Rotary drilling rig selection is governed by three hard spec gates — output torque, maximum hole diameter, and mast/mount geometry — because a rig under-specced for ground conditions stalls inside the first 20 m of tool string. Comacchio lists the CH 150 multifunction unit at 750 mm drilling diameter, marking the upper end of the compact rotary class typical of geotechnical and anchoring crews [S1]. SANY positions the SR235-C10 as a crawler diesel piling rig in the mid-250 kNm torque bracket, the workhorse band for cast-in-place piling and foundation work [S2].
The winch-mount study on the Scientific.Net paper index isolates a real engineering trade-off: the position of the main winch on the mast changes stress distribution in the drilling mast, and mispositioning is a documented driver of fatigue cracking in the upper-structure steelwork [S3]. That finding alone justifies treating mast strength and hoist line pull as first-class spec inputs, not afterthoughts. For context on how spec-driven selection looks in adjacent process equipment, the induction furnace sizing guide walks the same torque-class-vs-throughput logic on a different machine family.
Torque Class and Hole Diameter Bands
Rotary rigs cluster into three practical torque bands that map almost 1:1 to typical borehole diameters: compact units below 100 kNm for 400–750 mm anchoring and water-well work (Comacchio CH 150 sits in this band at 750 mm) [S1]; mid-range 150–280 kNm crawler units for 800–1500 mm piling (SANY SR235-C10) [S2]; and heavy 350+ kNm rigs for 1500–3000 mm large-diameter foundation shafts. The full hydraulic core-drill architecture common to modern rigs relies on a variable-displacement pump and a variable hydraulic motor to deliver stepless torque and RPM, which removes the conventional gearbox and trims machine mass by an estimated 15–25% versus a gear-driven equivalent of the same torque class.
Hole diameter is the single most abused spec on data sheets: vendors quote "maximum diameter under ideal soil" rather than continuous-duty diameter. A 750 mm Comacchio rating implies soft overburden, not coring into weathered rock, where sustained torque demand and bit life both drop sharply. A peer reference that covers torque-vs-throughput selection more broadly is the industrial gear buying guide, which shares the same duty-cycle and service-factor language.
Mast Geometry, Winch Mount and Structural Limits
Mast height, racking-board length, and main-winch line pull decide whether a given rig can pull long casing strings or triple casings without a crane assist. The Kang and Xu study in the Scientific.Net rotary-rig keyword index explicitly models how winch position alters mast stress concentration, and recommends positioning the main winch to keep the mast section modulus symmetric under hoisting load to avoid fatigue initiation at the crown [S3]. TR60 from the Chinese OEM catalogue uses an "advanced hydraulic loading back" architecture, with self-erecting capability intended to cut mobilisation time on congested urban piling sites [S5].
For crawler piling rigs in the SANY class, the undercarriage must support both transit load (typically 60–80 t for a 235 kNm rig) and the rotary's peak crowd-down force, which is where many mid-size contractors over-spec the carrier and under-spec the mast. Sizing the rotary drive without re-checking the mast's combined hoisting + crowd load envelope is the most common path to a cracked crown or bent kelly bar in the first 2,000 hours.
Power Pack, Hydraulics and Duty Cycle
Modern rotary rigs are full-hydraulic: a single diesel drives one or more variable-displacement piston pumps feeding a closed-loop swing and an open-loop main circuit, with a pressure transmitter on the rotary head line for crowd-load feedback. The CH 150 multifunction unit runs as a compact drilling module aimed at tight-site geotechnical work, where diesel-electric is rarely worth the capital premium [S1]. The SR235-C10 retains a direct-diesel crawler layout with hydraulic rotary head, a configuration that remains standard for the 200–280 kNm piling class because it keeps ground-bearing pressure low and service points accessible [S2].
Full-hydraulic architecture lets the operator vary rotary speed and torque independently across roughly a 10:1 range, which is what allows the same rig to switch from 6–10 RPM high-torque piling to 80–150 RPM coring without a bit change. Sourcing the variable pump and motor from the same hydraulic family is a common way to keep spare-parts inventory small; it is also why rotary hammer tooling specs are sometimes reused as a cross-check on hydraulic motor curves.
Site Constraints: Crawler vs Truck vs Modular
Site access decides carrier type before torque does. Crawler units like the SR235-C10 take 2.5–3.0 m of working width and can grade themselves onto soft pads; truck-mounted rigs need firm access roads and overhead clearance but mobilise in a single load. Modular self-erecting rigs such as the TR60 are designed to rig up inside a standard container footprint and are aimed at geothermal and small-diameter water-well crews who redeploy weekly [S5].
Ground-bearing pressure is the silent killer: a fully rigged 230 kNm crawler exerts roughly 80–100 kPa on outrigger pads, and most urban piling mats are rated for 150 kPa — fine on compacted stone, marginal on clay. The Comacchio CH 150 class is light enough to work off temporary track mats in alleys and inside existing structures, but trades off maximum hole diameter and crowd force for that portability [S1]. For projects where site access is constrained, the same logic of "carrier envelope first, then torque" shows up in the AMR selection 2026 piece.
Application Mapping: Piling, Geothermal, Anchoring, Core
By application: piling (800–2000 mm cast-in-place) maps to crawler diesel rigs in the 150–350 kNm class — SANY SR235-C10 is the centre of this band [S2]. Geothermal drilling uses modular or truck rigs in the 80–200 kNm range, often with high-RPM rotary heads for air-hammer or mud-rotary conversion, and the geothermal-drilling reference frame explicitly covers high-temperature downhole work that demands alloy tool strings and foam-circulation capability. Anchoring and micro-pile work under 400 mm diameter is the CH 150's natural habitat, where portability and quick set-up outweigh raw torque [S1].
Full-hydraulic core drilling — a separate machine family that shares many hydraulic sub-components with rotary piling rigs — uses the same stepless variable-pump/variable-motor logic to span low-RPM high-torque coring and high-RPM diamond-drilling modes, and is the architecture on which a lot of exploration-grade rotary tooling is now standardised. A useful cross-reference is the rotary drilling rig encyclopedia entry, which collects the torque, RPM and crowd-force envelopes side by side.
Limitations, Failure Modes and Sourcing Reality
Common sizing failures: picking a rig on maximum hole diameter rather than continuous-duty torque; ignoring mast strength when adding larger casings; under-sizing the hydraulic cooler for tropical sites; and pairing a high-RPM rotary head with low-RPM-only bit bearings. The Scientific.Net mast-stress work [S3] is the cleanest published reminder that structural, not hydraulic, limits most often end a rig's working life.
On sourcing, mid-class rotary rigs remain dominated by Chinese OEM lines (SANY, XCMG, Bauer-substitute suppliers), with European builders like Comacchio holding the compact and specialty end [S1][S2]. Independent rig providers such as Rotary Drilling Co. Ltd. continue to operate regional fleets across the EU [S4]. Buyers comparing 2026 quotes should anchor on torque class, mast section modulus and hydraulic pump family — not on the headline diameter number. For a related spec-first selection pattern on a different machine family, the induction furnace suppliers 2026 map uses a comparable torque-class / duty-cycle / sourcing-tier framework.
Trackable signals for the next spec cycle: (a) Chinese OEM moves into the 350–500 kNm class to close the gap with European heavy-piling builders; (b) wider adoption of single-pump dual-circuit hydraulics to cut parasitic losses; (c) revised mast-stress guidance from peer-reviewed rotary-rig papers building on the Kang/Xu winch-mount model [S3].