KENWORTH TRUCK COMPANY OFFERS ADVICE ON SPEC™ING A MIXER
| January 21, 2008 |
A mixer is a highly specialized piece of equipment which is expected to put in a number of years of service. To ensure efficiency over the life of the vehicle, it™s important to spec wisely.
A key consideration is the local length and weight regulations since these will dictate axle spacing needed to maximize payload. Some states require compliance with the Federal Bridge Formula; others don™t. This will have a big influence on how the axles are set up and spaced.
A Bridge Formula truck will tend to be longer to spread the weight and this will define where the steer axle goes and have an impact on frame dimensions, said Brian Lindgren, vocational sales director at Kenworth Truck Company in Kirkland, Wash. In most cases, you would spec a booster axle at the rear to increase the weight allowed. You need at least 427 inches from the centerline of the front steer axle to the centerline of the booster axle to be sure to qualify for a 36-foot bridge.
At the same time you need to stay within the 40-foot overall length limit, so the steer axle needs to be as far forward as possible. A 427-inch bridge length in a 480-inch overall length leaves only 53 inches for front and rear overhand. The rear mudflap is usually 25 inches from the center of the booster axle, so the steer axle needs to be within 28 inches of the front bumper, Lindgren said. Also, be careful not to spec a permanently-mounted tow hook which extends out front since it could make you over length.
In states where you don™t need to comply with the Bridge Formula, you can spec trucks shorter, making them more maneuverable on jobsites. But you may need to spec lift axles to carry the extra weight.
Depending on the weight carried by the steer axle and the laws in your state, you may also need to spec wide-based front tires. In the states where tire footprint is regulated, it™s typically 600 pounds per inch of tire width, said Kenworth™s Lindgren. That will require a wide tire such as a 425/65R22.5 size to get to 20,000 pounds on the two front tires.
Where you expect to be delivering concrete will also have an influence on specs. The heavier the load and the rougher the terrain, the more you will need to beef up the chassis and suspension.
Most mixer chassis often need to get into some pretty rough jobsites, said Lindgren, so be sure to spec a suspension with a lot of articulation.
The frame rail specs are impacted by the type of mixer barrel used. If the barrel has a subframe welded to the chassis rails that is approved by the OEM, a single 11-5/8-inch frame is fine, said Lindgren. You™ll want a transition plate bolted to the frame at back of the cab which goes as far as forward as possible. This eases the stress on the rails.
But if the mixer pedestals are mounted directly to the frame rails, you™ll need an inserted frame of at least 10-3/4 inches, he said. Remember, it™s not just strength you™re looking for, but stiffness, too. And, if you have more than one pusher axle, you should have an inserted rail to withstand the higher side loads.
Another thing to remember about the frame: Mixer supplier often request crossmembers in specific locations. It™s a good idea to check with them before the truck is built.
The type of construction sites you are typically visiting will affect the amount of traction you require. I always recommend side-to-side differential locks in both rear axles for traction off the pavement. In areas with sandy soil, such as Florida, all-wheel drive also known as 6×6 has been more commonly spec™d so that mixers don™t get stuck on jobsites, Lindgren said. More recently, we™ve seen operators spec™ing a 6×4 configuration with active tire-pressure control systems which allow the driver to reduce tire pressure when they™re off-road to get the needed traction in the sand.
Big horsepower is not a requirement for mixer applications. You should get just enough horsepower to do the job, said Kenworth™s Lindgren. Generally, 320 to 250 hp is plenty for most applications. Extra horsepower just uses more fuel, puts more strain on the rest of the drivetrain, and adds cost up front.
The transmission you put behind the engine needs a lot of ratio range. You need a low enough gear to inch along at a jobsite and a high enough top gear to attain decent highway speeds. Lindgren said that, The Eaton Fuller 9ALL is a common transmission spec for mixers. It has a really low ratio for crawling while pouring curbs or other continuous pours. But there™s also a high enough top end for traveling at highway speeds, he said.
We™re seeing more automatic transmissions being spec™d, he noted. With the electronic controls on the newer automatics, the shifting is very smooth and responsive. They™re easier on drivers and reduce stress on the drivetrain, and the truck can keep up in traffic much easier. The only downsides are higher up-front cost, and a slight decrease in fuel mileage.
The power take-off (PTO) must come directly from the engine in order to provide enough power for the mixer system. Whether you use front engine or rear engine PTO depends on the chassis configuration. Lindgren recommends rear engine PTOs any time you can make it fit.
