When replacing a clutch requires removing the transmission for access, it can be a costly job. Starting in the right gear can multiply the life of a clutch by a factor of three or more.
Throwaway item or longtime friend – your clutch can become either, depending on your choices of shifting, driving and maintenance.
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 clutches because they don’t enjoy holding it down and coordinating throttle and clutch for a long time. So, what do they do when starting in too high a gear? They increase engine rpm to speed up the process. Bad move.
In the most difficult driving conditions, the driveshaft’s tubing often becomes the safety valve that absorbs extreme torque. The resulting wear and tear can be startling. Ernie Fry, a driveshaft technician at Associated Truck Parts in Gilbertsville, Pa., once saw a driveshaft that resembled a pretzel.
“The driver got stuck in the mud,” Fry says. “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 six steps to follow in order to save your clutch:
- START 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 (leaving out 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 you to get through the “slip time” quickly and take your foot off the clutch pedal so it will lock up. Once lockup 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 as much as four times because the vehicle is moving twice as fast when the clutch locks.
Increasing the rpm from a normal clutch engagement, down near idle, to up near the torque peak greatly increases torque as well. Dave McKenna, marketing manager of Mack’s powertrain division, estimates that maneuver increases strain from 800 lb.-ft. to 1,500 lb.-ft.
“Remember that 1,500 lb.-ft. multiplied 10 times by transmission and axle gears is 15,000 lb.-ft.,” McKenna says. 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 you start two gears up from where you should, you increase torque to the driveshaft and wheels. This is followed by a harsh clutch engagement at high torque levels and a prolonged clutch engagement time.
“You need the mechanical advantage to start off smoothly,” McKenna says. “You then don’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 violently rock the cab, 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, Davis says.
Forcing a shift before the rotating speeds are properly synchronized is another source of damage. 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 shift with little or no shock
loading. The Eaton Autoshift and Ultrashift transmissions eliminate these problems, Davis says, even for novice drivers or fatigued veterans. - SHIFT PROPERLY. Heavy trucks have gearboxes with a “constant mesh” design, which means the gears are always 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 says. 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 says.
“When it comes to the clutch, use it,” McKenna says. “Don’t float-shift, ever, unless you’re a brain surgeon. Can you shift perfectly every time? What happens when you get tired? Especially with the power and torque of today’s engines, relieve some torque before you shift.”
- ALWAYS BACK CAREFULLY, AND BE WARY OF ICE. Other forms of drivetrain abuse include shock loads caused by backing into docks and coming off ice with slipping, says Brian Davis, sales support supervisor for Road-ranger and Eaton.
When backing, slowly reverse while depressing the clutch just before running up against the dock, Davis says. Depress the clutch pedal gradually, maintaining just enough throttle to eliminate the torque.
On ice, accelerate ultra-slowly and back off the moment you sense tire slippage, Davis says. One of the worst things a driver can do to his truck is spin the drive wheels on ice and suddenly find pavement.
- ADDRESS PROBLEMS EARLY. You should check out any leaks, shifting issues or noises in the shift lever, which often indicate driveline torsional vibration or bearing damage, says Mike Kidd, sales and technical training manager for Transmission Technology Corp.
It is vital to monitor the transmission’s temperature. Check your owner’s manual for the recommended maximum. If the lube gets hot going up a mountain, drop one gear, reducing torque and keeping engine rpm up for coolant and lube circulation. Mack’s McKenna calls this technique the “work release program.”
- DON’T CUT CORNERS ON MAINTENANCE. Use the right lube, urges Kidd. Extreme-pressure lubes designed for rear axles will cause the synchronizers used in many medium truck transmissions to fail prematurely.
“Maintenance of lubricants is critical to extending life of the components,” says Eaton’s Davis. “Follow inspection procedures and proper drain intervals. The use of synthetic drivetrain fluids will maximize performance of the drivetrain, as well as ensuring warranty protection.”
Eaton and Dana recently introduced new lube specifications to improve component protection, Davis says.
Keep the engine coolant topped off because the engine cooling system often handles the transmission’s cooling load, says McKenna. Gear lube has a distinctive smell, so you can sniff around for leaks.
McKenna recommends 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 says. “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 spec’ing, McKenna says. “It’s not just an engine pushing the truck down the road. Take 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 possible clutch dampening can help control drivetrain stress,” McKenna says. After all, even the best drivers make mistakes or poor choices sometimes.
In the case of a Mack clutch, McKenna says, “a nine-spring design is about a $250 upcharge from a six-spring design. It is a lot more forgiving and will last a lot longer.”
The extra protection is especially handy in vocational trucks. “If you spend a lot of time at a lower rpm, something typical of many vocational trucks, you get a lot of torque-generated vibration,” McKenna says.
Eaton has a new VCT (Vibration Control Technology) Plus PD (Pre-Damper) two-stage, soft rate damper. It’s designed to decrease idle gear rattle generated by the latest emissions-compliant engines.
“Idle gear rattle is caused by engine pulsations at idle speeds,” says Eaton’s Davis. “The firing of the engine’s cylinders causes the flywheel to accelerate and decelerate.”
In Eaton’s new damper, “Small springs around the hub dampen torsional vibration at idle speeds before they reach the transmission and cause gear rattle,” Davis says. “The larger springs dampen torsional vibrations at higher engine torques.”
Idle gear rattle won’t cause much damage, but you’ll be happier without the constant racket.
Careful attention to driveline spec’ing, maintenance and operation will greatly reduce your operating costs. Since driveline problems often result in unexpected downtime, your relationships with customers and ability to get repeat loads also will benefit.
GEAR-JAMMING ALTERNATIVES
Automated transmissions have been around ever since major transmission makers put a toe into the water with gearboxes that float-shifted between the top two gears, using the engine’s electronic control module.
ArvinMeritor had a system that brought the engine to synchronous rpm, but required the driver to move the gear lever, and another that moved the shift linkage, too, but only upon driver command. ArvinMeritor finally went with the completely automated Freedomline.
Today, Eaton has automated transmissions with both manual and automated clutch operation that can shift faster than the best driver on hills.
Gearmaster was invented as a way to help drivers shift, while avoiding much of the cost and potential maintenance issues of automated transmissions. It’s a digital device that shows drivers the relationship between rpm and the synchronous point for every gear.
As soon as the transmission starts changing rpm, the device’s tachometer arrow begins moving. When engine rpm reaches the synchronous point for a given gear, the arrow will be sitting directly under that gear’s number. You can then effortlessly – and silently – engage that gear.
