How to Select a Slew Drive: Torque, Load, Ratio, and Backlash

A slew drive (also called a slewing drive) is an integrated bearing-and-gear assembly that rotates a load while supporting axial force, radial force, and overturning moment. The slew drive gearbox has two jobs at once: it transmits torque, and it carries the bearing loads.

Selecting one therefore takes more than matching motor torque to a catalog number.

A high torque rating cannot compensate for insufficient bearing capacity. For a basic introduction to the mechanism, see what a slewing drive is.

Start with a Load Case, Not a Part Number

A common sizing error is to compare only the largest torque figure in each catalog row. Start from the machine instead.

Keep normal operation, emergency stops, wind events, and stationary holding as separate load cases. The worst torque and the worst overturning moment may occur at different times.

Calculate Output Torque

For a tangential force applied at a perpendicular lever arm, T = F × r. A 2,000 N force at 0.5 m produces 1,000 N·m.

Acceleration adds T_acc = J × α, where J is the moment of inertia about the rotation axis (kg·m²) and α is angular acceleration (rad/s²). With J = 500 kg·m² and α = 0.2 rad/s², acceleration adds 100 N·m. If a 1,000 N·m resisting torque acts at the same time, the simplified demand is 1,100 N·m.

For a real machine, evaluate the full torque-versus-time profile. Compare continuous operation with the manufacturer's nominal rating, and short transients with its applicable peak rating. Apply the manufacturer's service factor or application factor where specified; no universal factor suits every slew drive and duty cycle. For a deeper explanation of starting and running torque, see slewing bearing torque calculations.

Check the Bearing Loads Independently

• Axial force (Fₐ) acts along the rotation axis. A vertically mounted bearing supporting a 1,000 kg mass carries about 9.81 kN from gravity if it supports the entire weight.

• Radial force (Fᵣ) acts perpendicular to the rotation axis. Wind and lateral machine forces are common sources.

• Tilting moment (M) acts about an axis transverse to the rotation axis. For a perpendicular force and offset, M = F × e.

• Output torque (T) acts about the rotation axis and drives or resists slewing motion.

Torque and tilting moment share the unit N·m, but they load different parts of the assembly. A drive can transmit enough torque and still exceed its bearing's permissible combined loading.

Check simultaneous axial force, radial force, and tilting moment using the specific model's combined-load curve or the manufacturer-approved calculation. On LILY product pages this appears as a limiting load diagram: equivalent tilting moment plotted against equivalent axial load, with a bolt curve and a raceway curve. Static load limits, dynamic operating capacity, and fatigue life address different failure mechanisms, so a large catalog maximum does not establish acceptable service life.

A manufacturer’s limiting-load diagram is model-specific. Its axes may use equivalent loads and assumptions about radial force, bolt loading and mounting stiffness. Convert each actual load case using the stated method before comparing it with the permitted envelope; do not plot raw axial force and overturning moment against a curve unless its instructions explicitly allow that.

Worked Example: The Bearing Can Govern

Imagine an outdoor vertical-axis positioning structure. It needs 2,000 N·m of output torque to rotate, and its bearing carries a 12 kN axial load. In one lateral load case, a single 8 kN horizontal force acts 1.5 m above the bearing reference plane.

That one force produces Fᵣ = 8 kN at the bearing and an overturning moment of M = 8 kN × 1.5 m = 12 kN·m. There is no separate radial load to add.

The 2,000 N·m operating requirement cannot be cleared using a maximum-torque rating alone: the candidate’s nominal torque, permitted peak duration and duty cycle must first be checked. Separately, assume its manufacturer-approved combined-load assessment, accounting for the simultaneous 12 kN axial force, 8 kN radial force and 12 kN·m overturning moment, shows that this load case exceeds its permissible limit. The candidate is then unsuitable even if its torque rating is adequate. Increasing motor torque does not increase bearing capacity. This is a teaching example, not a test result or selection verdict for a LILY Bearing model.

