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Dennis

21st May 1914, Page 16
21st May 1914
Page 16
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Page 16, 21st May 1914 — Dennis
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Which of the following most accurately describes the problem?

E of THE SSIS

With a view to the introduction of as much variety as possible into the present series, our choice for the third article

has fallen upon a Dennis lire-engine, a type of machine which Dennis Bros. (1914 Ltd., has developed ab Indio, and with which it has secured a very large and growing volume of business in an entirely new field.

The motor fire-engine is unique, in respect of the nature of its employment, amongst commercial motors generally. It

is provided with an engine which is a great deal more powerful than would be necessitated for the mere propulsion of its load along the roadway, its actual mileage is relatively a very low one, and yet its pumping mechanism is liable to be called upon at intervals for long periods of full-power service.

There is, of course, a great deal more machinery about a motor fire-pump—and it is of that class of fire-fighting apparatus that we would write—than about the many more or less modern standard types of roods or passenger chassis. There is, therefore, a correspondingly inc reased number of adjustments and points of inspection to be systematically considered.

Worm and Turbine. The Dennis fire-engine, like its more ordinary brothers, is of course, fitted with worm and worm-wheel final drive. Dennis Bros. Ltd., it is hardly necessary to remind our readers, rightly claims to be the pioneer with regard to the use of this form of back axle for commercial purposes. The other outstanding characteristic of this company's firepumps is the use of the centrifugal type of pumping plant, concerning the claims of which, in distinction to those which are made on behalf of the reciprocating pump, there has been much controversy in the public Press as well as amongst professional firemen, during the past year or two.

Our Desire to Help the Designer.

We desire in the present article, of course, to follow the lines which we have laid down for treatment of our subject in previous numbers. That is to say, it is our wish to interpret, in as simple language as possible, the aims and desires of the designers as they have been translated from the drawing-board to the actual mechanical details of the chassis.

We need make no apology for again explaining that we often encounter instances in which the carefully-laid plans of designers an constructors, and sometimes painfully-evolved refinements to be found on a modern chassis, have proved fruitless when turned over to the care of men whe have known nothing of the trials and tribulations of the production stages.

Perhaps the most remarkable example in this direction that has come to our notice was the case in which the writer found a more or less intelligent driver with the spout of his oilcan D2 puked through a core hole in a cylinder casting, from which he had carefully removed the plug placed there by the makers. Questioned as to the reason for this extraordinary action, he said that he did not know what that particular plug was for, he had never been told, and he thought that as a precaution it would be as well to take it out and oil what ever it covered. That particular driver was, it must be admitted, lacking in the common sense and intelligence which is characteristic of the vast majority of drivers, but the example will serve to illustrate our point. We could tell of others equally startling. We are not concerned, in the present article, to describe in detail constructional features of the machine with which we are dealing, nor is it our desire to put into black and white instructions as to how the vehicle and its auxiliary machinery should be driven. We wish to take our share in ensuring that the mechanism which is put into the hands of men who cannot be expected to appreciate all the fine points in design shall have that amount of care and consideration which can only come of intelligent appreciation of the reason for the existence of all the means of adjustment, etc., which have been provided for very good purposes.

We may, perhaps, before proceeding to deal in detail with the individual features of the chassis, in a few words describe the machine as/ a whole. We have selected an example of the 400gallon model Dennis motor pumping chassis, and this has, as its prime mover, a very fine example of modern engine construction in the shape of a four-cylinder engine of White and Poppe design, of which the cylinders are 12'7 mm. in bore, or approximately 5 his., and of which the stroke is 180 mm,, or approximately 7 ins. ,ORDER 01-17RINC13.4, 2

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The Principal Characteristics of the Chassis. The ordinary propelling mechanism of the machine as a selfmoving chassis resembles in all respects the standard Dennis industrial machine. A cone clutch transmits the power to a change-speed box, and thence the final drive is to the familiar worm-operated back axle.

Over and beyond this is the propelling plant for the pumping installation. An auxiliary gearbox, situated between the clutch and the main gearbox. serves to take the drive, when the vehicle is at rest, through another layshaft, which drives direct to the big turbine pump mounted at the extreme end of the chassis frame. Immediately in front of the water pump proper, mounted again over this pump-driving shaft, is the small air-pump set, which is used to create a vacuum in the main suction duct of the water pump.

Farther detailed description of the chassis as a whole is unnecessary, but we would take this opportunity of drawing the attention of our readers to the carefully-prepared perspective line drawing, which is an integral part of the lubricating chart, which we shall reproduce in the second part of this article. This will serve better than any further description to illtistrate the general appearance and arrangement of the Dennis standard motor fire pump.

