temperature – trigger its release into the system. As couplings rotate, the centrifugal forces within the application force the grease to the outer circumference of the coupling’s inner cavity, where it engages between gear teeth to form a film and prevent wear.

How To Properly Lubricate Flexible Couplings

These flexible couplings contain metallic surfaces that slide and pivot against each other, requiring lubrication to prevent wear. The high loads induced over small contact areas demand the use of a grease containing a high viscosity oil.

The grease selected for these applications must also be carefully formulated in order to resist the effects of centrifugal force. These forces can vary greatly among couplings as a function of the coupling diameter and rotational speeds, and can negatively impact grease performance.

In fact, most general-purpose greases are not formulated to account for the centrifugal forces found in couplings. These greases often contain thickeners with a higher mass density than the oil that, if subjected to the forces in a rotating coupling, will separate from the oil and accumulate in the tooth mesh area. This forces the lubricating oil to the center of the coupling where it serves no useful purpose. As the thickener does not provide lubrication, this condition will result in coupling wear and failure.

Therefore, it is wise to select coupling grease for use in coupling service. Coupling greases will typically provide guidance as to the proper operating condition for which they are to be used.

To protect couplings from wear, it is important for operators to:

1. Select the proper grease lubricant rated for the type, speed and size of the coupling.
2. Install the correctly selected coupling within allowable tolerances for misalignment, peak torque and continuous torque conditions.
3. Frequently inspect the machinery for signs of leaking lubricant seals, keyways, filler plugs and flanges; missing or loose bolts; and vibration, periodically realigning the machine.
4. Determine the right amount of lubrication and inspection frequencies of the machine.

Finally, when relubricating a coupling, operators should periodically take the coupling completely apart for visual inspection and a thorough cleaning.

For reassembly of grid- and chain-type couplings:

• Hand-pack grease hubs by forcing grease between all grid channels (grid couplings) and chain links (chain couplings).
• For grid couplings, now install grid into the greased channels.
• Install all seals and covers.

For reassembly of gear-type couplings:

• Hand-pack all sleeve teeth with grease.
• Lightly coat seals with grease before reassembly.
• Remove all grease plugs and fill assembly until the coupling is 75 percent full with recommended grease for the application speed.
• Make sure to reinsert all grease plugs.

Note: Disassembly allows contaminants and spent lubricants to be removed. Relubrication without disassembly will contribute to premature coupling wear.