The driveline might be one of the most overlooked series of components on fire apparatus, yet it’s one of the most important. Your vehicle has to get members to the scene efficiently, it must function as needed, and it must return everyone to the firehouse with a minimum of downtime. None of that is accomplished without a quality driveline.
In this article, we’ll examine heavier driveline components for ladder trucks, which are the kind of drivelines that are used for vehicles that have a gross vehicle weight rating (GVWR) of at least 54,000 lbs. (Although they can be used on lighter vehicles, these drivelines often are less cost-effective in those instances.)
Engines
When comparing a 12-liter engine with a 15-liter engine, bigger isn’t always better. Many look at horsepower and displacement as the deciding factors for engine selection, but torque is more critical for typical fire apparatus driving. Stop-sign-to-stop-sign performance is based on torque. Horsepower comes into play when pumping water and at highway cruising speeds on a grade.
For example, a Cummins 12-liter (X12) engine that’s rated at either 455 hp or 500 hp will produce 1,700 ft.-lbs. of torque. The Cummins 15-liter (X15) that has a 450-hp rating will produce 1,750 ft.-lbs. of torque, or about 9 percent more than the torque that’s generated by the 12-liter engine; 500-, 565- and 605-hp ratings will produce 1,850 ft.-lbs. of torque.
There are valid reasons in certain cases to use a 15-liter engine. If your response district is very hilly and steep or if you cover several hundred square miles and your response is more like over-the-road trucking, these engines could be justified. However, you still might not need a 15-liter engine. Properly setting up the overall driveline will accomplish the same things at a more reasonable cost.
The engine’s physical size dramatically affects other features and components of the apparatus. The 15-liter engine is about 1,000 lbs. heavier than the 12-liter is and requires much more air flow for proper cooling. Several manufacturers successfully packaged the 12-liter engine in their 94- and 95-inch-wide cabs, but the 15-liter engine pushes you into the wider 98- or 99-inch-wide cab. That doesn’t sound like much until you try to back that vehicle into an older firehouse, navigate narrow streets or look at the additional costs that are associated with the larger cab.
Depending on your cab configuration (raised roof, overall cab length, number of seats), the 15-liter engine could push you into a larger front axle, larger front tires or both. This affects the cramp angles for steering and increases turning radius, which reduces maneuverability. Once the axle load is calculated, you might have to reduce the number of seats in the apparatus or reduce cab length, if the calculated load goes beyond the axle manufacturer’s maximum weight rating. The engine tunnel is larger to accommodate the larger engine and the air flow that’s required, so the seating space for the driver and officer could be reduced even in a larger cab.
A relatively small 3-liter difference between the engines has an exponential effect on operating cost. The 15-liter engine burns fuel at a much faster rate than the 12-liter version does, just to name one operational cost increase.
Transmissions
A popular transmission for the larger fire apparatus drivelines is the Allison 4000 Emergency Vehicle Series (EVS). There are four models in this series, but the most common is the 4000 EVS. The 4500 EVS is used more commonly with the 15-liter engine, because it can handle higher horsepower input and can be set up with torque-limiting for higher torque. The other two models that are in the 4000 Series are the 4700 EVS and 4800 EVS units. They are used in aircraft rescue and firefighting (ARFF) apparatus.
Both the 4000 Series and 4500 Series transmissions are six-speed units. In vehicles that have a GVWR of 50,000 lbs. (or more than 1,250 gallons of combined water and foam), the apparatus is speed-limited to a maximum of 60 mph by NFPA 1901: Standard for Automotive Fire Apparatus. Because the 12- and 15-liter engines easily can reach top speed without using sixth gear, it’s common to lock out sixth gear completely. It also is common to program the transmission to use first, second, third and fourth gear when the “D” is selected and to enable fifth gear with the “Mode” button (economy mode). This eliminates some upshifting and downshifting, particularly in urban and suburban driving environments. Limiting the shifting reduces wear and tear on the transmission as well as some of the jerking during the shift cycles, which is more comfortable for personnel who are in the apparatus.
Allison currently ships the 4000 EVS and the 4500 EVS with the economy mode enabled. You might consider having this feature shut off. Doing so will change the shift points slightly, and the transmission will react better to real-world driving conditions. You can specify (or request at your preconstruction conference) these changes, which will improve the life and performance of your transmission for certain.
The 4000 EVS is a close-ratio transmission; the 4500 EVS is a wide-ratio transmission. The gear ratios between the two differ. In first, second and third gears, the difference between the ratios in the two transmissions is greater than it is between fifth and sixth gear. Fourth gear in both transmissions is a one-to-one ratio. Different gear ratios react differently with the engine during driving evolutions. The wide-ratio gearing can be a viable option to pair with the 12-liter engine, particularly with heavier apparatus.
