Medium 7-15 inch (180-380 mm) slewing drives delivering 8,000-50,000 Nm output torque and 20,000-100,000 Nm holding torque are the dominant bracket for planters, grain carts, manure applicators, and large center-pivot irrigation booms [S1].
For lighter ag implements, hose carts, and small sprinkler heads, small 3-7 inch (75-180 mm) units in the 1,000-8,000 Nm output torque class are the workhorse, with gear ratios typically between 30:1 and 90:1 [S1]. Selection is driven by the combined axial, radial, and overturning moment load on the rotating structure, not by motor kW alone [S4][S7].
How Agriculture Loads Differ From Construction and Solar Loads
Agricultural slewing drives run in dust, fertilizer residue, manure splash, and high-humidity wash-down cycles, and they must hold position under wind gusts on empty planters and grain-cart augers [S2][S3]. The rotating structure is usually a single implement wing or boom, so the overturning moment comes from the boom's own weight plus the product being carried, not from a 30-tonne load chart [S4]. Enclosed-housing slew drives with sealed bearings are the standard recommendation for these conditions because the seals keep soil fines, ammonia vapour, and spray residue out of the worm gear mesh [S2][S6]. Double-worm configurations are reserved for the heavy end (centre-pivot drive towers and large manure applicator booms) where single-worm units would creep under static moment [S2].
Size and Torque Brackets Mapped to Ag Implements
SlewMaster and other ag-focused suppliers group farm applications into six implement families: manure pump units, hose carts, manure applicators, planters, grain carts, and tillage tools [S5]. TGB Group sizes its agriculture line for harsh field duty with sealed housings and corrosion-resistant finishes [S6]. Mapping these against the size tables from the size guide, the typical fits are: 3-5 inch units for hose-cart reels and small sprinklers (output torque 1,000-4,000 Nm); 5-9 inch units for planter row-unit drives, small grain augers, and tillage tool bar fold sections (output torque 4,000-15,000 Nm); 9-15 inch units for manure applicator booms, large grain-cart auger swing, and centre-pivot drive towers (output torque 15,000-50,000 Nm) [S1][S5]. The weight band tracks with size: roughly 5-25 kg for the small bracket, 25-120 kg for medium, with holding torque typically 2-3x output torque to hold position without continuous motor current [S1].
Selection Criteria: Load, Torque, Speed, Environment, Drive Input
Engineers should start from the actual rotating load, not from the implement's total weight, because only the boom or arm is rotating [S7][S8]. Compute the worst-case overturning moment from boom weight plus product weight at maximum reach, then pick a unit whose holding torque exceeds that moment with at least a 1.5x service factor for dust and vibration [S7][S8]. Speed is usually slow: ag slew drives typically run at 1-5 rpm output, so high-ratio worm gearing (50:1 to 150:1 in the medium bracket) is the norm rather than the exception [S1]. Environment drives the sealing decision: enclosed, IP65-class housings are appropriate for tillage and grain-handling implements, while fully sealed stainless or hard-coat units are specified for liquid manure and fertilizer exposure [S2][S6]. Drive input is split between hydraulic motors (common on tractor-mounted implements where the tractor's hydraulic remote supplies oil) and electric motors (common on centre-pivot irrigation towers and stationary grain handling) [S2][S4]. The selection guide at slewing drive sizing covers the full bracket interaction with the slewing bearing raceway as one integrated assembly.
Comparison of the Three Slew Drive Types Used in Agriculture
Three configurations cover the ag range, and the choice is a trade-off between cost, torque density, and sealing [S2]. Single worm slew drives are the most common and cost-effective option, used where load is moderate and a self-locking worm gear is acceptable for position holding. Double worm slew drives deliver roughly 1.5-2x the torque capacity of a single-worm unit of the same OD by engaging the worm wheel from both sides, and they are the standard fit for manure applicator booms and large centre-pivot towers. Enclosed slew drives prioritise sealing and contamination resistance over raw torque density and are the right pick for implements that see daily wash-down or constant dust, such as planters running in tilled soil. Open slew drives are the cheapest but are generally avoided in ag because dust ingress kills the worm gear mesh quickly [S2].
Where Slewing Drives Sit in the Ag Machine Architecture
On a centre-pivot irrigation system, each drive tower carries a slewing bearing that the drive wheel rotates around, allowing the span to track the pivot without skidding; the slew drive itself is what turns the span relative to the tower. On planters, slewing drives index row-unit fold arms and fertilizer hopper positioners, replacing the hydraulic cylinders and chain drives used on older designs. Manure applicator booms use slew drives to swing the distribution head, and grain carts use them to rotate the auger spout, which is why a grain-cart slew drive must hold position with a full auger of grain hanging outboard and a wind gust pushing on it [S5][S3]. For larger ag platforms, the slewing ring bearing inside the drive is sized separately from the gearbox, and the two are typically sourced as an integrated unit from the slew drive OEM rather than mixed-and-matched [S1][S8].
Limits, Failure Modes, and Standards to Watch
The dominant failure mode in agricultural slew drives is seal failure followed by worm gear wear from ingested dust or fertilizer, not bearing fatigue [S2][S4]. Holding torque is degraded once the worm thread shows visible pitting, and creep under static moment is the early warning sign. Lubrication interval is the single biggest controllable variable, with most OEMs specifying a re-grease every 500-1,000 field hours for ag units [S2]. For heavy ag platforms that share architecture with compact construction equipment, the construction machinery and equipment duty-cycle profile is a useful cross-reference. Standards to anchor on: ISO 6336 for gear load capacity calculation, and the OEM's own moment rating chart, which must be applied with the ag-specific service factor (typically 1.25-1.5) rather than the lighter solar-tracker factor [S7][S8]. When the rotating structure is driven by a hydraulic motor, the drive motor selection sets the continuous torque limit, and the slew drive's holding torque must exceed peak static load regardless of motor size. For electrically driven ag slew drives on stationary equipment, the servo drive sizing is driven by the acceleration phase of the boom, not the steady-state rpm.
For adjacent process coverage of sealing, material, and torque spec selection in hygienic environments, see slewing drive selection for food processing; for ag platforms that also cross over into cable and connector specification, the cable gland sizing guide is the next node.