Debate About Axles

Debate About the Piggins Axle

Efforts to improve Clément's internal-gear axle design began about the time Charles Piggins obtained his patent. As with the Piggins design, inventors hoped to simplify the Clément axle, so as to reduce manufacturing costs, reduce wear-and-tear damage to the gears and position the gear-wheels better so they would not have to be disturbed during vehicle repairs. There were soon dozens of US patents modifying the Clément jack-shaft-on-dead-axle design, and several implicitly also offered improvements to the Piggins design:

Filing Date	Invention	Inventors	US Patent
1916-02-05	Eliminating the heavy casing tubes around the jack-shafts	Viggo V. Torbensen, Torbensen Axle Co.	1380025
1916-08-26	Differential fully detached from dead axle, supported by jack-shafts like the Piggins axle	Albert W. Russel, Russel Motor Axle Co.	1258946
1917-02-10	Added insulation between the live and dead axles	Robert J. Burrows, Clark Equipment Co.	1354462
1919-05-10	Pinion attached to a spring so as to absorb torque reaction	Gabriel Midboe, International Motor Co.	1373142
1919-07-12	Means of avoiding deflection of the jack-shafts with consequent gear damage	Robert J. Burrows, Clark Equipment Co.	1479220
1919-09-29	Simplifies manufacture, differential may be removed as a unit	James A. Whitcomb and Raymond Koehler, Kenosha Wheel and Axle Co.	1370247

With the benefit of hindsight, we know that all of the jack-shaft-on-dead-axle drives, which had their heyday in the 1920s, were headed for a technological dead end. The succession of patents offer scathing judgements about their various predecessors, but also give some hint about why hybrid drives as a group ultimately disappeared:

• It has been found difficult ... to maintain the cooperating teeth of the gear and driving pinion in perfect mesh and alinement, owing to the prevailing methods of securing the gear ... to the wheel, whereby any distortion of the wheel has been communicated to the gear (Higinbotham, patent 1196864);
• Great difficulty has been experienced in providing for the efficient lubrication of the gear and pinion and in preventing the access of dust and dirt (Higinbotham, patent 1196864);
• In use, these axles are subjected to enormous strains which slightly twist and bend and otherwise distort the [rigidly connected casing tubes extending from the differential]: sometimes this breaks these casing tubes (Torbensen, patent 1380025);
• Great difficulty has been experienced ... with drive mechanisms of this character because the driving gears are exposed and rapidly accumulate grit in the lubricant so that a grinding compound is formed which wears out the gears (Fekete and Baits, patent 1319519);
• Repair shops do not have the equipment to easily handle the wheels (Church, patent 1401043;
• When running on low gear, as in starting, the jack-shafts have a tendency to buckle or be deflected intermediately, with the result that the outer end portions thereof are tilted or deflected ... so that the teeth of the pinions do not mesh properly with the teeth of the driving gears (Burrows, patent 1479220);
• Such a construction necessitates external supporting brackets for the eccentrically arranged drive axle sections and as the driving parts and their bearings should be enclosed, an axle casing is necessary.... The complexity of such an arrangement is obvious and employment of two distinct shafts transversely between the drive wheels of the vehicle is undesirable (Walker, patent 1420793).

The main explanation for the ultimate disappearance of the Piggins internal gear drive - it probably died with the Reliance light truck in 1927 - may well have had to do with the growing refinement of tubular axles and increasing scientific research, using test-beds, of the forces that wrecked drives.

Patents spanning the second decade of the 20th century suggest that an understanding of why axles broke varied widely. Francesco Pagliano of Turin stated in US Patent 1034497 in 1910 that he wished to prevent "the rapid change in the molecular structure of the axle, which renders it crystalline and weakens the axle, due to excessive shocks and vibrations". This early reference to metal fatigue may have been ahead of its time.

The emphasis of Charles Piggins and many others on the merits of a robust dead axle show they clearly believed that axle breakages were somehow caused by lateral strains arising from the downward pressure of the vehicle's weight and the upward forces coming from bumps in the road. It was not until the end of that decade that inventors such as Gabriel Midboe (patent 1373142) were clearly identifying torque and counter-torque as central issues. In time, automakers gained a better grasp of the forces that broke axles, particularly when carrying heavily laden vehicles on uneven roads. The varying torque effects on the drive-shafts from rough roads greatly added to the steady forces arising from engine thrust.

I would conjecture that hybrid axles were just as susceptible to drive-shaft failure as their contemporary rivals, the tubular axles, were. The strains did not originate primarily from the varying vertical distance between the load and road, but from the wrenching forces on the shaft as a vehicle crossed uneven surfaces. The shafts would have been subject to the torsion forces that cause "gear lash", which Don Piggins, a modern automotive engineer, explains as "the pulsing effect caused by the vehicle's wheels ... i.e. the vehicle propelling the motor/transmission rather than the other way around" as the vehicle descends and climbs out of depressions on the road. Car restorers today confirm that the axle shaft, not the casing, is what usually breaks in antique motor vehicles if they are driven too hard.

The Piggins Brothers' attempt to neutralize those forces by allowing more play around the axle assembly was ingenious, but did not ultimately succeed. In the end, axle breakage was to be solved by strength: the metallurgical industry learned to supply steel shafts that could withstand metal fatigue when used in tubular axles. Roads also became better engineered, to meet the needs of motor vehicles which travelled at higher speeds than the old horse-drawn traffic.