BOGIES AND SUSPENSION - South Central Railway

CHAPTER 6 – BOGIES AND SUSPENSION
WAGON MAINTENANCE MANUAL
Page 98 of 121
Tinning bath should be oil fired, pot type furnace, thermostatically controlled
and fitted with pyrometer. The tinning bath should be covered with a layer of zinc
chloride flux or tallow to prevent drossing. Immerse the fluxed shell with the concave
surface down in a molten bath kept at a temperature between 260 o C and 280 o C and
containing alloy of 50% tin and 50% lead. The bearing shell should be kept in the
bath till it has reached the temperature of the bath (about two minutes). Remove the
bearing shell, wire brush the surface immediately after removal, re-flux it and repeat
the above operation.
It is desirable to carry out the above tinning operation, on each shell, three
times but in any case the above operation must be repeated at least twice to ensure
satisfactory tinning of the shell. Wire brush is not to be used after the last dip in the
tinning bath. On the contrary, after the last dip, a soft hair-brush should be used
which would help in spreading the tin evenly over the entire surface and also to
remove the excess molten tin. A lime wash may be applied on the surface not to be
tinned as this will prevent the tinning of such surfaces.
When tinning has been successfully carried out, a continuous layer of alloy
covers the base metal (bronze metal) and this is covered with a smooth, bright,
uniform layer of tin. Inadequate preparation of the surface before tinning leads to a
lack of continuity or uniformity in one or both layers. The white metal when it is
poured, amalgamates or alloys with the top layer of the tin. Thus in the final bearing
there is a thin, continuous layer of alloy between the shell and the white metal.
Metalling should be carried out with the shell in the vertical position only.
A bearing should never be poured horizontally with the shell upper-most, as in
this position both air and oxide are inevitably trapped between the white metal and the
shell and a good and continuous bond cannot be guaranteed. When using a vertical
chill, pudding should be carried out with a steel wire to ensure escape of air and
gases.
A molten layer of tin to be available for amalgamating with the white metal to
form a strong bond. It is essential that white metalling be commenced as early as
possible and only about one minute time should be allowed to intervene between the
withdrawal of the bearing from tinning bath and pouring of white metal on it. It may
also be ensured that pouring of white metal is completed within 10 seconds.
The white metal bath should be a pot type, thermostatically controlled, oil
fired furnace fitted with pyrometer. The molten baths should be kept covered with
dry powdered charcoal to avoid oxidation. The white metal chill or mandrel should
be sufficiently hot (150-175 o C) to check premature freezing an yet not too hot so as
not to give a slight chill to the bearing surface. As far as possible, the running - gate
should be 6.5 sq. cm. in section and 33 cubic cm. in volume to avoid shrinkage. Use
bottom pour ladle of adequate capacity so that one shell could be completed without
interruption. Pour the white metal into the bearing with steady and continuous flow at
temperature between 350 o C and 380 o C so that no turbulence is created and the air and
gases escape from the mould. While pouring the metal, care should also be taken to
ensure that no dross runs with the metal into the bearing. Temperature of the metal in