Dealing With Pressure

Often overlooked and sometimes neglected, the air system on construction trucks is not only required to provide adequate pressure for air-brake operation, but also for numerous auxiliary components such as lift axles, gate cylinders, latches, tarp systems, and many others. Stop-and-go driving in urban material delivery applications and operating in a dusty environment also add to the demands on a construction truck's air system. While a properly spec'ed and maintained air system requires little attention, the alternative can lead to disaster.

Recently, we had the opportunity to talk with the experts at Bendix Commercial Vehicle Systems to learn more about maintaining air systems in the severe service environment of construction trucking. Starting at the air intake and moving through the compressor, discharge line, air dryer, supply lines, all the way downstream to the storage tanks, choosing the right components and maintaining them properly can ensure trouble-free operation.

Not surprisingly, one of the most important aspects of avoiding maintenance problems with an air system is to make sure it is spec'ed correctly for the truck's ultimate use in your fleet. All too often, construction fleets will purchase "stock" trucks from a dealer and place them in applications that are more demanding than they were built for, or worse yet, take a stock truck and upfit it with options that place additional demands on the air system without any regard to upgrading the air system.

In most cases, stock trucks at a dealership are spec'ed to minimum requirements, enabling the dealer to advertise the most competitive price on a particular model. Even though the stock specs may not fit your needs, most dealers (if pressed) will upgrade the necessary air system components to make a deal, so don't completely eliminate stock trucks from consideration, just make the right air system part of the package.

Similarly, a used truck may be otherwise spec'ed to fit your needs, but won't have an air system sufficient for a high-demand application. Again, thanks to the modular nature of the components, it's not very difficult to upgrade the air system to meet your needs before placing the truck in service. An added benefit to upgrading the air system at the time of purchase is that you'll be starting off with known good components, many with a warranty.

If you're ordering a truck and spec-ing it from the ground up, the air system is no place to try and save on weight or cost. Like the old oil filter commercial said, "You can pay now, or pay later." Bendix supplies the dealers and factory engineers with all the data necessary to ensure that your truck is built with a proper and capable air system, so be up-front with the dealer about the truck's intended use, and about any equipment you plan to add after the truck leaves the dealership.

The items to consider when determining the proper sizing for compressors and air dryers include: the total number of axles, the total number of lift axles, if the truck or trailer is equipped with an air suspension, and the truck's intended duty cycle (stop-and-go driving, central tire inflation, air-operated accessories). Bendix uses a point system to assign values to each of these factors, and the total point score is used to determine which compressor and dryer are best suited for the application.

Just as you wouldn't send out a compact excavator to dig the trench for a 36-inch sewer main or expect a skid steer to load quarry trucks, picking the right compressor for your truck's needs is critical to getting the job done safely.

Similarly, air dryers are sized based on their capacity to absorb moisture, oil and other contaminants between the purge cycles required to eject those contaminants from the system. Again, a standard duty dryer like the Bendix AD-SP is quite capable for the typical conditions found in over-the-road freight hauling, and stepping up to a higher capacity unit like the Bendix AD-9 will increase the amount of contaminants removed.

In more demanding applications, two of these dryer units can be run in tandem for still more capacity. When the supply of clean, dry air is mission critical, Bendix suggests adding its EverFlow module to the tandem configuration to ensure maximum system capacity.

Once the air is compressed, clean and dry, proper sizing of storage tanks is also critical. On most trailers that use air to operate gates, latches and other accessories, the air tanks have already been sized to provide ample air for operating these accessory systems, without impacting the supply of air required to operate the trailer brakes.

Unfortunately, there is a temptation on tractors and straight trucks to tap into existing storage tanks when adding air-operated accessories. Not only does this have the potential to reduce the truck's braking capacity, but if improperly plumbed, a failure of the accessory can lead to a total failure of the truck's braking system. For this reason, it is important that any equipment connected to your truck's air system be manufactured and installed by qualified personnel who can certify that the modifications have not affected the DOT minimum required braking capacity of the truck.

