Tendencies of the industry toward lower costs have been reflected in axle design. Large-volume business has made it worthwhile to introduce changes in the design of passenger-car and light-truck axles to increase production economy and improve design. For heavy trucks, the trend has been to keep costs down by making no changes that would involve added expense for tools, jigs, dies and fixtures. Front-wheel brakes for passenger cars have resulted in changing front-axle I-beam sections and front-spring design to take care of the increased stresses such brakes introduce.
In the design of rear axles for passenger cars, no fundamental change has occurred, although the change from the full-floating and three-quarter floating types to the semi-floating axle and a change in mounting the bevel pinion are two features that seem to be coming to the fore. As to the latter, the overhanging pinion formerly predominated; but many manufacturers have changed and some will change this year to a straddle mounting.
Mr. Rockwell expects the introduction of worm-drive passenger-car axles within the coming year. Over a period of several years, the bevel-gear axle has steadily increased in favor for a motor-truck rear-axle, due to low cost and efficiency for light trucks. A 5-ton truck recently brought from Europe has a bevel-gear axle and is equipped with solid tires. A straight-tooth bevel gear is used with a 7-tooth pinion and a 56-tooth ring-gear; this gives only an 8 to 1 reduction, whereas most 5-ton trucks in this Country require at least a 10 to 1 reduction.
For 6 years, the worm-drive truck-axle has been more popular than all other types; for any given reduction, it is possible to obtain a suitable gear-center, a favorable lead-angle and a low tooth-pressure, which will give high efficiency at normal operating speed. The worm-drive has a mechanical efficiency none too high, but it has justified its existence. The declining popularity of the internal-gear drive may be due to its increased price; the chain drive probably will remain in the field, as some prefer it to all other types.
All axle manufacturers are trying to reduce unsprung weight. The double-reduction axle unquestionably has shown the greatest increase in popularity under heavy-duty vehicles. In the spur and bevel type, it is possible to select spur and bevel pinions that have a sufficiently large number of teeth for high efficiency, together with a heavy pitch and satisfactory cross-sections for the ring-gears to hold distortion by heat-treatment within reasonable limits. Its efficiency is high throughout the working range, and changes in ratios can be made quickly without serious effect on the various components. This type of axle was chosen by leading English experts for the heavy-duty British subsidy war-trucks.
Future axle-design will be influenced by balloon tires. with a consequent reduction of the brake-drum diameters; by greater reductions and more steps in transmissions; by desires for greater road clearance and body clearance, and more satisfactory brakes; and possibly by the use of three or more axles, two or more being driving axles, to obtain greater safety, better load-distribution and better traction with less damage to roadbeds. Balloon tires may force the use of axles that provide a wider car-tread.
To date, transmissions have been designed to operate approximately 5 per cent of the time a truck is in service; additional requirements in this respect will necessitate changes in design. The interest in transmissions leads Mr. Rockwell to believe that the two-speed rear-axle will be demanded very soon. An axle for railroad use that is arranged for four-wheel drive, provides gear positions for idling and reverse driving and permits the use of all transmission speeds in either direction of travel is illustrated.