TRANSMISSIONS
Manual 'gearboxes' prevail, but shiftlessness is more available.
ED THOMAS
CONTRIBUTING EDITOR
Are your drivers "gear jammers" who still shift gears manually, or "shiftless" guys and gals whose work is made easier by some type of automated transmission? The big majority of transmissions in heavy trucks still are manuals, but there's a slow trend toward "auto trannies" because while they cost extra, they can save money. And more than half of all medium-duty trucks, and most of light trucks, get full automatics.
Whatever the type, a transmission is a complicated component. It houses a big collection of gears, and helps determine a rig's overall gearing. Rear axle ratio and tire/wheel size also affect gearing, much like pulleys. All are difficult to change after the truck is built, so buyers and builders look at them closely to make sure they're all correct.
TRANSMITTING THE POWER
The transmission is well named because it transmits power and torque from engine to driveline and beyond. The manual transmission does so with sets of gears that allow the driver to shift for changing situations. Gears are mounted on shafts, which are parallel to the engine's crankshaft; the transmission shafts slide fore and aft to take one set of gears out of the power flow and introduce another.
Light and medium duty manual transmissions are "synchronized."
Synchronizers are metal cones that squeeze together the varying speeds of two gears so a common gear on the mainshaft can disengage from one and slip onto the other. Synchronizers are used in European and Asian heavy truck transmissions, too. But in North America, the practice is to eliminate the synchros (which can require extra maintenance) and let drivers do the synchronizing by "double clutching."
Double-clutching is punching the clutch pedal twice during each shift, pausing in neutral to let the engine slow or speed up the gears. It is trickier during downshifts, when the lower gear must be speeded up by revving the engine to the speed it would be at if that lower gear were already engaged. Novice drivers usually learn this quickly.
Eaton Fuller's Low Inertia design, introduced in the early '90s, has a modified back section. This allows quicker speed changes and easier gear shifting. By properly feathering the foot feed, the driver doesn't have to double-clutch, or even use the clutch at all. Low Inertia models are designated "Super," as in Super-10, Super-13 and Super-18.
Going For More Ratios
Gearboxes with five to seven ratios are common in light and medium trucks, and feature a simple single-H shifting pattern. Eight or more ratios, as used in heavy trucks, usually require a repeating H-pattern, and these transmissions are sometimes referred to as "multi-speed."
In the old days, multi-speed performance to haul heavier loads was done by bolting a second "auxiliary" transmission behind the first or "main" transmission. The driver used two and sometimes three shift levers to "split" any or all of the gears in the main box with any or all of the ratios in the auxiliary box.
That's now done with one lever and one or two air switches controlling one or two auxiliary boxes. The second transmission is usually called the "back box" and the third, if used, is the "splitter box." This is the "range-type" transmission, which Fuller calls the Roadranger. Other manufacturers, including Spicer (now TTC) and Rockwell (now Meritor) in North America, adopted this principle, as did overseas builders. Nine-, 10-, 13-, 15- and 18-speed transmissions of this type are popular in North American trucks.
With a range-type transmission, the driver uses the repeating H pattern. Usually it's repeated twice, but sometimes a third set of ratios involves going through the pattern a third time. This is done with "LL" transmissions usually found in construction trucks, as with Fuller 15-speed trannies (which are increasingly rare). The driver throws an air switch to go from low range to high range and back (or from low-low to low to high; see below).
The multi-speed effect can also be accomplished with a "splitter" type of air switch. In a 10-speed, the lever is moved only four times (among five main ratios) and the switch moved in each of those. Mack once had that type of 10-speed, and Fuller has a Super 10 that uses it.
Two-Speed Axle
Another approach is the 2-speed axle, which was common is medium-duty trucks which hauled loads heavier than a standard 4- or 5-speed transmission could handle. It's still popular in Mexico. Here, too, the axle could be used to split the transmission's gears. This has given way to 6- and 7-speed transmissions with a single-speed axle, which are easier to drive.
Some of the 6-speeds (especially those in imported midrange trucks) have a 1st gear that's meant to be used for starting out under most circumstances. Others (usually in domestic trucks, and almost always in 7-speeds) have a "crawler" 1st gear, and the top five or six speeds are meant as "driving" gears.
On/off-road trucks need more low-end grunt, so specialized Mack "6-speed" or Fuller "LL" 11-speeds, with low-low ratios, are common. A "deep reduction" range provides the two or three lowest ratios; these are used in tough going off-road, but usually not on-road. A flip of an air switch gets the deep reduction range.
Eaton Fuller, Mack, Meritor, and Transmission Technology Corp. (which bought Dana Corp.'s Spicer Transmission business) all produce non-synchronized heavy duty transmissions. Mack trannies are available only in Mack trucks. Volvo once offered Volvo synchronized transmissions in its own trucks built in U.S., but no longer does.
Choosing Ratio Sets
Manufacturers offer a variety of ratio sets to suit various factors in the powertrain, as well as the owner's philosophy of operation. Among the considerations:
• Overdrive vs. direct-drive—The transmission's top gear ratio may be a 1:1 direct-drive, meaning the speed of its input and output shafts are the same. Or top gear will be an overdrive, which sends power through an extra set of gears so output-shaft speed is faster than that of the input shaft (which means the engine turns slower than the output and drive shafts). Overdrive ratios vary from 0.72 to 0.86 to 1. There are good arguments for either way, but "overdrive" and "double-overdrive" has come to mean a tranny that allows high road speeds.
• Startability—The lowest gears must be numerically high enough to get the truck moving on an upgrade (Low gear) and on the level (1st gear) without beating the clutch and driveline. A startability number (sometimes expressed as a percentage) is calculated by engineers at the factory. For on-highway use, the minimum number is 10.5 to 11; for on/off-road or extra-high gross weights, the number would be 14 or more.
• Gradeability—Top gear's ratio must allow the truck to proceed on a certain minimum grade before downshifting. The gradeability number for top gear should be 0.3 to 0.5 at peak horsepower, and 1.5 at peak torque. These, too, are calculated by the manufacturers.
• Ratio "steps"—These must be spaced correctly for the engine to effectively and economically pull the truck up the "ladder" formed by the load and road. Steps are described in percentages; for example, the step between a transmission's 13.41 Low gear and its 9.05 1st gear is 48% (the ratio difference of 4.36 is about 48% of 9.05).
Some engines and operating conditions require smaller or higher steps among all gears, so builders oblige. This, plus the overdrive vs. direct-drive issue, mean three to five gear sets may be offered within one transmission series. These are identified with a letter suffix (such as A or B) behind the transmission's model number.
• Torque capacity—Engine torque can kill gears, shafts and bearings, so suppliers manufacture them to withstand varying levels of torque output. Torque capacity is expressed in pounds-feet, and often will be part of the transmission model's nomenclature.
A buyer planning to uprate an engine for various reasons needs to order a stronger transmission and drivetrain. Uprating by reprogramming the engine's electronic controls is cheap, and can be useful to handle new operating conditions (e.g., heavier loads or running in hilly or mountainous terrain) and can add resale value at trade-in time. But the transmission must be able to take it. Automatic and automated mechanical transmissions often have lower torque ratings, so be sure to choose an engine rating that's safe for the transmission.
Related Article
Automatics and AMT's