Once commonly used in Northern climates to prevent icing of brake valves, alcohol injection systems have become obsolete as the quality and availability of air dryers have increased. When the compressor, dryer and the rest of the truck's air system is properly designed and maintained, adding alcohol to the air system to prevent freezing valves is unnecessary. Bendix has taken a stand, officially discouraging the use of alcohol in the air brake system as a means of preventing system freeze-up in cold temperatures.

Studies indicate that using alcohol and alcohol-based air system anti-freeze products removes the lubrication from the components of the air braking system. In addition, the materials used for the internal seals of the air system components may be adversely impacted by the residue that some anti-freeze additives leave behind. Both are detrimental to air system component life expectancy, causing premature wear.

Because of this, Bendix air system components' warranty will be void if analysis shows that alcohol was added to the air brake system.

According to Eric Weese, air treatment marketing manager for Bendix, "alcohol is not an acceptable substitute for having adequate air-drying capacity. If the air dryer is maintained in accordance with the manufacturer's recommended practices and moisture is found to be present in the system reservoirs, more drying capacity is required. Bendix has several viable options including extended purge air dryers, tandem dryers in parallel with common control, and air dryers arranged to provide continuous flow, as with the Bendix EverFlow air dryer module.

"To address concerns with contaminants in trailer air brake systems, products such as the Bendix Cyclone DuraDrain water separator and the Bendix System-Guard trailer air dryer are also available."

Bendix has also found some alcohol evaporators, which may be placed in series with a single air dryer, can be restrictive to the output of the compressor. This restriction can cause excessive compressor discharge pressures at the head of the compressor. This is especially true when the air induction to the compressor is pressurized (turbocharged). High head pressures can shorten the life of the internal components of the compressor and cause higher head temperatures, which may lead to carbonization of the compressor discharge and discharge line.

One disadvantage to the advent of inexpensive global manufacturing capacity has been the proliferation of counterfeit parts. Even though a fake part may look very similar to the authentic version, upon closer inspection it's rare to find similar construction or quality. Even if the part functions properly when installed, warranty support is typically non-existent, and there may be liability concerns if the part is found to have contributed to an accident.

Because air system and brake system components tend to be fairly universal in application, they have been a frequent target of the counterfeiters. Again, given the safety and liability concerns surrounding anything related to the truck's braking system, it's just not worth the risk to realize a minor saving on the purchase price.

Years ago, most air systems used a small filter mounted right on the compressor itself. Given the typical location for most compressors, this put the filter and the air intake right next to the front wheel well, a very dusty location on most construction trucks. Nowadays, virtually all new trucks supply the compressor with air from a source downstream of the truck's main engine air filter, taking advantage of better inlet locations and tremendously larger filter capacities. If your fleet still has any trucks with the older compressor-mounted filter, it may be worth upgrading to the newer filter configuration.

For most current trucks, there will be a hose connecting the compressor inlet to the engine air intake system. If the air source is between the engine air filter and the turbocharger, this line will operate at a slight vacuum, and if the source is downstream of the turbocharger, the line will be under pressure. In either case, a leak in this compressor inlet line may cause a problem.

For naturally aspirated (pre-turbo source) systems, a leak in this line will allow dust and other contaminants to reach the engine and the compressor. For pressurized (post-turbo source) systems, a leak in the compressor inlet line will allow engine boost pressure to bleed off, reducing engine performance.

At the opposite extreme, if this inlet line is kinked, restricted or if the filter is clogged, the compressor will be working much harder, reducing performance and sucking in more oil from the crankcase due to higher vacuum levels in the compressor.

So, the key items on the intake side are to ensure that the air filter is replaced before the manufacturer's recommended restriction levels are exceeded, route the compressor intake hose to prevent any restriction, and maintain the intake hose to ensure it doesn't leak or collapse. In those trucks still using a compressor-mounted air filter, Bendix recommends that polyurethane-type filters should be serviced every 5,000 miles, and that paper-type filters should be replaced every 20,000 miles.

The air compressor works closely with two other system components — a governor that cycles the compressor valves on (to increase system pressure) and off (once the system is up to maximum pressure), and a discharge line that routes the compressor output to the system's air dryer(s). As such, all three components play a role in the supply "package."

