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WEARING WORK

26th February 1954
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Page 46, 26th February 1954 — WEARING WORK
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WHEN Herbert Frood made his first brake block in a small hut at Chapel-en-le-Frith. Derbyshire, he little realized that within 60 years the largest factory of its type in the world would be built in that village, or that a team of expert drivers would be testing commercial vehicles for 200-250 miles a day over switchback roads in the Peak district—in endeavours to improve the standard of friction material. The routes planned for these tests include most of the longest and steepest gradients in the Peak district of the Pennines. All of them are descended at 20-30 m.p.h. in top gear, full reliance being placed on the brakes to hold the fully

loaded vehicles in check.

The Ferodo factory employed 650 people in 1939, but the present strength is 3,000, including over 100 research staff working full time to develop still better friction materials. The output of the factory is over lm. units a week, which includes brake and clutch facings, silent gears, drive couplings and fan belts.

My visit to the Ferodo factory was planned to observe

the work of the test department, which operates a fleet of motorcycles, cars and commercial vehicles, including an A.E.C. Matador and two Leyland Comet tippers loaded to 12 tons gross. and Bedford and Austin 5-tonners currying full load. Unlike The Commercial Motor, Ferodo technicians are not so concerned with actual brake stopping distance, which includes possible time delay in the system, as with recording the retarding effort at the drums under all conditions of heat, and the rate of wear and effect on the friction materials.

The equipment used to compare the retarding force of the materials includes a Tapley meter and U-tube decelerometer, and a pressure gauge is interposed in the braking system to determine the line pressure operating the shoes.

In the A.E.C. Matador, which has air-pressure braking equipment, blocks of various heights are fitted below the pedal to give definite line pressures of 25, 50 and 75 p.s.i. Thermocouples are introduced at four points on the braking path of each drum to detErmine drum temperature. .

BI2 It is usual first to test eXperimental portions of the friction material on the bench to determine its coefficient and characteristics within a temperature range up to 500' C., but if only a minor alteration is made in the constituents, a set of new brake facings may be made for trial on one of the" guinea-pig " lorries.

The perfect brake material for heavy goods vehicles has yet to be found, because whilst retaining a high coefficient of friction, it must be free from end-lift, fade, glide, cracking or glazing, and its rate of wear and effect on the drums should be negligible. Such virtues are easily satisfied individually, but collectively the problem is practically insuperable, especially when taking into account limitations imposed by vehicle and brake design and the loads and

speeds of the modern goods and passenger chassis.

Much of the material at present under test and development is of the moulded pattern, the quantities of the ingredients and methods adopted in production being necessarily the makers' secret. When experimental facings are made they are fitted, by riveting, to shoes which-are countersunk at 10 points on the rear face to provide locations for measuring the initial thickness and rate of wear of the material.

Micrometer readings are taken and recorded at each of the 10 points, and note is made whether the shoes are fitted in conjunction with new or used drums. The braking system of the test vehicles is standard and the drums are of the usual cast-iron construction.

Then follows arduous work on the road, both for the vehicle and its braking system. Two shifts of drivers make a morning and afternoon circuit of the Peaks and Pennines, the A.E.C. and Leyland vehicles covering 200 miles, and the Austin and Bedford, as 30 m.p.h. chassis, travelling about 250 miles a day.

Testing For Fade

The circuits are planned to include the maximum number of long and steep descents, followed by sectionS" where occasional use of the brakes is required to observe possible delayed fade or gliding. Gliding is similar to fade, but occurs during one deceleration, and is possibly caused by one of the ingredients being brought to the surface by heat thus temporarily reducing the coefficient of friction. • Tapley and U-tube decelerometer readings are taken initially with line pressures ranging in stages of 200 p.s.i. from 200 p.s.i. to 1,400 p.s.i., and a record is made of the hand-brake performance. After this the trials begin in earnest.

As an example, the Leyland Comet tippers, after leaving the works, make a short climb to the top of Mam Tor and begin the long descent with low-top gear engaged. These vehicles are equipped with five-speed gearboxes and Eaton two-speed axles.

Mam Tor is one of four gradients on the circuit at which pedal pressures are recorded to show the effort required to hold the vehicle in check at.20 On each of these hills the pressures are measured on gradients of similar degree at the top and bottom, which provides evidence of brake fade.

After Mam Tor there follows a winding road to Hathersage, which calls for frequent braking and provides further opportunity of tracing delayed fade. Within a few miles the course set for the Bedford and Austin joins the Leyland circuit, the petrol-engined vehicles being routed from Chapel-en-le-Frith via Glossop to include the Snake Pass and add a further 25 miles to their schedule.

Soon after leaving Hathersage there is a period of climbing and tortuous bends, followed by the recording of line pressures on the descent into Grindleford. The gradients around Stoke, Hassop, Bakewell and Matlock are renowned, and these are included in the trials before reaching Wirksworth and Cromford, which again provide observed hill tests for the Ferodo guinea-pigs. These two descents are almost adjoining, and, with some materials used, the drum temperatures have reached over 400° C.

