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ROAD AND WORKSHOP by Handyman

13th March 1964, Page 66
13th March 1964
Page 66
Page 66, 13th March 1964 — ROAD AND WORKSHOP by Handyman
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Self-lubricating Bushes

FREQUENTLY HEARD around transport repair shops are the words "premature failure" when some item has suffered rapid wear, seized up or parted company, and at the time it is all too easy to blame the component. Yet in many cases a careful investigation will show that somewhere along the fine the human element has slipped up, although it is no easy matter convincing anyone of this after the event. A recent example concerns the self-oiling bush, in particular the one at the commutator end of a goods vehicle dynamo.

The unit in question had served 97,000 trouble-free miles from new, when it erased to charge and chose a time when there was no service unit on the shelf, or an agent open for business. So a skilled fitter stripped the unit and discovered that the fault had started with a worn out bush allowing the armature to touch the pole pieces. No serious damage had been done, however, other than a little solder thrown from the commutator slots. This was rectified and the worn bush, pushed out. A new one was carefully pressed home in the end plate, the spindle oiled and the oil •feed wick also well Soaked in oil. .

On assembly the unit worked correctly. the vehicle returned to service on time c30 and all concerned were satisfied. However, 10 weeks later the same dynamo was again in trouble and once again the failure had sprung from a worn-out bush at the commutator end.

Self-oiling bushes are made of porous bronze and, rather like a sponge or washleather, they can soak up a remarkable quantity of liquid (in this case, oil) and can retain it for long periods if given correct treatment. However, a bush must he given a certain period to soak up its full amount of oil and this can take up to 24 hours if cold oil is used. To speed up the process it is necessary to warm the oil, and if the bush is immersed in oil maintained at around 80°C it will be fully saturated in two hours. When storing these bushes they should not be left exposed to the air on a shelf, as being rather slow-moving items they can be completely dried out when needed; also, they can accumulate dust or grit. Before being stored the bushes should be fully soaked and then packed in oiled paper or in a polythene bag and kept cool.

When fitting the bushes, no attempt should be made to ream or bore them • as this will close the pores of the material and oil would not seep out as required. The correct bush should be obtained for the particular model of generator or starter motor used, and it is very important to use a polished mandrel at dead shaft size. Make sure to push home the bush; do not hammer it, as this could fracture or crumble the bush. The premature wear found on our dynamo was because of wrong storage, plus inadequate lubrication; what little oil had entered the bush was by way of the small area of the feed wick, and without a good pre-soaking .the bush ran short of oil in the few weeks described. Once fully saturated, however, the bush can live off the wick seepage, and a few ' drops monthly to replenish the wick should give the bush a full life of around 100,000 miles.

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