Positioner Rated Capacity vs. Real Capacity: Eccentric-Load Derating

A positioner's rated capacity is a weight at an assumed center of gravity, and an off-center or tall load can exceed the positioner's bearing, gearing, or structure at well below that weight. UTEC Industrial designs, engineers, machines, fabricates, and installs custom material handling systems for aerospace and heavy industry from its Spokane Valley, WA facility, integrating Allen-Bradley PLC and motion control with in-house CNC machining, heat treating, and stress relief. This article works through the limits that set real capacity: how an eccentric load becomes axial load and tilting moment on the slewing bearing, how three bearing makers' load factors change the required rating, why the bearing's bolts have their own limit, what axis orientation and reducer overhung load do, and how sensing can keep the positioner inside its derated envelope. Capacity is decided along the whole chain, design → engineering → parts machining → fabrication → assembly → weld fatigue → stress relief → drives → controls → tuning → monitoring, because a bearing rating holds only on a flat, stress-relieved, machined mounting, and only while the controls keep the load inside the envelope it was rated for.

What does a positioner's rated capacity actually promise?

A single capacity figure promises less than it appears to. A weight rating is meaningful only with the CG offset from the rotation axis and the CG height above the table or faceplate that it assumes, and only for the axis orientations the machine will run in. Two positioners with the same headline weight can carry very different parts if one was rated at a 2 in offset and the other at 12 in.

No common rating convention closes that gap. No published standard was found that sets a convention for stating a positioner's rated load at a stated CG distance and eccentricity; that is the finding of the research for this article, not a statement made by any source. MHI publishes requirements for industrial tilters (ANSI MH29.2-2020) and industrial turntables (ANSI MH29.3-2023), and MH29.2's scope excludes invertors and rotators, and dumpers or upenders whose angular travel exceeds 110°, so a full-rotation positioner falls outside it. At the component level, Kaydon's slewing-bearing catalog states that there is no industry-wide standard for rating slewing ring bearing capacity, that vendors' ratings and service factors can vary while still producing about the same bearing, and that the equipment designer is responsible for choosing the correct service factor.

The practical consequence is that a positioner's real capacity has to be rebuilt from statics and from each component maker's own method. The worked sizing example derives the drive-torque limit, W_allow = T ÷ e, and tabulates payload against CG offset; this article covers the bearing, bolting, reducer, and duty limits that sit alongside it (Kaydon 2017, p. 16; MHI ANSI MH29.2-2020; MHI ANSI MH29.3-2023).

Which component limits set a positioner's real capacity?

Real capacity at a given CG is the smallest of several independent limits, and each one falls off with CG offset at a different rate:

• Drive torque and holding brake on any axis that is not vertical: allowable weight falls as 1 ÷ e.
• Slewing-bearing raceway: a combined limit on axial load and tilting moment, read from the maker's limiting-load curve.
• Slewing-bearing bolting: a second, separate curve for the same bearing, set by the bolts that hold it down.
• Reducer output shaft: a permitted overhung load that falls as the load moves out along the shaft.
• Structure and welds: fatigue of the table, spindle, and frame under a load that sweeps around once per revolution.
• Base stability: the combined CG staying inside the tipping edge.

The moment M on a bearing is the load times its perpendicular distance from the axis, M = F × d⊥. For a limit expressed as an allowable moment M_allow, the allowable weight is W_allow = M_allow ÷ e, so doubling the offset halves it. An axial-load limit F_allow does not change with e at all. The two cross at e* = M_allow ÷ F_allow: below that offset the axial limit governs, and above it the moment limit does. A rating quoted at a small offset can be set by an axial or torque limit, and then it says nothing about the moment case a large, off-center part creates.

One failure mode is a positioner checked against only one of these limits, such as drive torque, and then loaded with a part that satisfies the torque check but overloads the bearing bolts or the reducer shaft; that is engineering reasoning from the separate limits, not a cited finding. Each limit has to be evaluated with its own maker's method at the worst CG in the part family (Baker and Haynes 2026, §4.1 eq. 4.1.1; SKF 2019, pp. 20–21).

