Truck News


Clutch players

TORONTO, Ont. - Experienced truckers can make the simple act of shifting a thing to behold. There's never an unwelcome lurch or unexpected grinding sound as the lever moves from one position to the ne...

TORONTO, Ont. – Experienced truckers can make the simple act of shifting a thing to behold. There’s never an unwelcome lurch or unexpected grinding sound as the lever moves from one position to the next.

Still, these same drivers often become a little too over-confident, deciding that they don’t need to involve the clutch in the process.

The makers of driveline components are always quick to point out that there is no way to guarantee a proper shift without using the all-important pedal to the left.

“Is there a chance a driver can complete a good shift? Yes,” says Bob Langbo, manager of field and technical service at Eaton in Kalamazoo, Michigan, referring to the action known as float shifting. “(But) the likelihood of that happening is very slim.”

Automated transmissions may be able to allow shifts without the use of a clutch, but they rely on electronic controls to precisely match the engine speed with the rotation of the drive shaft. While automated equipment can perform a shift within 40 RPM of this “synchronous” position, an expert float shifter would consider any change within 100 RPM to feel like a smooth shift, explains Lon Miller, Eaton’s product service manager.

Think of the clutch as a type of lock between your engine and the transmission. When the pedal is not depressed, a pressure plate pushes against a clutch disc that, in turn, pushes against a flywheel to lock the engine to the transmission’s input shaft. As the engine turns at a higher speed, so does that shaft.

At a certain point, however, you need to change to another gear, and that requires breaking torque by pushing down on the clutch pedal, which pulls the pressure plate from the clutch disc and releases it from the spinning engine. The speed of the engine then needs to be changed to the appropriate number of revolutions before the clutch pedal is pushed a second time (aka double-clutching) and the lever can slide from the neutral position into the appropriate gear.

The problems begin if the clutch disc and flywheel are allowed to turn at different speeds. If the next gear is selected before the two are matched, the clutch is forced to absorb the difference in the form of excess heat.

The very action of float shifting also requires a driver to preload the shifter fork (also known as the sliding yoke) as they prepare to slide it into gear, explains Miller. The danger in the process, however, is that they’ll wear out the yoke slot in the sliding clutch, or the shifter fork itself.

“The obvious problem going into gears is (that) excess raking will round off the clutching teeth,” he adds. And as the front corners of these teeth wear out, they lose some of the depth needed to properly engage a shift. “Then it starts wanting to walk or come out of the gear, and you have washer problems and slip-up problems.”

The practice known as “snubbing” is one of the most common causes of wear at the corner of the clutching teeth, Langbo adds, referring to the gentle push of a lever just before the gear is engaged. If a driver fails to synchronize with the engine speed, the clutch rakes against the gear, leading to a grinding sound.

When a driver pre-loads the shift lever, the teeth around a sliding clutch actually work like a mill cutter, wearing away at the deflecting shift fork, Miller says.

Of course, today’s clutch designs are more robust than ever. For example, there is added surface area where the fork comes in contact with the teeth. Clutching teeth have also been lengthened, and pads are added between the yoke and clutches. Automatically adjusting clutch designs ensure that the clutch brake is always being operated, improving the performance of engaging and disengaging start gears.

Yet the use of the clutch isn’t the only step required in a smooth shift.

Unlike their predecessors, for example, today’s 13- and 18-speed transmissions require drivers to pre-select range shifts before actually moving the lever in the first place.

“The pre-selection keeps you from getting the front box in the wrong gear,” Miller explains.

After the range lever or splitter control button is moved into the proper position, the change in the synchronizer range happens as the lever moves through the neutral position.

Meanwhile, there should be no need to apply the throttle to coax a properly geared engine into motion. If the wheels are allowed to hop, the resulting spike in torque that occurs when they hit the ground can be seven to eight times higher than the levels the system is designed to tackle, Miller says.

“The shock loads when tires come back to the ground are horrendous.”

That being said, some of the greatest wear-related problems can be found between the fourth and fifth gears, he says. It’s why tapered teeth that are wider at the front than in the back are better able to handle the wear, and are less susceptible to pulling out.

So, too, have there been advancements in the lengths and bends of levers.

But in the face of extended automation, are the days of manual shifting numbered?

They could be in North America, Langbo says, referring to the continuing shift toward shift-by-wire designs. “Some types of automatic or automated product is likely to (account for) the majority of the market in the future.”

Consider Eaton’s current line of products in development. All of them involve some form of electronic controls.

That’s not to suggest that the manual shifts will disappear entirely. For example, manual transmissions are likely to remain key components in vocational trucks, where shift algorithms are “a little trickier” to plot, Miller says, referring to the mathematical calculations used to automate, well, anything that can be automated.

“Automation can react to what has hit the wheels. It cannot predict a downshift ahead of you,” Langbo says, “but the driver can still anticipate.”

Miller, for example, refers to how truckers in Canadian logging applications still have a specific idea about the speed at which they should descend a hill.

So, too, do other countries continue to demand manual equipment, since electronic engines are hardly the norm. It seems every jurisdiction has its favorites. China prefers synchronized gear boxes, while South America prefers crash boxes. And synchronized transmissions have long been the norm in Europe.

“Big over-the-road fleets, they’re turning drivers a hell of a lot faster (than vocational businesses),” Langbo says.

Whoever is at the wheel, however, will need to pay close attention to any clutch pedals that remain.

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