by Michael Jermann, Assistant Editor
A new development in gear couplings allows downsizing without loss of performance.
Gear couplings handle motion functions that other coupling types are unable to, or that need modifications and de-rating to function. Plus, they can be modified for shear pin service, floating shaft type, vertical applications, electrical isolation, limited end float, and can have a brake drum or disc added.
Gear couplings use the meshing of gear teeth to transmit torque and provide for misalignment. External gear teeth are cut on the circumference of the hub. Two of the toothed hubs fit inside of a tubular sleeve with matching gear teeth cut around its interior circumference, with each tooth extending axially the full length of the sleeve. Hub and sleeve teeth mesh, so torque transfers from the driving hub’s teeth to the sleeve teeth and back to the driven hub’s teeth.
Gear couplings achieve their misalignment capability through backlash in the teeth, crowning on the tooth surfaces and a major diameter fit. Backlash is the looseness-of-fit that results from gear teeth being narrower than the gaps between the teeth. In addition to contributing to the misalignment capabilities, the backlash provides space for the lubricant. The loose fit provides misalignment capability by allowing the sleeve to shift off-axis without binding against the hub teeth.
Crowning, or curving the surface of the hub teeth, further enhances this capability. The crowning can include tip crowns, flank crowns and chamfers on the sharp edges. This also helps improve tooth life by broadening the contact area along the “pitch line” (where the gear teeth mate and transfer torque), thereby reducing the pressure of torque forces. In addition, it prevents the sharp squared edges of the tooth from digging in and locking the coupling. Variable Crowning, which varies the curvature radius along the tooth flank, maintains greater contact area between teeth during misalignment compared with standard crowning, and reduces those stresses that cause wear. This crowning applies to hub teeth only; sleeve teeth are straight except for a chamfer on the minor diameter edge.
While the hub and sleeve teeth are cut to fit loosely side to side, they fit closely where the tip diameter of the hub teeth meet the root diameter of the gaps between the sleeve teeth. This is called a major diameter fit. Minor diameter fits (where the tips of the sleeve teeth meet the root diameter of the hub teeth) are purposely avoided, because a close fit would prevent suitable misalignment and torque transmission capability.
Gear coupling sleeves can be a single piece, termed a “continuous sleeve,” or can be split radially into two half sleeves, one for each hub. The split version is termed a “flanged sleeve,” because each half has a flanged end, which is drilled for connecting fasteners.
Gear couplings use the AGMA standard naming convention to specify the size of the coupling starting at size 1 and increasing to size 30; with a corresponding increase in size (a flanged size 1 gear coupling is approximately 4 ½ in. in diameter, while a flanged size 30 gear coupling can approach 78 in. in diameter). AGMA specifies that flange gear couplings from size 1 to 9 will match up half for half with other flange type gear couplings made to the AGMA standard dimensions. However, while the dimensional standard ensures compatibility of the face-to-face match between sleeve flanges, it does not assure matching torque or bore capacity.
Gear couplings are power intensive, meaning more torque is transmitted per coupling mass and space consumed than other coupling types. The resulting relatively small size of the gear coupling allows the addition of attachments without having the coupling grow to excessive proportions.
Despite the current gear coupling marketplace offerings, opportunities exist for more options and more capabilities. Increased torque capacity is often requested, but that requirement is usually linked to a need for increased coupling service life. Interrelated is the request for more design varieties, and varieties that retain interchangeability with existing gear coupling sleeve flanges per the AGMA standard dimensions.
Based on customer input, engineering was concentrated on developing a robust gear coupling design. The first phase focused on increasing the torque capacity while simultaneously enhancing the fatigue life. A design of experiments (DOE) was commenced to understand what factors influenced the design and to the extent a change would impact the desired characteristics. Parameters in the study encompassed variations in the crowning of the gear tooth profile, lubricant effectiveness, grade of material, permitted manufacturing tolerances, fastener grade and clamp load generation.
The study also incorporated additional design features based on the analysis. The seal geometry was modified to reduce the influence of wear on the seal, as well as a modification of the elastomer material to reduce loss of performance from aging. The cavity area around the gear teeth was increased to provide additional grease volume to ensure lubrication effectiveness in the harshest environments. Fasteners were modified to increase the generated clamp force while still permitting interchangeability with existing designs.
The resulting gear coupling design had undergone numerous FEA iterations, statistical fatigue life studies and performance analysis. To link the theoretical with the actual physical performance, validation testing was required. The validation testing program underwent a similar enhancement process as the design phase.
Validation testing consisted of a series of individual component experiments in addition to subjecting fully assembled couplings to a battery of worst-case conditions. High cycle coupling fatigue, high misalignment tooth fatigue, ultimate torque capacity, high torque reverse loading and seal contaminated environment life performance were a few of the tests performed. Comparing the results achieved during the physical testing to the predicted analytical performance confirmed the validity of the analytical models.
With the multiple upgrades in the HercuFlex gear coupling, including increased torque and bore capacity, most applications have the ability to downsize the coupling and still meet system requirements. This allows the end user to obtain a gear coupling that is physically smaller while still achieving an increase in service life.