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:

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.

Click a picture to see a larger view.

This page is a work in progress and will never be quite finished. If you have a comment or a contribution, please write to me.

© Jean-Baptiste Piggin 2000-2009
Creative Commons License
This page is licensed under a Creative Commons Attribution-No Derivative Works 3.0 Unported License.