The Crawler Track: Laying Down a Road to Carry the Machine

Sheet 6 of US 874,008 shows the track in outline: an endless belt of links round two sprocket wheels, with the transverse shoe that each link carries. What the specification specifies, in order: - **Links** made male and female, pinned with bolts turning in sleeves, "entirely protected from grit and dirt", with channels to inject lubricant. - **Transverse shoes** riveted across the links, "a sufficiently broad support to prevent the apparatus … from sinking into the ground"; for very soft ground, wider supplementary shoes bolted on, with upturned flanges for grip on slippery ground. - **Bearing wheels** running on the inside edges of the chain between the sprockets, arranged on a curve convex downwards so the machine turns more easily than on a flat run. - **A differential or clutch mechanism** so that power "may be transmitted to either of the driving sides", assisting turning; the front of the machine rides on a steering wheel.

Fill a tray 60 mm deep with dry fine sand and level it with a straightedge. Build two carriers of the same weight on balsa decks: one on four toy wheels, one on two 'tracks' — loops of nylon webbing round a pair of 12 mm dowels each, with a balsa slat under each loop so the webbing is supported along its length, as Holt's bearing wheels support his. Load both to 2 kg with weights. Stand each on the sand for one minute and measure sinkage with the depth rod of a caliper at four points. Then push each 300 mm with a spring scale and read the force. The wheels sink deeper and need more force to push, because they are always climbing out of the rut they have just made; the tracks leave a shallow print. Calculate each carrier's ground pressure from its contact area and set it beside the sinkage.

**Benjamin Holt** of Stockton, California, built combine harvesters and steam traction engines. He tested his first track-type machine in 1904 and was granted **US 874,008**, *Traction-Engine*, on 17 December 1907. Holt's company adopted the name Caterpillar, and merged with its rival C. L. Best in 1925 to form the Caterpillar Tractor Company. Tracks were proposed before Holt, but his was a working machine built in numbers for soft farmland. **Honest limits.** Tracks are heavier, slower and dearer than wheels, and they wear fast on hard roads. Skid steering scuffs the ground and wastes power in the turn. Low average ground pressure does not make compaction vanish: the pressure under each roller is higher than the average, and total load still compacts deeper soil layers.