How is an eccentric load resolved into slewing-bearing loads?

Every load on a slewing bearing reduces to four resultants acting on or about its center: axial load, radial load, tilting moment, and torque. Kaydon notes that an axial load applied away from the rotation axis also creates a moment, and that a radial load applied above or below the rolling elements creates a moment too. SKF's selection method writes the result for a bearing carrying a working load and several weight fractions:

• Resulting axial load: Fa = Qa + G1 + G2 + G3
• Resulting tilting moment: Mt = Qa × L + Fr × Hr + G3 × L3 − G1 × L1 − G2 × L2

Here Qa is the working load at radius L, Fr a radial force acting at height Hr, and each G a weight fraction at its own radius, with weights on the opposite side subtracting. SKF says that where the working radius varies, the maximum working radius must be used, that radial loads up to 5 percent of the axial load may be neglected, that where a radial load acts at any point other than the bearing plane the resulting tilting moment should be calculated and taken into account, and that above Fr ÷ Fa = 0.6 its application engineers should be consulted.

For a positioner, Qa is the part and L is its CG offset. An illustrative turntable case, every input an assumption: a 20,000 lb part with its CG 15 in off the axis, a 3,000 lb fixture with its CG 5 in off the axis on the opposite side, and a 2,000 lb table centered on the axis.

• Fa = 20,000 + 3,000 + 2,000 = 25,000 lb (111.2 kN)
• Mt = 20,000 × 15 − 3,000 × 5 = 300,000 − 15,000 = 285,000 lb·in (23,750 lb·ft, or 32.2 kN·m)

The fixture's counter-side weight reduces the moment here only because it is always present. If the same table sometimes runs with a lighter fixture or none, the design case is 300,000 lb·in, and the maximum working radius is the largest offset any part in the family will have (SKF 2019, p. 20, Fig. 1; Kaydon 2017, p. 14).

How do bearing makers' load factors change the required rating?

The loads from the statics are not compared with the catalog curve directly. Each maker multiplies them by an application factor first, and the factors and the rules for applying them differ from maker to maker:

• SKF multiplies the resulting axial load and tilting moment by a load factor fL from its Table 1, Far = fL × Fa and Mtr = fL × Mt, and lists fL = 1.15 for welding positioners and for turntables, against 1.33 for aerial platforms, 1.5 for mobile cranes, and 2 for carrousels.
• Rothe Erde multiplies the loads by a static factor fstat and checks two load combinations against the bearing's static limiting curve. Its Table 1 puts "welding turntable" in a group with fstat = 1.10, fL = 1.0, and a basis of 30,000 full-load rotations. With no radial load, combination I is Fa′ = Fa × fstat and Mk′ = Mk × fstat, and combination II multiplies both by 1.225 before applying fstat, and for the bearing series the catalog applies them to, the bearing is statically suitable if either combination falls below the curve.
• Kaydon draws its load-rating curves at an application service factor of 1.00, and the required rating is the service factor from its Table 2-4 times the applied loads. The table has no welding-positioner line; for an unlisted application the catalog says to select a comparable one for initial sizing. Its index and turnstile tables line gives 1.00 for occasional use with intermittent rotation, 1.25 for frequent use with intermittent rotation, and 1.50 for frequent use with intermittent rotation and impact loads.

