By Scott Kwas on 2019-02-07
Greased couplings are a critical element in industrial operations, and if couplings aren’t properly maintained, the result can be equipment failure, higher maintenance costs and lost productivity. One key to avoiding unnecessary shutdowns is to use a grease designed specifically for couplings.
Couplings are designed to connect and transmit torque between the driver to a driven piece of equipment while accommodating for any minor shaft misalignment. While some couplings do not require grease, when grease is required, it is important to use a grease designed specifically for couplings.
When the equipment is in operation, metal surfaces of greased couplings are locked together and theoretically have no relative motion. However, in practice there is always some misalignment that creates slight movement between the mating surfaces – hence why grease is required. Because loads on couplings are high, with little relative motion between adjoining surfaces, the lubricant must have a high viscosity.
The three main types of greased couplings are gear, chain and grid. They all take the same type of grease, and it’s important to note that coupling grease is specially formulated to protect couplings, and should not be replaced with multipurpose grease. Here’s why: coupling grease is designed specifically to hold up in the high centrifugal conditions seen in couplings. These conditions require a high viscosity of 3200 centistokes or greater, which is much greater than the base oil viscosity in multipurpose grease. Additionally the oil and the thickener in the grease are designed to not separate, even under high speed conditions.
The faster a shaft spins, the more it increases the likelihood that the grease’s base oil will separate from its thickener. If separation occurs, the oil may leak out of the coupling seals, resulting in a loss of lubrication and ultimately, the coupling may become damaged or even fail.
One common misconception is that coupling grease must be tacky to prevent being “flung off” from the spinning shaft. In fact, it’s the housing of the coupling that keeps the grease in place. And this performance is not dependent on the “tackiness” of the grease.
In addition to using specially formulated coupling grease, some other good maintenance tips are:
- Use a dedicated pump and hose for coupling grease, so that it isn’t mixed with other types of greases.
- Use short lines with a large diameter, since the viscosity of the oil in a coupling grease is very high.
- Pump grease into the coupling under non-pressurized conditions, to prevent blowing out the seals.
- Do not fully fill the coupling with grease but rather follow the fill levels dictated by the coupling OEM.
Couplings also should be routinely inspected. If leaks are seen, it is an indication either that the seals have been compromised or that an improper grease has been used.
To see data sheets for Shell’s coupling greases, click on the red "Shell LubeChat" tab at the bottom right of your screen and search “Shell Gadus S2 High Speed Coupling Grease.”
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