Machine Design

Vehicle Transmissions

Automatic and manual vehicle transmissions have always been competitive with each other. While this is still true, each has found its niche in truck and bus markets.

Advantages of manual over automatic transmissions are one-third to one-fourth lower cost, lower weight, and a greater number of forward speeds. This is offset somewhat by greater driver skill requirements. Payoff is higher fuel economy because the clutch provides lock-up with each engagement.

Manual transmissions:Recent advances here include shorter lever throws that require less effort. Gears have increased contact ratio, are smaller, and are hard finished to reduce operating weight and noise. Weight is further reduced, in some cases, with aluminum housings.

A new assembly technique called end-case loading simplifies maintenance and rebuilding. It allows assembly of components and gears with the transmission on end and outside the case. The housing is then lowered over the assembly to hold everything together.

Another new concept is twin layshafts. Torque is transmitted to gear shafts, or layshafts, on parallel fixed axes. Twin layshafts in 10 and 16-speed transmissions allow more teeth in simultaneous contact for smooth power flow. Other advantages are reduced bearing loads by balancing axial and radial gear forces and reduced noise with better tooth contact mesh.

Automatic transmissions: Generally, these have defined their role as assistants to less-skilled drivers. Even though they may cost more than similarly sized manuals, the higher cost is offset by operating simplicity, easier driver training, and improved road safety. In addition, some bus fleet maintenance supervisors report overall fuel economy actually increases 2 to 3% when automatics replace manuals.

Typically, these transmissions are hydraulically controlled; thus, their operation can be optimized with electrohydraulic controls that accept instructions from on-board computers. Electronic transmission controls typically use PROMs that contain the program for transmission functions. Programmability then allows, for example, a school bus transmission to operate differently than an identical one in a rural delivery truck. More often now, closed-loop algorithms govern transmission functions for greater efficiency.

Other automatic transmission developments include retarders, torque converters that lock-up. Retarders act like an internal brake to dissipate vehicle speed through a fluid coupling-type rotor and oil-cooled friction clutch packs. Retarders can operate from the brake pedal, a separate foot pedal, or a selector lever on the dashboard. Advantages are reduced brake maintenance and improved vehicle control in severe or long downhill conditions.

Torque converter lock-up is programmed for upshifting from second gear and higher. This simple feature allows automatics to mimic manual transmissions by eliminating torque converter slip common to earlier designs.

Multispeed transmissions have intrigued vehicle manufacturers with their promise of greater fuel economy. One of the latest developments in this area is a continuously variable transmission with a steel belt operating on variable sheaves. Intended for auto applications, the drive has teased automakers for a number of years but so far has proven useful only in low-power or stationary applications.

A transmission with many speeds, or increments, can approximate a ,continuously variable device. With a high initial ratio, the torque converter can be replaced with a simpler clutch. Computer control can maintain engine speed to avoid torque/speed points that produce the worst emissions. Most efficient motor speed can be maintained while the transmission selects an acceleration rate from driver input.

Binary-logic transmission: This design consists of several stages, each with two speeds or ratios: 1 and "not 1." For a speed increaser, the nonunity ratio is less than 1; for a decreaser, greater than 1. With each stage shifted independently of others, a transmission would have 2n speeds or increments, where n = number of stages. Therefore, a two-stage transmission can have four speeds. Cascading several stages produces a multiple-speed transmission.

The stage ratio can be found from logx = logR/(2 N-1), where x = basic stage ratio, R = highest or overall ratio, and N = total number of stages. Ratio of the nth stage of an increment is rn=xm, where m = 2n - 1. Therefore, a six-stage transmission with an initial ratio of 9:1 has 26 = 64 ratios, where x, the basic ratio stage, is 1.0355.

Relative change in vehicle speed is given by 1 - 1/x, about 3.4% from shift to shift, when the motor is run at constant speed. With all stages shifted to the 1 ratio, the final drive configuration has no meshing gears.

To contrast transmission complexities, a state of the art manual has 12 gear elements, and four forward speeds. A binary logic transmission with the same number of gear elements would have six speeds.

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