Applied to the 25,000 lb and 285,000 lb·in case above, the required values are:

Method (maker's own)	Factor	Required axial	Required moment
SKF, welding positioner	fL = 1.15	28,750 lb	327,750 lb·in
Rothe Erde, welding turntable, combination I	fstat = 1.10	27,500 lb	313,500 lb·in
Rothe Erde, combination II	1.225 × 1.10	33,688 lb	384,038 lb·in
Kaydon, index table, frequent use with intermittent rotation	1.25	31,250 lb	356,250 lb·in

Each row is valid only against the same maker's curve. Kaydon notes that vendors' ratings and service factors vary while still producing about the same bearing, so a factor belongs with its own maker's ratings and is not carried over to another maker's curve; Rothe Erde states that its fstat values may be undershot only in exceptional circumstances and with its prior written approval (SKF 2019, p. 21, Table 1; thyssenkrupp Rothe Erde 2016, pp. 60–61, Table 1; Kaydon 2017, pp. 16–17, Table 2-4).

Why does a slewing bearing have both a raceway and a bolting limit?

A slewing bearing can fail in its raceways or in the joint that holds it down, and SKF rates those separately. SKF's static limiting-load diagram carries two curves for each bearing: a solid line for raceway capacity, the maximum static load the bearing accepts without harming its running behavior, and a dotted line for bolting capacity. The point where the factored axial load and factored moment intersect must lie below both.

The bolting curve holds only under stated conditions. SKF's bolting capacity assumes grade 10.9 bolts and nuts, threads coated with a thin layer of light oil, and tightening to its tabulated values. All of SKF's load ratings and capacity data apply to supported bearings, with suspended arrangements referred to SKF. To fully use the bearings, SKF also requires the rings to be fully supported by strong, rigid components, with flat support surfaces and guideline support-flange thicknesses set as a fraction of the mean raceway diameter. Kaydon's "normal application" likewise assumes mounting surfaces machined and reinforced to limit deviation from a true plane. Elsewhere the catalog holds the designer responsible for the mounting arrangement, and it warns that failure to follow its installation and maintenance guidance can severely limit the ability of the bearing, its bolts, and the adjacent structure to safely transmit the indicated loads.

Two failure modes follow. A bearing selected on its raceway curve alone can have its bolts loosen or fatigue under the moment case, and a bearing bolted to a frame that is out of flat carries load on only part of its circumference; Kaydon adds that a distorted mounting can need excessive turning torque. It also warns that welding the bearing, or welding near it, can damage the bearing, which matters on a welding positioner whose table is later modified.

This is where the build chain sets the rating. The mounting face has to be flat after the weldment's residual stress is gone, so heavy positioner frames are stress-relieved before final machining, as covered in stress relief for machine bases and frames. UTEC Industrial stress-relieves welded frames in-house and machines to tolerances as tight as ±0.001 in, so bearing mounting faces are machined after that stress is removed.

A bearing is rated for the joint it is bolted into, not for the bearing alone (SKF 2019, pp. 21 and 23–24; Kaydon 2017, pp. 15–16 and 20).

What changes when the rotation axis is horizontal or tilting?

The catalog curves assume a particular orientation. Kaydon's "normal application" is a vertical axis of rotation with predominantly compressive thrust and moment, radial load no more than 10 percent of thrust, intermittent rotation within its pitch-line speed limits, an operating temperature of −20 °F to +140 °F, machined and reinforced mounting surfaces, an installation procedure that assures the roundness of both races, and provision for periodic lubrication and bolt checks. Conditions outside those need special attention in selection. SKF's application data sheet asks whether the axis is vertical, horizontal, or changing.

A positioner can break each assumption:

• Horizontal-axis faceplate. On a headstock, the part's weight acts across the axis, so the bearing sees mostly radial load and little axial load. The ratio Fr ÷ Fa is then far above SKF's 0.6 consultation threshold and Kaydon's 10 percent line, and the standard selection path does not apply.
• Tilt-and-rotate table. As the table tilts, axial load turns into radial load and the CG height adds moment; the worked sizing example shows the bearing moment peaking near 77° at about 4.4 times its table-flat value.
• Tilt past 90°. Beyond vertical, the payload's weight stops pressing the bearing rings together and starts pulling them apart. That last point is engineering reasoning from the statics, not a catalog statement, but it lands on a distinction both makers draw: Kaydon says a suspended (tension) axial load is concentrated predominantly in a smaller area around the bolts and calls this a critical difference that must be considered in the design, and SKF's capacity data are valid for supported bearings only.

