Reinventing the brake

April 1, 1998

Reinventing the brake

Nearly a decade ago, the EPA outlawed using asbestos in commercial products — the result is a decade of brake product improvements

By Greg Napert

April 1998

It wasn't very long ago that the Environmental Protection Agency (EPA) outlawed the production of any products incorporating asbestos. The EPA's Asbestos Ban and Phase-out Rule, which was implemented in 1989, as well as the National Emission Standards for Hazardous Air Pollutants (NESHAP) rules implemented in 1990, eliminated the production of products that contain asbestos. The laws allowed brake pad suppliers to sell the remaining products they had in stock but not manufacture any new ones with asbestos as an ingredient.

Unfortunately, asbestos was the primary ingredient, as it is resistant to heat and is also flexible enough to resist cracking.

The new laws sent the brake pad manufacturers and suppliers scurrying off to find a replacement material that could be used for brake pads.

Cleveland Wheels and Brakes was the first to introduce a non-asbestos replacement material that it uses to this day. John Bakos, distributor sales manager for Cleveland and Airborne, says that the company has been using the product ever since they introduced it in 1988, and it has performed quite well. Like the asbestos pads, he says, they still have to be conditioned properly in order to work as they should.

Other manufacturers, such as Rapco in Hartland, WI, had some growing pains with the replacement material. Many problems were primarily with cracking of the pads. Although cracking was often the result of placing too much pressure on the rivets during installation, the material should have been able to hold up to traditional brake pad installation techniques. Rapco continued to experiment with new combinations of materials to replace asbestos.

Pat White, VP of sales and marketing for Rapco, explains that the company continued to invest in research with new materials that would perform as well as asbestos. Nine years and several brake pad generations later, the company is finally satisfied with the material it's using. In fact, they believe the new formula for their pads is better than asbestos.

Mike Lotzer, sales manager for Rapco, says, "The material we are using today is as good as the original asbestos in terms of its performance. I believe the new material will actually outlast the original."

Rapco went through several generations of linings and has arrived at what it considers to be a superior product (right, with notch) to the older asbestos lining.

Lotzer explains, "The first replacement material we used after asbestos was outlawed did quite well on the dynamometer, but there was a consideration that we didn't take into place — it was too brittle in operation. Asbestos was a much better material for binding and flexing. The new material was brittle, and many people were cracking the linings upon installation, or they would crack if the disc or back plates were not perfectly flat. So we discontinued that lining. We then changed the material and added a metal back plate. We still weren't satisfied, so we moved to encasing the material in aluminum to try to prevent cracking. As time progressed, we finally discovered a material that was quite good, and this allowed us to eliminate the backplates."

With the exception of Cleveland's brake lining material (Cleveland does not use Kevlar in its linings), one component of the newest generation of brake pads is Kevlar, a fiber commonly found in many of today's composites, including bulletproof flack jackets.

Jeff Kelly, president of AVPRO, says his company introduced a specially designed Kevlar-based non-asbestos organic brake linings, which offer many improvements over non-asbestos pads available in the marketplace today. "The primary improvement is a 25 percent service life increase over non-Kevlar pads with no reduction in the coefficient of friction or otherwise stated braking power. This means for every four non-Kevlar lining changes, you will only need to provide three AVPRO changes."

Lotzer from Rapco says that Rapco also uses Kevlar in its brake linings. He cautions, however, that too much Kevlar in the brake lining formula will cause the brake linings to degrade. "We do put a small amount of Kevlar in our brake lining material because it works good as a good bonding material. You can't use too much Kevlar; it has a low melting point. The brakes get up to 1,200¡F on the surface of the linings. Kevlar does give the brake pad some good qualities. It's great for holding the material together, for instance, but it is really critical that the Kevlar be used in the right amount."

Improving brake discs
As the need for finding a new brake lining material arose, the search for better brake discs also came into play. This search came about because the market was demanding a better disc — one that would have better wear properties and one that would not rust.

Cleveland was the first to answer at least part of this equation with a new one-piece forged disc it introduced about six years ago. Bakos says that the company's introduction of this new disc resulted in a much stronger disc with better wear properties. The one-piece evolution eliminated the weld at the base of the flange, and the forging produced a grain structure that significantly improved the strength of the disc.

