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FILE NAME: General Motors (GM) DATE: 1980 July DOC#: GM033 DOCUMENT DESCRIPTION: Trade Journal Articles - Fleet Owner - Brakes Caught in a bind between regulations that mandate better brake perform ance and government studies that could lead to a ban on the use of asbestos, fleets, vehicle makers, and brake-system manufacturers are investi gating alternative materials and methods of stopping a vehicle safely, consis tently, and economically. This special report looks at new friction materials, improved air disc brakes, the "reborn '' automatic load proportioners and slack adjusters, and various auxiliary braking systems. . . . ThdeifgfeoBrvReenArntKmmINeantGet:rwiaalnsts Asbestos, the traditional brake material, may have to go, say OSHA and EPA. But asbestos got its reputation by surviving a lot of heat V nder pressure from environmentalists and federal regulators to eliminate asbestos from automotive brake and clutch linings because of its suspected carcinogenic hazard, the friction-materials industry has been working hard to develop effective non-asbestos linings trucks, and buses. Not all segments of the industry agree that asbestos brake linings pose a health hazard and should be elimi nated. At a recent press conference, officials of Johns- Manville Corp. said they were aware that the Environ mental Protection Agency and environmentalists wanted to ban all uses of asbestos because of the possibility of lung cancer and other health problems in workers exposed to excessive amounts of fibers, mostly in indus trial and architectural insulation jobs. But the J-M officials state that automotive brake lining is one of the safest, and most important uses of asbestos. They claim that virtually no fiber is released into the atmosphere as linings w ear. Rather than breaking up into airborne fibers, brake linings, they claim, undergo a change while wearing aw ay that makes the resulting material harmless. All you get, they say, is a non- carcinogenic powder and an insignificant release of fiber--and they claim to have data to prove it. But while the EPA has been anxious to get rid of all asbestos, other concerned government agencies, d o t and its nhtsa, have said little or nothing. The j-M officials by jack Lyndall, senior editor/equipm ent think that m ay b e due to the DOT/NHTSA stopping- distance requirements. )ohns-Manville reports that it is trying to find substitute materials for asbestos. But to date, it hasn't found a substitute as heat resistant and tough as asbestos. for caArcst,ually, non-asbestos brake and clutch linings have been available for many years. M ostly, they were used for very severe applications and sold for a premium. Those products included clutch plates with ceramicmetallic facings, and brake linings made from sintered iron or other metal particles. But for the majority of vehicles,the popular and economical asbestos-based lin ings gave good service at reasonable cost. The quest has been a new family of friction materials without asbestos that would give the same good service at low over-all cost. The non-asbestos materials hitherto available have often been expensive and had some application restrictions. There are some non-asbestos brake lining materials in use on certain Mack trucks, according to Irvin G. Detra, vice president, product engineering for Mack Trucks Inc., Allentown, Pa. He points out, however, that asbestostype brake lining and blocks are still standard on all Mack on-highway trucks. The friction ratings on those linings range from EE on smaller front axles to GH on some rear axles. (See box for SAE friction ratings.) All brake linings on OEM vehicles must still meet FMVSS-121 stoppingdistance requirements, which means that friction levels are higher than on pre-121 vehicles. H ow ever, some semi-metallic lining has been released 72 FLEET O W N ER / jU LY 1980 by M ack engineering for certain on/off-road axles. And test w ork is under w ay with a non-asbestos lining con taining ceram ic fibers for on-highway axles. Detra reports that the Molded Materials Co. division of Carlisle Corp. is the lining supplier. Few changes w ere required in the foundation-brake system on the Mack axles to accept the non-asbestos linings, according to Bryson C . O cker )r,, supervisor engineering for highway vehicles. Some air-brake-cham-' ber sizes were increased for the high-friction FF rated linings used on the 44,000-lb. tandems, but no changes w ere needed in chambers on 34,000- and 38,000-lb. on-highway tandems. New lining technology is developing Altough exact lining formulations are proprietory, Robert Kickel, former vp-engineering and now vp-OEM sales for Carlisle's Molded Materials division, says