We don't want to insult you‚ we know you probably know how to shift already. What might be less obvious, however, are the fine points on heavy truck transmissions that might help you maximize the life of your equipment and minimize repairs.
Country and Western songs about trucking often refer to truck drivers as "gear-jammers", and you might have figured out why. Shifting takes a little skill, and when you miss a shift, the transmission grinds.
In this article, we explain a brief history of transmissions, why they're designed like they are, and best practices for shifting.
A Brief History of Transmissions
The heavy truck manual transmission is one of the best engineered, purely mechanical devices ever made. Its basic design was created by the Fuller Transmission Division of Eaton Corp. of Kalamazoo, Michigan. Today, most manuals are made by Eaton/Fuller and there are millions out on the road. Through at least the early 1990s, this basic design dominated the market, with 13-speeds, 9-speeds, and later 10-speeds being used in most heavy trucks. After that, the 12-speed automated transmission - originally made by Volvo - began to take over, and other manufacturers, including Eaton-Cummins, followed suit with fully-automated 12-speeds.
One of the issues that needs to be handled when designing and operating a transmission with multiple gears is shifting between those gears. Different gears rotating at different speeds must be locked, one for each ratio, to the transmission’s main shaft when changing ratios. In order to do this, the speeds of the rotating parts must be "synchronized" or brought to a similar RPM before the gear can be selected.
While in Europe, manuals evolved to use synchronizer clutches or "synchronizers" like car transmissions have, they never made it into the heavy truck market in the US. European trucks eventually had manual transmissions that had good synchronizers and shifted fairly easily, but American fleet owners refused to buy transmissions that had them in Class 8 trucks‚ the ones that the most skillful truck drivers operate. This is because:
- The gears and bearings in a manual transmission will last almost forever, but synchronizers, designed to work on friction, inevitably fail long before the primary parts of the transmission wear out. Typical European manual gearboxes would have to be taken apart and would receive new synchronizers long before a complete overhaul.
- The secondary reason is ease of shifting. As heavy truck transmissions weigh at least 600 lb. in order to handle very large amounts of torque, car transmissions may weigh only 120 lb.
As a result, the typical big truck synchronizers tried briefly in the early days of trucking required a lot of fatiguing effort to operate. With the typical modern "constant-mesh" truck transmission, the engine does the work of synchronization instead of the driver. The result is easy and lightning fast shifts‚ shifts that are easier and quicker even than those with the highly developed synchronizers developed for European trucks by the 1990s.
The Design of the Transmission
The modern truck transmission is called a "constant-mesh" design because all the gears remain engaged or in mesh with one another all the time. This design prevents drivers from damaging gear teeth during shifting, as happened with early manuals where the gears slid along the main shaft when engaged. What happens in a modern truck transmission is that the gears are engaged and disengaged with what are called "shift collars", a small device which is slid along the mainshaft when the driver moves the gear lever. Shift collars are "splined" to the mainshaft‚ which means they are attached with male teeth and female grooves‚ that allow them to slide, but ensure they remain turning at the same speed as the shaft at all times. To engage a gear, the shift collar is slid forward or backward along the shaft by the shift linkage until it slides inside the gear being engaged. The outer diameter of the collar has male teeth or ‚Äúsplines,‚Äù and the gear has female grooves or splines and, if the gear and shift collar are turning at about the same RPM, the collar will slide easily into the gear, locking it to the shaft and enabling the engine to apply full torque to the main shaft of the transmission. This is made especially easy by some subtle parts of the design.
Best Practices for Shifting
Let's move into the fine points of transmissions for optimal performance.
Synchronize the gears
The typical manual transmission of today is a "win-countershaft" design. This means the gears you are shifting into actually float, held up by their teeth on either side. Their teeth mesh with the teeth of gears on either side that are integral with, meaning a solid part of each of the two countershafts. The design came out of a desire to make short transmissions during the days when the overall length of a truck and tractor was limited, but it remains because the design is especially strong and easy to shift. The front ends of the male splines on the shift collars are cut at an angle, as are the female splines in the gears. This means that the gears are self-centering when you shift, which makes it easy for the collar to slide right in.
The collars are much smaller than the gears themselves (only a few inches in diameter) which means the teeth or splines move a lot slower than the teeth on the gears.
So, if the RPM of the gear and the shift collar are even reasonably close, the collar will slide into the gear quite easily, without a lot of clashing, and easily lock the gear to the shaft.
In addition, if there is a difference in the RPM of the gear and collar, any wear or damage that occurs when the collar is forced into the gear will occur to the teeth on the collar and the grooves inside the gear. Since these parts don't have to roll over one another the way gear teeth do, slight damage will have no effect on the smooth operation of the transmission. As long as the teeth and grooves remain mostly intact, the collar will remain inside the gear, locking it to the shaft.
Double clutching
Double-clutching or "float shifting" by bringing the engine's RPM down to what it will be in the next gear when shifting up, or up to what it will be in the next gear when shifting down, while in neutral, will ensure easy engagement of the next gear. The point is (if you don't know this already), know the rpm drop or increase between gears. Typically, if you upshift from 2nd gear to 3rd gear at 1,400 RPM, you are likely to need the engine at about 1,100 RPM to engage 3rd. When downshifting, you’ll see about the same change in RPM, but going up. Obviously when going uphill, you need to wait a little longer for the RPM to drop a bit more as the rig will slow during the shift. Likewise, when going downhill, you need to shift faster. When downshifting on uphill grades, you need to raise RPM, but not quite as much as when on the level. You probably have this pretty well figured out already.