Two previous solutions, chromed discs and stainless discs, were aimed at addressing corrosion problems and are still available. They do the job in terms of corrosion protection, but they have some problems in the performance area. Cleveland recommends that chromed discs should be used on aircraft that are flown less than 200 hours per year. Bakos says chrome is primarily designed for aircraft owners that leave the aircraft sitting for long periods of time and don't want corroded discs.

Stainless discs are not recommended by Cleveland, as well as other manufacturers. Bakos says that stainless discs should be avoided as they don't have the capacity to dissipate heat. "The thermal conductivity of stainless is 30 percent less than carbon steel," he says. Because of this, there is a strong possibility that the brake system will overheat and fail.

Lotzer agrees. He says, "One thing that we put on our boxes of our organic pads is not to use them against stainless steel discs. It's not that the stainless gets hotter than regular steel, it's just that it takes longer for the heat to dissipate and exposes the pads to heat for a longer period of time. Stainless has insulating properties, relative to other steels, so it does not work well. With longer exposure to heat, the materials used to adhere the organic material together in the break pads can actually be melted and the brake pads or disc itself breaks down or wears down quicker. The type of stainless used in the industry is 305 stainless which is relatively soft.

An"The challenge with brake pads and discs is you want a good coefficient of friction in order to stop the airplanes. Consequently, you are going to have a lot of heat build-up from that friction. Your average disc is red hot during the average stopping process. And I'm not talking about a full power landing, I mean that the disc is normally glowing red hot even when slowing the airplane during an average taxi.

"Another option for a non-corrosive disc is a chrome disc," Lotzer adds. "I would recommend this over the stainless, but it's still not as good, performance-wise, as stainless. The trouble with chrome, also, is that it's only put on .003 thick, and it's impossible to get an absolutely even coating. Because of this, after a period of time, it will wear through the chrome unevenly. Now you have ridges where the chrome is torn away, and it will chip away at the lining and wear it away really fast.

"We feel that there might be a better way to develop a non-corrosive disc in the future. But from a metalligurcal point of view, we just don't feel there's a suitable material out there yet. The only time I would recommend a chrome disc is if you have an airplane that's going to sit for most of the year, and you aren't going to use it much."

AVPRO, on the other hand, feels it has solved the corrosion problem with its new disc named "Black Steel," which is descriptive of its color and type of material used. Kelly says, "The case-hardening Melonite process creates the corrosion protection similar to chrome and the service life improvement over competitive products. The single-piece forging construction provides for improved strength and durability during braking operations throughout the life of the disc."

Reinventing the brake

Nearly a decade ago, the EPA outlawed using asbestos in commercial products — the result is a decade of brake product improvements

By Greg Napert

April 1998

continued from page 1.....

Rapco's brake rivet tool helps to eliminate lining cracking problems, as it allows you to control the pressure you apply to the rivets.

He continues, "The Melonite process performed on our disc product line reduces the corrosion and heavy pitting rust through the process of impregnating a corrosion resistance material into the molecular structure of the disc to a depth below the wear limits. The reason a standard steel disc rusts and corrodes so badly today is that the steel has open pores in which moisture can be retained, which eventually creates the surface corrosion and pitting of the disc. The impregnated material in the black steel disc fills all of those pores, preventing moisture from coming in contact with the steel. This significantly reduces any corrosion or pitting of the disc."

Rapco has also invested a great deal of engineering and expense into redesigning its brake disc. It's new line of brake discs are a forged one-piece hardened proprietary alloy material disc that it says significantly improves the strength and life of the disc. The forge process results in a grain structure that follows the shape of the disc — making the disc very strong. White says that Dyno tests have shown that these new discs are virtually indestructible under normal loading.

A need for improved maintenance practices
Lotzer says that regardless of the type of pads you're using, the key to successfully installing brake pads is having a flat back plate and pressure plate before you install new pads. He explains, "You should clean the plates with a wire brush or bead blasting machine to remove any dirt or corrosion and run a file lightly over the plate where the pad sits against and see if the pad is warped. If a plate is warped, you can touch it up with a file, but always check it against the minimum limits. This should be done especially on the pressure plate because they get much warmer than the back plate. The pressure plate doesn't have as much exposed area for cooling, plus it's much thinner. If they get too hot, they can warp. You're really pushing the limits of the brake assemblies for what it's designed for. The brake assembly is actually quite light, and you're asking it to do a very large job."