that one of the principal ingredients replacing asbestos is a mineral fiber that is produced by melting steel-mill slag combined with limestone and other materials. Those materials are melted together, formed into a ribbon, and then air-cooled and shredded. The end product resem bles fiberglass in appearance but differs in friction charac teristics, of course. Those metallic fibers are then combined with friction modifiers and a phenolic resin binder. Engineering tests and field experience show extended life characteristics--often 30%--for the non-asbestos lin ings, Kickel reports. Drum life has been twice that attained with conventional asbestos linings in some cases, and there is little or no brake noise. There is a price premium for the new non-asbestos lining because of the higher cost of the materials and limited production. But the higher life expectancy for the lining and drums will make it cost effective, Kickel claims. Although production of the non-asbestos linings is limited at present, Kickel reports that Carlisle is now supplying it to some OEMs and also supplying some aftermarket replacement linings. The OEM lining is identi fied by the Carlisle number NAB-9M, which is a mediumfriction FF lining. Afterm arket linings are designated K-79. Development work is also going on at Carlisle for other asbestos-free linings. A semi-metallic lining using steel wool as a friction material for extra-heavy-duty applications in both drum and disc brakes is under active development. Kickel says some of that lining is being used by Mack on 18x7 brakes in extreme-heavy-duty applications such as on/off-road dump trucks. The Abex C orp., manufacturer of American Brakeblok linings, has followed two approaches in developing non asbestos linings, according to Robert E. Nelson, manager, technical services, friction-products group at Abex. One formulation uses metal particles (ferrous oxides with iron and steel fibers) in a resin binder. This lining formulation is designed principally for use in disc brakes. For drum brakes, a variety of fibers, including fiber glass, have been tested. (Processed mineral fibers and carbon fibers are reportedly expensive.) Nelson says that the non-asbestos lining can be applied by bonding or riveting. The linings for disc brakes have been integrally molded with the backing material. And for trucks with drum brakes or disc brakes, the linings have been molded to the shoes. For disc brakes, a semi-metallic, resin-bonded lining seems to work best, while in drum brakes, a non asbestos, non-metallic lining appears best, according to Nelson. Because of the limited number of heavy-duty air disc brakes in service, Abex has had little experience with the wear rate that can be expected. But in drum brakes, says Nelson, the life expectancy of the non-asbestos lining is equivalent to or somewhat better than that of asbestos linings. And he agrees with Kickel that improved drum life can be expected. Abex expects to market non-asbestos lining to meet the friction ratings that have been in use for the asbestos Ledinieint gonmaasnbnelsatoetsairntrhsosuesaehthorldeagtoionds The federal government has requested some 1,200 com panies to describe in detail how they use asbestos in a wide range of products. The Consumer Product Safety Commission said that about 30 categories of products would be affected, including ranges and ovens, toasters, oven mits, potholders, clothes washers and dryers, dishwashers and refrig erators, portable electric heaters, asbestos paper, pipe insulation, iron rests, and burner mats. All the information must be supplied within 60 days after the order is published (that should take place in mid-July), and the commission will then decide whether any of the affected products should be regulated. Although brake-lining manufacturers are not affected, the industry has felt the same sort of pressure to elimi nate the use of asbestos. There is nothing at the present time mandating such elimination o f asbestos from brake and clutch linings, but the implication is that if the indus try won't do it voluntarily, the government will attempt to force it to do so. lining. Nelson foresees no significant problem in mixing the asbestos and non-asbestos linings on a vehicle during the transition period when stocks of asbestos lining are being used up and before the industry has capacity to produce all linings in non-asbestos types. He emphasizes, though, that the linings must have the same friction rating and that it is best not to mix the tw o lining types on a vehicle, if it can be avoided. Nelson also predicts that, initially at least, the non asbestos linings will sell at a premium. But with longer drum life, he says, "They will be cost-effective." Wagner Electric