The Axle

The Piggins Axle

Piggins Brothers were proud that their trucks did not have chain transmissions, as was common in heavy-duty vehicles of the day, but used gear wheels to propel the trucks' rear wheels. The rest of the trucks' drive-train appears to have been quite conventional by the standards of 1911, and indeed by the methods of today, consisting of an engine, clutch, gearbox, universal joint and drive-shaft in that order from front to back.

Getting power from an engine which is rigidly fixed on the chassis to rear wheels that are tossing about on springs is a technological challenge. The bumps on the road cause the rear axle to sway violently and also pitch, yet the power has to be delivered smoothly and constantly. The intermediate point between the fixed and suspended parts of the drive-train is usually the differential gear, a device which splits the power to left and right wheels, ensuring that these rotate at different speeds but with identical traction while the vehicle turns a corner. A differential has been an essential part of rear-wheel-drive set-ups almost since the automobile was invented. In the Brass Era of automotive design, the power transfer through the differential to the road wheels of trucks could be rigged in at least three different ways:

• Through a tubular axle, with left and right transverse driveshafts fixed to the road wheels: this arrangement was used from the beginning in cars and is used universally in motor vehicles today: sophisticated designs for this kind of live axle were already available in 1911 (see for example Thomas Lindsay's US patent 1097653 filed 1911-08-07);
• With a "dead" non-rotating axle to hold the road wheels, which were driven via chains from sprockets mounted mid-way along the vehicle on its left and right sides: these sprockets were spun by jack-shafts, or transverse rods, emerging from a differential gearbox rigidly attached under the chassis (photo): these chain-driven transmissions remained in manufacture till the 1950s;
• In a hybrid of the above two systems, a solid-steel dead axle could be utilized for greater strength and reliability, but the chain could be eliminated by relocating the differential and jack-shafts onto the axle structure, rising and falling along with the sprung axle as it bumped along the road: Russel's US patent 1258946 sums up the principle like this: the hybrid "consists of the combination of a dead non-rotating axle for carrying the load and a power-transmitting jack-shaft assembly, comprising drive-shafts provided with pinions which engage and mesh with ... the wheel hubs." Vain efforts to perfect jack-shaft-on-dead-axle designs consumed more than a decade of attempts by American engineers.

While a hybrid axle had been patented earlier, by the French auto industrialist Adolphe Clément in 1907, the Piggins brothers were the fathers of a new subtype which came to be known in later years as the "external gear" design. The technical details will be discussed in greater detail below. In 1912, customers were crying out for powerful trucks capable of carrying very heavy loads. The weak point in car designs had always been the axles: the driveshafts tended to snap when carrying laden weights of several tons, or else the casing gave way where it was fastened to the differential. The brothers were the forerunners by a margin of five years of intense efforts to solve axle weakness problems.

Chain-drive was attractive as a solution because it meant the back, load-bearing axle could be a solid steel bar. According to Whitcomb and Koehler in US patent 1370247, this was usually a forged I-section steel beam. It was believed that this set-up kept most of the road-shocks out of the jack-shafts, or to use an equivalent term in engineering vocabulary, the "counter-shafts": both these terms describe rotating shafts used in an engine as intermediary mechanisms to combine or distribute power from the energy source to the place where the work must be done.

Charles Piggins gave considerable thought to ways of shifting the jack-shafts from the vehicle chassis to the axle structure so as to eliminate the chains and use pinion gears instead. Like Clément, he was convinced that the entire output end of the drive-train, including the differential, had to be part of the floating, road-hugging assembly if the jack-shaft-on-dead-axle idea was to work. We do not know why he set himself this challenge, but his experience cutting gears may have created an interest in geared systems.

While the noisiness of chain drives and their crude appearance in comparison to the mechanisms sleekly built beneath cars made chain drives unattractive to many buyers, they were a solid technology, proven in the bicycle industry. Even Clément in France continued to equip his larger Clément-Bayard cars with chain drives until 1911, four years after applying to patent a supposedly "better" technology in the United States. But as Eugene William Lewis remarked, "Many old timers have vivid memories of driving along dusty roads or coasting down a hill and wondering why the car was running so smoothly, only to find ... that the chain was missing... If one retraced every foot ... the chain could be found... The next step was only the small matter of having enough cotter pins and extra links to restore the chain..."

The reception for Piggins's idea of 1912 obtained shows that it deeply impressed the entire American truck industry. Within a decade, dozens of manufacturers were applying similar hybrid-axle techniques and refining them, and only diehard chain-drive companies such as Mack Truck were to continue making the old-fashioned sort of trucks.