Tame That Torque

Transmission makers gear up to handle a new generation of high-torque powerplants.For the past several years, fleets -- particularly truckload carriers -- have been on a horsepower and torque binge that shows no signs of easing. Eaton Truck Components Operations-Americas, which claims 75% of the North American heavy-duty transmission market, now reports that 53% of its transmission production is rated

Transmission makers gear up to handle a new generation of high-torque powerplants.

For the past several years, fleets -- particularly truckload carriers -- have been on a horsepower and torque binge that shows no signs of easing. Eaton Truck Components Operations-Americas, which claims 75% of the North American heavy-duty transmission market, now reports that 53% of its transmission production is rated at or above 1,550 lb.-ft. of torque. As recently as 1993, such ratings accounted for just 13% of its sales.

There are several reasons for this shift to higher input ratings. A growing number of fleet managers believe that with intense competition, a shortage of drivers, increasing customer expectations, and heavier trucks, they need more performance from engines. To begin with, engines with loftier torque ratings help rigs sustain higher average speeds. They also help meet driver demands for ever-better vehicle gradability and reduced shifting.

New families of engines are waiting in the wings to supply the higher torque levels that fleets are demanding, and possibly without fuel penalties. In fact, improved performance and greater efficiency of the new engines suggest that the torque race may not top out at 1,650 lb.-ft., or even 1,850, but may one day reach beyond the 2,000 lb.-ft. level.

The increase in torque presents some handling problems for transmission makers, however. Truck gearboxes have to extract top performance and economy from engines, but they also must be durable enough to withstand the stresses of elevated torque levels for 750,000 mi.

A move to 1,650 lb.-ft. engines would most likely be accomplished with today's gearboxes. In fact, a corresponding move down in rear-axle ratio might enable fleets to use the added power to achieve desired cruise speed in top gear at reduced engine rpms, which could offset some of the fuel penalties. But there are limits as to how far down axle ratios can go.

With Caterpillar Engine Div.'s 15.8-liter 3406E announcement last March, the extent of the torque challenge that's confronting transmission makers comes into focus. Cat's 600-hp. big bore, which puts out 2,050 lb.-ft. of peak torque, is just the first of an ultra-high-torque breed.

Next March, Cummins Engine Co.'s Signature 600 arrives. The 2,050 lb.-ft. rated engine reportedly can deliver up to 0.5 mi. better mpg due to advanced turbo and combustion technology. Detroit Diesel Corp. is said to have a new engine program under way that also includes higher torque ratings.

But how do transmission makers propose to produce products that can handle elevated levels of engine torque? One way is to improve existing gearbox designs and/or manufacturing processes. The second involves adoption of electronic management of engine torque delivery so that boxes would not be forced to operate beyond their input limits. And third, in what some might consider a radical structural change, an additional countershaft could be added to provide more input capacity.

Numerous steps to strengthen the muscle mass of twin-countershaft gearboxes have already been taken. For example, sections of gear cases are redesigned for additional rigidity. Bigger bearings, advanced manufacturing techniques, stronger steel alloys, and enhanced gear- and shaft-hardening processes, as well as better gear-finishing processes, are being used.

Conspicuously absent is talk of increased center distances (the dimension from the centerline of the mainshaft to the centerline of the countershaft). The advantage to widening center distances is that it would enable transmission engineers to accommodate significantly higher torque loadings because gear teeth could be realigned and tooth counts increased.

The drawback, though, is that the transmission "space envelope" would have to be widened, and today's boxes are already a very close fit between the frame rails.

There is an electronic countermeasure, however: torque management. Through the J1939 data link, transmission controllers can "talk" to engine controllers. Software can be written that instructs engines to reduce torque delivery at certain times, particularly in the lower gears when torque multiplication in transmissions is greatest. This will prevent engine torque delivery from overwhelming gearboxes and thus shortening their life.

Torque management works by exerting control over fuel delivery. The transmissions could signal engines when it is safe to release full torque -- generally when the vehicle is operating at road speeds in the upper two gears.

Downshifts would still be required on steep hills, but torque delivery could be micro-managed so that momentary cutbacks in torque would protect gear teeth and shafts. As useful as the torque management option may appear, however, its acceptance by drivers is a question mark.

