A slewing ring bearing is a rotational rolling-element or plain bearing that supports a heavy but slow-turning or slow-oscillating load — typical host structures include conventional cranes, swing yarders and the wind-facing platform of a horizontal-axis wind turbine [S3][S4].
Standard product diameter bands run from 200 mm up to 5000 mm across the major Chinese tier-1 makers, with internal-gear, external-gear and non-geared race configurations offered as catalogue options and pitch diameters typically one metre or larger compared to other rolling-element bearings [S3][S4][S7].
Load Ratings: Static and Dynamic Curve Points
Selection starts with the static load rating Co and the dynamic load rating Cr, both published on the manufacturer's dimension chart and expressed in kN for a given raceway diameter [S4]. A short coming-down to a single number is misleading: the same 1000 mm pitch-diameter slewing ring will appear in at least three Co/Cr pairs across catalogues because raceway cross-section, ball complement and heat treatment each shift the curve [S4][S7].
For tower-crane and crawler-crane slewing rings, the controlling load case is usually an overturning moment (tilting) rather than a pure axial force; the published moment rating M and the axial/radial ratings are the three numbers that must be compared against the worst-case reaction at the slew bearing plane [S4].
Wind-turbine yaw and pitch bearings cite pitch-diameter values from roughly 1 m to 3 m, with 4-point contact ball preferred for yaw (slow rotation, high moment) and crossed-roller selected for blade pitch (higher tilt stiffness, lower frictional hysteresis) — see the encyclopedia entry on slewing ring bearings for the load-curve geometry [S5].
Rolling-Element Family: Four Decision Gates
Four rolling-element families cover roughly 95 % of the catalogued Chinese supply: 4-point contact ball, crossed-roller, three-row roller (slewing), and double-row ball — each with a different load/tilt/stiffness profile that maps to a host-machine class [S3][S4].
4-point contact ball: highest speed, lowest tilt stiffness, lowest cost; standard for crane booms, man-lifts and medium-duty wind yaw. Crossed roller: roughly double the moment stiffness of 4-point contact for the same pitch diameter, lower limit speed, used on CT scanners, robotic jibs, slew drives with precision position-hold; more sensitive to moment overload [S4].
Three-row roller: separates axial, radial and moment loads on three discrete raceways, used on heavy excavator and large crawler-crane upper works; the most expensive per kN of rating, and the only family that genuinely lets the engineer design axial, radial and moment capacities independently [S4].
Double-row ball: a cost compromise between 4-point contact and three-row roller; popular on small to mid-size truck-mounted cranes. The deeper encyclopedia read on slewing bearings maps the same four families to ISO 281/ISO 16281 dynamic-equivalent load factors.
Gear Integration: Internal, External or No Gear
Slewing rings with internal gear drive the platform relative to the base through teeth cut into the inner or outer race, and a pinion meshes on the stationary side — this is the dominant configuration on small- to mid-size crawler cranes, man-lifts and harbour cranes [S3].
External-gear slewing rings are preferred where the pinion sits in a tight arc envelope (excavator cabs, some knuckle-boom loaders) and where debris shedding away from the gear matters; pinion mesh forces are lower per tooth than internal gear at the same module because the pinion sits on the larger effective radius [S3].
No-gear (plain race) rings are specified where a separate slew drive (worm or planetary) carries the pinion, or where the structure itself rotates — typical on large wind-turbine nacelles and on turntables with hydraulic slew motors mounted off the bearing. Across the seven Chinese factories surveyed, internal-gear is the most-published variant, external-gear second, plain-race third [S3][S4][S6][S7].
Mounting, Hardness and Tolerance Class
Mounting structure stiffness is the variable most often missed in spec sheets: a slewing ring is only as stiff as the supporting rings on the host, and a 1 mm flatness error across a 2 m bolt circle can deflect the race enough to drop dynamic load rating by a double-digit percentage in service [S4].
Raceway hardness is typically quoted in the 55–62 HRC band for through-hardened chrome steel, with case depth on induction-hardened rings usually 3–6 mm depending on pitch diameter; manufacturers like LYXQL, Fenghe and Xuzhou Wanda publish these values on request rather than on the standard data sheet [S5][S6][S7].