The compressor itself is the "heart" of the air system, but in a properly designed and maintained system, it is fairly trouble-free. Because the compressor is driven with engine power (via gear or pulley), lubricated with engine oil and cooled with engine coolant, merely following the engine manufacturer's guidelines for maintenance of lubricant and coolant takes care of most of the compressor's needs.

Bendix recommends regular inspection of the compressor and its connections at six-month or 50,000-mile service intervals. This includes inspecting for kinks, restrictions or leaks in the fittings and lines for lubricant, coolant, inlet air, discharge air, and unloader signal. It also includes checking for noisy operation that could be due to the drive gear or pulley.

Additionally, Bendix recommends a thorough compressor inspection every 200,000 miles. Depending on the outcome (and on the manufacturer's warranty), the compressor can be disassembled, cleaned and rebuilt, or the compressor can be replaced with a remanufactured unit.

If the compressor is the heart of the system, then the air governor can be considered as the brain. The compressor's duty cycle is controlled by the governor. In order to maintain the air system pressure within the range required by safety regulations, the governor sends an air signal to the compressor unloader port. The governor itself can be mounted onto the compressor or connected to the compressor with an air line. Bendix recommends that the governor be inspected for proper functionality at six-month service intervals.

In spite of its simple appearance, the discharge line is more than just basic plumbing. Even under the best of circumstances, the process of compressing air will generate heat and moisture. By providing a single, unrestricted discharge line of the proper diameter and length between the compressor and the air dryer, the compressed air is allowed to cool, which helps to condense the moisture and makes it easier for the dryer to do its job efficiently.

Obviously, a leak in the discharge line will allow system pressure to escape and cause the compressor to run non-stop. Still visible but maybe not quite as apparent, improper routing and/or abuse can kink the discharge line, restricting the compressor's output. Not visible without disassembly, restriction caused by a buildup of oil and carbon inside the discharge line can be just as problematic.

According to Nicholas Petek, engineering supervisor for Bendix Commercial Vehicle Systems, "Under normal operation, small amounts of engine oil are entrained in the compressor discharge air. Also, the discharge air is hot due to the process of compression. The combination of heat and oil can cause carbon deposits on the inside of the discharge line. Bendix recommends that the discharge line be inspected at regular service intervals, and replaced if deposits cause restriction."

Diagnosis of any less-than-obvious air system problems should include a thorough inspection of the discharge line before other components are suspected.

The air dryer is installed between the compressor and reservoirs to collect and remove solid, liquid and aerosol contaminants before they enter the air brake system and jeopardize efficient operation. Again, if the air dryer is maintained in accordance with the manufacturer's recommended practices and moisture is found to be present in the system reservoirs, more drying capacity is required.

Because the dryer's desiccant cartridge acts much like a filter to remove moisture and other contaminants from the air system, it must be replaced as a regular maintenance item. In severe service applications, the desiccant should be replaced annually, or when more than a trace of moisture is found in any of the air system reservoir tanks.

One of the benefits of a managed two-dryer system such as the Bendix EverFlow configuration is that since each air dryer operates only half the time, replacement desiccant cartridges last longer than with a single dryer cartridge.

According to Eric Weese, "Bendix recommends replacing the entire dryer every three to five years depending on its condition and vocation, while other maintenance items such as check valves and heater/thermostats are only called for on an as-needed basis."

Given the severe service nature of construction trucking, it's conceivable that sooner than later, it will be necessary to repair or replace air lines somewhere on a truck or trailer. Although it should go without saying, air lines should always be replaced with material of equal or better quality than that supplied by the factory.

A popular way to repair a small leak is to cut the line and insert a union fitting to couple the newly cut ends. (Note: Never do this on the compressor discharge line.) However, as each fitting in a line makes a cumulative addition to restriction levels, this type of "patch" is a one-time deal. If a section of line between two factory fittings gets a second leak, it's better to just replace the entire section of line.

Similarly, in a new installation of an air-operated accessory, it's better to use a sweeping curved section of line than to use one or more angled fittings to achieve the desired routing. Not only does this minimize restriction, but it also reduces the number of potential failure points.

Speaking of failure points, every inspection of your truck should include special attention to those points where air lines are subject to rubbing, chafing or vibration from adjacent components or from the truck chassis.