Self-Loading Log Truck

Overview

A self-loading log truck carries its own knuckleboom crane so it can load and unload roundwood without a separate loader at the landing. This independence matters in forestry, where harvest sites are remote, access is poor, and bringing a dedicated wheeled loader to every landing is uneconomic. The driver parks at the roadside, deploys the outriggers, and uses the crane to pile cut logs onto the bunks of the truck and its pole trailer. At the mill the same crane unloads the stack.

The vehicle combines a high-mobility all-wheel-drive driveline for forest tracks with a robust hydraulic system powerful enough to run a heavy timber crane. The crane folds onto a frame between the cab and the load when travelling on public roads.

Knuckleboom crane

The crane is a knuckleboom design: a slewing column carries an inner boom that is jointed to an outer (knuckle) boom, and a hydraulic telescopic extension reaches the far end of the log pile. This jointed geometry folds compactly for transport yet unfolds to 8 m or more of outreach. The booms are fabricated from high-strength S690QL plate, stepping up to S960QL for the thin-walled extension where weight matters most.

Two lift cylinders raise the booms, and the slew drive rotates the column. At the boom tip a grapple rotator lets the timber grapple turn freely to align with a log. The grapple closes its two Hardox tines around a bundle of stems using a grapple cylinder. Every articulation point pivots on a hardened pivot pin.

Hydraulic power

A timber crane needs both high flow for fast cycles and pressure for lifting moment. The truck drives a variable axial piston pump of around 90 cc per revolution at 320 bar from a transmission power take-off. The pump feeds a load-sensing valve bank so the crane draws only the oil flow the operator actually commands, which saves fuel and reduces heat. Oil circulates from a 200 L reservoir through filters and an oil cooler because sustained crane work generates significant heat. Reinforced four-spiral hoses (EN 856 4SP) carry oil to the moving booms.

Chassis, axles and trailer

The chassis is a 6x6 tractor with an extendable reach pole connecting to a pole trailer, so the wheelbase adjusts to the log length being carried. The rear drive axles use hub reduction gears for the high torque and ground clearance forest terrain demands. The front steer axle and trailer bogie are sprung on leaf spring packs, and all run on on/off-road tires.

Log bunks and securing

The payload sits on bunks: cross bolsters with upright stanchions that cage the log stack. The stanchions are S690QL to resist the sideways thrust of a shifting load. Once loaded, ratchet load binders strap the stack down so logs cannot roll off in transit.

Driveline

The driveline starts with an inline-6 turbo diesel engine near 13 L, feeding a 16-speed transmission with crawler ratios for steep tracks. A transfer case splits drive to front and rear axles. Twin propeller shafts carry torque to the tandem bogie. A large 400 L fuel tank supports long days far from refuelling.

Cab and controls

The driver has a normal day cab, and an elevated crane operator seat gives a clear view over the bunks during loading. Two proportional joysticks meter crane motions, with signals processed by the crane controller. The electrical system runs at 24 V with an alternator charging two batteries and powering work lamps for night loading.

Materials and construction

Forestry work is abrasive and the loads are heavy, so material selection focuses on strength and wear resistance. The crane booms use quenched and tempered S690QL plate, which combines high yield strength with the toughness needed to survive shock loads when a log drops into the grapple. The grapple tines step up to Hardox 450 wear plate because bark, grit, and stones grind away softer steel. The chassis main rails are micro-alloyed S700MC. Pivot pins are 42CrMo4 with an induction-hardened surface so they resist the wallowing wear that loose, dirty joints suffer in the field.

Variants

Configurations vary with the type of timber and the terrain. Shortwood operations carry cut-to-length logs in stacked bunks, while a tree-length or pole layout uses the extendable reach pole to carry full stems on a steered rear bogie. Crane size is matched to the wood: a smaller crane suffices for plantation thinnings, while old-growth stems need a higher lift moment and longer reach. Some units mount the crane behind the cab, others at the rear over the trailer drawbar, which changes weight distribution and the operator's loading view.

Braking and use

Stopping a 60 t combination relies on a dual-circuit air brake system with eight chambers acting on cast-iron drums. In daily use the truck shuttles between cutting sites and the mill, loading and unloading itself at each end. Maintenance centers on greasing crane pivots, inspecting hoses for abrasion against bark and branches, and checking grapple tine wear.