Contributions from Drivers and Mechanics.
Page 19
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TEN SHILLINGS WEEKLY for the Best Communication Received, and One Penny a Line of ten words for any
thing else published.
Drivers of commercial-motor vehic'es and tralors, and mechanics and foremen of garages or shops, are invited to send short contributions on any subject which is likely to prove of interest to oar readers. Workshop tips and smart repairs ; long and successful runs ; interesting photographs : all are suitable subjects. Send a post-card, or a letter, or a sketch to us—no matter how short, or hole/ written, or how worded. We will "knock it into shape" and prefiaresketchas, where necessary, before publication. The absence of a sketch does not disqualify for a prize. When writing use one side of the paper only and mention your employer's name as a guarantee of bona fides. Neither your ozein nor your employer's name will be disclosed. Paynient will be made immediately after publicatiou. Address your letters to The Editor, THE CONI ERCI.11, MOTOR, 7-75. kosebery A Veil le, London, E.C.
A MechAnic's Optical Indicator.
The sender of the following communication has been awarded the .I0s. prize this week.
Lb691J " G.S.C." (Slough) writes :—" I enclose certain sketches, together with a written explanation of an optical indicator which I have adopted for some three years. [We have had these sketches redrawn.—En.] I was induced to try my hand at the construction of a device of this sort, owing to the fact that we had had sev
oral engines out of which no one had ever been able to get good results ; they were suplmsed by the men who had had them to require some special setting. I was not prepared to accept this explanation, but, at the time it was made, I was not in a position to prove what I thought, namely, that the exhaust cams were not correctly profiled. Afterwards, with the aid of the indicator which I propose to describe. I was able to prove that, at the completion of each exhaust stroke, even when the silencer was disconnected, there remained in the cylinders burned gases at a pressure of about (3 lb. per sq. in.
"I first of all set to work to study the details of all the various indicators of which I could get particulars, and as a result of this I constructed one on the lines of a well-known manograph, but on that particular design I did not like the idea of a length of small copper tubing, through which the gas had to pass before reaching the diaphragm which operated thsi mirror. Secondly, I had a great amount of trouble with various types of diaphragms with which I had experimented. I tried flat ones with indifferent success. I also obtained some corrugated ones from Messrs. Schaffer and Budenburg, but these also cracked. The makers kindly replaced the cracked discs, hut at the same time they advised me that they
thought, steel diaphragms were not likely to be successful for the purpose I had in mind. I then finally discarded the disc type, and set to work to design one according to my own idea. I do not claim that the result gave me a perfect instrument, of course, by a long way, but, as a means of obtaining a rough dia. grant for testing purposes, it did its work splendidly. " Referring to the sketches, you will see that the instrument which I constructed is of the steam-engine type, at any rate, so far as its basic principle is con cerned. A small piston working against a spring is used, but the movement is restricted to a very small amount and the moving parts are made very light indeed in order to minimize the inertia effects. V ith reference to the sketches, Fig. A is the recording con cave mirror. This is of 4 in. diameter and has a focus
of about 18 in. I used this when I was taking directvision records, but, when I wanted to secure photo
graphic diagrams, I substituted a 12-in.-focus mirror.
it will be seen that, when the mirror is pivoted horizontally so that it may be oscillated by the indi
cator piston, its vertical oscillations are secured from the crankshaft. Fig. B is a side-sectional elevation of the complete instrument ; whilst C is an elevation looking at the back of the device. D is a plan of the
base-plate. a is the body piece, turned with radiating fins to assist cooling ; this part is fitted with a tap
which may be shut off when diagrams are not being
taken. b is the piston which is turned complete with its rod in order to save weight. c is the spring ;
these are made from flat spring steel of varying
strengths for different diagrams. d is the frame carrying the mirror. e is the frame in which the mirsoisframe itself is pivoted. f is the small arm to
which is connected an arrangement of Bowden wires in order to obtain the oscillating motion. The small crank and connecting rod, which are worked from the engine crank, must be in the same proportion to each other as the main crank and rod.
" When it is desired to use the instrument for directvision work, it is screwed into the cylinder head in
any convenient position. The reciprocating gear is
coupled up, and a cardboard screen is set up at about 18 in. from the mirror, in such a position that the light from the latter is reflected on the centre when the
engine is stationary. The light is acetylene and is enclosed in a tin box.
" On starting up the engine a horizontal line appears on the screen. When the tap is opened, the complete
diagram will be seen. In my own case I found that this indication was all that I needed, but for special work T have a box to fit over the indicator in order to obtain a photographic reproduction.
"In conclusion I must not forget to say that darken the test shed in order to render the diagram on the screen as visible as possible."