Quality system references that recur on supplier data sheets include ISO 9001 (factory certification), plus Chinese industry standards JG/T 66-1999, JG/T 67-1999, JG/T 68-1999 and JB/T 2300-1999 covering slewing ring geometry, tolerances and testing — these are the most-cited domestic references for procurement QA clauses [S7].
Capacity Sizing: Quick Method vs Formal
The quick (catalog) method: match host machine class to a maker's selection table, where one row lists bore, OD, height, ball circle, ball diameter, tooth module, Co, Cr, M, weight, and the engineer picks the first row whose Co and M both clear the calculated worst-case load with a 1.25–1.5 service factor [S4][S7].
The formal method: build a load spectrum (axial Fa, radial Fr, moment M) for one full duty cycle, convert to equivalent dynamic load P using ISO 76/ISO 281 formulas, then check L10h against the design life target — typically 20 000 h for industrial cranes, 50 000–120 000 h for wind turbine gearboxes [S4].
Two frequent sizing errors: (1) ignoring the moment-induced axial reaction on the far side of the race, which can roughly double the effective Fa at extreme tilt; (2) selecting a 4-point contact ball for a high-moment, slow-tilt duty and discovering in service that the race plasticised — at which point the design should have stepped up to a crossed-roller or three-row roller [S3][S4].
Lubrication, Sealing and Service Intervals
Grease lubrication is the catalogue default; oil-bath is reserved for large three-row roller rings on heavy excavators where heat dissipation matters. Re-lubrication interval is normally 100–500 operating hours depending on speed, load and ambient contamination [S4][S7].
Sealing comes as nitrile-rubber NBR contact seals in most catalogue lines, with fluoroelastomer FKM optional above 120 °C or where chemical resistance is needed; the seal groove geometry is part of the dimension print and must not be remachined on-site [S4].
Bolt pre-load and re-torque is the single highest-payoff field practice: the bolt-circle preload determines whether the ring carries moment as a tension/compression couple or as a clamped flange, and a re-torque at 100 h and again at 500 h is the common maker recommendation for new installations [S4][S7].
How the Main Families Compare on Four Decision Criteria
On cost per kN of dynamic load rating, 4-point contact ball is the lowest, double-row ball next, crossed-roller third, three-row roller the highest — the ratio between cheapest and dearest for a 1500 mm pitch-diameter class is commonly 1 : 2.5 to 1 : 3 across the supplier base [S3][S4].
On moment stiffness for the same pitch diameter, 4-point contact ball is the baseline, double-row ball roughly 1.4×, crossed-roller roughly 1.8–2.2×, and three-row roller the highest by a clear margin because the moment load is carried on its own third raceway [S3][S4].
On limit speed (DN value), 4-point contact ball is highest, double-row ball next, crossed-roller lower, and three-row roller the lowest — so a slow-oscillating host (blade pitch, radar) is comfortable on any family, while a high-speed slew drive (rotary index table) should default to 4-point contact or double-row ball [S4].
On lead time and MOQ economics for procurement, tier-1 Chinese makers publish MOQ around 1 set on catalogue lines and 10 sets on custom variants, with US$200-set entry pricing on 4-point contact ball and proportional step-ups for internal-gear and three-row roller builds [S1][S3][S7]. For a related read on how a slewing ring sits inside a crawler-crane upperworks, see the crawler crane sizing field guide, and for truck-mounted crane bolt-circle and axle loads, see truck-mounted crane sizing.
To size a slewing ring on the first pass: lock the host-machine class, fix the rolling-element family from the moment/speed gate, set pitch diameter from the moment rating M with a 1.25–1.5 service factor, choose internal/external/no-gear from the drive layout, then verify Co, Cr and bolt-circle flatness against the actual structure [S3][S4][S7].
Trackable signals for the next spec revision: ISO 16281-edition updates for dynamic-equivalent load factors on large slewing rings; published moment-rating curves for wind-turbine pitch bearings at the 3–4 m pitch-diameter scale; and any maker moving 4-point contact ball catalogues to a higher Co/Cr pair at constant pitch diameter [S4][S5][S7].
For component-level specifications, see linear guide.