. Disc Brake versus Drum
Page 48
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Would Disc Brakes Overcome the Problems of Heat Checking and Scoring ?
WHETHER the disc brake was likely W to replace the drum type for road vehicles was a question often asked, said Mr. I. M. Waller, B.A., M.I.Mech.E.. of Small and Parkes, Ltd., addressing a meeting of the Institute of Road Transport Engineers at the North Western Centre. He considered it of interest to study whether the disc brake could be expected to mitigate any of the troubles actually experienced with drum brakes.
These fell into two main classes—heat checking and scoring. There was no reason to suppose that the disc brake would be any less prone to these two forms of trouble, which actually occur in disc clutches as well as m drum brakes. Possibly the disc type might permit the use-of alternative metals or alloys which would be better in these respects, but it was unlikely that cast iron would he bettered on economic grounds. On the other question of whether heat checking and scoring were less objectionable in a disc than a drum, clutch experience would suggest an answer in the negative.
One drum problem remained, namely, rumbling—a harsh vibration which sometimes accompanies the formation of patches on the drum and is attributed to a hammeringeffect on the shoes when the drum goes out-of-round because of thermal deformation of the rubbing surface. It was possible: but by no means certain, that the disc brake would be better in this respect.
Disc brakes lent themselves better to centrifugal air cooling of the rubbing surface and where the disc took the place of a narrow drum the surface area could be greater, added to which the effect of the drum expanding away from the shoes was considerably less than with the disc brake.
Thus, the latter type could be expected to be better for fade and wear c10 than narrow drum brakes because of surface area, cooling and the effects of thermal expansion. Whether discs were in practice superior to wide drum brakes with the same surface area remained to be seen.
Brake facings which would operate satisfactorily under higher temperature conditions were already available in the form of heavy-duty moulded blocks. but such materials were not easily produced in large quantities in the form of thinnee facings. Where flat segments were used, as in disc brakes, the very high density materials could, however, be employed economically.
For squeal to occur in brakes two essential conditions must be present; the rubbing surfaces must have the right kind of polish to initiate the vibration and the mechanism must be capable of being vibrated at a frequency within the audible range.
Moulded material appeared to be less prone to develop a squeal-producing surface so that the disc brake would show an advantage over the smaller drum brakes which did not ordinarily employ this material. Whether the mechanism of the disc brake would be any less easily excited within the audible range was difficult to say, but it was significant that squeal from disc clutches was rare.
Regarding service problems associated with clutches, Mr. Waller mentioned that slip could be due to oil on the friction faces. insufficient spring load, facings with too low a friction value. fade caused by overheated facing material, friction in the withdrawal mechanism, no free pedal travel or the driver dragging the pedal. Except for oil on the facing, diagnosis was impossible by just examining the driven plate after it had been removed from the vehicle.
Judder on engagement, caused by a geometric quarrel between the pedal linkage and the flexible engine mounting, was usually worse for one direction of the vehicle travel than when moving in the opposite direction. If eqbally prominent in both directions and the linkage was not influenced by engine movement, then the judder might be due to insufficient cushioning between the friction facings.
Whilst this cushioning was primarily intended to give a progressive rather than an abrupt engagement, it had a marked effect on juddcr. and if this still persisted when both the linkage and cushioning were correct, then it was necessary to try a better type of facing. A compromise always had to be accepted because wear value and heat resistance needed consideration, as well as ease of manufacture, and a material good for judder might be bad in other respects.
Rapid facing-wear could always he associated with slip and heat build-up. Spline wear, and certain types of driven plate breakage, were often due to misalignment between crankshaft and clutch shaft, the symptoms being recognizable by a tapered wear of the splines. Other mechanical failures of the driven plate were associated with the flexible centre. where such a unit was fitted.
Where steel springs were used they fatigued and the ends of their locating slots wore away, whilst rubber centre,. were also liable eventually to collap,:-. The weakness in both cases could he attributed to insufficient space in which to house highly stressed parts.