A positioner that tilts through 135° therefore needs its bearing selected with the maker for the changing-axis case, not read off a vertical-axis curve (Kaydon 2017, pp. 14 and 16; SKF 2019, pp. 20–21 and application data sheet).

How does an off-center load derate the gear reducer's output shaft?

The reducer that drives a positioner axis can carry a pinion, sprocket, or faceplate directly on its output shaft, and that shaft has its own overhung-load limit. SEW-EURODRIVE's gear-unit catalog shows how that limit derates:

• Catalog permitted overhung loads, FRa, assume the force acts at the center of the shaft end, 0.5 × l, at the least favorable force application angle and direction of rotation.
• When the force acts elsewhere, at a distance x from the shaft collar, the permitted overhung load at x is the smaller of a bearing-life limit and a shaft-strength limit, and it falls as the force moves farther out along the shaft.
• The resulting overhung load is the tangential force times a transmission-element factor fZ: 1.15 for gears with fewer than 17 teeth, 1.40 for sprockets with fewer than 13 teeth, 1.25 for fewer than 20, 1.50 for toothed belts, and 2.00 for a pretensioned rack and pinion.
• Some mountings cut the permitted value to 50 percent of FRa, and one hollow-shaft mounting variant permits no overhung load at all.

An illustrative case, with assumed values: a 15-tooth pinion of 3 in pitch radius drives a slewing-ring gear with 24,000 lb·in at the reducer output. The tangential force is 24,000 ÷ 3 = 8,000 lb, and with fZ = 1.15 the resulting overhung load is 9,200 lb. That figure is compared with the catalog FRa only if the pinion sits at the shaft-end center; mounted farther out, it is compared with the smaller, reduced limit.

A faceplate bolted straight to a reducer shaft is the extreme case. The part's weight acts well beyond the shaft end, so a heavy or off-center part can call for its own outboard bearing, with the reducer carrying torque only; that is engineering practice, not a catalog statement. The reducer itself is rated as an industrial enclosed gear drive under ANSI/AGMA 6013-B16 (SEW-EURODRIVE 2026, Overhung and Axial Loads; ANSI/AGMA 6013-B16).

Which build-state, process, and test loads belong in the rating case?

The design load is the worst load the positioner ever sees, not the finished part. For a structure built up on the table, the CG moves as each component is added, and the composite CG, x̄ = Σx̄ᵢWᵢ ÷ ΣWᵢ, has to be checked at each build state. In an illustrative case, an 8,000 lb base weldment centered on the axis is joined by a 4,000 lb bracket 30 in out:

• x̄ = (8,000 × 0 + 4,000 × 30) ÷ 12,000 = 10 in, and M = 12,000 × 10 = 120,000 lb·in
• When a matching 4,000 lb member is later added 30 in out on the other side, x̄ returns to 0

The intermediate state sets the moment, and the balanced finished part would hide it. The same applies to a weldment that grows through tack-up, root passes, and final welds: AWS D1.1/D1.1M:2025 is the structural welding code for welded structures of commonly used carbon and low-alloy constructional steels, and parts held on welding positioners may be built to it, but the positioner has to carry every stage of that build, not only the finished assembly.

Test loads count too. Rothe Erde states that the static check must include the maximum loadings, including additional and test loads. Kaydon's selection procedure lists loads imposed during overload or testing situations among the items to consider when determining the maximum bearing loads, and its gear step names overload testing as well. A proof or acceptance test at more than the rated load is therefore a bearing load case, and it has to fall below the maker's curve with the factor applied. The positioner's own welded frame and table are designed for the same loads; AWS D14.4/D14.4M:2019 is the specification for welded joints in machinery and equipment (Baker and Haynes 2026, §7.2 eq. 7.2.2; thyssenkrupp Rothe Erde 2016, p. 61; Kaydon 2017, p. 13; AWS D1.1/D1.1M:2025; AWS D14.4/D14.4M:2019).

