THE ACCESSIBILITY OF PA MR-CARRYING CHASSIS.
If you've noticed an error in this article please click here to report it so we can fix it.
Points in the Design and Layout iassis Components which Affect ance. Ease o
91 HE maintenance of a passenger-carrying chassis I in good running order is of the greatest importance because a mechanical breakdown, even if it does not endanger human lives, discredits mechanical road transport in general and the fleet of vehicles concerned in-particular. Furthermore, a vehicle which is laid up for repairs at a busy period represents a loss of revenue, and the need for such repairs can often be obviated if periodical inspection and adjustment are made easy by the design of the vehicle. In short, ease of maintenance and repair are both largely dependent upon tile accessibility of the chassis.
Recent development in design—notably the forward control position and the low loading line—have not helped to make the chassis accessible. Indeed, there are many vehicles at present being sold which are impossible to overhaul without first entirely removing the bodywork.
Apart from this question of general overhauls, however, there are many details which require attention from time to time, the ease of access to which might well be more carefully studied. The engine is a case in point, and here, components requiring more or less frequent attention are the carburetter, magneto and valves.
The magneto should preferably be placed transversely, with the contact-breaker cover facing outwards, so that the points can readily be seen. In some modern magnetos the distributor is placed above the instrument instead of at one end ; a position in which it is much more accessible.
The location of the carburetter is often faulty in that the instrument is placed too low, the designer of the engine overlooking the fact that in most cases jets must be removed from beneath, whilst, for overseas work where fords have frequently to be crossed, it is essential that the magneto and carburetter be set high.
As regards valve tappet adjustment, much depends apon the location of the valves, and the engine in which these components are placed side by side, Is often an offender. In such an engine the space around the tappets is sometimes very cramped, so that the use of two spanners to operate the tappet adjustment becomes a matter of considerable difficulty. The overhead-valve engine scores in this respect, thetappet adjustment usually consisting of accessible screws fitted to the ends of the rockers.
Then there is the matter of the lubrication system. The operator is told that the crankcase must be drained and replenished at regular intervals, and yet this operation is often rendered extremely difficult by the design employed. There seems to be no valid reason why it should not be possible to drain the crankcase by operating a lengthy handle extending upwards from a tap, but too often the only means of letting the oil escape consists of a small plug, to reach which the mechanic must crawl underneath the engine.
As regards the process of removing the cylinder head for decarbonizing, the side-valve engine is easier to deal with as the valve gear can be left in situ. The operation is simplified if cast lugs are provided on the head and on the cylinder block, between which a lever can be inserted, or if the head be drilled and tapped in a couple of places for a stud which, bearing on the upper surface of the block, lifts up the head. This latter is a good method, as the gasket is not likely to be injured. It would also he advantageous to the operator if more makers would adopt the scheme of fitting 'a separate water connection between the head and the block instead of providing passages sealed by the same gasket which is used to make the cylinders gas-tight.'
The way in which the engine is constructed has a considerable effect upon the ease of access to important bearings such as big-ends, connecting rods and pistons. The use of detachable heads has, of course, become practically universal, and when such a head is removed
the piston crowns and the combustion space become available for decarbonizing; the mere removal of the head does not, however, enable the mechanic to get at the insides of the pistons, or, of course, to reach the connecting rods.
In passing, it may be mentioned that it is quite good practice to provide two independent heads on an engine of considerable size, even although the cylinders may be cast in one block, because the separate heads are not only easier to handle, but are also less liable to leak at the joint than is a single head of large area.
In large engines the crankcase is almost invariably a separate casting of aluminium, so that the working parts can be reached fairly readily by removing the cylinder block ; alternatively, the sump can be taken off, the big-end bearings unbolted from below and the pistons and connecting rods then pushed out upwards. In many ways this procedure is more convenient than removing the cylinder s block, provided that the engine is so designed as to facilitate the removal of the sump, but, in too ninny instances, the sump is secured by an extremely large number of finicking bolts and nuts, so that to take it off is quite a lengthy process.
It would really seem that there is quite a good case for the marine engine practice of fitting large doors to the sides of the crankcase. Such doors enable the big-end bearings to be undone from the side instead of from beneath, and if the cylinder head be removed the pistons and connecting rods can then be pushed out upwards. It is, however, possible to go a step farther in large engines by making the doors of such a size that the connecting rods and pistons can actually be withdrawn through them, leaving the cylinder block and head undisturbed.
Small four-cylinder engines, such as are used •to propel chassis of the 16-20-seater type, often have the cylinder block and the upper half of the crankcase cast in one piece. This system leads to great rigidity of the power unit and consequent freedom from vibration; but it is in some ways objectionable from the point of view of maintenance. With the cylinder
block and crankcase formed in one piece in this way, it becomes more than ever important to make sure that the sump is easy to remove, so that the pistons and connecting rods can be withdrawn entirely from below. This job is, in any case, an awkward one owing to the difficulty of manceuvring the parts past the crankshaft, and yet the process of decarbonization cannot be said to be complete unless the pistons have been taken out for the purpose of cleaning the rings and removing the deposit from the inner surfaces of the crowns.
