Document Qk1DoVxOpONbQp5YrZq0QMwJE

.I 4 28 D. Hatch operating conditions. As the durability or brakes improves, farther reductions will occur in the percentage of asbestos which .survives the wear process, because each particle of asbestos is then subjected to more heat treatment. Of the approximately 1 per cent of asbestos remaining in wear produets, a large proportion b ground down during the wear process to much less than the $ u length at which particles are judged to be fibres. This is essy to understand when one considers that undcr'normal condi tions a lining will lose about one-millionth of an inch of thickness for every 20 ft of sliding. Over recent weeks I have had regular tests performed in which employees' private care, and trucks from the Company fleet have been brought into the garage for brake cleaning. The drums have been removed and in a garage with no forced ventilation, all loose dust has been removed from the drums and back plates by the use of a compressed air jet. As each brake was cleaned, instantaneous atmospheric samples were rapidly taken by a hand-pump from the centre of the dust doud. Several of the dust douds were visibly filthy, and would, in my view, be avoided by the operator; the dust counts token at the point of maximum dust density were as given in Table 1. Tabu i. Dusr counts duuno maxs clbamno ur COMMEND AIMU Test Non-fibitna Fibre count Fibre count No. above 2 p 2-5 p Above 5 p Simples 1 2 3 4 S 6 7 JO min vcjige 2000+ >09 402 eo? 475 >313 62* 181 306 11 n. 21 . 37 49 3S 5-0 5-4 2*1 2-5 3-7 5-3 *2 3-2 0-8 The interesting points from this exercise are, firstly, the ratio of the number of particles -of.asbestos to .the -total-slust panicles .ofa -similar-sur, Ahowiag.a .very-small percentage of asbestos, and, secondly, the distribution of size of the fibrous element, suggesting that those particles over 5 u must have been only just over. 1 am not suggesting for a minute that the practice of blowing out brake drums with a compressed air line is to be recommended, merdy that an intense dust doud of this composition is a lot less harmful than one might think. Unfortunately, at the present time, no corresponding data is available from dutches but one would expect the situation to be better than in brakes rather than worse, because on the one hand, surface rubbing speeds arc higher, giving rise to higher point contact temperatures, and on the other hand, wear rates are very much lower resulting in an even finer subdivision in the wear products. I i in. in Pf oil to ne Cl Su M a Si