Short answer
A hydraulic auger is an attachment that turns an excavator’s hydraulic flow into rotation and bores holes in the ground. What a breaker does with impact, an auger does with rotation and axial thrust: the bit cuts the ground, the flighting carries the cut material up and out of the hole. Because the working principle is simple, the choice comes down to three things: the diameter and depth of the hole, the type of ground, and the hydraulic flow the machine can deliver. Once those three are set, the remaining detail is the bit.
How it works
Hydraulic oil turns the auger’s hydraulic motor. Motor speed is usually high and its torque low, so a gearbox sits between: it reduces speed and raises torque. It is that torque that does the work in the ground.
The shaft passes the rotation from the gearbox to the auger flighting. The cutter at the end of the flighting cuts the ground; the flighting itself works like a conveyor and carries the cut material upwards.
Axial thrust comes from the boom, but too much of it does damage: an auger pulls itself in, and the boom’s job is only to maintain contact. Pressing too hard loads the flighting, clogs it and leads to sticking.
The main parts
- Hydraulic motor: the part that produces rotation.
- Gearbox: the drive that reduces speed and raises torque. The most sensitive and most expensive part of an auger.
- Output shaft: passes rotation from gearbox to auger and carries axial and radial load too.
- Auger body and flighting: the helix that carries cut material upwards.
- Cutting teeth and pilot bit: the parts that enter first and do the cutting. They change with the ground.
- Bracket: joins the auger to the machine. As with other attachments, it is made to the machine.
Choosing a bit for the ground
The single most important choice governing an auger’s output is the bit. A bit designed for earth and clay dulls quickly in stony ground; one designed for rock is needlessly expensive in soft ground and advances more slowly.
In soft and medium ground plain cutting blades are enough. In stony and hard ground carbide tipped cutters are needed; they resist wear but are more sensitive to impact.
In rock an auger is not always the right tool. For massive rock a rock drill or another method should be considered; persisting with an auger in rock strains the gearbox.
The pilot bit is the whole of the alignment
The pilot bit is the part that enters the ground first and holds the centre. Working with a worn or broken pilot, the auger loses its centre, the hole goes off vertical and the shaft starts seeing off-axis load. Far cheaper than the cutters themselves, it is the first item to check regularly.
Where it is used
An auger’s advantage appears in repetition: where many holes of the same diameter are needed, the difference against hand or bucket work is large. For a single hole, the set-up time eats that advantage.
- Post, fence and lighting holes: the most common use.
- Tree planting and landscaping: fast, repeatable holes in landscaping and agriculture work.
- Small diameter holes in foundation and ground improvement work.
- Installing signage, railings and safety barriers.
- Vertical holes for drainage and cable entries.
What to look at when choosing
- Hole diameter: the auger diameter follows it, and the torque required changes with diameter.
- Depth: where the required depth exceeds one auger length, an extension shaft is needed.
- Ground type: it sets the bit type and the torque required.
- The machine’s hydraulic flow: the motor and gearbox selection depend on it.
- Machine weight: an auger is a light attachment, but its rotational reaction goes back into the machine; on a very small machine that creates a balance problem.