How does duty cycle change the derating?

The factors in these tables carry duty assumptions. Rothe Erde's welding-turntable group is based on 30,000 full-load rotations, and it states that its fL values relate to the maximum operating load and come from practical experience and test-rig tests. Kaydon bases its service factors primarily on the frequency of use at higher versus normal loads and the potential for extreme or overload. Its index-table line goes from 1.00 for occasional use to 1.25 for frequent use, both with intermittent rotation, and for continuous rotation it gives no factor at all, only "alternate criteria", which call for an alternative method of evaluation and selection. For more frequent operation with definitive duty cycles, Kaydon says the fatigue life of the bearing and gear may dictate the design and that service factors should not be the sole selection criterion.

An illustrative duty shows why. A positioner turning at 2 rpm for 4 hours a shift, on two shifts for 250 days, completes 2 × 60 × 4 × 2 × 250 = 240,000 revolutions a year, eight times the 30,000 full-load rotations behind Rothe Erde's table. Not all of those revolutions carry full load, so the life check needs a load spectrum, the fraction of revolutions at each load, rather than one factor; that point is engineering reasoning, and Rothe Erde's utilization-period method works from a duty cycle and time slices.

In this example speed does not govern. Kaydon limits single-row bearings to a pitch-line speed of 500 ft/min and multi-row bearings to 300 ft/min for intermittent rotation. A 60 in pitch-diameter ring at 2 rpm runs at π × 5 ft × 2 = 31.4 ft/min, far below either limit, so on a busy positioner the revolution count, not the speed, is what moves the bearing and gear out of the static-selection regime (thyssenkrupp Rothe Erde 2016, pp. 60–61; Kaydon 2017, pp. 16, 17, and 20).

How can sensing and PLC logic hold a positioner inside its derated envelope?

A derated envelope is only useful if the machine enforces it. The controls can measure the two quantities every limit depends on, weight and CG, and compare them with the envelope before each move:

• Part recipes. The PLC stores, for each part number, the weight range, CG envelope, and allowed tilt range, so an operator selects a part rather than a capacity.
• Weight. Load cells in the table supports or fixture measure the payload directly.
• CG offset. On a tilted or horizontal axis, holding torque divided by weight gives the offset; the sizing example works that check. The PLC can then compute Fa and Mt from measured values and block rotation or tilt that would move the point outside the factored bearing curve.
• Running torque. Kaydon estimates the bearing's friction torque as Mw = μ × (4.4 Mk + Fa × Dp + 2.2 Fr × Dp) ÷ 2, with μ = 0.006 or 0.004 depending on bearing series. For the 23,750 lb·ft, 25,000 lb case with an assumed 3 ft pitch diameter and μ = 0.006, that is 0.006 × (104,500 + 75,000) ÷ 2 = 539 lb·ft. Kaydon calls the equation an estimate that is not valid at zero load, so the commissioning measurement is the baseline; a later rise in running torque for the same load points to a distorted mounting or lost lubrication.
• Bolt checks. Kaydon's normal application assumes provision for periodic checking of mounting-bolt tension, which belongs on the maintenance schedule the controls track.
• Drives and safety. Allen-Bradley Kinetix 5700 servo drives support DSL and Hiperface encoder feedback, offer position-loop, velocity-loop, and torque-loop axis configurations with a current regulator loop, and feature a safe torque-off function. ISO 13849-1:2023 specifies a methodology for the design and integration of safety-related parts of control systems that perform safety functions, and applies to high-demand and continuous modes of operation.

UTEC Industrial, a Rockwell Automation Recognized System Integrator, builds Allen-Bradley ControlLogix and CompactLogix control, VFD and servo drives, and UL 508A panels into the positioners it fabricates.

