There are two main types of crawler tracks: “Christie suspension” uses oversized road wheels, laying the tracking on top of the wheels creating a banana-like appearance as the tracks droop after running over the driving wheel and idler. “Vickers suspension” uses smaller return rollers holding the track straight from the idler to the driving wheel, making the tracks look like a sideways D-shape.
Source(s): Crawler Tractor
 So Many Inventors, So Many Patents
The first seeds of crawler tractor history were planted more than two and half centuries ago. In 1713, Frenchman M. D’Hermand created a crawler tread trailer to be pulled by goats. In 1770, English writer and inventor Richard Edgeworth patented the first steam-driven moving tracking tread system. In 1826, another British inventor, George Cayley, developed a continuous track system he called the “universal railway.” In 1837, the Russian inventor, Dmitry Zagryazhsky, was working on a “carriage with mobile tracks.” However, none of theses inventors were able move far beyond the paper or prototype stage.
Alvin Lombard of Waterville Iron Works in Waterville, Maine was the first to institute the crawler track system into vehicle production. The Lombard log hauler, patented in 1901, was designed to improve traction in the snow. Initially these steam-powered tractors were steered by horses until Lombard installed a steering wheel and sled system to the front of the machine. Though the Lombard log haulers were capable of towing up to 300 tons, they were not equipped with brakes. They pulled logs on sleds driving four to five miles (6.4 to 8 km) per hour. In total, 83 Lombard log haulers were manufactured. With the majority of sales in Maine and New Hampshire, three were sold to Russia and one each to Wisconsin and Michigan.
Despite the initial successes of Alvin Lombard, it wasn’t until construction pioneer Benjamin Holt entered the crawler tractor industry that the machine became truly popular. As head of the Holt Manufacturing Company in Stockton, California, his vehicles were suffering from the tenderness of the rich soil in the western states. His company began experimenting with a variety of wheel systems to stop tractors from sinking into the soil. Holt Manufacturing added more and more wheels until its vehicles were 45 feet (13.7 m) wide, with wheel diameters of 12 feet (3.7 m). Holt eventually started experimenting with crawler tracks in 1904. After a number of successful tests his company sold its first crawler in 1906. Lombard believed Holt’s crawler tractor was a reproduction of his log hauler and was upset Holt failed to pay any sort of royalties.
Meanwhile, in England, David Roberts, chief engineer of R. Hornsby & Sons, patented his own track design in 1904. He attached his tracks to an oil-powered tractor, but sales never succeeded like they had for Holt Manufacturing. He sold his patent to Holt in 1914.
 Gas-powered Engines and the Bulldozer
Holt quickly began developing a gasoline-powered motor to replace steam driven tractors. He and his nephew Pliny established Aurora Engine Co. in Stockton in October 1906. Within two months, they were testing their first motor. By 1908 testing had progressed and Holt sold their first gas-powered “Caterpillar” crawler tractor; it was designated Model 40. The engine was a four-cylinder, 6x8 bore-a-stroke, valve-in-head engine rated at 25-drawbar horsepower. Initially, these gas-powered tractors were primarily used for agricultural projects. However, by attaching a large blade to the front of the tractor, its usability increased tremendously. The bulldozer was born.
Weaker versions of the bulldozer had been achieved in the nineteenth century by attaching shovels to horses, but the crawler tractor could increase its power exponentially. The first shovels fixed to tractors had to be powered by manually moving the blade with hand wheels. Experimentation with bulldozers improved in the early 1920s, with the first hydraulically operated dozer blade manufactured in 1925 by LaPlant-Choate Manufacturing Co. of Cedar Rapids, Iowa. This blade was attached to the tractor on a rectangular frame, pivoting on the crawler’s frame, and controlled by a hydraulic cylinder at the rear of the tractor.
Movement of the blade was enhanced with the development of the Power Controlled Unit (PCU), introduced by Robert Gilmore LeTourneau in 1928. The new unit was controlled through the use of clutches and brakes, and was available with four winches. The Power Controlled Unit was also used in a wide range of other attachments including pull-type scrapers and rippers.
