Grapple Skidder

Overview

A grapple skidder is a forestry tractor that drags felled trees from the stump to a roadside landing. Unlike a cable skidder, which throws out a winch line that a worker hooks to each log, a grapple skidder backs up to a bunch of logs and clamps them in a hydraulic grapple, so one operator works the whole cycle from the seat. The machine is built around an articulated four-wheel-drive chassis: it steers by bending at a centre joint rather than by turning the wheels, which keeps the front and rear tyres tracking in the same ruts and gives a tight turning radius on soft ground. The example here pairs a 177 kW Diesel Engine with a powershift Powershift Driveline, a rear Grapple Boom, and large flotation tyres on planetary Axle Assembly. Operating weight is about 17.2 tonnes, most of it carried low and rearward so the machine stays planted when it lifts a loaded grapple.

Engine and cooling

The Diesel Engine is a 6.7-litre inline-six with a wastegated Turbocharger and a common-rail Fuel Injection System system running up to 1,800 bar. A forged Crankshaft rides on seven main bearings, and each Piston uses a steel crown with an oil-cooling gallery to survive the sustained high loads of skidding uphill with a full drag. The single Cylinder Head carries four valves per cylinder. Forestry duty is hard on cooling because chaff and bark dust pack the cores, so the Cooling Package mounts the radiator, charge cooler and hydraulic cooler side by side rather than stacked, and the Cooling Fan reverses on a timer to blow debris back out. Emissions are met with a Exhaust Aftertreatment stack of oxidation catalyst, diesel particulate filter and selective catalytic reduction.

Driveline and traction

Power leaves the engine through a Torque Converter that multiplies torque at low ground speed and locks up for road travel. Behind it sits a Powershift Transmission giving four forward and four reverse ranges through hydraulically engaged Clutch Pack sets, so the operator can shuttle between the stump and the landing without clutching. Two Driveshaft assemblies carry drive to the front and rear Axle Assembly. Each axle uses a Differential with an automatic no-spin No-Spin Gear Set lock that ties both wheels together the moment one starts to slip, and a planetary Final Drive at each wheel end multiplies torque close to the tyre to keep half-shaft loads down. Drawbar pull reaches about 215 kN, enough to break a heavy bunch loose from mud or snow.

Articulation and steering

The chassis splits into a Front Frame and a Rear Frame joined at the Articulation Joint. Two Steering Cylinder units bend the frame up to 42 degrees each way, and a rear Oscillation Bearing lets the back frame rock so all four tyres stay loaded over uneven cut-over ground. Because the joint carries the full steering and twisting load, the Pivot Pin pins run in tapered roller bearings sealed against grit. Steering by articulation also keeps the drag line straight behind the machine, which matters when the rear arch is towing several tonnes of timber.

Grapple and boom

The working end is the Grapple Boom. A swinging Swing Arch raises the front of the load clear of the ground to cut drag, while two Boom Cylinder units position the Grapple Head over the bunch. The head closes two curved Grapple Jaw tines through a pair of Grapple Cylinder rams, gripping up to 1.6 square metres of cross-section and opening to 2,440 mm to pick up a wide spread of stems. A Grapple Rotator turns the grapple through 340 degrees so the operator can line up scattered logs without repositioning the machine, and a Swivel Joint passes hydraulic flow across the rotation. A front Dozer Blade doubles as a counterweight, a decking tool, and a brace for stacking logs at the landing.

Winch and hydraulics

Some stems sit beyond grapple reach, behind a stump or down a bank, so the machine also carries a Line Winch. Its Winch Motor spools 16 mm Winch Cable off the Winch Drum, and a choker hook lets the operator drag a single log into grapple range. All of this runs off the Hydraulic System system: a variable Hydraulic Pump delivers about 160 L/min at 24 MPa, and a sectional Control Valve Bank meters that flow to steering, boom, grapple and winch in priority order so steering never starves under load. A Pressure Sensor on each main circuit feeds the controller for load limiting.

Cab and electrical

The Operator Cab is built around a certified ROPS/FOPS Structure cage with Cab Glazing polycarbonate windows that resist branch strikes. An air-suspension Operator Seat swivels so the operator faces the rear grapple during the loaded run, and two Control Joystick controls handle drive and implement functions. The Electrical System system runs at 24 V from two batteries in series, charged by a sealed Alternator. A sealed Machine Controller machine controller, built on an Microcontroller and tied into the Wire Bundle harness through weather-pack Connector terminals, coordinates the engine, transmission and hydraulic limits and drives the cab LCD Panel display.