Stone picking regularly ranks as one of the most hated jobs in the farming calendar, but for ex-farmworker Carl Hobbs it is proving a profitable side-line business.

Keen to break away from a career of farm labouring, three years ago he took a punt on Finnish firm Peltuote’s tractor-towed Kivi-Pekka stone collector with a view to starting his own country-wide self-drive hire service.

See also: Stonebine rock picker is 2020 inventions winner

He was already wise to its appeal, having previously slogged away for a couple of days each year clearing stones before drilling as part of a three-man gang on a local farm.

After gauging interest among other businesses in north-west Somerset, where limestone rocks are the scourge of arable and grass farmers alike, he pulled the trigger on a £32,000 investment through Northumberland-based importer Dales Agri Sales Agency.

A steady stream of work followed its arrival in September 2020 across various sectors, including farming, sports ground developments and construction site clear-ups.

The soaring price of agricultural machinery has helped his cause, with farmers more concerned than ever about the cost implications of putting a stone through a combine or forager, and the wear and tear to mowers, cultivators and drills.

As Carl tends to lease it out, rather than operate it himself, it means he can run the operation alongside his current role as a partner on his in-laws’ farm.

Limited choice

After settling on stone picking as his sector of choice, Carl trawled the internet to consider his options.

“It turned out that there were only two machines to pick from, and the Kongskilde Stone Bear is basically an old version of the Kivi-Pekka; although more or less identical, it isn’t as strongly built.

“There was nothing on the second-hand market either, which suggested they were pretty reliable, but it meant I had to buy new.”

He settled for a 4m model as a reasonable balance between output and cost.

This has fold-down wings angled forwards, under which are 1.9m-long rollers with steel bar tines that protrude by 12cm to form a prickly rotor designed to tease stones to the central pick-up.

Wear is minimal, as they purely work to flick loose surface material rather than doing any intensive digging or cultivating, and after three seasons’ work there is little sign of deterioration bar a slight smoothing of the tips.

Dales Agri Services estimates they should be good for at least a decade.

Power comes from the tractor’s 540-speed pto via a central gearbox and driveshafts that run to the end of each wing.

Steel sieves form the floor and rear wall of the hopper © MAG/Oliver Mark

It’s here that the only minor problems have occurred, with Carl having replaced five universal joints (UJs) so far.

“The pto should be put into neutral before folding up, otherwise there’s no give in the driveline. I suspect customers don’t always do this, as I’ve replaced a handful of UJs since I’ve had it. The obvious solution would be to swap the mechanical drive for a hydraulic system, and it’s something I might look at doing in the future.”

Five V-belts then transmit the torque to the spiked rotors. These effectively work as a slip clutch, so they’ll stop driving if they encounter too much resistance.

“It’s a rake rather than a cultivator, so getting it working at maximum efficiency relies on working the ground properly beforehand,” says Carl.

“Ideally, it should be ploughed and then spring-tined in relatively dry conditions, leaving a loose finish with the stones at the top – if it’s too cloddy then the machine will pick up big lumps of soil.”

Depth is set via a pivot on the semi-mounted linkage and turnbuckles for the small wheels behind the wings, with the tines designed to work 50-75cm deep. 

Fancier models with isobus control have an ultrasound sensor to automatically adjust the working depth.

Carl Hobbs © MAG/Oliver Mark

Pick-up and hopper

At the centre is a 70cm-wide mouth, fed by a belt-driven lifting drum that carries 18 heavy-duty sprung cultivator-style tines.

These push the material over a steel sieve and into the 3cu m hopper.

The tine spacings allow it to consume anything up to 35cm in size – about that of a breeze block – and a grille underneath sees small stones and topsoil drop back to the ground.

Though this can leave a slight ridge in the field, it’s never significant enough to require another cultivation pass, so drilling can commence straight after.

Particularly big or awkward-shaped boulders can occasionally pinch between the front of the pick-up and the tines.

As there’s no reverse-drive function, these are freed using a steel bar and a bit of elbow grease.

More sieves are fitted on the floor and back wall of the hopper, the latter providing a further degree of cleaning as the load is tipped.

There’s no fill alert on the hopper and it’s impossible to see inside from the tractor cab but, says Carl, operators can soon tell when it’s full as stones start tumbling noisily out of the front of the pick-up.

The up-and-over unloading system sees a long-stroke ram lift the hopper upwards to clear a 2.9m lip, allowing it to empty straight into a dump trailer.

Rotor tines tease stones from the soil and flick them towards the intake © MAG/Oliver Mark

Output

The 4m model is rated to collect up to 700kg/min, with Carl’s best day yet returning 500t of stone from a 1.5ha game plot in Exmoor.

This is a potentially valuable product, either for forming farm tracks or selling on to the building trade – particularly in the case of flint – and can help cover the £600/day or £2,500/week hire cost.

Working speed ranges from 1kph to 6kph, depending on the stone burden, which equates to about 0.4ha/hour.

Given this snail-like progress, it’s best run on a tractor with a stepless transmission to avoid constantly riding the clutch.

However, the power requirement is modest, so a tractor of little more than 70hp should be adequate.

This should also help minimise compaction, with the weight of the 4.3t implement, and up to 4t in the hopper, spread by a set of 560/60 R22.5 flotation tyres.

Driveshafts on each wing transmit power to the rotors via five V-belts © MAG/Oliver Mark

Carl now has plans to add a smaller second model to the Hobbs Hire fleet.

The 3.3m Kivi-Pekka can pick up the same size stones, but its simpler design features small fold-out wings with tines rather than rotors, and it has a 2m-wide central pick-up with 53 lifting tines.

This increases collection rates by as much as 200kg/min compared with his existing version, and it also has a bigger 5cu m tank.

“It’s a good deal cheaper too – the 4m model has gone up in price by £10,000 since I bought it.

“And because the design is simpler still, with no folding wings, I won’t have the running costs of replacement UJs and pto guards, which are getting expensive.”

Harvested stone can be unloaded straight into a dump trailer © Carl Hobbs