Easy means for draining and replenishing oil should be provided on the gearbox and back axle, it being usually necessary to provide trap-doors in the floor of the body above these parts. Another part of the mechanism which should be easy to reach consists of the means provided for adjusting the brakes.
Turning to the question of overhauls, it is in many ways the best plan to remove the body entirely before commencing operations, but this system will not appeal to the man who is only running a small fleet of two or three vehicles and who cannot afford a costly installation of jacks or other lifting appliances for the purpose of dismounting the body. Consequently the layout of the chassis should be arranged, so far as possible, to facilitate access to the chassis when the body is in place.
In cases Where the engine and gearbox are fitted in the -frame as separate units, it should be possible to lift out the former from above, after removing the radiator and disconnecting the clutch shaft, etc., without disturbing the dash. Even if a direct lift be
not feasible, the engine bearers should be fitted in such a way as to allow the power unit to slide forwards along the frame members while it is being lifted out, the forward movement being sufficient to enable it to clear the dash.
An alternative scheme, which is sometimes possible when the engine and gearbox form a unit, consists of running the vehicle over a pit, placing slings around the engine unit, and dropping the whole job through the frame into the pit beneath. The difficulty here lies in the fact that in most chassis the front axle im 534 pedes the operation, skilful manceuvring and the removal of parts such as the timing cover being necessary to get the power unit past it. In fact, it is usually necessary to jack up the frame and remove the front axle.
There remains the scheme, quite often adopted, in which the engine is supported by jacks and packing pieces, the bearer bolts are removed, and a crane is employed to lift the front end of the vehicle upwards, leaving the engine below. As a rule, however, it is necessary first to remove the front axle, as this and the steering rods will prevent the frame from being lifted clear. This plan is often useful in the case of vehicles in which the engine and gearbox are mounted in a sub-frame, this being, in turn, bolted to the main frame. The whole of the sub-frame assembly can then be packed up and left behind.
A separately mounted gearbox, if of any size and weight, is a very difficult component with which to deal. With a closed body left in position, it is usually impos sible to reach the gearbox with a crane from above, and to get it out from below entails the use of joists and jacks with which it must be painfully lowered on to a trolley. Preceding this operation, of course, comes the job of disconnecting the clutch and propeller shaft, a matter which some universal joints make impossible without first disconnecting the rear springs and moving the back axle. From this point of view the Hardy disc type of joint has many advantages, as it is usually very easy to 'dismantle.
There are many pros and cons to consider when determining the vexed problem of whether to build the gearbox in a unit with the engine or to. make it a separate component. There is no question that the former simplifies manufacture considerably, and, if the design of the whole vehicle be carefully considered, the removal of the entire engine and gearbox unit should not present undue difficulty. On the other hand, unit construction almost inevitably leads more or less to a loss of clutch accessibility.
Furthermore, the distance of the gearbox from the back axle becomes so great that even in a small bus chassis, such as the 20-seater type, it is usually necessary to employ two transmission shafts joined by a flexible coupling supported by an intermediate crossmember. This being the ease, it would, in a way, seem logical to place the gearbox in the centre of the chassis with a transmission shaft at each end of it. On the other hand, as already pointed out, a centrally mounted gearbox is very difficult to reach or remove when the body is in position.
Turning to the back axle, this component is wormdriven in the majority of cases and should preferably be so arranged that the cross-driving shafts and the differential unit can quite readily be removed. It is, as a matter of fact, common practice nowadays to arrange matters in such a way that the cross-shafts can he pulled out from the ends, after the removal of the dogs through which they drive the wheels, without jacking up or taking the load off the wheels. With regard to the removal of the differential and final drive unit, this is rendered very simple in the case of the pot type of axle, as the differential cage, worm wheel and worm are all mounted in a single housing which can be lifted out. In a lighter form of underslung-drive axle construction, in which the axle tubes are secured by flanges and bolts to a divided central portion, the worm wheel can be taken out when the upper half of the central casing is removed. Whatever its other advantages may be, the built-up type of axle which is divided. vertically is, 'perhaps, one of the worst types from the point of view of accessibility.
We have, in this article, dealt with the subject more particularly from the point of view of the operator whose fleet is a small one and who cannot afford expensive systems of repair and maintenance. Of course, one of the best plans, when the number of vehicles justifies the expense, consists of keeping one or more of each of the chief components in stock as spares.
Should a fault in the engine develop, for example, the whole unit is lifted out over-night and replaced by a spare engine, in which case the fault can be found and corrected at leisure in the shop. When this system is in vogue, matters are probably simplified by making the gearbox form a separate unit, provided that some fairly cheap and efficient contrivance can be devised to facilitate its removal from the rest of the chassis.