C o rp ., St. Louis, M o., is still working with its lining suppliers to develop a line of non-asbestos friction materials for drum brakes, according to Eugene E. W allace, vp-engineering. He notes, how ever, that non asbestos pads for automotive disc brakes are already available. (Also see p. 151 of this issue.) Some non-asbestos drum-brake lining materials tested by Wagner engineers w ere somewhat brittle, according to W allace. Raybestos-Manhattan is also in the development stage on non-asbestos linings, but none has been marketed yet, according to a R-M spokesman. And several fleets are either using or testing non asbestos brake linings produced by Scan-Pac Mfg. C o ., Mequon, W is., which in 1973 purchased the industrial brake-lining business of Johns-Manville. H.A. (Skip) Scandett, president of Scan-Pac, says his firm's non-asbestos lining contains brass chips, zinc, resin binders, and friction modifiers, and uses fiberglass pri marily as a strengthener. "W e have many thousands of sets of the non asbestos truck blocks in service--most of it rated FF. It has shown excellent stopping ability and significantly improved lining and drum life," says Scandett. Among the fleets testing the Scan-Pac non-asbestos lining is that of Safeway Stores Inc., Oakland, Calif. E.C. Hill, manager-transport, purchasing, and maintenance department at Safew ay, confirms that three Safeway fleets are testing the linings: At Portland, O re., Robert W iseberger, superintend ent of maintenance, has the material on one tractor and one trailer. The trailer brake lining has been in service for 6,000 miles with no driver complaints and no reports of brake squealing. The non-asbestos lining first installed on the tractor was involved in a recall as a result of a production problem. It was replaced by a second set, and that is reported to have perform ed very well. W iseberger points out that the Scan-Pac lining costs about $30 more per set than asbestos linings. At Phoenix, A riz., Elmer Cow an, superintendent of maintenance, has equipped one tractor with the ScanPac linings, and he reports no driver complaints, fade, or squeals during the short time the linings have been in service. (He notes, incidentally, that his fleet's tractors are equipped with jake Brakes and normally get 500,000 miles on a set of brake linings). At San Diego, C alif., Neil Hill, superintendent of maintenance, reports four sets of Scan-Pac linings now in service--on one trailer and three tractors. "No prob lems, and no com plaints," he says. Scandett says that the Chevron USA fleet in Phoenix, A riz., reports a 40% increase in lining life with Scan-Pac non-asbestos linings (compared with conventional asbes tos linings) and no observable drum w ear. And a fleet in Florida reports a 50% increase during tests It's still too early to predict a complete changeover to non-asbestos linings. Because of production capacity limi tations, such a switch would take several years, accord ing to most engineers contracted by FLEET OWNER. And should the )ohns-Manville data hold up, asbestos linings may be in use indefinitely. W Irtkrfrietlon identifleition lyitem Designed to eliminate the former confusion that pre vailed over friction characteristics of different brake lin ings, the SAE )866a Recommended Practice has been accepted throughout the industry. Formerly, brake lining manufacturers identified linings as "lo w friction," "medi um friction," or "high friction." Because there was no uniformity of those friction ratings between manufactur ers, a medium-friction lining from one manufacturer could be quite different in its performance from a medi um-friction lining from another manufacturer. The SAE practice changed that. Coding--The code will consist of two letters reflecting the friction coefficients as follows: e letter C D E F C H Z Coefficient of friction Not over 0.15 Over 0.15 but not over 0.25 Over 0.25 but not over 0.35 Over 0.35 but not over 0.45 Over 0.45 but not over 0.55 Over 0.55 Unclassified The first letter will represent normal friction coefficient and the second letter will represent hot friction coefficient. Normal friction coefficient--Normal friction coefficient is defined as the average of four points on the second fade curve, located at 200, 250, 300, and 400F. Hot friction coefficient--Hot friction coefficient is defined as the average of 10 points located at 400 and 300F. on the first recovery; 450<, 500, 550, 600, and 650F. on the second fade; and 500, 400, and 300F on the second recovery. Note: If any temperature point or points required to calcu late friction coefficients are not reached in the prescribed time limit, the coefficient of friction value at 10 minutes shall be used to give the full number of points required. Example: A lining having a normal friction coefficient of 0.29 and a hot friction coefficient of 0.40 would be coded "EF." Location of code--The appropriate code designation will be marked on an external noncontacting surface in letters not less than 0.125 in. in height. 