At this time, only one box, Mack's 18-speed T-2180A, is built to survive on a diet of 2,000 lb.-ft. of engine torque. This transmission owes its huge input capacity to its third countershaft.

Eaton, Rockwell, and Spicer heavy-duty transmissions are all twin-countershaft designs. But nothing is forever. If fleets were ever to demand more than 2,200 lb.-ft. of unrestricted torque in all gears, the addition of a third countershaft would probably be mandatory.

For the near term, though, Eaton, Rockwell, and Spicer are working on less radical solutions. As recently as a year ago, Eaton Fuller had only a single 1,850 lb.-ft. rated box, the RTLO-18718B, a product that offered much vocational flexibility. Recently, a second 1,850-lb.-ft. box was added. And in January of next year, a new Eaton Fuller transmission family will appear. It is slated to have 13- and 18-speed models, including an 18 with 2,050 lb.-ft. input capacity. The electronically controlled Top 2 option will be available on all members of the new family.

Although many industry observers think the new engines will be so powerful that drivers will no longer need multi-speeds, Eaton disagrees. Its analyses of the North American heavy-duty market show that as a percentage of sales, the popularity of 13- and 18-speed transmissions (all brands) stands at record levels; and that multis are continuing to take market share from everything, including 10-speeds.

Thirteen- and 18-speeds will continue to be an advantage, reasons Eaton, because they'll enable drivers to cruise tomorrow's prodigious torque producers at the most fuel-efficient rpms, regardless of whether state speed limits are 55, 65, or 75 mph. A tractor equipped with a 10-speed, Eaton notes, could be optimized for most economical operation at one or two of these road speed limits, but not all three.

Rockwell Automotive, which produces 9- and 10-speeds, as well as a 13, says its plan of attack is to introduce a 2,150 lb.-ft.-rated 10-speed with computerized controls. The company is actively pursuing torque-control concepts up to the 2,550 lb.-ft. level.

Rockwell, which has already announced that it is going to provide a line of high-torque multis because some fleets will want them, believes that with flatter horsepower and fuel consumption curves most operations are not going to need multi-speeds, except in special situations.

Ten-speeds, it contends, will meet most fleet requirements at lower initial cost. Moreover, 10-speeds reduce the need for driver training and require less maintenance.

According to Rockwell, all drivetrain components, including the clutch, U-joints, and rear axles, will be impacted by the higher torques. When engine torque levels rise above 2,150 lb.-ft., the standard 15.5-in. clutch is inadequate. A 17-in. two-plate is needed.

Spicer Transmission Div., which makes 9- and 10-speeds, as well as 16- and 18-speeds, is working on a program to increase the torque capacity of its flagship 10-speed PSO 165-10S. Advanced manufacturing processes are being used to enable this transmission to handle 1,750 lb.-ft. of torque. Spicer also predicts that many of its fleet customers will be moving to ultra-high torque-producing engines. Therefore, a crucial Spicer option is full exploitation of its AutoMate technology, with torque tailoring. In a Class 8 box with full AutoMate technology, a conventional gearbox and clutch setup is made to respond like an automatic.

Allison Transmission's HD Model, which carries a 1,435 lb.-ft. torque rating, can handle engines putting out 1,525 lb.-ft. of gross torque. But the ability to handle 1,850 lb.-ft. would be a big jump. Such a move won't be made until the need is more clearly identified. At that point, a two-pronged approach would be required. There would be a need to develop higher capacity torque converters, as well as additional software to enable the transmission computer to converse more widely with the engine ECU over the J1939 data bus.

Sensing vehicle road speed and gear, torque management would enable the HD transmission to signal the engine to cut back on torque production in the lower gears. At higher road speeds, torque would be reduced momentarily during shifting to ease the loading on clutch packs. After shifting was completed, the transmission would signal the engine that it was safe to restore full torque.

Fleets will clearly benefit if the high-torque engines help reduce driver turnover and increase rig productivity. And they will be particularly pleased if the new transmissions, like the new engines, are fully upratable, thus further increasing resale values.

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