Gear couplings are among the most commonly used methods for connecting process equipment. When properly selected, installed and maintained, they can provide long life and good reliability. Gear couplings offer several advantages over other couplings, including moderate misalignment capacity, exceptional torsional stiffness and very high torque density.
However, when it comes to gear coupling reliability, there are many areas where failures may be initiated. Often these failures begin because of a lack of knowledge or a lack of execution of certain fundamentals, which are necessary for these couplings to run reliably.
Selecting the correct coupling for the application is critical for gear coupling reliability. Use the following steps to help make the selection process easier:
Some couplings don’t get much of a chance at a decent life due to their installation. Just like other components that experience infant mortality, often times these parts don’t die but are murdered. Certain elements of gear coupling installation must be considered if optimum reliability is to be obtained, including:
Different coupling styles have different lube and bore capacities. (Ref. Kopflex)
Perhaps the most important operating factor for a gear coupling to be reliable is lubrication. Selection of the proper lubricant is the first step. Many coupling manufacturers supply their own lubricants for their couplings. Gear couplings may either be grease- or oil-lubricated depending on the design. Oil-lubricated couplings will not dry out like grease couplings, while Fast-style couplings have smaller bore capacities.
It is fair to say that most gear couplings are grease-lubricated. Coupling greases have special properties, so general-purpose greases should never be used in gear coupling applications. Gear couplings can be subjected to very high centrifugal forces, and oil separation is a critical element of coupling greases. Since greases are comprised of oil and mostly a thickener, special considerations must be made regarding the selection and application of coupling greases.
Soap thickeners typically are heavier than the oils, so centrifugal forces tend to deposit the thickener at the gear teeth. Generally, a grease with a high oil content of high-viscosity oil and a grade 1 rating from the National Lubricating Grease Institute (NLGI) is preferred. A higher consistency grease may be considered for high-speed applications but should be avoided at low-speed applications.
Grease specifications may include speed limits or certain tests such as the K36 separation factor. Any grease will have oil separation based on time, temperature and centrifugal force. The K36 factor determines the maximum oil separation of the grease while running at 36,000 Gs. A K36 factor of 8/24 means the oil separation was 8 percent in 24 hours. In comparison, a grease with a K36 factor of 3/24 would mean that it did not separate as much as the grease with a K36 factor of 8/24.
Higher oil separation is desirable at lower speeds (lower G forces), while lower oil separation is preferred at higher speeds and higher temperatures. High-vibration equipment can also enhance oil separation and induce failures. Studies have even shown that gear coupling wear rates decrease as coupling speeds increase.
The main function of a lubricant in a gear coupling is to reduce the friction between the gear teeth as they slide against each other. The relative motion between the mating gear teeth occurs in the axial direction due to slight shaft misalignment. This motion is oscillatory, low amplitude, relatively high frequency and a function of the magnitude of angular misalignment.
This sliding axial motion between the gear teeth can generate lots of wear if lubrication is not sufficient. This is why the gear coupling lubricant plays such a critical role in the reliability and life of a gear coupling. Poor lubrication between the gear teeth generates higher friction between these teeth, resulting in gear coupling wear, heat generation and high axial loads to mating equipment bearings. The higher axial loads on the bearings will then decrease the life of the equipment.
The pump shown on the left had a dry coupling that was operating in a torque-lock condition and creating high axial forces on the equipment. The coupling was replaced without making any adjustments to the pump or motor. The only change was a coupling with good lubrication, which reduced tooth friction and decreased the axial forces from the coupling to the pump and motor. The result was a noticeable decrease in the operating temperature of the pump bearing.
Maintenance is the final factor to ensure gear coupling reliability for long equipment life. While the first three factors have more to do with a lack of knowledge, maintenance often comes down to a lack of execution. Unfortunately, this requires discipline by operations and maintenance groups as well as managerial courage to dedicate the resources to ensure that it can happen.
Typical recommendations from gear coupling manufacturers require regreasing at a minimum of 12 months. A regreasing procedure would include breaking, cleaning, inspecting and hand-packing the coupling with fresh grease. Using a grease gun typically is not recommended when the coupling has been broken and ready to receive new grease. When a gear coupling is greased through a fitting instead of hand-packing, it can result in overgreasing, and a hydraulic lock condition can occur, causing high axial forces on the equipment. A hydraulic lock condition can even make alignment difficult, as shafts may be hard to turn.
Some applications require regreasing at six months to ensure good reliability. These applications may include high speeds (high G forces), high temperatures, misalignment or vibration. Smaller lube sump capacity can also be a factor in regreasing intervals. However, deciding to go longer than 12 months without grease replenishment on a gear coupling is a high-risk move that is not recommended.
Regular maintenance of gear couplings should involve special care with respect to many of the installation factors discussed previously. When inspecting gaskets and O-rings, ensure the lubricant stays in the coupling until the next maintenance task is scheduled. Grease fittings should be removed before completing maintenance. These fittings have been known to leak lubricant and can hit guarding, causing loss of lubricant. Under high centrifugal forces, the grease must be completely sealed within the coupling. Guarding should also allow enough access so it does not have to be completely removed for normal coupling maintenance.
Remember, reliability is not for the faint of heart. Most all of these factors must be executed correctly to achieve good gear coupling reliability. This is why the work of maintenance and reliability professionals is rarely ever finished.
Randy Riddell is a senior mechanical reliability engineer for International Paper in Courtland, Ala. He is a certified lubrication specialist (CLS) by the Society of Tribologists and Lubrication Engineers and a certified level I machinery lubrication technician (MLT) by the International Council for Machinery Lubrication (ICML). He is also a certified maintenance and reliability professional (CMRP) by the Society for Maintenance and Reliability Professionals (SMRP).