Truck Checks: Air-brake maintenance

Air-brake maintenance
December 06, 2007
Written by Don Henry
donhenryThere are a lot of myths, rumours and superstitions about the maintenance and operation of air-brake systems. First, let's get the math out of the way. The engine may be required to accelerate the truck to 100 km/h from 0 km/h in 30 seconds. To accomplish this may require 200 horsepower from the engine. That same truck may be required to slow from 100 km/h to 0 km/h in just six seconds; this would require the brakes to absorb more than 2,000 horsepower. The brakes accomplish this by converting the forward energy of the vehicle to heat through fiction. This heat causes all kinds of problems. A highway truck may use its brakes only every few hours, as the driver knows where the next rest stop is and can begin to gear down before the stop, thus using the brakes less often and extending their operational life. Fire apparatus often make many aggressive stops. This causes the brake lining material and the brake drums to overheat. This heat has two effects; first, the drums expand and get larger and, second, the brake material losses its coefficient of friction. When this happens, the brake lining does not grab the drum as well. This condition is referred to as brake fad and it causes the truck to not be able stop as fast. The first few stops out of the firehouse are very normal because the brakes are still cold but as the brakes overheat the truck will not stop as quickly. This next bit of information is just plain physics: If you double the weight of the truck you will need to double the stopping power. If you cannot double the stopping power then you will need twice the distance to stop. This is just one reason we should not overload the apparatus. The real problem comes with speed. If the speed of the vehicle is doubled then the stopping force will increases by a factor of four. If both weight and speed are doubled then the increase is by a factor of eight. While we can not do much about the laws of physics we can take measures to ensure the truck can stop.

1. Know the route; do the planning and understand the grades of the hills you will need to deal with.

2. Ensure your brake-slack adjusters are correctly set with regard to angle and adjustment. As the brake drums heat up, they expand. If the brake-slack adjustment was marginal when the brakes were cold, it may be out of adjustment when the brakes are hot. This is probably the greatest cause of air-brake accidents. Make all your drivers/operators complete an air-brake course and ensure they know how to inspect the slack adjuster for correct adjustment and operation. It may be your department's policy to allow only qualified emergency-vehicle technicians to adjust the brakes and that's all right, but the driver must be able to recognize when the brakes need adjusting.

3. When coming to a stop, do not pump the brake pedal (also called fanning the brakes). This wastes compressed air. It is better to make a steady brake application. Myth one: You can pump up the air brakes. This is a holdover from hydraulic brakes. The first application of the brake pedal will give the most amount of air pressure. Any additional pumping of the brakes just wastes valuable air pressure. To make sure the truck can stop repeatedly and not suffer from brake fad, the truck may need an auxiliary braking device. NFPA 1901 says all apparatus with a gross vehicle weight rating of 36,000 pounds (16,330 kg) or greater shall be equipped with an auxiliary braking system. You could, of course, install the auxiliary-braking devices on lighter weight units.

Drain air tanks daily
Air contains moisture. This humidity is measured as a percentage and it affects the air-brake system. Moisture-laden air will cool in the truck's air tanks. This moisture will settle out and collect in the bottom of the tanks. Water, of course, is not compressible and will accumulate in the air tanks of your braking system. It needs to be drained out. If it is not drained then the effective size and capability of the air system is reduced and less air available for aggressive stops. The second problem will come during cold-weather operation. The water in the tanks, lines and valves will cause the brakes to malfunction and freeze up. You can ensure this does not happen by draining the air tanks daily. Drain the supply tank first, then the primary and secondary tanks. This order will ensure that excessive contamination in the supply tank does not get forced into the other tanks. The senior personnel in your department may have called the supply tank the wet tank. Myth two: You don't have to drain the air tanks because you have an air dryer. Wrong. Air dryers are great devices and I recommend them on each truck I help departments spec out. An air dryer ensures that moisture does not reach the air tanks. But air dryers do fail, even though they may be purging when the air compressor cuts out (that's the bush sound you hear), this does not mean the dryer is removing the moisture from the air. The air dryer needs its desiccant cartridge changed regularly. How often? A cement truck in a major city may need the desiccant changed every six months to one year. A fire truck could go longer without a change, it just depends on the amount of use. It needs to be changed when the daily draining of the air tanks reveals water in tanks.

Compressor build-up test
Your air-brake system was sized according to the Canadian Motor Vehicle Safety Standard 121. This standard determines how large the compressor needs to be and the size of the air tanks. I have noticed that some departments have allowed this test to slip over the years. It is very important for the following reasons: NFPA 1901 says "a quick build-up section in the air-reservoir system arranged so that if the apparatus has a completely discharged air system, it is able to move within 60 seconds of startup. The quick build-up system shall provide sufficient air pressure so that the apparatus has no brake drag and is able to stop under the intended operating conditions following the 60-second build-up time. On a chassis that cannot be equipped with a quick buildup air-brake system, an onboard automatic electric compressor with an automatically ejected electric shoreline or a fire station compressed-air shoreline hookup shall be permitted in order to maintain full stand-by air pressure while the vehicle is not running." In other words, the truck needs to go from no air pressure, out the door and to a full stop at the end of apron before you reach the street. Think your truck can do that? I have never been a big fan of onboard electrical compressors or the use of shorelines. I feel these give a false feeling of security - you think the air system on the truck is operational just because the building's air compressor is functioning. If you are going to use a shoreline, make sure someone in building maintenance is draining the water from the building's air compressor. Often, the building's maintenance department thinks the fire department is draining this moisture. Another problem is that many onboard air compressors don't remove moisture from the air before it is pumped into the air tanks. These onboard compressors do not route through the truck's air dryer and may be continually pumping in moisture-laden air.

Combine this with failure to drain the air tanks daily and you very quickly get an air-brake system that is full of water. The compressor build-up time also gives us much more information. If the time is too fast, it may be because the air tanks are full of water, thus reducing the volume. If the time to build up is too long then there may be an air leak in the system or a worn air compressor. A common method of checking compressor build-up time is as follows: With the engine cold, time the air buildup of the tanks from 50 psi to 90 psi. This should be done with the engine running at 1,200 r.p.m. The system should build up pressure in less than three minutes and, in many cases, a lot less then three minutes.

Let's say your truck takes one minute, 30 seconds and has done so from the day it came from the manufacturer. If today it takes two minutes and 30 seconds, I would want to find out why. Besides air leaks, one common cause is worn compressor piston rings. If the compressor piston rings are worn then the compressor will pass oil into the air tanks and be slow to pressurize the tanks. Of course, you will catch that with the daily draining of the air tanks. A small amount of this oil is normal.

Excess amounts will foul the air dryer. Warning: Make sure you wear eye protection when draining the air tanks - any contamination can exit the drain at a very high velocity causing eye damage. If the tanks have not be drained for years (Don't laugh, I have seen it!) then the drains maybe plugged with rust or scale. Don't use a device like a nail to make a hole in the drain. The compressed air could drive the nail into your hand. I often use an old coat hanger. This works well (and the world has too many coat hangers anyway!). If you continue to get rust and scale from the tanks, the tanks need to be replaced.

Anyone who has taken a basic air-brake course should know everything covered above, with the exception of the NFPA 1901 standard. I am amazed that we have people who operate fire apparatus with air brakes who have not taken an air-brake endorsement course. Oh yes, and before I forget, did I mention that it's very important to drain your air tanks daily. In future articles we will cover anti-lock brakes and adjustment of slack adjusters.

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