Tech Track: Essential drivetrain care

The hardest working part of the drivetrain is the clutch. It’s also the most vulnerable. At best, it lasts half as long as other drivetrain components, depending on how often and how hard it’s used. Even the smallest change in operating technique can make a huge difference in a clutch’s longevity.

Many drivers wear out their clutch because they don’t enjoy holding it down and coordinating throttle and clutch for a long time. So, what do they do when they start in too high a gear? They increase engine rpm to speed up the process. Bad move.

In the most difficult driving conditions, the drive shaft’s tubing often becomes the safety valve that absorbs extreme torque. The resulting wear and tear can be startling. Ernie Fry, drive shaft technician, Associated Truck Parts, Gilbertsville, PA, once saw a drive shaft that resembled a pretzel.

“The driver got stuck in the mud,” Fry said. “Even after he noticed that the vehicle had started violently hopping up and down, he didn’t back off.”

If a vehicle shows signs of erratic traction, Fry has one suggestion: Get a tow.

Below are a few steps your customer should follow in order to save his clutch.

• Begin in the correct gear. This makes more difference than most drivers realize because it reduces stress on the rest of the drivetrain.

  The normal starting gear for a fully-loaded truck on a level road or slight upgrade is the first position (omitting Low) on a 9-, 13- or 18-speed and the second position on a 10-speed.

  Starting in this gear, and not one or two gears above, enables a good driver to get through the “slip time” quickly and take his foot off the clutch pedal so it will lock up. Once lock up occurs, clutch wear stops.

  Starting in third in a multi-speed transmission or fourth in a 10-speed, rather than the correct starting gear, will multiply clutch wear by as much as four times because the vehicle is moving twice as fast when the clutch locks up.

  Increasing the rpm from a normal clutch engagement, down near idle, to up near the torque peak greatly increases torque as well.

  Dave McKenna, powertrain sales and marketing manager, Mack Trucks, Inc., estimated that that maneuver increases strain from 800 lb.-ft. to 1500 lb.-ft. “Remember that 1500 lb.-ft. multiplied 10 times by the transmission and axle gears is 15,000 lb.-ft.,” McKenna explained.

  This happens because the electronic control module cuts fuel back below the torque peak to minimize emissions and ramps it back up as the engine nears peak torque and the turbo speeds up.

  When a driver starts two gears up from where he should begin, he increases torque to the drive shaft and wheels. This is followed by a harsh clutch engagement at high torque levels and a prolonged clutch engagement time. “The driver needs the mechanical advantage to start off smoothly,” McKenna said. “He then doesn’t need to use an excessive amount of torque at the engine to get enough at the wheels.”

  This maneuver stresses not only the clutch, but other drivetrain parts and the shaft tube as well. This kind of action can rock the cab violently, as when a novice driver attempts to start out on a steep hill.

  Bottom line: A clutch that would last 1 million miles with a careful driver may last just 100,000 miles with a driver who consistently starts in the wrong gear.

  Some drivers even start out in high range, Brian Davis, sales support supervisor, Roadranger and Eaton Corp., said. Here, forcing a shift before the rotating speeds are synchronized properly is another source of damage. Tell the driver to take the time to double-clutch and bring the engine rpm close to what it will be in the next gear, and the transmission will align with little or no wear or shock loading.

• Shift properly. Heavy trucks have gearboxes with a “constant mesh” design, which means the gears always are engaged. This reduces the tooth damage that occurred in earlier boxes when gears violently knocked into each other. Today’s gearboxes allow the driver to slide toothed collars along the mainshaft and lock the gears via internal teeth.

  Most drivers don’t realize, however, that though the shift collar’s teeth take most of the abuse, vibration still can travel through the metal and fracture gear teeth, McKenna said.
  When a driver float shifts, the shift collars are chewed up because the torque is far higher than they were designed for. When a driver misses, even by a little, “the vibration migrates through the drivetrain and causes all sorts of troubles,” McKenna explained.

  “When it comes to the clutch, tell the driver to use it,” McKenna said. He should never float shift. No one can shift perfectly every time.

• Always back up carefully, and be wary of ice. Other forms of drivetrain abuse include shock loads caused by backing into loading docks and coming off of ice with slipping, said Davis.

  When the driver is backing up, tell him to slowly reverse while depressing the clutch just before running up against the dock, Davis said. He should be sure to gradually release the clutch while maintaining just enough throttle to eliminate torque.

  On ice, the driver should accelerate ultra-slowly and back off the accelerator the moment he senses tire slippage, Davis said.

  One of the worst things a driver can do to his truck is to spin the drive wheels on ice and then suddenly find pavement.

• Address problems early. When the vehicle is in your shop, check out any leaks, shifting issues or noises in the shift lever, which often indicate driveline torsional vibration or bearing damage, said Mike Kidd, sales and technical training manager, Transmission Technology Corp.

  Also it is vital to monitor the transmission’s temperature. Check the owner’s manual for the recommended maximum.

  If the lube gets hot while going up a mountain, tell the driver to drop one gear, reducing torque and keeping engine rpm up for coolant and lube circulation. McKenna calls this technique the “work release program.”

• Don’t cut corners on maintenance. During maintenance, use the right lube, urged Kidd. Extreme-pressure lubes designed for rear axles will cause the synchronizers used in many medium-duty truck transmissions to fail prematurely.

  “Maintenance of lubricants is critical to extending life of the components,” Davis said. “Follow inspection procedures and proper drain intervals. The use of synthetic drivetrain fluids will maximize performance of the drivetrain, as well as ensure warranty protection.”

  Keep the engine coolant topped off because the engine cooling system often handles the transmission’s cooling load, said McKenna. Gear lube has a distinctive odor, so you can smell for leaks.

  McKenna recommended synthetics for “better cold flow.” Gears dry out during shutdown, and improved flow gets lube to them faster after cold starts.

• Spec the right stuff. “Choosing the proper transmission and engine combination is vitally important,” Kidd explained. “If the engine has 800 lb.-ft. of torque, the transmission should be rated at 800 lb.-ft.”

  Owner/operators should take a holistic approach when specing, McKenna said. “It’s not just an engine pushing the truck down the road. Put the whole situation in perspective. Is it a tractor or a dump truck? What will it be doing? Even with the same horsepower, the ideal spec won’t be the same.” Vocational trucks, for instance, may require a robust specification.

  “The most effective clutch dampening possible can help control drivetrain stress,” McKenna said. After all, even the best drivers make mistakes or poor choices sometimes.

  “If the driver spends a lot of time at a lower rpm, something typical of many vocational trucks, he will get a lot of torque-generated vibration,” McKenna said.

  “Idle gear rattle is caused by engine pulsations at idle speeds,” said Davis. “The firing of the engine’s cylinders causes the flywheel to accelerate and decelerate.”

  Idle gear rattle won’t cause much damage, but the driver will be happier without the constant racket.

  Careful attention to driveline specing, maintenance and operation will reduce your customer’s operating costs a great deal. Since driveline problems often result in unexpected downtime, your relationships with customers also will benefit.