Match the Gear Ratio to the Motor and Speed

For reduction ratio i = n_in / n_out, a 60 rpm input into a 60:1 reduction gives 1 rpm at the output. The steady-state output torque estimate is T_out = T_in × i × η, where η is the efficiency of the transmission. With T_in = 20 N·m, i = 60, and an illustrative η = 0.45:

T_out = 20 × 60 × 0.45 = 540 N·m

The 0.45 efficiency is an arithmetic assumption for the complete reduction train, not a measured value for any LILY Bearing drive. For motor sizing, obtain the efficiency and permissible speed at the actual operating point. Do not infer self-locking or safe holding from the assumed efficiency; check the model’s published holding and back-driving behavior, and specify a brake or lock when the application requires one.

If the motor runs much faster than 60 rpm, for example 1,800 rpm, an upstream gearbox or a larger total ratio is needed to reach a slow output speed. Check the motor's speed-torque curve, its thermal limits, and the drive's permitted input speed. UC San Diego's machine-design explanation of gear ratios covers the underlying speed, power, and torque relationships.

Specify Backlash and Positioning Error Separately

Backlash is lost motion caused by clearance in the gear mesh when the direction of transmitted torque reverses. It is only one component of total positioning error.

At a 2 m radius, 0.1° (6 arcminutes) corresponds to an arc distance of s = r × θ = 2 × (0.1π/180) ≈ 3.49 mm. This is a geometric conversion, not a measured machine error.

To compare drives fairly, state the allowed output backlash, the test torque, and the measurement method. Structural deflection, bearing clearance, mounting stiffness, and encoder location can matter as much as the gear mesh.

Check Duty, Holding, Protection, and Life

Before choosing the final part number, resolve these questions:

• Static versus dynamic loading: Can the bearing support the worst stationary load, and does the operating duty give acceptable fatigue life?

• Duty and service factor: How often does the unit start, stop, reverse, or take shock loads? Apply the model's specified duty limits and factors.

• Holding and back-driving: Is a brake or independent lock required? Self-locking depends on geometry, friction, lubrication, and wear, so it cannot be assumed for worm gearing.

• Ingress protection: What dust and water exposure will the drive, motor, seals, and connectors see? An IP rating applies only to the assembly or component it was tested on; do not infer it from a sealed housing.

• Lubrication and environment: Are the grease, relubrication interval, temperature range, and corrosion protection suitable?

Worm Gear vs Spur Gear Slew Drives, and the LILY Bearing Series

Both gear types are used in slew drives. These are general design tendencies, so confirm the details against each model's data.

LILY Bearing publishes worm-gear WD-H and WD-L families and straight-tooth (spur) SP-H and SP-I families. The following two configurations of one verified WD-H size illustrate why nominal and maximum torque must be kept separate.

Published WD-H 0300 figures are catalog ratings, not approval for a particular load case. The corresponding bearing-load diagram, installation, motor, duty and holding requirements still need separate review. Series-level tilting-moment ranges must not be used as a substitute for a model-specific combined-load assessment. Source: WD-H 0300 / 1 drive technical specifications.

The WD-H series catalog marks these configurations “Self Locking Gears: No”. Where unintended rotation is hazardous, evaluate a suitably rated brake or independent locking arrangement rather than assuming worm gearing will hold the load.

Slew Drive Selection Checklist for an RFQ

Send the completed input table above, plus the mounting drawing, a force diagram showing where each load acts, the motor preference, and the environmental conditions. For technical selection support, contact us.

Frequently Asked Questions

Can a slew drive rotate continuously?

Yes, if the chosen model is rated for the speed, duty, temperature, and lubrication involved. Look for a continuous-duty rating in the model's data; an intermittent-positioning rating does not imply it.

Is a slew drive the same as a slewing ring bearing?

No. A slewing ring bearing supports rotation; a slew drive integrates the bearing with a drive gear arrangement so torque can be applied through its input. The terms slew drive and slewing drive refer to the same type of assembly.

Can an encoder remove backlash?

No. An encoder measures position. Closed-loop control can compensate for some repeatable lost motion, but the mechanical clearance remains.