Sizing a Heavy Duty Slewing Bearing Starts With Getting Your Combined Load Data Right

How do you know if a catalog bearing rated for 500 kN axial load will actually survive your application once radial force and tilting moment are added at the same time? This is the question most design engineers run into once they move past preliminary layout and start writing a real technical specification. A single-axis rating number from a catalog tells you almost nothing about how a heavy duty slewing bearing behaves under combined loading, and sizing errors here are expensive because they usually surface after the machine is in the field, not during design review.

This article walks through how to organize operating load and duty-cycle data so a supplier can run a proper combined-load calculation instead of guessing from a single number you happened to have handy.

Why Combined Loads Break Simple Catalog Selection

Slewing bearings rarely see one load direction in isolation. A crane boom, excavator upper structure, or wind turbine yaw system applies axial force, radial force, and a tilting moment simultaneously, and these three interact inside the raceway contact zone. Picking a bearing because its published axial capacity exceeds your axial load — while ignoring the moment component — is one of the most common sizing errors we see in RFQs.

The reason this matters mechanically: the tilting moment generates a non-uniform load distribution across the ball or roller set. Balls on one side of the raceway carry far more load than the catalog's "rated axial capacity" assumes, because that number is usually published for pure axial loading only. Add radial load on top, and the peak contact stress point shifts again. Static capacity (C0) and equivalent load calculations have to account for all three components together, not sequentially.

How to Convert Axial, Radial, and Tilting Moment Data Into a Sizing Input Set

Before any supplier can size a bearing correctly, your operating loads need to be organized into a format that separates static design loads from dynamic duty-cycle loads. Here's the procedure we ask engineering customers to follow.

Step 1 — Identify Every Load Case, Not Just the Worst One

List each operating condition separately: maximum lift, maximum slew with partial load, transport/stowed position, wind load case, emergency stop, and seismic case if applicable. Common failure here: engineers submit only the "worst case" load and skip the intermediate cases that actually dominate fatigue life over the duty cycle.

Step 2 — Record Axial Force (Fa), Radial Force (Fr), and Moment (M) for Each Case

For every load case, record three numbers at the bearing centerline: axial force in kN, radial force in kN, and overturning moment in kN·m. Checkpoint: confirm all three are referenced to the same coordinate origin — the bearing's geometric center — not to the structure's base or a remote sensor location. Mixing reference points is the single most frequent data error we catch during technical review.

Step 3 — Convert Moment Into an Equivalent Load Where Required

Some suppliers request raw Fa/Fr/M values directly; others want an equivalent axial load (Fa-eq) that folds the moment contribution in using the bearing's pitch diameter. If you don't know which format your supplier needs, send the raw three-component data — it's always convertible downstream, while a pre-converted equivalent load can't be unpacked back into its original components if the supplier needs to check ball spacing or raceway hardness separately.

Step 4 — Attach Duty Cycle Information

Static load cases size the bearing against static capacity (C0). Fatigue life needs operating time at each load level: percentage of operating hours at full load, partial load, and idle slew. Without this, a supplier can only check static safety factor — they cannot confirm expected service life in hours or cycles.

Step 5 — Flag Shock Loads and Mounting Stiffness

Note any shock loading (bucket impact, sudden stop, transport over rough terrain) and describe the stiffness of the supporting structure — thin flange vs. thick cast housing. A flexible mounting surface changes how load distributes across the raceway, even if the nominal loads are identical to a rigid-mount case.

Once these six fields are filled for every load case, you have a usable sizing input set. This is the format we recommend attaching to any RFQ, and it is also the basis of the worksheet referenced in our bearing selection guide.

Worked Example — Mobile Crane Slew Ring

A 50-ton mobile crane upper structure applies, at full boom extension: Fa = 420 kN, Fr = 85 kN, M = 650 kN·m, at a pitch diameter of 1,600 mm. At stowed/transport position, loads drop to Fa = 310 kN, Fr = 40 kN, M = 120 kN·m, but shock factor from road transport is applied at 1.5x.

Submitting both cases — not just the full-extension case — lets the supplier check whether the transport case with shock factor actually governs static safety margin, which happens more often than engineers expect. In this example, the transport case controlled contact stress at the raceway because the 1.5x shock multiplier pushed the effective load close to the full-extension case despite lower nominal values.

Duty Cycle and Operating Conditions That Change the Rating

Two machines with identical peak loads can need different bearing designs once duty cycle is factored in. A tower crane slewing 20 times per hour, 10 hours a day, needs a different raceway hardness and ball complement than a fixed platform that rotates twice a shift under the same peak load.

• Continuous slow rotation (wind turbine yaw, solar tracker): low speed, high cycle count — lubrication retention and seal design matter more than peak static capacity.
• Intermittent high-load slewing (excavator, crane): moment load variation across a wide range — four-point contact ball designs are common here.
• Fixed-position with occasional indexing (turntables, test rigs): static capacity usually governs, not fatigue life.

This is also where gear selection interacts with bearing selection — if your application integrates a slewing ring gear on the outer race, tooth load adds a tangential force component that should be included in the radial force field, not treated separately.

Common Mistakes When Preparing Load Data for Suppliers

• Submitting only peak load, no duty cycle — results in a bearing sized for static safety but with unknown fatigue life.
• Mixing load reference points — loads calculated at the structure base instead of the bearing centerline distort the moment arm.
• Ignoring mounting flange stiffness — a thin, flexible flange needs a different raceway hardening approach than a rigid cast housing, even under identical nominal loads.
• Treating a thin section bearing load case the same as a standard slewing ring — thin section designs have different stiffness and deflection behavior under moment load and should not reuse standard load formulas without adjustment.
• Skipping shock and impact factors — a 1.3–1.5x multiplier on transient loads is common in mobile equipment but frequently left out of the submitted dataset.

Buyer's Checklist Before Sending Specs to a Supplier

• All load cases listed separately, not just the worst case
• Fa, Fr, and M recorded at the bearing centerline for each case
• Duty cycle percentage attached for fatigue life calculation
• Shock/impact factors flagged where applicable
• Mounting flange stiffness and bolt pattern described
• Operating speed and environment (temperature, dust, washdown) noted
• Decision on four-point contact, three-row roller, or crossed roller geometry — or left open for supplier recommendation

If the application needs rotation under load beyond manual positioning — ring gear plus motor and gearbox integrated with the bearing — the same load dataset applies directly to selecting a Slewing Drives unit, since drive sizing depends on the same moment and radial data used for bearing selection.

Matching Load Data to Bearing Geometry Options

Non-standard bore diameters, flange patterns, or raceway hardness profiles are common once a customer's load dataset doesn't match an off-the-shelf series. Because our production runs with no fixed MOQ, customized designs built around non-standard bearings specifications are handled the same way as catalog orders, with delivery typically running 30 to 90 days depending on size and complexity.

Installation and Verification After Sizing

A correctly sized bearing can still underperform if bolt torque, flatness, or grouting tolerance on the mounting surface is off. Checking flange flatness and bolt preload against the supplier's installation instructions is a required step once the bearing arrives — see our bearing installation notes for torque sequencing and flatness tolerance guidance before final assembly.

Organizing your axial, radial, and moment data this way before contacting a supplier — rather than during back-and-forth clarification emails — typically shortens the quotation cycle and reduces the chance of a mismatch discovered after fabrication. Our engineering team reviews combined load data under ISO9001:2015 quality procedures before confirming any heavy duty slewing bearing design, so a well-organized dataset also means a faster, more accurate first-pass quote.

If you have load case data ready, or need help structuring it into the format above, send your drawings and operating conditions to our engineering team for a sizing review and quote.