Slewing bearing

01

Choose the rolling-element design for the load and stiffness

NREL's wind turbine design guideline DG03 describes yaw and pitch bearings as two continuous ring forgings with mounting holes, so the bearing bolts directly to its supporting structures, and balls or rollers inserted through a radial hole that is closed by a loading plug. Four-point contact ball, eight-point contact ball and cross-roller types all carry simultaneous radial, axial and overturning moment loads, and a spur gear may be cut into one ring. The eight-point (two-row) ball bearing costs more to make than the four-point bearing but gives lower ball loads, lower Hertz stresses, less required case depth and longer fatigue life. [1]

Kaydon's slewing ring range shows the same family: four-point contact ball bearings with gothic arch raceways for cranes, aerial lifts and lift truck rotators; an eight-point ball bearing it describes as giving up to 80% more capacity than a single-row four-point design with a smaller profile than a three-row roller design; cross-roller bearings for high stiffness and low rotational torque; and three-row roller bearings offering the highest capacity for a given diameter, used in cranes, excavators, stackers and reclaimers. Its custom rectangular-section bearings reach 218 in (5,500 mm) outside diameter. [2]

DG03 states that the rings are hardened to a core hardness of 250–300 HB so that gear teeth and bolt holes can be machined, and that the raceway is induction hardened to a minimum surface hardness of 58 HRC, with case depth defined to 50 HRC. [1]

Give the supplier the machine and duty, the load cases (axial, radial and overturning moment for operating, test and extreme conditions), speed or oscillation pattern and duty cycle, required life, mounting structure stiffness and flatness, bolt circle and bolt size, gear (internal, external or none) with module and pinion torque, rotation accuracy and stiffness needs, operating environment, lubrication and sealing, and any classification or certification rules.

02

Rate the loads, life, static capacity and friction torque

DG03 rates slewing bearings by turning the radial load Fr, axial load Fa and overturning moment M acting on a pitch diameter dm into a dynamic equivalent axial load, Pea = 0.75 Fr + Fa + 2M/dm, from which L10 life (10% probability of failure) follows from the dynamic axial load rating, with exponent 3 for ball and 10/3 for roller bearings. For static capacity it takes a permanent deformation of 0.0001 of the rolling element diameter as tolerable, corresponding to maximum Hertz stresses of 4,200 MPa for balls and 4,000 MPa for rollers. [1]

Editorial calculation: DG03's yaw bearing example has a 1,454 mm pitch diameter and, in its first operating condition, a radial load of 30.60 kN, an axial load of 226.0 kN and a moment of 89.27 kNm. Then Pea = 0.75 × 30.60 + 226.0 + 2 × 89.27 / 1.454 ≈ 22.95 + 226.0 + 122.79 = 371.7 kN, the 371,700 N the guideline tabulates. The moment term is more than half the axial load here, which is why quotations must be checked against the moment, not the axial load alone. [1]

For oscillating, grease-lubricated bearings DG03 notes that no meaningful oil film forms, recommends clean grease with good boundary additives and a seal system, and cites one maker's suggested Hertz stress limits against fretting corrosion of 3,200 MPa ultimate, 2,800 MPa maximum operating and 2,400 MPa mean operating. It gives friction coefficients of 0.003 for ball bearings with cage and 0.004 for ball bearings with spacers and cross-roller bearings, warns that starting friction may be up to 1.5 times running friction, and requires mounting bolts with enough preload and strength to keep the rings in contact with their mounting surfaces at all times. [1]

A 2023 Science Progress paper from Henan University of Science and Technology, written with engineers from two Luoyang bearing companies, shows how a static load-limit (carrying) curve is built and what moves it. It plots the combinations of axial load and tilting moment at which the peak ball-raceway contact stress reaches the allowable raceway contact stress, which it sets at 3,600 MPa for a 42CrMo slewing bearing, and checked the curve against an ANSYS finite element model. Its shipyard-crane example is a double-row bearing with different ball sizes: 173 balls of 45 mm on a 2,990 mm pitch diameter in the main raceway and 196 balls of 39.6875 mm on 2,980 mm in the auxiliary raceway, both with a 0.525 groove radius coefficient and a 60° contact angle. The authors report that capacity falls as the groove radius coefficient rises from 0.515 to 0.530 and rises as ball diameter goes from 0.90 to 1.05 times the initial size. [4] Editorial calculation: the 3,600 MPa limit is about 14% below the 4,200 MPa ball-raceway Hertz stress DG03 accepts for static capacity, so ask which limit and material the supplier's load-limit curve uses. [4] [1]

03

Compare what suppliers offer

Suppliers linked to this page list slewing bearings; each card links to the page reviewed.

Listings include single- and double-row ball slewing bearings with external, internal or no gear, cross-roller and three-row roller slewing rings, slew drives, and slewing rings sold through bearing wholesalers. Ask each for the load-limit curve, life calculation basis, raceway hardness and case depth, gear data, bolting and seal design.

04

Specify mounting flatness, bolting and wear checks

Leonessa Brevini's slewing ring catalogue sets maximum out-of-flatness for the upper and lower supporting surfaces, counting the sum of circumferential error and radial error due to conicity, by rolling diameter and ring design (table below). The maximum value is to be reached only once within a 180° section. Brevini warns that if the connecting structure is not rigid and flat enough, an irregular rotation torque may occur, causing lockout of the slewing ring and premature wear of the raceways, and it recommends circular reinforcing ribs close to the raceway in place of radial ribs, plus final machining of the supporting surfaces. [3]

For bolting, Brevini recommends class 10.9 or 8.8 bolts to ISO 898-1 with nuts of equal or greater class, and forbids welding and any elastic washer. Its bolt limit curves are valid only if all holes are used, bolts are pre-tensioned with a torque wrench to its table values, the joint has an average friction coefficient of 0.14, and torque is checked again after testing. The tightening length must be at least 5 times the bolt diameter, thread depth at least 1.5 times, with at least 6 free threads in the stressed portion. [3]

For maintenance it recommends greasing the raceway after the first 50 hours and then every 100 hours, checking bolt tightness after the first 100 working hours and then yearly, and measuring machine clearance after mounting with a 0.01 mm comparator under maximum load at different angle positions, then once a year; when clearance reaches four times the initial value, Brevini asks to be contacted for a wear evaluation. [3]

05

Trace the bearing components

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your machine

A slewing bearing quotation needs values for your machine and duty. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the load cases, duty and mounting data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Machine: [type; duty]
Loads: [Fa; Fr; M for each load case]
Motion: [speed or oscillation; duty cycle; life]
Design: [ball or roller; rows; stiffness]
Gear: [internal, external or none; module; pinion]
Mounting: [bolt circle; bolts; structure flatness]
Environment: [temperature; lubrication; seals]

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