Hot Air

Twenty degrees doesn't sound like much at first, but it can really make a difference. If we're talking about outside temperatures, it's the difference between a coat and a short-sleeved shirt. Move that same 20 degree shift under the hood of an already hot heavy-duty truck, and you're talking some serious heat.

It's those 20 added degrees in underhood heat, brought about by the new 2007-standard diesel engines, that have some fleet maintenance managers worrying about their brake air system components.

We talked to some brake experts to find out if worries about premature wear and failure on 2007-spec' trucks are legitimate—and, if they are, to find out what fleet maintenance managers can do to address the issue.


"I'm not sure how widespread the fear is," says Randy Petresh, VP of technical services for Haldex. "I'm concerned about it, and I've had a number of conversations with a variety of people both at fleets and at the OEMs themselves, and I know there are quite a few of us who are concerned. We know the underhoods are higher, the operating temperatures of the new engines are higher, and we know there's a lot of stuff under the hood that is affected by higher temperatures."

To a certain degree, Petresh is fine with that—his company has already done a great deal of product development and testing in preparation for the 2007 heat wave—but there are limits to what is physically possible.

"We all have thresholds of pain, so to speak, when it comes to upper temperatures," he explains. "If you're below those thresholds, you're happy. Once you start creeping up closer and closer to your threshold, then the potential exists that you're going to breach it at some time or another. And we're running close."


"There are some things that fall in the category of capability, like seals, for instance; those are one of our biggest concerns," Petresh says. "There is no such thing as a rubber that will handle all circumstances. You can have one rubber that works well in high temperatures, one that works well in cold, one that will work well in oil, one that will work in water, but there isn't anything that will work in all of them. So everything is a very complicated compromise when it comes to seals. And of course there are millions of seals in there, and a lot of them are in the brake system."

"Our biggest concern is the foot valve, because it's got multiple seals," says Paul Johnston, senior director of ArvinMeritor's compression and braking business unit. "Many of those are what we call dynamic seals, which are very sensitive to compression and loss of sealing capability. The tractor protection valve is a little more static, and we've got room to play with some of the effects of that high temperature. As the temperatures get hotter the rubber gets harder and starts to shrink. You start losing that contact with the metal bores.

"Five or seven years on the valves will eventually get to a point that they have what we call compression set, where the rubber gets less elastic and you get more heat wear," Johnston says. "Eventually you get to the same level of leakage, but that's five to seven years out instead of the first year, let's say, under these higher operating temperatures."


Ken McKibben isn't as concerned as Johnstone or Petresh... yet. "Oh, I'm not concerned about it now," he says. "But I could be, because it's going to jump up 20 degrees."

The senior vice president for field maintenance for Penske Truck Leasing, McKibben hasn't seen any reason for concern so far. With 213,000 trucks spread out over 700 locations, he just makes sure his technicians stick to the basics.

"I'd break it down into three parts," he says. "Number one, know the system. The technicians need to know and understand the system and how it works.

"Number two, you need to understand how to keep it dry and keep it well-serviced," he continues. "Now, there aren't a lot of service points on these, as long as you've got the right specifications.

"Number three: you have to spec' it with the right air flow and the right size compressor," he goes on. "We're using a BA921 now, from Bendix, and when we have to go with a bigger one we use a BA922. And we make sure we spec' the Bendix air dryer on it. Now we keep it clean without having to rely on the drivers to drain the air tank, like we did years ago. We rarely have to do that now."


McKibben's strategy falls right in line with the advice of the worried experts: check your brake air system thoroughly, and check it often.

"The first checkpoint is opening up the manual drain valves on the reservoirs, whether that's the wet tank or the primary or secondary tanks," says Johnston. "The first checking point is either pulling the lanyard on the drain valve cable or getting under there and opening the drain cock. The mechanic can clearly see if there's moisture coming out.

"Then I would back up and look at the dryers," he continues. "Each dryer, regardless of design, has a purge valve—a discharge, an exhaust, coming out of the dryer, typically on the bottom. A good indication of the health of the dryer and compressor is looking at how much moisture and evidence of oil is coming out of the purge area. If there's more oil than normal, there's something going on with the compressor or possibly the dryer.

"By using my foot valve or reservoir drain cocks, I'd make sure that the compressor loads and unloads on a smooth and consistent basis," he concludes. "That can be done by lowering the pressure in the system and then letting the compressor start to compress air again and watch to see that it unloads properly and doesn't quickly cycle back and load again."


"The compressors are dependent on lubrication oil from the engines, and they also depend on coolant from the engines," says Petresh. "You need to have enough coolant flow, and with these new engines there's a big question mark in our minds as to whether the coolant levels and lubrication levels are going to be adequate. Some configurations of engines provide less flow, or the oil coming in is as hot as the oil going out. That translates into significantly reduced life durability for the compressors.

"Compressors deteriorate and wear out over time, they're going to pass more oil, and that's going to contaminate your air dryers," he continues. "And if the air dryers are totally contaminated, they'll pump that oil downstream."

"The SAE (Society of Automotive Engineers) has a standard of 160 degrees F for air dryer inlet temperature," says Leslie Kern, senior product manager—heavy duty, for SKF. "The higher the temperature, the quicker you're going to have oil carbonizing and coking up the lines before the air dryer, which will make the compressor work that much harder. It will also affect the components within the air dryer; you will get a problem with oil contamination in the desiccant, shortening the service life of the air dryer.

"For us, the key would be to have plenty of pre-cool line between the air compressor and the air dryer, and that can be done through spec'ing or before the vehicle is delivered to the fleet," she says. "The standard right now is at least six feet. If you're going to run hotter you may want to go to eight feet, and you must have at least a steel braided line."


"The OEMs have three options once it is determined that a particular component is at risk," says Sharon Seitz, product manager, Trailer Systems, Bendix Commercial Vehicle Systems LLC. "They can relocate the part. In some cases, the component can be shielded from the heat source. They can upgrade to a component designed to tolerate the higher temperatures. For instance, Bendix released a brake valve that is more tolerance of high temperatures—the Bendix® E-6™ HT valve design which has been released by some OEMs."

Seitz notes that the E-6 HT is specifically designed to handle higher temperatures, and is marked with a metal tag stating 'High Temp Product.' "To prevent premature degradation, it is important to replace that valve with a genuine Bendix HT valve, NOT a standard brake valve," she cautions.

"We're looking at all our designs," Petresh says. "We're looking at different rubber formulations, different sealing compounds, and have been for quite a while. Even 20 to 30 degrees is significant. If you're 20 or 30 degrees below your threshold, and now you're at your threshold, then 20 or 30 degrees is life-or-death."


According to Petresh, Haldex has also been talking to engine and truck OEMs since 2002, discussing ways to move the brake air dryers out of the engine compartment and under the seats, or between the frame rails or behind the cab. "But as you know," he says, "most of them are still in the engine compartment."

Will that be a problem as 2007 trucks start to chalk up significant road miles? Despite their best efforts to be proactive, the component suppliers aren't resting easy on this one.

"This isn't getting a lot of press or exposure, and it could really become something serious down the road," Petresh says.