74 FLEET O W N ER / JULY 1980 ti.at better heat dissipation was obtained The next step in dis< -brake evolution was a system oi ventilated rotors and hydraulically actuated calipers tor heavv-dutv vehicles Trucks and buses equipped with air systems, however, still required air-hydraulic converters to supply the hydraulic pressure to the calipers From that point, however, it was a logical step to try to -nstall disc brakes on tront, rear, and trailer axles with air as the initial actuating force The result of that etfort, oi course', was the development of the present air disc brake, using the air system and a mec hanical actuator Counting the blessings Whereas hydraulic disc brakes ottered many advan tage's over conventional drum brakes ot several years ago, including increased fade resistance, improved recovery, better directional stability, and reduced stop ping distances, air disc brakes provided those same pluses and much more For example, as mentioned, oniy a.r is required as the actuating force Stanciard trailer air hoses and couplings can be used \ o brake bleeding is required The system is compatible with spring brake's \nd. rotor surfaces and linings can be checked without removing the wheels or rims Disc brakes utilize two designs -- external or internal The former is generally preferred, however because the disc broking surfaces are exposed to outside air exc epi .it tne point where they travel through the caliper The internal disc-brake consists basically ot two cylin drical discs earned on the hub assembly Between those discs is a mechanism that applies external torces to tne shoe and lining assemblies th re e basic types ot calipers are assoc i.ited w tr d .brakes -- fixed, sliding, and fixed with a slicing c m fixed caliper straddles the rotor, w hich is attached to (nub assem bly and is solidly at'ixed to the verse ,. suspension spindle or axle tla.oge The sliding caliper also straddles the rotor a-, l r-zy .-s but slides axially n an ant nor plate ib e fixed i m e' requires an actuating m echanism on cue h s a l e -> .. rotor, w hile the slicing m o d e 1utilizes but one I n.e o n caliper w ith slicing disc slides Torn one shoe and ..- v y assem bly to the other and' is usee vvth a tixed c a; n c straddles the doc (This type' s almost exc .o :vn v ,mm crated w ith aircraft appiic ations ) Although external automatic slack adjusters pe"n---satisfactorily, iniernal automatic adjusters are g e o -'a , used w ith disc brakes :n the interests ot cost ot proem-non, sealing the adjuster, and reliability This com on-em is assem bled in a com m on housing with the n raxe ac m .r ing mechanism The' autom atic adjuster adjusts tne brake 'or e x c e s s , e clearances created by lining vvea- and at the s u r e tenprovides a design clearance so the disc yvi-i -otate t'e e v upon release Kelsey-H aves ( o one o' the true U 'i doc n oise m anufacturing oioneers, [mints out " 'at neavv-dutv a : disc brake' s arc' up to V ) II) lighter pe- a\;e tha" -to /" brakes 'n audit on. ma-ntenanrc> time is said to nc ; drastically For exam ple, w h en drake -tracts nave to :>e tep laced . only the tires, runs, and disc w neeis nave to ticrem o ved to get to the air disc brase assem bly File"- ~ no need to rem o ve the axle snatt. w h e e bearings o' seals as w ill' many older drum tv lies Also the Kelsey -Hayes rotor is n o : rem o ved c u rn g pact reolacem c' nt nut s inspected tor excessive w na vy hen the caliper is luted olf the anchor plate BFG oodrich otters tafiic' s ciesigneci to h e p a r 'iM - imamtenanc c' savings ot disc \ s drum bra-ves th ro i.C " three relines O n e tatiulation pits the <ost of eiu.m se's m B F G o o d m h air dis< b ra k e , show n here on a dynam om eter, features a solid steel rotor u  *, linings, drums, foundation parts, and seals against the disc brake's required eight sets of linings, springs, and nuts. The result is a $664 savings for disc parts--$1,518 vs. $2,182. To complete the equation, 16 mechanics hours for drum brakes, as opposed to four hours for disc models, must be added in and multiplied by each individual company's hourly mechanics' shop rate. The bottom line is arrived at by multiplying total labor and materials costs by the number of tractor-trailer units being serviced. Drum brake labor and materials costs do dip considerably on the second reline, but are said climb back to the original figures on the third. W hy, then, if disc brakes effect substantial