Rotary drilling rig (kelly rig for bored piles and drilled shafts)

01

Check torque and crowd across the speed range

The FHWA drilled shafts manual (GEC 10, FHWA-NHI-18-024, 2018) states that "the capacity of a drilling rig is often expressed in terms of the maximum torque that can be delivered to the drilling tools and the 'crowd' or downward force that can be applied"; tool type and details also strongly affect efficiency, but torque and crowd are important factors in drilling rate. It notes that "most excavations for drilled shafts in the United States are made by some type of rotary-drilling machine", which transmits force from the power unit through the rotary to a kelly bar and the tool. [1]

The manual explains that hydraulic drive can give a higher torque range and lets the rotary move up and down the mast, and that "more often the crowd in a hydraulic system is applied through the rotary", while "mechanical drive systems often apply crowd through a pull-down system applied to a drive bar atop the kelly"; hydraulic rigs are sometimes heavier and more expensive than mechanical machines of similar size. Rigs may be mounted on trucks, crawlers, excavators or cranes, or work as top-drive units on casing. [1]

SANY's SR285R data sheet gives a rated output torque of 285 kN·m, rotation speed of 5–24 rpm, crowd force of 260 kN, line pull of 335 kN, a 6,000 mm crowd stroke, a 330 kN main winch and an Isuzu engine of 300 kW at 1,800 rpm. Its standard-mode torque-speed curve runs from 285 kN·m at 5 rpm to 102 kN·m at 24 rpm. [2]

Editorial calculation: rotary power is torque × speed × 2π/60, so 285 kN·m at 5 rpm is about 149 kW and 102 kN·m at 24 rpm about 256 kW, within the 300 kW engine; quoting only the peak torque hides how much is left at drilling speeds, so ask for the full curve. [2]

Give the supplier the pile diameters and depths, ground profile (soil, gravel, boulders, rock strength), whether casing or slurry will be used, the tools needed (augers, buckets, core barrels, casing drive), site access and ground bearing, transport limits, emission standard required, and the installation plan the contractor will submit.

02

Choose the kelly bar for depth and ground

FHWA's manual describes the kelly bar as the steel drive shaft that transmits torque and crowd to the tool; kellys are round or square, either single-piece (up to about 60 ft long) or telescoping with inner sections to reach greater depth. "Square kelly bars often require a worker to insert a pin to lock the outer bar to the inner telescoping kelly piece, whereas round kellys often include an internal locking mechanism"; in some rigs the weight of the kelly and tool provides the crowd. [1]

SANY lists the SR285R's maximum depth as 94 m with a friction kelly and 61 m with an interlocking kelly, with Φ508 mm kelly options in 3, 4 and 6 sections weighing about 13.4–15.8 t, a maximum drilling diameter of 2,300 mm and an operating weight of 100 t. It describes the rig as suited to soil, pebble, rock and mixed layers. [2] Its product range lists rigs from 1,000 mm to 3,200 mm maximum pile diameter and 23 t to 180 t operating weight. [3]

Editorial note: because the quoted maximum depth depends on the kelly fitted, confirm with the supplier which kelly the depth refers to, its weight, and the drilling tools that come with the rig.

03

SANY's kelly rig range by pile diameter, depth and weight

SANY, the one card shown, lists rotary drilling rigs from about 1,000 mm to 3,200 mm pile diameter alongside hydraulic grabs and trench cutters.

Ask SANY, or any other rig maker you approach, for the torque-speed curve, crowd force and stroke, kelly options and weights, winch capacities, engine and emission stage, and transport dimensions.

04

Compare torque per gear and kelly weight against depth

Liebherr's technical data for its LB 28 drilling rig (US units) splits the rotary drive into two gears: 210,950 lbf-ft at up to 30 rpm in 1st gear and 109,160 lbf-ft at up to 59 rpm in 2nd gear, from a 350 kW (469 hp) engine rated to ISO 9249 that meets EPA/CARB Tier 3 and EU Stage III. Its rope crowd system gives 90,000 lbf push and pull with 60 ft travel of the working tool. Maximum kelly drilling diameter is 6.2 ft uncased and 4.9 ft cased. Its twelve MD 28 kelly bars, all 16.5 inches in diameter, run from 72.2 ft drilling depth to 229.6 ft; the quoted operating weight of 209,450 lbs includes the MD 28/3/30 kelly and 33,730 lbs of counterweight. [4]

Editorial calculation: 210,950 lbf-ft × 1.3558 ≈ 286 kN·m; at 30 rpm that torque would need 286 × 30 × 2π / 60 ≈ 898 kW, against the 350 kW engine, so the 1st-gear torque and speed are separate limits, not one operating point; ask for the torque-speed curve, as for the SANY rig above. The deepest kelly, 229.6 ft, is about 70 m (229.6 × 0.3048). [4]

05

Carrier, mast, rotary drive, crowd, kelly, winches and drilling tools

Carrier, mast, drive, crowd and kelly rows follow FHWA's description of rotary rigs; the winch row comes only from SANY's SR285R data, and the right-hand column names what to fix for your piles.

06

Depth, winch pull, speed, power, height, diameter, crowd, torque and weight for a piling-rig quote

Published values are the SANY SR285R's unless the row names the Liebherr LB 28; check the maximum depth against the kelly it was quoted with.

07

Send the pile, ground and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Piles: [diameters; depths; number]
Ground: [soil; boulders; rock strength; water]
Method: [dry; casing; slurry]
Rig: [torque curve; crowd; kelly type]
Tools: [augers; buckets; core barrels]
Site: [access; bearing; transport]
Engine: [power; emission stage]

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