The transmission is designed so the RPM of the gear and shift collar does not need to be perfectly matched. The mass or weight of the involved gears and shafts is relatively small, so if the collar on the mainshaft is turning 60-80 RPM faster or slower than the gear, you'll still be able to engage the next gear with ease. The collar will just grab and since all the teeth and grooves all the way around share the load, the spinning parts will easily be forced to the same RPM.
As you know, you need to double-clutch‚ releasing the clutch while the transmission passes through neutral‚ so you'll be able to use the engine, which you can hear or the RPM of which you can see on the tach, can be used to bring the parts to the same RPM. Where problems occur is when a driver either shifts without double-clutching, or perhaps without dropping or raising the RPM an appropriate amount, and just forces the transmission to shift. Or when he or she attempts to "float-shift" or shift without depressing the clutch, and the parts are not rotating at almost exactly the same RPM. When float shifting, the shift collar would have to bring the entire rotating mass of the engine to the right RPM to force the shift‚ unless the driver can get the parts to exactly the same RPM‚ with a minimal tolerance.
Good drivers can float shifts with great skill, no doubt about it. And while transmission makers refuse to encourage float shifting, in the real world drivers do it and do it successfully every day.
The secret the transmission engineers won't normally let out is that there is just a very small amount of looseness in the splines‚ just enough so that if the RPM is really close, you can float shift without damage.
Poor shifting will tend to gradually chip the ends of the teeth and grooves. Up to a point, this doesn't matter much. It may put a very small number of tiny metal chips into the transmission lube that will drain out when the oil is changed. But, this kind of contamination is hardly ideal. The less the better. If a transmission is really abused by careless shifting, especially careless float shifting, the result will be that it will slip out of gear on a hard climb or any time, and need to be taken down and rebuilt, replacing gears and shift collars.
Avoid float shifting
The transmission makers recommend that you avoid float shifting when you are fatigued or under stress from the pressures of the road, or in difficult situations îlike when climbing a steep hill. Shifting with the clutch, if done with just a bit of care, even if every shift isn‚Äôt perfect, should never yield a lot of trouble. But, don't just jam the gearbox into the next gear except when necessary for safety. And don't attempt to float shift when you're not up to doing those shifts perfectly.
The second part of this is clutching‚ the clutch in a heavy truck with a manual transmission will last a very long time‚ if the rig is driven properly. But, it can easily be abused, as you may know. Damage can extend to the rest of the drivetrain, too.
There are two things to control when it comes to using the clutch‚ torque and RPM. And, because of the torque characteristics of a diesel engine, they sometimes go together.
The point is‚ you always need to engage the clutch at the lowest possible RPM.
Shifting is a chore, and many drivers, especially when they've had a long day, will want to start in a relatively fast gear, skipping the first few. Heavy-duty diesels will produce gobs of torque, and the torque increases, more than doubling, from idle speed to 1,100 or 1,200 RPM. Today's fleet engines often develop maximum torque way down at 1,000 RPM. So you can start several gears up from the bottom of the transmission, and the truck will pull like mad—no problem! Don’t do it, if you can possibly avoid it.
Utilize the "idle-away" technique
The transmission and clutch makers recommend that you use a technique called "idle-away". Since the diesel's governor will increase throttle and keep the engine at 600 RPM (or whatever speed it idles at) when load is applied, it's best to start out without touching the throttle, if you can. While when empty it's often OK to use higher gears, when loaded you should really be starting in say 2nd gear in a 10-speed, or the 1 position on a 13-or 18-speed (without needling Low). When loaded on a hill, the lowest gear you have‚ like 1st in a 10-speed is ideal. One reason 10-speeds became popular in the ‚Äò90s is that you can gracefully shift between the lowest gear and 2nd.
Why does this matter? Well, first of all, a powerful starting gear means that you can probably idle away, or use very little RPM above idle. It‚Äôs the difference between the RPM of the engine and that of the drivetrain at the beginning of moving off that determines roughly how much the clutch will slip. Obviously, at times, you may get moving much more quickly when starting on a hill by applying a little throttle‚ that's why the clutch engagement torque of an engine is rated at 800 RPM instead of idle speed.
But, the laws of inertia also apply. 600 RPM seems like it would cut the slippage by half from 1,200 RPM. But that's not the whole story. If in 2nd and allowing the clutch to engage at 1200 RPM versus 600, you are actually doing four times the work with the clutch.This is true because the law of inertia tells us that the energy needed to accelerate an object goes up as the square of the speed, not in direct proportion. This means that it will take 4 times the amount of energy (and four times the clutch wear) to accelerate the truck to, say, 6 mph versus 3 mph before fully releasing the clutch.
And, if you were to start in, say 3rd or 4th and apply heavier throttle, things get much, much worse. The diesel's torque increases tremendously above even the 800 RPM at which clutch engagement torque is rated. Drivers sometimes start in 3rd or 4th gear and increase throttle to get moving, which actually is quite effective. But, the fact is that starting in 3rd or 4th gear on a hill and using a lot of RPM may actually greatly increase the torque placed on the driveshafts in spite of the transmission being in a higher gear. That's why you'll often see less skillful drivers with their cabs rocking back and forth like crazy when they go for an aggressive start on a hill. The end result is often drivetrain damage along with excessive clutch wear. Add to this the fact that the temperature of the clutch increases when it slips. A lot of slippage makes the friction surfaces get what the engineers call "grabby" so the engagement won't be smooth.
So you can see why starting in the wrong gear is especially hard on the drivetrain, and why using a low gear with a minimum of throttle will greatly extend the life of all of the components that get you moving every day.
Again, you probably have most of this already pretty well figured out. And we want to make sure you have the context around why skillful operation, especially making the best possible use of the lower gears in your transmission, can pay big dividends when it comes to keeping your rig on the road and doing so at minimal cost.
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