Front engine PTOs can reduce the size of the radiator, Lindgren said. With 2007 engines rejecting more heat, you need a larger radiator to cool the same horsepower, not to mention the added heat load of transmission coolers, power steering coolers, air conditioner, and so on. And with a front engine PTO, the pump is out front in a vulnerable spot. The hydraulic lines have to snake past the engine, and often close to the exhaust, to get back to the mixer drive motor. Rear engine PTOs make a cleaner installation.
But in those areas where the front pedestal must be mounted immediately back of cab, front engine PTO is the only alternative. A front-mounted PTO makes sense in Florida where the weight laws give you an incentive to get more load on the steer axle, he said.
If you spec an automatic transmission, you definitely want a rear-engine PTO because it will provide 30% higher output speed due to the gearing at the flywheel. This allows much less slip in the torque converter when you are crawling along and want to keep the drum speed up to pour the concrete. Too much slip in the torque converter can lead to overheating the transmission or the engine™s cooling system.
According to Lindgren, the rear axle ratios should be evenly matched with the transmission so that engine speed is around 1,600 rpm at highway speeds. You should also be able to go as slow as 1.5 mph at 1,400 or more rpm, he said. A 4:30:1 ratio with 11R22.5 tires and the -9ALL transmission works well. With a 6-speed automatic transmission, use a 4.88:1 or slower ratio.
Another thing to remember is air filtration. Lindgren recommends a pre-cleaner on underhood air cleaners to filter out large particles and plenty of element area on the main filter. Dual external air cleaners are better, and many like the look. A little money spent up front on a better air cleaner is cheap compared to a dusted engine, he said. And better filtration will usually mean longer life for the filter elements.
Weight is an issue with mixers, but you have to save a lot to be able to haul an extra half yard of concrete. The cost of lightweight components has to be considered.
You can slim down by spec™ing components such as wheels, air tanks, and clutch housings in aluminum rather steel. Use the smallest fuel tank you can get away with, Lindgren added. Most mixer applications burn about r to 4-1/2 gallons per hour. So a 55- to 60-gallon tank is usually plenty of fuel for one shift.
Lindgren suggests engines such as the Cummins ISL or Caterpillar C9 to save up to 800 pounds compared with a 12-liter block. With 350 horsepower and 1,250 lb-ft of torque, these 9-liter engines have all the power most operators need.
Lindgren also advises selecting an aluminum cab for its light weight and corrosion resistance.
You can save valuable pounds by choosing the right suspension. The difference can be as high as 400 pounds, Lindgren said of the different suspension options. You can also leave shock absorbers off the rear because the weight of the mixer means you will never be running light.
Using wide-base super single tires instead of duals on the rear can save hundreds of pounds in wheel and tire weight. The trade-off is that you get a narrower track if you stick with standard track axles, said Lindgren. You can go with a wider track, but then you may not have the option to go back to duals later.
DRIVER PERFORMANCE ITEMS
Finally, let™s look at a few driver performance-related items. To get the best turn performance and road feel from steering, Lindgren recommends dual small gears rather than a large single steering gear with an assist ram. The dual system usually requires less maintenance than a single system with assist.
Try to spec as many windows as possible. Consider Kenworth DayLite® doors and rear corner windows behind the doors, Lindgren said. It™s a good idea to mount the exhaust behind the cab rather than at the side to keep out of the way of the corner windows.
While we™re talking windows, Lindgren suggests picking low-replacement cost windshields when they are available. Most mixer fleets replace at least one windshield side per truck per year. Two-piece flat-glass windshields with roped-in seals can be replaced in half an hour for a total cost of under a hundred dollars. This can save thousands of dollars over the life of the truck.
With lift axles, it™s smart to get a six-channel ABS system. Lift axles, especially steerable ones, are normally over-braked for the load, Lindgren says. By including them in the ABS system, it™s much easier for the driver to avoid locking them up and flat spotting the tires.
Two last driver performance items that Lindgren recommends: For a small price in cost and weight, treat your drivers to Kenworth™s Extended Day Cab and Kenworth QuietCab® package. The Extended Day Cab gives the driver 6 inches more room fore-and-aft, and 5 inches more headroom, and the QuietCab option reduces in-cab noise by two decibels, meaning it cuts perceived noise by almost 50 percent. These options can dramatically reduce driver fatigue, and that helps keep them more productive.