Skip to main content

Full text of "Modern Mechanical Engineering Vol-I"

See other formats

264                             HEAVY  MACHINERY
laid on, makes an excellent joint. For jointing together large, flat, machined
surfaces of condensing plant, red and white lead, thoroughly mixed together
into a thin cream by the addition of gold size, is excellent, but it is essential
that the ingredients be thoroughly mixed together; the joint can be improved
by laying in a strand or two of tubular cotton packing. For very large,
flat joints a ribbon of asbestos from i to 2 in. broad, laid on a smearing of
red and white lead applied to both top and bottom surfaces, makes a very-
tight joint.
Use of Cranes and Lifting-tackle.—For light lifts, hemp-rope slings
are the handiest to use; it is also an advantage to have several lengths of
rope of different sizes (not slings), so as to be able to make a sling to any
given length, but in tying the two ends together care should be taken to
always insert a wooden pin (preferably 3 to 4 in. diameter and tapering
down) in the knot; unless this is done it may not be possible to open the
knot again, once a heavy weight has been lifted and the knot pulled tight;
the taper-pin is easily knocked out and does not lessen the security of the knot.
For heavier lifts, wire-rope slings should be used in preference to chains;
the latter are liable to crystallize when in constant use, and eventually snap
off short; this is particularly the case in frosty weather. If chain slings are
used, they should be annealed at regular intervals, e.g. every three months,
by heating to a dull red and allowing to cool slowly. Another objection to
chain slings is that the links are liable to press into and damage the machined
surfaces of the plant being erected, and in this respect particularly wire-rope
slings are much to be preferred, though even wire ropes will press into and
indent highly finished surfaces, such as the journals of heavy shafts, and it
is usual, therefore, in designing such shafts, to make special provision of
space at each end where the lifting slings should be placed.
In lifting a heavy piece the following precautions should be observed:
*i. When the crane or lifting-block has been tightened up, so as to just
put some tension on the slings, see that the crane ropes are vertical when
viewed both from the front and side; unless this is done the piece will swing
to one side when lifted, and, in the case of a heavy lift, considerable damage
may result.
2.  The slinging should always be arranged so that the centre of gravity
is below the point of support, i.e. the point or points from which the piece
is suspended.   If the centre of gravity is above the point of support, the
piece may capsize and fall out of the slings when being raised or lowered.
3.  If one end of the piece lifts before the other, the sling at the end
which lifts first should be lengthened, or the other end shortened.   The
most convenient way to lengthen a sling is by means of a series of shackles
each with removable pin.   For shortening a sling, pieces of timber or wooden
wedges can be placed between the sling and the piece.
4.  Whatever type of sling is used, it should always be protected from
damage where it passes over sharp edges, corners, &c.; fillets of sheet iron
or thick lead are used, or several thicknesses of sacking.
5.  When the piece has been lifted, say, an inch clear, the lifting operation