In the course of our references to the adjustments and other items fur which care is asked, we shall of necessity draw attention, incidentally, to many detailed mechanical features of the various chassis units.

In oider to deal with the chassis under the headings which we have indicated, we cannot do better probably than to gareN the main components much in the same manner as that which we have adopted for previous examples of the present series. First we will write of the engine as a complete unit, and with it we will include the clutch gear, then the eearbox, next the final drive, and then we may group the steering gear and controlling mechanism together, and we may collectively consider the springs, axles and wheels, whilst the brakes claim attention for themselves.

A motor fire-engine, naturally, has a lot of subsidiary mechanism on board, and we will, therefore, devote a portion of the pnesent article to the care of the Dennis pump, and of the gear which drives it.

An Engine that Wants Little Attention.

The sub-heading which we have adopted for the present series is particularly applicable to engines of high-grade design such as are used on Dennis chassis, and, indeed, the makers rather encourage the attitude that interference with the engine, unless it be well authorized, may lead to trouble. Nevertheless, we persist, as usual, in our contention that it is absence of knowledge which, more often than not, leads to senseless derangement, and that it is good policy to make available, for those who desire it, all the information which can be produced concerning not only the engine, but the other principal, components on modern chassis.

This engine is not dissimilar to other modern examples in that. the possibilities of necessary adjustment. concern principally: the valves, the carburetter, and the magneto. Yet, even in these three cases, nothing more than periodic inspection and occasional adjustment should be necessary if the engine as a whole be not abused. With its very complete lubricating arrangements we will deal when we come to consider the care for the lubrication of the chassis as a whole.

The Diagram on page 284.

First., then, of the engine. This, as is well keown in the case of all Dennis chassis, is of the famous White arid Poppe design, and the 60 h.p. example which is used in connection with the 400-gallon Gwynne pump is a specially-produced model for fire-engine work. For those to whom it may fall to take this engine to pieces for repairing and cleaning, we ieprocluce our special valve-setting diagram, by the aid of which any ordinarily intelligent mechanic should be able to reproduce the acted valve setting, which his been definitely etandardieed by the makers. The diagram is drawn up in

sech a way, that it will probably best speak for itself. We have also, it will he noted, added the official magneto setting.

The Care of the Valves.

Much has been written, and we presume will be written, of the way in which to take care of mushroom valves, but we have not the space, even were it necessary, to reprint hints and tips of this kind. As to the actual setting of these valves, the accuracy of the machining of the camshafts will insure their correct closing if provision for their proper opening he made by correct adjustment. If at any time doubt should arise as to the proper following of the intended valve cycle and its periods, it is a simple matter to cheek these against the diagram to which we have already referred it is drawn op in a manner to facilitate such reference.

t hi considerable exactitude. Large nuts arid lockouts for this purpose will be found at the head of each largediameter tappet. A special spanner is provided f or the valve cap, and a quick-action cotter

of pecial design holds each valve spring in position. Another way of testing their proper adjustment is to insure that, when the engine is running slowly and a finger is placed on the valve collar, no concussion is noticeable. The most likely cause for the necessity of any adjustment of this kind is the lowering of the exhaust valves through periodic grinding on to their seats. Should it be necessary to remove the cylinders, this is easily effected by the removal of the accessible holding-down nuts, and the dieconnection of pipes and leads. The big. cud bearings and mains, if it at any time becomes advisable to take them up or reline them, can readily be inspected by the removal of the bottom half of the crankcaee, which, as a matter of fact, can he taken away without disturbing the crankshaft journals or the ease for the two-toone gear.

The Mileage of a Fire Engine is Very Low.

As a general rule, we advise that valves should be inspected miceineverv 1900 miles. But mileage, as we have said, is nothing to the fire-engine, and with ample time in which to carry out inspection and adjustment, it, will be good advice to insist, that they be looked to after every long pumping run, and, failing Chat, once a month.

The ignition is of a type which is familiar to all users of modern commercial vehicles. As a matter of fact, two completely separate systems are employed ; one uses the Bosch ZR4, whilst the ether calls for battery and coil with hightension distributor, It is well to remember that the magnetoignition leads are placed in the lower fibre tube, whilst those fur the coil system are collected in the upper one.

When Necessary to Adjust the Magneto.

An interesting provision of which all users of this engine should be aware is that by which, if the oil pump be disconnected, the distributor is also out of action, so that the engine cannot be inn. Each of these ignition eystems has its own witch, and is, in fact, in every respect separate. With regard to the care of the magneto, we can summarize by strongly advising the avoidance of interference with it se lung as possible. Adjustments and replacements to it are very seldom necessary, and when they are required should be turned over to the hands of an expert. The timing of the magneto itself may for some reason or other become slightly altered, and this can be remedied by means of the little caste]. hied differential coupling which we illustrate by a drawing The turning of it in the correct direction produces slig,ht advancement or retardation, respectively. The actual position in which the break should take place is shown on our valve diagram.

