A cable-tool drill rig is a tripod of three poles 5-6 m long lashed at the top, spread so the base is about 60-70% of the pole length, with a pulley at the apex and a 6-10 mm rope running over it to a 30-50 kg bit. In normal use each leg carries only a few hundred newtons, far below what a straight pole can bear, so the parts that fail are the rope, the lashing and the feet spreading, and the rope must be tested and rated at ten times the static weight of the bit.
Part of Deep Wells
A drill rig is the hoisting frame that raises and drops the bit and bailer in percussion drilling. A hand-worked rig is a tripod, a pulley and a rope, and its loads can be worked out before it is built.
The Poles Are Not The Weak Part
The loads on a tripod are easy to calculate, and they show where to put the effort. With a pulley at the apex, the apex carries the sum of the rope on each side. If the free end of the rope is hauled straight down, the apex load is twice the rope tension T. For a 50 kg bit held still (bits run 30-50 kg), T = 50 × 9.81 = 491 N, and the apex load is P = 2 × 491 = 981 N.
Leg force. Each of three legs of length L is spread so that its foot is a horizontal distance r from the centre. The angle from vertical is θ = arcsin(r ÷ L). For poles 5.5 m long spread to a base diameter of 3.76 m (68% of 5.5 m, within the 60-70% the Tripod Lashing guide recommends), r = 1.88 m, sin θ = 1.88 ÷ 5.5 = 0.342, θ = 20°.
- Force along each leg = P ÷ (3 × cos θ) = 981 ÷ (3 × 0.940) = 348 N
- Outward push at each foot = P × tan θ ÷ 3 = 981 × 0.364 ÷ 3 = 119 N
Buckling. A pole 5.5 m long and 0.10 m across, with timber stiffness E = 10 GPa (a typical value for sound timber, recalled rather than measured), has I = π × 0.10⁴ ÷ 64 = 4.9×10⁻⁶ m⁴ and Euler buckling load = π² × E × I ÷ L² = 9.87 × 10¹⁰ × 4.9×10⁻⁶ ÷ 30.25 ≈ 16 kN. The leg load of 348 N, a few hundred newtons, is far below this, so poles that are straight and sound do not buckle in this use.
A stuck bit changes the picture. If the crew pulls 2 kN on the rope, the apex load is 4 kN, each leg carries 4 ÷ (3 × 0.940) = 1.4 kN, and each foot is pushed outward with 4 × 0.364 ÷ 3 = 0.49 kN. The legs still hold, but the feet will slide and the lashing may slip long before the poles give.
What You Need
- Three straight poles, 5-6 m long — sound, without splits, similar in thickness
- Lashing cord — for the top; see Tripod Lashing
- A pulley — a hardwood sheave on an axle between two cheek plates (Single Pulley)
- A rope of 6-10 mm — natural fibre or synthetic, tested (Rope Strength Testing)
- Ground ties or foot pits — to stop the feet spreading
- A collar plank — a plank with a hole at the mouth of the borehole to guide the tools and stop them falling in
Method: Build, Test The Rope, Then Load
Step 1: Lash and raise. Lash the three poles loosely at the top so the legs can spread, add frapping turns for the grip, hang the pulley from the lashing, and raise the frame over the borehole mark.
Step 2: Set the spread. Spread the feet until the base diameter is 60-70% of the pole length; for 5.5 m poles that is 3.3-3.85 m. Make the apex plumb over the hole.
Step 3: Anchor the feet. Dig each foot into a shallow pit, or tie the three feet together at ground level with a rope triangle, so the 0.12-0.5 kN outward push cannot spread them.
Step 4: Rate the rope. Hang weights from a length of the rope until it breaks (Rope Strength Testing). That corpus guide uses a working load of one-fifth of breaking strength for general lines and one-tenth for anything that supports a person, and warns that shock loads of two to ten times the static weight are common. A drilling rope may snatch tight and whip, so take the one-tenth rule: for a 50 kg bit (491 N) the rope must break at no less than 10 × 491 = 4.9 kN, about 500 kg-force. If your tested rope breaks lower, use a lighter bit, double the rope, or add a block and tackle to share the load.
Step 5: Check the sheave. The pulley wheel must be large enough that the rope does not bend sharply (10-15 cm is the sheave size in the Single Pulley guide), run freely, and be fastened so that it cannot jump off the axle.
Step 6: Test with a dead load first. Hang a weight about equal to the bit from the rope for a few minutes, walk round the rig, then lift it a short distance. Look for slipping lashings, sinking feet and a leaning frame.
Common Mistakes
- Rope never tested. A rope that looks strong breaks at a fraction of the weight you assume.
- Feet unanchored. A tripod on a smooth or wet surface spreads and drops.
- Apex not over the hole. The bit then drags on one side of the borehole and the frame leans.
- Letting the bit run on the rope. Letting a falling bit snap the rope taut adds a shock load many times its weight.
- Hand-gripping the rope. A rope under load drags a wrapped hand into the pulley.
Safety
A rope under load stores energy, and a falling bit can break bones
Stand clear of the line of the rope and the legs of the tripod when the bit is lifted or stuck; never wrap rope around your hand or body; keep fingers away from the sheave. If the bit sticks, do not heave on it with more than the rope’s rating, and do not put anyone under the bit or the tripod. A frame that tilts or a rope that creaks means stop and unload.
What’s Next
- Learn next: Bailer Operation — working the other tool on the same rope
- Combine with: Percussion Drilling — the full drilling method