Exposing Undesirable Characteristics

In general, most brake facings char at such heat and unsuitable material is soon disclosed by temperature readings, fade, wear and loss of efficiency when next tested against the performance meters. A detour is made through Middleton to descend the Cromford gradient.

Returning to the factory by Newhaven, Longnor, Axe Edge and Buxton, the brakes are in constant and prolonged use, and it is not surprising that unsuitable materials may demand the adjustment of brakes every 300 miles. Immediately on arriving at Chapel, a second driver takes over and the vehicles are dispatched on the second lap of the course.

Each driver completes a daily log sheet on the performance of the brakes. A log sheet applying to one of the Leylands discloses that the vehicle had new drums and had covered 1,000 miles without intermediate adjustment for facing wear. Medium to heavy line pressure was recorded for effective retardation at high, moderate and low speeds, but there was no evidence of fade and recovery after descending hills was immediate. The brake efficiency between' hills was stated to be below average and there was an occasional increase of pedal travel.

Brake squeal was noted at one point and there was a tendency to pull to the off side soon after descending Cromford Hill. Line pressures recorded were 200250 p.s.i. on Mam Tor, 150-180 p.s.i. on Grindleford, 250-300 p.s.i. on Wirksworth and 250-350 p.s.i. for the Cromford gradient. The reports make it obvious that the drivers are most experienced testers capable of writing explanatory reports and have interest in their work.

Intermediate Trials

After 500 miles' road work the two drivers team up to conduct the second performance test, using the meters, and on the afternoon shift the vehicle is back on its routine circuit. At 1,000 miles. which is the completion of five days' driving, the crew remove the drums for inspection and to measure facing wear. A good material should not show more than 0.015 in. wear in the 1,000-mile test, and end-lift, which is one Of the difficulties of moulded facings, should be less apparent with use.

Again referring to the drivers' report, comment is made on the condition of the drum and on the appearance of dark rings or heat spots. In addition to measuring the facing thickness and deducing the amount of wear, the degree and type of any cracking in the material is recorded and end-lift is measured by using feeler gauges. A good material, which was well fitted to the shoe, should show no more than 0.015 in. end-lift R14 at any stage of its life. Slight surface cracks can be ignored, but separation of the material, shown by laminated cracking, results in its rejection.

The routine 200 miles per day, with efficiency checks at 500-mile intervals and inspections at 1,000-mile stages. continue until a uniform rate of wear is found, or a fault is traced in the material. ; Contrary to the layman's expectations, the rate of wear in the first 1,000 miles is often low and, once bedded down, the facings have a uniform rate of wear which is slightly higher than in the initial period. The efficiency is often low at first, possibly requiring the use of indirect gears to keep the road speed at a safe pace on the hills, but improves at 400-500 miles and is at its peak at 1,200-1,500 miles. Such information I gleaned from the explanatory reports compiled by the drivers.

Once the rate of wear is established and the potential life of the facing is computed. the vehicle is taken to a three-mile descent between Oldham and Ashton, who-e further fade tests are conducted. Extended trials may be conducted at high speed on the M.I.R.A. proving ground, Nuneaton, to represent overseas conditions.

The next stage is to determine the degree of facing glazing by reducing the payload to 50 per cent. and driving extensively in town with low line pressure for every brake application. If the experimental materials have survived these tests satisfactorily, the vehicle is reloaded to full capacity and taken to the M.I.R.A. proving ground for water innoculation tests.

After initial brake tests to find the line pressure for an 0.5g. stop, the brake drums are submerged in the deep watersplash for live minutes. The vehicle is then subjected to five emergency stops from 30 m.p.h., staged at approximately one-minute intervals, and these are followed by 0.5g. stops every three minutes until the initial efficiency, with dry drums, is restored. If 10 applications are required the facing material is rejected. A good substance will recover much sooner.

The search for the perfect brake facing is continued

n this manner every day and there is close alliance with he proprietary component makers. The work to which he guinea-pig vehicles is subjected over the test circuits s far more arduous than in commercial operation, which shown by the drum temperatures. Whereas on the :ircuits the drums may reach 250-450° C., a fast trip )etween Chapel-en-le-Frith and Nuneaton indicated 100° C. as a normal level and 150 C. after descending into Ashbourne. Any commercially used vehicle having :barred brake facings has been abused and its driver should be reprimanded.

The need for frequent adjustment for wear in a braking system may be caused by using an incorrect type of facing, and the makers are always willing to give their recommendations to improve performance. Problems of fade, glazing or laminated cracking experienced in normal service might also be solved by the Fero& technicians. It is estimated that the test courses mentioned produce rates of brake wear at least five times greater than those of service vehicles, and the experts usually have an early and broad view of the problems experienced by operators.

Tags

People: Herbert Frood
Locations: Austin, Bedford

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