Slewing ring bearings — the bearings that find use on cranes, wind turbines, and other smaller designs — are sometimes cut with teeth to double as gears. Let’s review the benefits and challenges of applying these units.

Drive systems can be simplified by incorporating a gear on either the inner or outer bearing race of a slewing ring. These gear details can take on many styles and features: Fellows stubs, full-depth involutes, straight, helical, hardened or unhardened, and ground or unground, but what factors make for a good slewing-ring gear? Gear static strength, resistance to pitting, gear fatigue, and gear/pinion interfaces, or mesh.

A pressure angle of 20° is most common because 20° cutters are standard, but 14.5°, 25°, and special pressure angles are also used. Machine designers select slewing ring bearings based on load capacities. (See attached load charts.) The tooth size and form is then selected using the following Lewis equation:

L = SFY P Where L = Tangential tooth load

S = Allowable bending stress Y = Tooth form factor (from tables )

P = Diametral pitch F = Face width

The stub tooth form is often used in large gears; economics is the main reason. The form requires less material in the ring forging and less gear cutting time. Full depth tooth forms, on the other hand, provide greater contact ratio for smoother operation, but have lower bending strength.

With core hardness of 23 to 30 Rc, allowable bending stress is 34,000 psi. With 262 to 302 BHN (27 to 32 Rc) this is 37,000 psi, and 29 to 34 Rc allows 40,000 psi. These stress approximations are for maximum or stall torque conditions — so when shock is included in the loading, higher stresses may occur.

Induction-hardened gear teeth with a minimum surface hardness of 55 Rc should be considered when high tooth surface pressures are constant. One example is an excavator or logger that undergoes high acceleration rates and rapid deceleration during a swing cycle.

A full root radius with root hardening is also recommended; the tooth pattern and depth of hardness are critical here.

Backlash

All gears need backlash room. This is especially true of bearing gears, in which large diameters and large center distances require greater manufacturing tolerances. Other factors can determine whether adjusting the center distance between gear and pinion is appropriate to make room for backlash. The cost advantages of adjustment should be kept in mind: Gear size tolerance can be greater (and life extended) with takeup for wear.