The new federal mandate requiring shorter stopping distances dictates more brake torque for the front axle of trucks and tractors. The regulation can be accommodated by bigger drums, but it may stimulate a faster acceptance of the air disc brake for steer axle brakes.

But even without this, there are good reasons for adopting disc brakes. There's the better stopping, legislation or no; there's the freedom from fade on long downgrades; there's the better straight line stability with no side-to-side pull. And then there's the maintenance saving in some applications.

Air disc brake systems (pictured) don't look anything like drum brake systems. Inspection standards and procedures are not the same either.
Air disc brake systems (pictured) don't look anything like drum brake systems. Inspection standards and procedures are not the same either.

Of course, duty cycle is everything when it comes to brake maintenance. A hard-used braking system, such as on a trash truck or a bus, cries out for a brake that not only goes the distance but is quick and easy to service when the lining material is eaten up or the mating metal surfaces damaged by heat. The fact that the friction material on the pads on disc brakes can be viewed without taking off the wheels is a big plus. And when it's time to renew the worn pads, pulling a retainer, winding off the slack adjuster and removing the pads has the usual procedure for drums beaten in a New York minute.

In a Technology and Maintenance Council presentation several years ago, the relative times for a drum brake reline and a disc pad change was estimated at around four to one, with 15 minutes for the pads against one hour for conventional S-cam brakes.

Admittedly, scored or heat-checked drums from the harsh duty cycle can be very easily replaced. But rotors generally go at least to bearing and seal inspection periods in the latest disc designs.

In the on-highway, truckload segment, the disc brake may be good for the first life or even the entire life of the tractor. Possibly one brake job may be necessary, especially as fleets look like they may extend trade cycles in the post-2010 environment. And while disc brakes are easily maintained, they do have to be inspected. So the maintenance savings don't really add much to the plus side of the disc brake cost equation. For these fleets, it's performance that would draw them to the disc brakes, as one S-cam brake job in three to five years is not going to break the bank.

So the most likely candidates for maintenance savings with disc brakes are in the medium to heavy duty cycles, from regional line-haul through urban distribution to refuse and bus applications.

With all the consolidations in brake and driveline component manufacturers and suppliers, we are down to Bendix and Meritor as domestic suppliers. Haldex and more recently SAF Holland have also come to offer discs, in the latter case a trailer setup on its revolutionary CBX-40 slider.

Regular maintenance

According to Bendix Service Data for its mainstream product, the ADB 22X, there are a number of preventive maintenance steps necessary to ensure that air disc brakes perform properly over their long lives.

For over-the-highway vehicles, the wear indicator on the caliper/pad assembly should be inspected every four months. As experience grows with the brakes, this interval can be changed so that the checks are made four or five times between pad changes. More extreme duty cycles should be inspected more frequently initially so that the interval between inspections and pad changes can be established.

At the same time, the discs must be inspected. In line-haul applications little interaction can be anticipated, but in severe service some cracking may become apparent. Understanding how much cracking and other surface damage can be tolerated is important for these inspections.

A service interval should be established for checking the running clearance between pads and rotor, and for correct adjuster function. These procedures vary and can be found in the service literature from the relevant brake supplier. Caliper travel should also be checked. This may be done on every pass through the safety lane. Techs should check the sliding calipers to ensure they slide freely on the pins or guide sleeves. This ensures pads press evenly on both sides of the rotors and wear evenly when the brake is applied.

On the Bendix calipers, tappets and boots and the general appearance of covers, caps, hoses and brake exterior should be examined during this check. The same applies to the appearance and general condition of seals on other makes of disc brake.

If there's any binding of the slide or uneven clearance, or too much clearance, a wheels-off inspection should be made.

Pad and rotor change

If optional electrical wear indicators are present, a dash light will illuminate or an alarm buzzer sound to indicate a brake problem. But in this case, or when the indicators on calipers and pads align, the pads need checking. If the friction material on the pads is worn down to 2 mm, says Meritor, they are due for replacement. If the pads measure more than 2 mm or there are significant chunks of material missing, replacement is required. And pads should be replaced on both ends of an axle.

This is obviously a wheels-off procedure and the pads are replaced according to the manufacturer's service literature. In general, this means removing the pad retaining clip, backing off the adjuster mechanism and withdrawing the old pads. This is a very quick operation and, depending on what is included in the new pad kit, pads and retainers, some seals, etc., are discarded.

Technicians should be protected from brake dust. They should be sure to wash hands after handling brake components, certainly before eating or smoking. While not specifically carcinogenic, brake dust is still a dangerous irritant.

In Meritor's case, the recommendation is to remove the old pads, back off the adjuster until the outboard pad can be inserted, push the caliper across until the pad contacts the rotor and continue to back off the adjuster until the inboard pad can be installed.

The adjuster should be tightened until pads are on the rotor, than backed off for the appropriate running clearance.

Obviously, the rotor should have been inspected before reassembly commences so that any extensive damage can be dealt with. Bendix says small radial cracks no more than 1.5 mm wide and 1 mm deep are OK, providing they don't extend across more than 75 percent of the disc face. Any crack that goes to either edge of the rotor means the rotor should be discarded. Circumferential grooves are OK, providing they are no deeper than 1.5 mm.

Changing out a rotor is no more time-consuming than a drum, once the wheel-end is exposed. Obviously the caliper has to be removed to get the hub off, and there is always the danger of damage to the seals. The rotor in all but the severest service applications will outlast the periodic maintenance for wheel-end bearings and seals, so this should not be a problem.

Bendix has a cute fix for disc replacement on its European sliding disc products. Angled blocks are substituted for the pads and a brake application cracks the disc in half. The two pieces are removed and a new, pre-cracked disc is re-installed on the splines and reassembled using a pair of circumferential setscrews. There's no removal of the caliper or the hub. This feature has been promoted in the past, but not offered in North America. That may change. If it does, it will likely be very popular with refuse fleets that are looking at annual rotor changes.

Adding up

The maintenance savings benefit of the air disc brake is the way fleets will try to pay back the cost of the option. As we see here, duty cycle determines what those savings may be and whether there is an economic case to be made for the new technology.

But the brake manufacturers do anticipate a gradual acceptance of the disc brake for its performance benefits that enhance safety. If a fleet makes a decision to adopt air disc brakes for this reason, management will likely hear a small cheer from the service shop