Rapco's latest lining includes wear indicators on the side which tell you when you have worn .100 of an inch off the pads, which is .020 inch above the rivet head.

In addition to finding the right material and advocating the correct procedures for installation of brake linings, Rapco also noticed that many technicians in the field were installing brake pad rivets with a simple hammer and punch. They found that this procedure, if not done correctly, could easily result in excessive stresses being placed on the lining and could result in cracking.

To solve this problem, the company decided to manufacture a riveting tool which would be inexpensive and effective. The new riveting tool makes it possible to feel the rivet being squeezed so that you apply just the right amount of pressure and apply it evenly.

Another problem that Lotzer points out is not enough attention to pad wear or minimum disc dimensions.

To solve the brake pad wear limit problem the company decided to build a wear indicator into its pads. The indicator is a small step, which is molded into the pad when it is manufactured. It is designed to indicate that there is still pad life left. As long as the notch is visible, the pad can continue in operation. When the notch disappears, this indicates that you have worn off .100 of an inch off the pads, which is .020 inch above the rivet head.

Lotzer says, in general, the cleaner you keep a brake disc and pad, the longer it will last. If you're taking off and landing on dirty strips, you're going to have more brake wear automatically. He recommends blowing out the brakes with compressed air whenever you can.

Bakos agrees. Cleanliness of the brake system is important. He recommends covering the wheels and brakes, however, when you pressure wash the aircraft, so as not to get the linings wet. This prevents water and solvents from getting onto the pads and past the wheel bearing seals.

The following information is taken from Parker Hannifin Corporation, Cleveland Wheels & Brakes Manual AWBCMM0001-1/USA

Brake Disc Minimum Thickness
Under average field conditions a brake disc should give years of trouble-free service. However, unimproved fields, standing water, heavy industrial pollution, or infrequent use of the aircraft may necessitate more frequent inspection of discs to prolong the life of the brake lining.

Generally the disc faces should be checked for wear (Dim. "A"), grooves, deep scratches, or excessive general pitting or coning of the brake disc. Coning beyond 0.015 inch (0.381 mm) in either direction would be cause for replacement.

Single or isolated grooves up to 0.030 deep should not be cause for replacement, although general grooving of the disc faces will reduce lining life. Discs are plated for special applications only; therefore, rust in varying degrees can occur. If a powder rust appears, one or two taxi braking applications should wipe the disc clear. Rust allowed to progress beyond this point may require removal of the disc from wheel assembly to properly clean both faces.

Wire brushing, followed by sanding with 220 grit sandpaper, can restore the braking surfaces for continued use.

Lining Conditioning Procedures
When new linings have been installed, it is important to condition them properly to obtain the service life designed into them. The metallic and organic linings are not conditioned in the same manner because they have different operating characteristics. Separate conditioning procedures are given for metallic and organic linings.

1. Taxi aircraft for 1500 feet with engine at 1700 rpm applying brake pedal force as needed to develop a 5-10 mph taxi speed.
2. Allow the brakes to cool for 10 to 15 minutes.
3. Apply brakes and check for restraint at high static throttle. If brakes hold, conditioning is complete.
4. If brakes cannot hold aircraft during static run up, allow brakes to completely cool, and repeat steps 1 through 3.

1. Perform two (2) consecutive full stop braking applications from 30 to 35 knots. Do not allow the brake discs to cool substantially between the stops.
2. Allow the brakes to cool for 10-15 minutes.
3. Apply brakes and check for restraint at high static throttle. If brakes hold, conditioning is complete.
4. If brakes cannot hold aircraft during static run-up, allow brakes to completely cool, and repeat steps 1 through 3.

This conditioning procedure will wear off high spots and generate sufficient heat to create a thin layer of glazed material at the lining friction surface. Normal brake usage should generate enough heat to maintain the glaze throughout the life of the lining. Properly conditioned linings will provide many hours of maintenance free service. A visual inspection of the brake disc will indicate the lining condition. A smooth surface, one without grooves, indicates the linings are properly glazed.

If the disc is rough (grooved), the linings must be reglazed. The conditioning procedure should be performed whenever the rough disc condition is observed. Light use, such as in taxiing, will cause the glaze to be worn rapidly.