Flexible gearbox?
Not a problem with Extended Flank!

How to achieve >85% effective face width in flexible gear drives using ATA X-Gear™

The ever-increasing quest for more compact, cost-effective gear units with higher performance often results in gear drive designs with significant elasticity and thermal effects. To ensure desired gear performance in such an environment, bevel gears need to be designed with highly application-specific, optimized tooth geometries.

Many factors make modern gear drives a demanding environment for bevel gears to work in, especially in continuously moving equipment such as Azimuth thrusters and heavy vehicles. Apart from high loading, bevel gears must also cope with remarkable amount of displacements. These misalignments are due to both elastic deformations and thermal expansion occurring in the driveline components and housing. To successfully handle such situations, ATA Gears now introduces Extended Flank as part of the ATA X-Gear™ concept, which is applied for maximum reliability and performance.

This is basically “business as usual” at ATA Gears: The behavior of the customer’s application and gearbox is modelled and simulated, and the bevel gear geometries are optimized to ensure proper operation in that environment.

However, in some cases, achieving satisfactory outcomes may be tricky even with such sophisticated design methodology. In applications with high mechanical/thermal displacements, the contact pattern – without load – needs to be positioned right on the edge at the toe side of the teeth. And in loaded conditions, the contact may still move all the way up to the flank edge on the heel side (see example Fig A, upper part).

From the design perspective, such a situation is challenging because the contact pattern cannot be moved further to the toe as in low load situations it would go out of the flank. Thereby, in order to keep the contact on the flank in high load situations, the designer has no choice but to use relatively high crownings. But high crownings also mean higher tooth stresses and reduce the effective contact width (beff) under load. Gear ratings are often based on standard calculations assuming certain value of beff (typically 85%). If this condition is not actually realized, the calculations may be considered invalid. This might cause significant problems in, for example, classification acceptance processes. In essence, high amount of displacements can lead to increased tooth stresses and smaller contact area, thereby limiting the performance of the gears and also complicating the acceptance processes.

Extended flank provides a simple and effective solution to this problem. The tooth is extended at the toe, however keeping the nominal tooth width (b) unchanged (see illustration in Fig A, lower part).

This procedure creates “extra room” for the contact pattern, which can then be moved beyond the nominal tooth width (b) in low load situations. Consequently, the crownings can now also be reduced and effective face width increased under load.

At first this sounds just like normal increase of tooth width, but there is a clear difference: As the shape of the nominal tooth and the location of the mean pitch point are not changed, all gearing parameters and gear rating calculations remain unaffected, enabling possibility for higher performance with the same gearing.

Another benefit is that the contact pattern movements cover a larger area of the tooth flank. When the gears work in varying loading conditions, the fatigue cumulation in a certain location is less. In essence, the utilization of the available tooth flank area is more effective.

At ATA this feature is called Extended Flank and applied as part of the X-Gear concept when necessary. An actual example of the improvement achieved by Extended Flank is shown in Fig B.

The benefits of Extended Flank include
– higher effective contact width under load (beff %)
– lower tooth stresses
– outer diameters of gears unchanged
– manufacturability by conventional bevel gear machines maintained
– gear rating calculations not affected negatively
– gearing parameters unchanged

So what? No worries about changing plans, simply less stress. Good news in today’s highly competitive business environment.

Published by Mr. Jesse Rontu, Chief Gear Engineer, Ata Gears Ltd. on October 14, 2019.