Any young engineer who will make use of what he has read will never get
his engine into much trouble. Manufacturers of farm engines to-day make
a specialty of this class of goods, as they endeavor to build them as
simple and of as few parts as possible. They do this well knowing that,
as a rule, they must be run by men who cannot take a course in practical
engineering. If each one of the many thousands of engines that are
turned out every year had to have a practical engineer to run it, it
would be better to be an engineer than to own the engine; and
manufacturers knowing this, they therefore make their engines as simple
and with as little liability to get out of order as possible. The
simplest form of an engine, however, requires of the operator a certain
amount of brains and a willingness to do that which he knows should be
done; and if you will follow the instructions you have already received,
you can run your engine as successfully as any one can wish as long as
your engine is in order, and, as I have just stated, it is not liable to
get out of order, except from constant wear, and this wear will appear
in the boxes, journals and valve. The brasses on wrist pin and
cross-head will probably require your first and most careful attention,
and of these two the wrist or crank box will require the most; and what
is true of one is true of both boxes. It is, therefore, not necessary
to take up both boxes in instructing you how to handle them. We will
take up the box most likely to require your attention. This is the
wrist box. You will find this box in two parts or halves. In a new
engine you will find that these two halves do not meet on the wrist pin
by at least one-eighth of an inch. They are brought up to the pin by
means of a wedge-shaped key. (I am speaking now of the most common form
of wrist boxes. If your engine should not have this key, it will have
something which serves the same purpose.) As the brasses wear you can
take up this wear by forcing the key down, which brings the two halves
nearer together. You can continue to gradually take up this wear until
you have brought them together. You will then see that it is necessary
to do something, in order to take up any more wear, and this "something"
is to take out the brasses and file about one-sixteenth of an inch off
of each brass. This will allow you another eighth of an inch to take up
in wear.
Now here is a nice little problem for you to solve and I want you to
solve it to your own satisfaction, and when you do, you will thoroughly
understand it, and to understand it is to never allow it to get you into
trouble. We started out by saying that in a new engine you would most
likely find about one-eighth of an inch between the brasses, and we said
you would finally get these brasses, or halves together, and would have
to take them out and file them. Now we have taken up one-eighth of an
inch and the result is, we have lengthened our pitman just one-sixteenth
of an inch; or in other words, the center of wrist pin and the center of
cross-head are just one-sixteenth of an inch further apart than they
were before any wear had taken place, and the piston head has
one-sixteenth of an inch more clearance at one end, and one-sixteenth of
an inch less at the other end than it had before. Now if we take out the
boxes and file them so we have, another eighth of an inch, by the time
we have taken up this wear, we will then have this distance doubled, and
we will soon have the piston head striking the end of the cylinder, and
besides, the engine will not run as smooth as it did. Half of the wear
comes off of each half, and the half next to the key is brought up to
the wrist pin because of the tapering key, while the outside half
remains in one place. You must therefore place back of this half a thin
piece of sheet copper, or a piece of tin will do. Now suppose our boxes
had one-eighth of an inch for wear. When we have taken up this much we
must put in one-sixteenth of an inch backing (as it is called), for we
have reduced the outside half by just that amount. We have also reduced
the front half the same, but as we have said, the tapering key brings
this half up to its place.
Now we think we have made this clear enough and we will leave this and
go back to the key again. You must remember that we stated that the key
was tapering or a wedged shape, and as a wedge, is equally as powerful
as a screw, and you must bear in mind that a slight tap will bring these
two boxes up tight against the wrist pin. Young engineers experience
more trouble with this box than with any other part of the engine, and
all because they do not know how to manage it. You should be very
careful not to get your box too tight, and don't imagine that every time
there is a little knock about your engine that you can stop it by
driving the key down a little more. This is a great mistake that many,
and even old engineers make. I at one time seen a wrist pin and boxes
ruined by the engineer trying to stop a knock that came from a loose
fly-wheel. It is a fact, and one that has never been satisfactorily
explained, that a knock coming from almost any part of an engine will
appear to be in the wrist. So bear this in mind and don't allow
yourself to be deceived in this way, and never try to stop a knock until
you have first located the trouble beyond a doubt.
When it becomes necessary to key up your brasses, you will find it a
good safe way to loosen up the set screw which holds the key, then drive
it down till you are satisfied you have it tight. Then drive it back
again and then with your fist drive the key down as far as you can. You
may consider this a peculiar kind of a hammer, but your boxes will
rarely ever heat after being keyed in this manner.