For Practical Workers
���How to Make an Efficient Boiler-Patch
THE usual method of patching a boiler cannot be relied upon for high efficiency. Suppose there is need of a patch at the check-\alve hole of a locomotive-boiler. The radial cracks, most often along the length of the plate, start out from the hole. If a disk of boiler- plate is applied with a single row of ri\ets, as shown in F"ig. I, there is danger of tearing the boiler-plate or the patch, be- tween the rivets; also of shearing the rivets.
The more rivets are used, the weaker the i)atch, since e\ery hole weakens the plate. This may be seen by examining Fig. 2. The first piece of cardboard is solid all the way across ; the second has a hole in it, making the resistance to tear- ing just that much less. The only way
���Fig. 1. The usual form of boiler-patch
���Fig. 2. Obviously the hole in the plate shown at the right weakens it
��to prevent shearing the rivets is to add more rivets opposite the defect, but this increases the number of holes and actual- ly weakens the plate.
This difficulty can be avoided by the use of a patch like the one shown in Fig. 3. Two rows of rivets are used. The additional number of holes does not weaken the patch, howev'er, because the force tending to pull the plate apart does not act at right angles to the line of
���Fig. 3. An improved patch can be made by placing the rivets so as to prevent the strain from acting at right angles to them
rivets. The component of this force is to be reckoned with and the greater the angle of the rivets the less this force will be. By multiplying the number of rivets it is possible to mass more metal opposite the defect than there would be in the original plate, and the efficiency in shearing will be e\en greater than one hundred per cent. The obsersance of this simple expedient, which simpK- takes into consideration a princii)lc of physics, will result in far less danger from weak s[)ots in a boiler.