"A Shift for the Better," Feature Article, April 2008

a shift for the better

A manufacturer fine-tunes its automatic transmissions for heavy trucks.

by John Reschovsky

Moving a Class 8 tractor-trailer down the road with efficiency is a tricky business. With a rig that weighs 33,000 pounds or more, the driver has to shift 10 to 18 times just to get the truck up to highway speed. Once cruising velocity is achieved, the driver is constantly shifting among as many as four top gears as traffic or terrain mandates.

It's a constant battle of using the transmission to match the engine torque to the load. Eaton Corp., which makes many of the manual heavy-duty truck transmissions in North America, is working to automate the gearshifting, for a variety of reasons.

According to Kurt Gerlofs, principal reliability engineer for Eaton's Heavy Duty Transmission Division, "If you have a pure manual transmission, a great driver can achieve the maximum fuel efficiency. But now, senior drivers are getting harder to come by, and average and beginner drivers are very fuel inefficient. They don't optimize the cycle."

Trucking companies are very much interested in achieving optimal fuel efficiency. A gain of 4 or 5 percent could mean annual savings of hundreds of thousands of dollars for a company with a large fleet of trucks. In addition, companies would like to increase driver retention, and one way to do so is to make the big rigs easier to drive in congested traffic.

What's more, the strength and size of drivers varies greatly, and not all of them are happy using manual transmissions. Taken together, these factors add up to the increasing demand for automatic transmissions for heavy trucks. The trucking companies want them for fuel efficiency and driver retention, and the drivers want them for ease of use.

For the past 10 years, Eaton has offered automatic transmissions for heavy trucks. The latest generation of Eaton automated transmissions, the Fuller UltraShift, uses servo motors to execute the gearshifts and an electronically controlled clutch, all under the management of an electronic control unit, or ECU. The engine also has an ECU. The two units talk to each other so that engine speed and shifting work together.

According to Arnold Woldhuis, senior technologist for the Eaton Truck Group, "These trucks need fast shifts. But you don't want it to be harsh, either; you want the transmission to shift smoothly so the driver doesn't get thrown around and you don't break things in the drivetrain."

For Smoother Moves

As a result, the research and development team in the Truck Group is continuously trying to improve shift quality. "They're constantly monitoring and tweaking software to control the transmissions, and the torque measurements are an important part of a whole group of issues that we look at," Woldhuis said.

Our company, Accumetrics Associates, has supplied Eaton with a telemetry system that it uses to gather data on its transmissions. Eaton owns six of the devices, called AT-5000 EasyApp. It includes a miniature, battery-powered digital transmitting module that can be strapped to rotating shafts of diameters greater than 0.9 inch to work with devices that measure strain, torque, and temperature, and with other bridge-conditioned sensors.

A test truck (top) at Eaton's facilities carries a receiver (middle) in its cab, and the transmitter of Accumetrics' AT-5000 system is strapped to the truck's driveshaft. The transmitter is connected to a strain gauge to test the shifting quality of a transmission. The brass loop surrounding the shaft is an antenna.

The transmitting module, which includes a lithium battery, mounts opposite an equally weighted adjustable yoke to maintain balance. The combination requires about 0.7 inch of radial clearance around the shaft. The Kevlar strap has a tensile strength of 3,000 pounds and can withstand over 12,000 rpm on a 4-inch driveshaft. Battery life can be as much as 150 hours.

Other components of the telemetry system include a receiving antenna and a receiver. By gathering data on the rotor and digitizing it before transmission, the AT-5000 system maintains high data integrity and noise immunity. It is crystal-controlled to require no tuning, and has a sample rate up to 11,718 Hz.

Fred James, electronics lab supervisor at the Eaton Truck Group, said, "From my standpoint, I have a bunch of customers who want to understand some aspect of our product's usage. They want to know what's happening to the product in the field, or they want to know that the test conditions they think they are running in the field are really being run."

Tough Questioning

Among the questions James has faced are: How big is the torque spike on the driveline when you back into a loading dock? What is the magnitude of the torque and the oscillations in the driveline when you're snowplowing, you hit a patch of ice, and then your wheels hit dry pavement again? And, of course, what kind of torque and torque peaks are being experienced by the driveshaft of a heavy truck equipped with an Eaton automated manual transmission?

Woldhuis said that most of the R&D is focused on Class 6, 7, and 8 trucks. (Class 6 starts at 19,501 lbs. Class 8 includes all trucks over 33,000 lbs.) Some of the testing is done at Eaton's R&D facility, some on a test track at the Marshall Proving Grounds in Marshall, Mich., about 25 miles away, and some at customer sites.

A strain gauge, to measure torque, and the AT-5000 EasyApp transmitter module are applied to the driveshaft. Technicians first mount the driveshaft on a fixture in a test cell and apply torque to calibrate the measurement system against a known standard. The driveshaft is then mounted on a truck. The next step is to mount a brass loop receiving antenna in the vicinity of the transmitter.

Coaxial cable from the receiving antenna is then routed into the cab of the truck to the receiver. The receiver's output is fed into a laptop computer with a National Instruments card to collect data. Once the transmitter is turned on, the engineer looks at the receiver, and if both red lights are off, a good signal is being received. Then the engineer and the driver (sometimes the engineer is the driver) go for a ride to collect data.

Woldhuis said Eaton once used FM telemetry, which required tuning the receiver to the right frequency. He added that, since driveshafts are not all the same size, the strap-on system saves time and the expense of making custom rings for different models.

According to James, "We have a mission to continuously improve our products and to get the design right on new products the first time. Our goal is to release it once, release it fast, and release it right. That's very challenging to achieve. It's a tall marching order, and we're going to try our best to achieve it, but that means we need a lot of information.

"The information has to be reliable and fast, and easy to collect. A high-resolution telemetry system that is easy to apply helps us to be flexible, responsive, and get the job done," he added.

John Reschovsky is president of Accumetrics Associates Inc. in Schenectady, N.Y.