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IS THE FIRST CUT THE CHEAPEST?

10th June 1993, Page 92
10th June 1993
Page 92
Page 93
Page 92, 10th June 1993 — IS THE FIRST CUT THE CHEAPEST?
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Which of the following most accurately describes the problem?

Does it make sense to skim brake drums and re-profile linin9s, or is replacement the way to go? Colin Sowman investigates.

If you've ever stripped a brake you'll be familiar with the signs of wear to the drum. I laving to back the adjusters off before the drum can be removed, and the tell-tail ridge once you have removed it, are common symptoms of the problem. Further inspection of the drum can reveal other horrors like crazing, while a rolling-road test can detect defects such as ovality.

When the truck first left the factory its drums were sound and round and the contact radius on the drum and linings were identical. Bearing in mind the effort involved in soaking up the level of kinetic energy generated by an LGV wear is inevitable, and after many thousands of kilometers the question has to be asked, is it sensible to put new linings of the original diameter into a drum that is probably worn, crazed and oval?

Not only will the brake be less efficient until the linings bed in, but any crazing canincrease the wear rate by acting like a file on the new friction material.

Many operators will reline the brakes a couple of services prior to the annual test to allow for the bedding-in period. But what would happen if the vehicle were involved in an accident before the linings had fully bedded in and was deemed to have deficient brakes?

Imbalance across the axle is also a worry In extreme cases a build-up of tolerances could prevent the relined leading shoe on one side and the trailing shoe on the other from contacting the warn drum, giving a theoretical difference of 40% between the brake effort of two wheels on the axle. This would be so serious that the situation could not be allowed to solve itself as the linings bedded in.

On the Continent these arguments would not arise as brake drum skimming and lining profiling are so widely practised as to be almost standard practice. So who has it right—us or them? In pure engineering terms there is no doubt that the skimmingprofiling lobby is right. Most vehicle, brake and lining manufacturers are firmly in this camp. To make up our minds we set up a test with the aid of Central Trailer Rentco to look at each stage of the process. To start with we took a 1990 tri-axle trailer with RO-R axles that had over 181,000km on the hubodometer. It was loaded with a test weight of around six tonnes and brake tested on CTR's rolling road (a VL Test machine as found in the VI test stations).

The performance and ovality of each wheel was noted and the worst axle was selected for a reline. As the trailer stood it would have passed the annual test on performance (all wheels locked) and ovality (26°c nearside, offside 110).

Then CTR's fitters got to work stripping the hubs to change the linings, return springs and rollers. With the drums removed the crazing was evident. The nearside had 0.6mm of ovality and the offside 0.4mm. Having fitted the new linings the axle was retested. Again it would have passed the VI test but this time the measured ovality had come down to 1000 on the nearside and 6° on the offside. The reason for this became evident when the drums were removed and the lining contact area could be assessed.

With two linings on each shoe there were eight linings on the axle. On the nearside, one lining was not contacting the drum and its partner on the same shoe was making contact only over about 20% of its length. The other shoe in that drum had 100'. and 20% contact on the two linings. The longer the contact area, the more the ovality will be exaggerated. The shortened contact area of the new shoes minimised the ovality seen on the brake test result.

SKIMMING Now the removed drums were taken for skimming. First job was to remove the remaining taper roller race from the inside of the drum (the outer had already been removed to get the drum off). The drum was then set up on a Hunger brake-drum skimming machine, using the remaining bearing races for location. Around 2mm was taken out (of an already worn drum) before the crazing and ovality was removed.

The machine works in a similar way to a traditional lathe. Having set the cut, the single-point tool traverses automatically across the inner face of the rotating drum. On a final cut a single traverse can take 30 mM utes, but a grinding attachment allows a simultaneous roughing cut and final grind, speeding the process considerably.

Machining the two drums took over three hours as there was no grinding attachment and hard spots in the casting proved a problem the grinder would have removed these without further work. The drums were then refitted for another brake test On the nearside, the ovality had come down to 6% while the offside remained at 6%. Even though the drum is perfectly circular, the tester is bound to indicate some ovality due to running clearances on the bearings and tolerances on the brake assembly. That said, the improvement was readily apparent on the rolling road.

Next the drums were removed again and the linings cut to match the machined drum diameter. This is achieved by clamping collars on to the bearing diameters on stub axle, putting a closely machined hub over the collars and attaching the profiler to this hub. A single-point tool travels round a ring arrangement cutting the lining; it is traversed across the width of the shoe. All the dust is gathered directly from the cutting tool by a vacuum cleaner.

The technique here is to measure the new diameter of the drum and the cutter's swept diameter is set to just below that figure. The shoes are adjusted until the cutter is just removing material from all the friction blocks and the cutter is taken to the outer edge of the shoe. The machine then traverses the lining.

Given the variations in contact area on the nearside linings it was not surprising that a large cut had to be made to clean them up. In fact, after removing 3mm from the bottom shoe, the top one was machined over only 60% of its length. By contrast the offside required a cut of only lmm to clean up all the way round. This is not a quick process as the shoes are held in place only by springs so cuts have to be small—but then large cuts should not be necessary We were unable to make a final brake test on the fully machined brakes but nothing had been done to change the ovality readings.

From a practical and legal point of view, workshop manage ri could be forgiven for asking "where's the problem?". The trailer would have passed the test straight after a reline, despite its crazed, oval drums, so

why bother to do more? It is difficult t argue against this except to say that whil the trailer would pass the test as it is now.

The rolling road tester used by the Vii able to measure ovality and brake delay a well as imbalance. It is likely that ft required standard will change, causing fai ures on items like ovality without recourE to a road test. But until that happens brali drum skimming and lining profiling wi remain a minority sport in the UK haulag industry.

Even with a tighter test, considering ft relatively low cost of drums, replacemet might still be seen as more cost effecth than machining.

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