weight savings and reduced downtime and maintenance costs, aren't more OTR fleets using them? Part of the answer has to lie in the number of brake applications per mile. A trash hauler could stop 100 times in three miles; a linehaul unit three times in 100 miles. For that reason, many maintenance directors don't feel a need to equip long-distance vehicles with disc brakes in spite of lower weight, less downtime, and what the manufacturers declare is better stopping with better directional stability. Back in 1977, worry over mechanics' unfamiliarity with disc brakes and the unavailability of parts, except at the OEM level, were listed as valid reasons for fleet caution concerning disc brakes. Now, however, most disc-brake manufacturers say their parts are available at hundreds of factory branches and distributor outlets As for mechanics' unfamiliarity with disc-type brakes, manufacturers and current users alike call it a poor excuse, saying maintenance is just as easy to understand and easier to perform than that of drum types. Lack of aftermarket parts and mechanical know-how may be a thing of the past, mainly because disc brakes are now big business rather than "the new kid on the block.'' BFCoodrich, for example, went from zero sales in 1977 to $6-million in 1979, with a projection of $25million for the current year. Disc brakes are performing dramatically on emergency , vehicles such as heavy fire trucks. They are standard on 1 Mack OEM fire trucks, and the City of New York's Fire Department has experienced outstanding success with them. In fact, 100 new pieces of American LaFrance fire apparatus, now on order by the City of New York, will be equipped with Kelsey-Hayes disc brakes. Check each manufacturer Disc brakes are produced by over two dozen manu facturers (see FO -- 10/79 Buyers Directory). And, while the principle remains largely the same, different manu facturers offer different models, features, specifications, and total weights. Individual companies should be con sulted to determine their formulas for brake sizing and their product's compatibility with service brakes, spring brakes, and antilock systems. Brake balance is ultra-important--balancing the w ork load proportionately between tractor brakes and trailer brakes. Combinations of disc brakes on tractors and S-cam o r dual wedge brakes on trailers provide different levels of balance or imbalance. A rig leaving the yard with a balanced tractor-trailer brake combination often interlines with a trailer that throws the balance ratio completely out of kilter. The rolling radius of various makes of tires is also a factor. There's no doubt, though, that discs have arrived. They run cooler, stop within shorter distances, and pro vide better stability. Such companies as Carl Subler Trucking Inc. and CF Industries report outstanding suc cess with BFCoodrich power screw discs. Kelsey-Hayes notes enthusiastic acceptance by many truck and bus companies, including Ruan Transport Corp. Still, the manufacturers feel the disc brake is not yet being used to its full advantage. m Ft.EFT OWNER IU1Y m o ThloeardeBtpuRrroAnpKooIfrNatiuGotn:oimngatic It seems that, if you want legal stopping confined to the same lane, and no electronics, you may find yourself using a system developed in the 60s The rumored rebirth of FMVSS-121's court- stop within the parameters of 121's tough stopping- voided 60-mph stopping-distance rule and its distance (293 ft. at 60 mph) and lateral stability (within a anticipated inclusion in the National Highway 12-ft.-wide lane) requirements. Traffic Safety Administration's proposal for The potential of load-proportioning as it relates to new federal air-brake standard 130 is renew balanced braking is such that NHTSA, in its advanced ing interest in all-mechanical, automatic load-sensing-raunledm- aking on future truck-brake systems issued earlier proportioning to balance the brakes of medium- and this year, included a section on the "prevention of wheel heavy-duty trucks. lockup by use of load-proportioning systems that adjust Balanced braking by means of mechanical devices that the pressure to the brakes in proportion to the weight deliver braking torque to the wheels in direct proportion on each axle." The agency called the braking concept to a full range of variable axle load weights is being "one of the most efficient means of maintaining lateral pegged by many truck-brake engineers as the only feasi stability and avoiding wheel lockup." ble way to achieve better and safer truck-stopping per The idea of balanced braking through automatic load formance in the future--without returning to the compli proportioning is not new. The system is widely used in cated and controversial electronic antilock systems