Th.e platinum points of both magneto and coil will seldom want touching, but from time to time it will be advisable to see that they are clean and flat, We cannot do better than refer those of our readers who are interested in the magneto as a machine and in its care to one of the capital little handbooks which are issued by the Bosch Magneto Co., Ltd_ The London headquarters are at 204, Tottenham Court Road, W., and a copy will always be sent to responsible applicants who quote the name of the chassis md the type of magneto in which the inquirer is interested.

The high-tension distributor cover should be removed occasionally and carefully dusted, as with high-tension current, of course, there is an additional risk, if dirt be present, of shorting, and a little care in this respect is nwessary.

It is hardly requisite to reTobeeree/ mind our readers that all leads 4theeedrecbleih should be kept clean and free Irqhttrocnwe from oil, plugs should be oec.a sionally withdrawn to insure that they are free from soot, and that their gaps are as they should be—that is, 1-50 in. or .5 mm.

This big engine can always be started up on the coil and then switched over to the magneto at convenience. The magneto should be used all the time whilst pumping is going on. It is not advisable to run on both systems together, and rio advantage accrues from such a course, but it is wise periodically to run on the alternative systems, as only in this way is it possible to feel sure at all times that the two systems are equally available and in good working order. There is always a tendency, when two systems are fitted, for a driver to rely especially on one installation, to the detriment of the other.

The carburetter is the well-know n White and Popp°, and it will be recalled that in this speciality not only can the air supply be varied, but the petrol jet hole is also opened and closed by the same movement that controls the throttle. The jet lies a hole drilled out of centre, and over this is fitted a regulator, which is a tight fit for petrol, and which also has a hole drilled out of centre. It is obvious that the relative motion of these two parts will vary the jet opening. as the action is simultaneous, by virtue of the coupling of the control rods, with the opening of the throttle.

Before proceeding to consider the necessity of any carburetter adjustment, it is imperative to search for the possibility of any leakage on the inlet side of the fitting, We perhaps cannot do better than quote from the excellent handbook concerning White and Poppe carburetters with regard to their general adjustment. We also advise those who contemplate making any alteration in the carburetter only to do so after very carefully assuring themselves that it is necessary, and after having carefully studied this little

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booklet, a copy of which there will be no difficulty in obtaining from the makers at their Coventry works. The quotation we make from this booklet with regard to adjustment is as follows:—

" Start the engine and allow it to run slowly, then unscrew the small brass plug which will be found in the aluminium mixing chamber. This will allow the engine to take in an extra supply of air. lithe engine picks up any speed, after a moment or two, and maintains the increased speed, it is a sign that the mixture has been too rich_ Return the plug to its position and adjust the carburetter as mentioned hereafter_ After adjusting, start the engine again, and make the same experiment until removal of the plug will cause the engine gradually to slow down and to atop. If the engine stops immediately the plug is re

moved, the mixture is pro

section/ram Aem,o bably on the weak side. If a tilifsconnecteoy weaker mixture be required, the short lever which regulates the area of the air port should be moved very slightly in the same direction as the operating lever moves when closing the throttle. A movement equivalent to thethickness of a piece of paper is sufficient to make a decided difference in the mixture."

Carburetter Dont's.

A few White and Poppe " Don'ts" may conclude our consideration of this important part of the power plant_

Don't alter the size of the jet. Don't have a leak anywhere betweencarburetter and engine.

Don't use asbestos for making joints. Fit a dowel tube, tight fitting each side of joint, or make the joint with boiled oil.

Don't leave off the warming pipe.

For the English climate make the sleeve round the exhaust pipe half the length of the engine.

Always provide a cut-uut to reduce the temperature of the warmed air in summer.

Don't forget that a good filter must be fitted between the petrol tank and engine for straining petrol, and note that the throttle must be nearly closed when starting the engine.

It will be noticed that the air supply is taken from between the two front cylinders. The main petrol supply is carried in a tank under the driver's seat, and will carry up to 25 gallons. As a rule, it is fitted with some fire-resisting device like the Snereold. On its way to the engine the petrol runs down to an Enots filter, the strainer in which should be periodically inspected and cleaned. Over and above this we always advise the use of a funnel fitted with a gauze when petrol is being put into the tank. After a pumping spell at a fire or on teat, all petrol pipes should be run clear and the filters cleaned.

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Organisations: eta
Locations: Coventry

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