Measured weight and CG turn the rating table into an interlock (Kaydon 2017, pp. 16 and 21; Rockwell Automation 2198-UM002E-EN-P, Kinetix 5700; ISO 13849-1:2023).

What should a buyer ask for instead of a single capacity number?

A capacity statement that can be checked names its assumptions. A useful request, and a useful supplier answer, covers:

• Rating envelope per axis: allowable payload against CG offset and CG height, with the governing limit marked in each region.
• Bearing basis: the bearing maker, the selection method, the factor used and its source, and the load combination checked, since factors from different makers are not interchangeable.
• Mounting basis: bolt grade, thread lubrication, and tightening method, since SKF's bolting capacity is valid only for grade 10.9 bolts and nuts with lightly oiled threads tightened to its tabulated values; and, as a separate item, the support structure's rigidity and the flatness of its support surfaces, which SKF lists among its general design considerations for fully using the bearing.
• Axis orientation: vertical, horizontal, or changing, and whether any tilt angle puts the bearing in suspension.
• Reducer check: pinion or sprocket position on the output shaft, the transmission-element factor used, and any outboard bearing.
• Duty: revolutions per year and the load spectrum, so the maker's life check can be run.
• Test load: the proof or acceptance load, included as a bearing load case.

Kaydon makes the equipment designer responsible for determining the correct service factor, so this article recommends that the envelope and its basis be written into the purchase documents rather than left to a catalog. UTEC Industrial performs factory acceptance testing and on-site commissioning, so a loaded test at the maximum CG offset can be witnessed before the positioner ships.

A rating stated this way can be verified at acceptance and enforced in service (Kaydon 2017, p. 16; SKF 2019, pp. 21 and 23–24; ANSI/AGMA 6013-B16).

Related Articles

• Sizing a Positioner: Payload, CG Offset, Overturning Moment, and Torque — the worked torque and overturning calculation
• Headstock-Tailstock vs. Trunnion vs. Turntable Positioners — the axis orientations that decide which bearing case applies
• Stress Relief for Machine Bases and Frames Before Final Machining — keeping bearing mounting faces flat after welding
• Machining Oversized and Heavy Workpieces: What It Takes Beyond Standard CNC — machining the large mounting faces a bearing rating assumes

References

• Kaydon Catalog 390: Slewing Ring Bearings. Kaydon Bearings, 2017.
• MHI ANSI MH29.2-2020: Safety Requirements for Industrial Tilters. MHI, 2020.
• MHI ANSI MH29.3-2023: Safety Requirements for Industrial Turntables. MHI, 2023.
• Baker, D.W. and Haynes, W. Engineering Statics: Open and Interactive. Colorado State University and Massachusetts Maritime Academy, 2026.
• SKF PUB BU/P2 06115/3 EN: Slewing Bearings. SKF Group, 2019.
• thyssenkrupp Rothe Erde: Rothe Erde Slewing Bearings. thyssenkrupp Rothe Erde GmbH, 2016.
• SEW-EURODRIVE: DRN.. Gearmotors (IE3), catalog, project planning section 'Overhung and Axial Loads of R.., F.., K.., S.., W.. Gear Units'. SEW-EURODRIVE GmbH & Co KG, 2026.
• ANSI/AGMA 6013-B16 (R2021): Standard for Industrial Enclosed Gear Drives. AGMA, 2016.
• AWS D1.1/D1.1M:2025: Structural Welding Code—Steel. AWS, 2025.
• AWS D14.4/D14.4M:2019: Specification for the Design of Welded Joints in Machinery and Equipment. AWS, 2019.
• Rockwell Automation 2198-UM002E-EN-P (2018): Kinetix 5700 Servo Drives User Manual. Rockwell Automation, 2018.
• ISO 13849-1:2023: Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design. International Organization for Standardization, 2023.

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