While initial blade development was done separately from tractors, individual companies soon began teaming together with tractor manufacturers creating a more unified, stronger product. Baker joined with Allis-Chalmers, Bucyrus-Erie with International, and LeTourneau with Caterpillar.
By the 1940s, tractor manufacturers integrated blade development into their facilities. Ten years later, the tractors and blades were no longer individual pieces, but were built to create one seamless vehicle.
 The Crawler Tractor During Wartime
The success of the crawler tractor spawned the development of the tank. Its tracks could traverse almost any condition making it perfect for the elements of war. It made its first worldwide appearance in World War I. The Americans' development of the tank lagged behind their European allies because they didn’t become involved in the war until quite late, 1917. However, after viewing the success of tracked vehicles in war, the U.S. instituted mass development and production.
By World War II, crawler tractors and bulldozers had increased in versatility. They were used for combat construction all over Europe, Asia and the Pacific. With the muddy conditions of the jungles, no piece of machinery was better equipped to traverse the terrain than the crawler tractor. It was responsible for moving artillery while the bulldozer leveled ground for roads and airfields and cleared debris.
 Post-war DevelopmentsInternational Harvester introduced the largest tractor yet, the TD-30, in 1962. A year later Allis-Chalmers continued the trend by building the HD-41, which had an operating weight of 70 tons and was equipped with a 524 horsepower engine.
Competition among companies led to a series of trend-setting developments. Caterpillar established and patented sealed and lubricated tracks that extended the life of pins and brushings by 30 percent. JCB and Liebherr introduced hydrostatic drive crawlers in the early 1970s. Caterpillar pioneered the “high drive” undercarriage with its D10 model in 1978.
 The Age of Comfort
While engine and blade development continued, a new concern took focus: the comfort of the tractor operator. Early tractor and bulldozer operators were required to move machinery with heavy hand wheels and were subject to the natural elements. Modern operators are equipped with all hydraulic controls and seated in covered air-conditioned cabs with multi-position seats, radios, and cup holders. As new developments were introduced, the power and versatility of the crawler tractor/bulldozer increased.
 Other Developments
Crawler tractors have been fitted with a variety of attachments and the crawler track system has been adapted to number of different construction vehicles.
One common attachment is the toothed ripper. It is a claw-type device, usually mounted to the rear of the vehicle. Rippers can either be equipped with a single-shank (meaning one tooth) or multi-shank (two or more teeth). The ripper is mainly used to rip large objects from the ground (e.g., large rocks, tree stumps) or to loosen soils so they can be excavated.
 Features/How It Works/Types
The tracks are made up of modular-chain links that form an enclosed chain. Each link is broad and often made of manganese alloy steel to increase strength and durability. The track is set to the ground by inner road wheels, called bogies or track rollers. When mounted on suspension, the bogies can cushion the ride. The tracks move on a toothed drive wheel, or drive sprocket, that connects to holes in the track links. Also, a non-powered wheel, known as an idler, is mounted at either or both ends of the tracks to increase tension, allowing tracks to move more smoothly.
The bulldozer blade is operated with hydraulics and comes in three different models: a straight blade is short with no lateral curve or side wings, and is used for fine grading. The universal blade is tall and curved with large side wings enabling it to carry a heavy load. The combination straight-universal blade melds the two styles, retaining size and curvature, but less extreme in both. It is used mainly for pushing piles of large rocks.
Additional types of blades include the angle dozer in which a removable angling blade is pinned to a "C frame" which is bolted to the crawler tractor.
Smaller crawler tractors can have a six-way blade which, in addition to raising and lowering, can also angle and tilt.
 Common Manufacturers
- Hunan Sunward
- John Deere
- New Holland
 Different Names For The Same Thing
This is a list of alternate names for crawler tractors.
 Additional Photos
 Ritchie Bros. Marketplace
- A History of Crawler Tracks. Darlind. 2008-09-24.
- Continuous Tracks Have Changed the World. Teknoxgroup. 2008-09-24.
- Logger. Mainerec. 2008-09-24.
- Logger. Mainerec. 2008-09-23.
- Cars. Is It A Lemon. 2008-09-24.
- Haddock, Keith. The Earthmover Encyclopedia. MBI Publishing Company, 1998.19.