During a recent project on a tower ladder quint that has a GVWR of 84,400 lbs., we paired an X12 500-hp engine to a 4500 EVS transmission and a 6.14 gear ratio in the rear axles. We were weight-limited on the front axle and couldn’t use a 15-liter engine, so the design required some creativity. Road test at final went very well: There was plenty of power (the vehicle accelerated well and didn’t lag on moderate inclines), and the pump test went flawlessly. This all was possible because of collaboration with the engineers, and it serves as a valuable lesson in selecting driveline components that work best for a department’s needs.
Something to keep in mind regarding engine offerings from Cummins and transmission offerings from Allison: The hardware doesn’t change across the different families in the product lines. The 4000 EVS transmission that’s used in a fire apparatus is the same as an Allison 4000 Series transmission that’s used in construction vehicles. The X12 and X15 engines that are used in fire apparatus are the same as the ones that are used in commercial trucks. What does change is the electronics and software, which affects characteristics and operation.
When looking at the Allison transmission from the rear, the power take-off (PTO) locations are situated at 8 o’clock and 1 o’clock. The ports for the PTOs are cast into the transmission housing and aren’t movable. In an aerial device, one PTO is needed to operate the aerial hydraulics; often, a second PTO operates a hydraulic generator. To accomplish this, most manufacturers use a piggyback PTO arrangement that’s mounted at the 8 o’clock position, which allows the floor in the rear of the cab to remain flat.
If your specifications call for separate PTOs or if your apparatus setup needs a third PTO, keep in mind that this will necessitate putting a hump in the rear cab floor for clearance of a PTO that’s mounted in the 1 o’clock position.
Driveshafts
Once you move to an engine that’s 12 liters or larger, the driveshaft will be the 1810 series. This reflects the rating for the shaft tube, the yokes and the universal joints. The exception to this is on a tandem-rear-axle vehicle. The interconnect driveshaft between the forward tandem axle and the rear tandem axle will be a 1710 series, which makes the driveshaft the sacrificial link between the two axles, because it’s the easiest and least expensive component to replace.
Don’t forget to include driveshaft brackets in your specifications in case of component failure. They’ll retain the driveshaft and help to limit damage.
Rear axles
Tandem rear axles are most common on aerial apparatus. Although single-rear-axle aerials have been around for a long time, generally, they are found on smaller aerials (70–75 feet). Advancements in aerial engineering and axle and suspension engineering expanded the use of single-rear-axle offerings. Some design changes and operational options in pumpers, tankers, heavy rescues and aerials rely on a single rear axle and suspension that’s rated for 35,000 lbs. In higher gross vehicle weight (GVW) apparatus, you must ensure that the single-rear-axle configuration of the driveline will serve your needs and carry the tools and equipment that your department’s operations require. We speak for many when we say that there’s nothing worse than finding out that putting everything on the apparatus that you just bought gives you an overweight vehicle.
In addition to the weight rating, there are other options that you should include in your driveline specification. Some internal gear assemblies are more robust and have a larger contact area between the gears than other assemblies do. These can withstand higher levels of use and abuse with less downtime.
A driver-controlled differential lock is accessed by a switch that’s located on the dashboard to lock both sides of the axle into a traction mode. It assists with driving in low-traction situations, such as mud and snow, and is available on both single- and tandem-axle assemblies.
Tandem axles also have an available inter-axle differential lock. In low-traction situations, the inter-axle lock engages power to both axles, not just to the front.
Do you work in a flood-prone area or one where water often ponds on streets during storms? An extended axle breather-tube will help keep water out of the axle if you must drive through flooding.
You also can specify rear axle gear ratio. The rear axle ratio tells you the number of rotations that the driveshaft must make to turn the rear wheels one full turn. A rear axle ratio of 5.52 means that the driveshaft must make five-and-a-half rotations to make the rear wheels turn one revolution. Generally, lower gear ratios are better suited for long-haul trucking on paved roads with moderate elevation changes. Higher gear ratios are best for towing, steep terrain and heavier loads. Most manufacturers use a standardized ratio unless the purchaser tells them otherwise, generally in the high 4s or low 5s. This ratio is good for many mixed driving applications, but you can customize your vehicle’s ratio for your response district.
It’s very easy to speed-limit a truck—a simple alteration in the computer program—but more time-consuming to figure out the best combination of gear ratios. You must limit the speed to meet NFPA 1901 mechanically, make your vehicle more operationally compatible to your response district and ensure improved performance over the life of your vehicle.
Work with your salesperson and your manufacturer’s engineering department. If you believe that you need more information, you shouldn’t hesitate to contact the manufacturers of the respective components. The time that’s invested in making these decisions will pay dividends over the life of the apparatus.
