Document 5oLeewj2Ojq7rJL2adLrVekz

28 D. Hatch operating conditions. As the durability of brakes improves, further 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 I percent of asbestos remaining in wear products, a large proportion is ground down during the wear process to much less than the 5 u length at which particles are judged to be fibres. This is easy to understand when one considers that under normal condi* lions a lining will lose about one-millionth of an inch of thickness for every 20 ft of sliding. Over recent weeks t have had regular tests performed in which employees' private cars, 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 cloud. Several of the dust clouds were visibly filthy, and would, in my view, be avoided by the operator; the dust counts taken at the point of maximum dust density were as given in Table 1. Tagle I. Dust counts during drake cleaning by COMPRESSED AIR-JET Test No. Non-fibrous above 2 |i Fibre count 2-5 + Fibre count Above 5 ft Samples 1 2 3 4 5 6 7 10 min avenge 2000 + 109 402 607 475 1513 62S ISI 306 H 17 21 37 49 35 50 5-4 2-1 2-5 3-7 5-3 8-2 3-2 0-8 The interesting points from this exercise are, firstly, the ratio of the number of particles of asbestos to the total dust particles of a similar size, showing a very small percentage of asbestos, and, secondly, the distribution of size of the fibrous element, suggesting that those particles over 5 + 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, merely that an intense dust cloud of this composition is a lot less harmful than one might think. Unfortunately, at the present time, no corresponding data is available from clutches hut one would expect the situation to be belter than in brakes rather than worse, because on the one hand, surface rubbing speeds are higher, giving rise to higher point contact temperatures, and on 4hc other hand, wear rales are very much lower resulting in an even finer subdivision in die wear products.