that Europe, where it has been mandated on trucks and were blamed for many vehicle safety and maintenance buses for nearly 20 years. Load-sensing brake-balancing problems in the late 1970s. devices were looked at seriously by several U S. manu Mechanical load-proportioning-type systems, which facturers (including Berg Manufacturing Co. and Midland- are claimed to be simpler and less expensive than 121- Ross) in the mid-1960s, and some were even produced related electronic hardware, already can be found on However, the lack of fleet-market interest and the certain trucks and tractors being sold in the U S. In advent of 121 in 1969 before load-proportioning valving addition, research-and-development work and field test really had a chance to be refined, caused those early ing being done by several truck manufacturers indicate projects to be placed on the shelf, while the government that more load-proportioning systems soon may be in production. and industry turned their attention to electronic controls. In fact, some truck-brake engineers say that without A simple mechanical design load-proportioned braking, it would be nearly impossible Basically, a load-proportioning system consists of a to bring a lightly loaded or unloaded truck or tractor- simple mechanical lever arm and valve assembly trailer combination (or bobtail tractor) to a controlled mounted on the vehicle frame and linked to the rear by Thomas W . Duncan, Western editor axle. The linkage senses suspension or spring deflection 78 FLEET O W N ER / JULY 1980 as the vehicle is loaded or unloaded, and the weight on the rear axle increases or decreases. Braking pressure to the rear wheels is delivered in proportion to the loadweight on the axle. The concept can w ork with full-air, air-over-hydraulic, or hydraulic brake systems. Although the need for such a system has been appar ent for several years, current trends toward longer and heavier trucks, more fuel-efficient engines (with lower engine-retarding capabilities), and drag-reducing aerody namic vehicle designs have renewed interest in rigid stopping performance standards. (NHTSA estimates that new truck and engine designs have reduced vehicle rolling resistance by about 25%.) The more balanced a vehicle's brakes are, the better its stoppingdistance and lateral-traction (direc tional stability) capabilities will be, reasons NHTSA-- and the more ag gressive a driver can be in braking to avoid an accident, without fear of losing control of his rig. lightly loaded stops are a problem Braking systems on most U.S.- made trucks are designed to stop a fully loaded rear axle (at maximum allowable gross weight). Fully loaded, most current-production brake systems can meet 121's 60-mph stopping-distance requirements. H ow ever, the braking perform ance of a truck or semi at full load is entirely different from what it is under partial-load or no-load conditions. Load-weight variations are a critical consideration in truck-braking perform ance. Although a vehicle may leave a plant or terminal with a full payload evenly distributed over the axles, it may drop some of its cargo enroute and make a portion of its trip partially loaded, then return home empty or bobtail. Under less-thanfull-load conditions, brakes will grab first at the most lightly loaded wheels, causing them to lock and skid. In addition to being unsafe, braking under imbalanced axle-load conditions can be costly as w ell. Rear-wheel lockup can cause excessive tire w ear and lead to flat spots on tire surfaces that eventually can result in vibra tion and unbalanced-wheel conditions. W heel lockup also can cause axle- or bogie-hop in single- or tandemaxle vehicles, resulting in a violent shuddering effect that can shorten suspension life and crack frame members. Front-end tipping when front brakes grab first on lightly loaded vehicles can cause unequal brake-lining wear and cargo-shift damage. IH testing brake-proportioning system Some trucking engineers feel that without proper load proportioning, brake balance for two or more different types of load conditions would be nearly impossible to achieve. O ne truck-brake expert who agrees with that idea is Al C . Beier, chief engineer/brakes. International Harvest er Truck Croup, Fort W ayne, Ind. IH currently is working on a mechanical brake-proportioning system to improve vehicle control during braking and reduce vehicle stop ping distance under all road-surface conditions. The sys tem is designed to stop air-braked empty and lightly loaded tractor-trailers and straight trucks and bobtail tractors within 121's 60-mph stopping guidelines. Preliminary tests already have shown that the IH sys tem will reduce bobtail stopping distances by at least 20%, says Beier, with an "appreciable" improvement in In Budd anti-skid unit, when brakes lock, manifold (1) attached to wheel stops rotating; inertia wheel (2) keeps rotating; valve (3) senses relative motion, signals slave valve to close, unlocking brakes. vehicle lateral control during braking. The IH system consists of specially designed frame- mounted proportioning valves. W hen the valves sense that a vehicle is lightly loaded or unloaded, they reduce air pressure to the brakes on the lightly loaded axle. In addition, the valves have an override feature to provide full braking capabilities to the previously lightly braked axle. Conversely, when the axle load is increased, and the frame rail deflects toward the axle, the valve will open and increase air pressure to the brakes. Up to now , Beier points out, load-proportioning devices have been difficult to apply to U.S. trucks because of the many different suspension systems in use. In Europe, most suspensions are leaf-spring types with 3-in. to 4-in. deflection. Many U.S. suspensions offer little more than an inch of deflection, making mechanical deflection-sensing difficult. Beier feels, how ever, that the potential for improved vehicle control during braking will promote brakeproportioning systems from the conceptual to full- FLEET O W N ER / JULY 1980 79 Load proportioning valve (below) used on new Iveco Z 100 trucks is mounted inside the frame rail and connected through a series of levers to the rear axle. In unloaded condition, the frame rail rides high relative to rear axle, and the valve restricts pressure to the rear brakes, directing more braking effort to the more heavily loaded front axle. As vehicle is loaded, the frame rail rides lower, and the levers act on the valve so as to direct increased braking effort back onto the rear axle. At fuH-load, of course, maximum brake effort is available at all axles. IH developmental system uses frame-mounted load-sensing valves. Tests straw a 20% reduction in bobtail-tractor stopping distances. Volvo also has developed a mechanical load-propor tioning valve for the full-air brake system of its heavy F720 diesel-powered, two-axle straight truck (39,58041,800-lb. G VW ), which is used primarily for urban-area refuse and fuel-oil hauling. The F720 just went into production earlier this year; no models with load-proportioning systems installed have yet been sold in the U.S. but will be soon. The system is a static-type, which senses only empty or fully loaded conditions; it cannot compensate for rear-to-front-axle load-weight shifts. Rick Beavan, Volvo engineer, says mechanical load sensing equipment costs only about $80 per vehicle, compared to $500-$600 for electronic antilock. He claims they are easier to maintain--just routine air-system checks for corrosion and m ositure--and do not fall prey to vibration, corrosion, and water damage as easily as did the 121 electronic system. Further installations of load-proportioning equipment on U.S.-sold Volvos will depend on what the govern ment does with the new air-brake standard, says Man chester. "W e're optimistic about the future of this brak ing concept," he adds. "The trucking industry is not about to go back to electronic black boxes." Another European truck manufacturer, Iveco Trucks FLEET OWNER / JULY 1980 81 ThiesB`sbRlaoAcrnkKaaINgdGajui:nst'er The problem may have been in design, and maybe in early maintenance practices, but the industry needs the new,________ trustworthy slack adjusters Tfirmly he first automatic slack adjusters were intro duced in the early 1960s, and they started running into trouble right away The new device tightened up blindly and could adjust itself to an over-heated, expanded brake drum. When such a drum cooled down, it contracted over the brake shoes, locking up a wheel. Often, that meant a road call to manually back-off the slack adjuster and get a vehicle back on the road. Later, more problems appeared. As road salt and grit made their way into the slack adjusters, for example, the inner works on some corroded, out of sight and often undetected. Operators assumed the automatic slacks were doing their job when in fact, some had ceased functioning. Although almost everyone agreed the new technology was a good idea, things were off to a bad start. Various manufacturers entered, then left the field. Some disap peared altogether, but no one gave up on the automat ics entirely In 1971, a new round of automatic slack adjusters began to hit the market, but even the new generation wasn't fool-proof. Holland Motor Express tried the new equipment early in the '70s. "It was a time of shortages,'' said Bob Brackenridge, Holland's vp-equipment, "and some low-quality materials and poor workmanship got into the automatic mechanisms " Trouble for Holland began in 1973 when "all hell broke loose,'' according to Brackenridge In some cases, it wasn't apparent that a In addition to actuating the S cam in an air-brake system, the Bendix by john Bendel, associate editor Sure Stroke slack adjuster monitors shoe-to-dmm clearance and auto matically takes up the slack as the brake lining wears. With Rockwell drive- and trailer-axle unit, adjustment occurs when the air at the brake chamber is released and forces in the brake-shoe-actuating system approach zero. There is no reliance on friction clutches, which are subject to wear. Manufacturers are counting on many fleets to recon sider automatic slack adjusters, and most find that the economics of brake maintenance point toward the even tual widespread acceptance of the devices. That, in turn, has led to substantial investment in developing durable, finely tuned products. The National Highway Traffic Safety Administration also is interested in automatic slack adjusters. According to Duane Perrin of NHTSA, the agency wants to get some information from fleets using automatic slack adjusters before proposing any rulemaking. "W e're particularly concerned about reliability," he said. One manufacturer's spokesman, though, told FLEET OWNER, "I hope they don't make automatic slacks man datory. W e're happy with the way our market is grow ing. W e're growing along with the demand very nicely. If automatic slacks were to become mandatory, we would have to greatly increase production over a short period of time, and that could mean real quality problems. W e don't need that." Among the manufacturers currently offering automatic slack adjusters to the OEM and after-markets are Bendix, Borg-Warner, and Rockwell International. Kelsey-Hayes is currently field testing its own automatic slack adjuster design and plans to go into production soon. Meanwhile, acceptance of the device is growing, and it could just be that automatic slack adjusters are here to stay FUFF OWNER 'H l\ " i;o Nwl i nil BrakeSaver unit is silent in operation, is made only for Cat engines. involved in a new-vehicle standard pertaining to retard ers. At this point, it looks like enforcement falls under the purview of BMCS or state governments. There is a range of approaches to auxiliary braking. Some manufacturers restrict exhaust flow to retard speed; some alter valve motion in the engine to trans form a diesel into a power-absorbing air compressor. Some manufacturers have developed devices that work to hydraulically retard a vehicle's forward motion; others have found ways to harness magnetic forces to achieve desired speed control. The least expensive auxiliary brake is the exhaust brake, essentially a butterfly valve installed in the exhaust manifold. When the driver steps on a special pedal, the valve closes. Trapped gases back up, causing pressure to build up in the engine's cylinders. That pressure, or retarding force, is transmitted back through the driveline to the rear wheels to slow the truck. The exhaust brake, standard equipment on European trucks for over 20 years, has proven to be relatively easy to service; however, it does not offer as great a degree of retardation as the more elaborate auxiliary braking devices. In the mid-1970s, Williams Air Controls began market ing an air-operated version of the exhaust brake, which it calls the "Blue O x." The Williams brake is installed on the exhaust manifold in such a way that a sliding gate-valve positively blocks exhaust gas flow from the engine The better seal improves retardation, and Williams claims a longer life for the gate-valve mechanism. Although exhaust brakes are the least expensive of the auxiliary brake devices, they are only second in populari ty to engine brakes, devices made by lacobs Manufac turing Co. and Mack Trucks Inc. Sandwiched between the engine block and head, the Jacobs engine brake weighs about 75 lb., and features a modulated control that permits the driver to select the degree of retardation to best match conditions. Because engine brakes are dependent on engine rpm for efficiency--the faster an engine is turning the more retardation energy it can produce--drivers running the newer low-rpm, high-torque-rise engines should raise revs beyond the fuel-efficient 1,700-rpm level when 88 Jake Brake w orks hydrauRcaly t o , convert a power-producing (Sesei to a power-absorbing air com pressor. W hen solenoid is energized engine ofeA passagew ay to control valve raising it and unseating b a l check valve. Engine oB then A s passagew ay, forcing mast piston against injector rocker-lever V adjusting screw (o r exhaust rockerteverM ^pstingscrew j p--v9-lQnrOfOCf crossbead dow n, FLEET O W N ER / JULY 1980 FLEET OWNER / (ULY lVHf) H9