How to Keep Your Lorry. Fit.---II.
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A Series of Articles Principally Useful to Drivers, but Well Worthy of Perusal by Owners, Mechanics and Foremen. This One Deals with the Thornycroft j•Type Chassis.
As stated in the opening and introductory article of this series, the attention which the modern commercial vehicle needsslight • indeed. To none of he chassis marketed to-day, does that remark apply to a greater extent than it does to the Thornycroft type J, 3i-tonner. This machine is a. typical example • of up-to-date British design. With the exception of the final drive, which is by worm and wheel, its construction is sthiilar to the standard War-Officesubsidy model, it embodies a four-cylinder engine, with pair-cast cylinders and three-bearing crankshaft ; a four-speed-and-reverse gearbox ; and, as stated, wormand-wheel final transmission. It is designed to carry a gross load of • ,q tons, and as the weight of the chassis in running order is 3 tons. 6 cwt., the gross weight Of the corn, plete vehicle, with body and lead, must not exceed 7 tons 16 cwt. These particulars are essential, in view, of the 'strictures which were passed in the previous article on the question of overloading. The engine is a governed one, and provided the governor is not tampered with, the vehicle cannot be driven at a speed exceeding 16 miles per hour, and even this-may be, on rough roads, excessive.
Engine Lubrication an Important Care.
Concerning the power unit ; it will generally be admitted that the most important point bearing on its fitness is its satisfactory and timely lubrication. The lubrica-•
tion system on the Thornycroft engine is very full and complete. It is effected by a pump fixed at the front of the engine and driven off the end of the inlet camshaft The oil is drawn by this pump from the base chamber and distributed to the bearings by the three connections which are marked on our
illustration. From the main bearings, this oil liasses to thebig-ends by means of holes drilled in the webs of the cranks. Any excess of oil is returned to the sump through a relief valve situated over the engine timing wheels. The oil level in the crankcase should be tested every morning ; this operation is effected by opening the level cock near the bottom of the lower half of the crankcase. Oil should be poured into the case until, when this tap is open, the oil just overflows ; take care not to
put in too much. In the -event of the gauge on the dashboard not registering the correct pressure (10 in.) when the engine is running, the cause Will prove to be one of the following. There is n.ot sufficient oil in the engine, the remedy for which is obvious. There may be .a breakage of an oil pipe, if this is external and on the delivery side, it will be similarly obvious ; but, if it is on the suction side, it will make itself felt by the presence of air bubbles in the oil. The simplest way to test for this is to disconnect the union at the pressure gauge and watch the stream of oil issuing from the end of the pipe for bubbles. The pressure gauge itself may be broken ; if this is thought to be the case, disconnect the gauge, press the thumb over the open end of the pipe while the engine is running, when it will.be readily ascertained whether or not there is any pressure in the system. Finally, the cause may be that the pump refuses to work because the strainer or the pipes are choked. The system should be cleaned out every two months, and the pipes themselves taken down and cleaned out with-paraffin. Access
• may be had to the strainer by removing the main union on the pipe where it joins the strainer (see illustration), and then, after removing the two nuts, taking out the strainer.
It is recommended that, unless there is particular need for investigation, the relief valve be left severely alone. In the event, however, of it being necessary for any.
reason to take it down—this may occur owing to trouble with the lubrication for which no other cause is apparent; and which will probably manifest itself by failure of the oil pressure, which is indicated by • the pressiire gaugeon the dashboard—after replacing it, the small set-screw (see our illustration) should, beslacked back, and the pressure Varied by turning the.hexagon cap. Screwing down the ,cap; increases the pressure on the internal spring, and therefore puts additional load on the relief valve. Correct pressure can only be found by trial, running the engine until the oil is warm and thin enough to run freely, and noting the indication on the gauge. Screwing down the small set-screw, locks the valve and spring correctly.
Grind Valves and Adjust Tappets.
The valves require grinding in occasionally, and, when this operation has been performed, the clearance between the • stems and tappets should have attention. The proper distance between these when warm, and when the valve is properly closed and the tappet in its bottom position, is about the thickness of a visiting card..
On some Thorny-croft engines,
the tappets are fitted with thin steel discs, contained by a cap over the tappet—these)may be.removed or added to as required ; on others the tappets are fitted with screwed beads, and it is only necessary to slack back the locking nut, screw the head to the correct clearance and tighten up the locknut once more.
Clean Pistons Inside and Out.
Don't forget when the engine is down for cleaning and carbon is being removed from the outside of the pistons, to clean it off the inside of the pistons as well. Carbon left inside the pistons has a habit of dropping. into the base chamber and causing trouble by choking the oil strainer.
The Pump. and Fan Belt.
Occasional attention is required for theOelt driving the celirifugal pump and fan. 'This should be kept just tight enough to drive without slipping. It should be dressed occasionally with neatsfoot oil or castor oil. The gland nut on the pump spindle should only be screwed up until there is an occasional drip of water, which will serve as a lubricant to the pump spindle. If the leak is more than this and cannot be stopped by tightening up the gland nut; pack the gland with a cotton grease
packing; on no account should asbestos packing be used The small tap on the top of the pump is an air cock, and should be opened when filling the radiator -with water, so as to obviate any air hick.
The Clutch.
In the ordinary way, all that the Thornycroft clutch needs is lubrication and occasional dressing of the leather, or adjustment. It is the clutch lubricators on most chassis that are forgotten and neglected. On the Thornycroft there are three : one for supplying grease to the ball thrust which comes into operation when the clutch is withdriwn ; the other two lubricate the main clutch pivotbearing. The adjustment of the dutch spring is attained by screwing or unscrewing the nuts on the two studs in the rim of the flywheel at the end of the spring, by screwing these in, the clutch is caused to grip more tightly by loosening them, it is eased. Fierceness may usually be cured by dressingthe clutch leather with engine oil • this method should be tried before slacking the adjusting screws.
The Gearbox.
In the ordinary way, the gearbox needs no attention beyond maintaining the oil at its proper level ; that is, up to the centre of the bottom shaft, and being drained and washed out once every six months. If, owing to a few unfortunate cases of bad gear changing, small chips may have been broken from the edges of the gears, then the gearbox should be cleaned out on the earliest convenient occasion, as otherwise there is a probability that these chips may get into the ball bearings and do considerable damage.
Rear Axle and_ Steering Gear..
As regards the need for atten7 tion, the rear axle compares with the gearbox, insomuch as the oil level is the only feature that needs occasional attention. There is a filling plug provided on the axle, and the easing should be filled until the oil is ready to overflow front this hole. The makers recommend a special brand of oil for this axle, known as Amber Gear Oil, which is supplied by Prices Co., Ltd., Battersea, S.W. On no account, they say, must grease . be used in the back axle, and it is recommended that the casing be cleaned out and fresh oil put in once every
month. • Having now dealt with the main units of the chassis, attention may
be directed to the minor, but no less essential features, such .as steering and brake gear. With regard to either of these, the neeessity for keeping them in good order cannot be too strongly emphasized. The steering gear joints on the engine are encased in leather gaiters, and there is a tendency towards "Out of sight, out of mind " ; this is a dangerous attitude which should be avoided. The gaiters must be removed from time to time and filled, with grease, and the joints examined to seethat there
is no slackness-; if there is, it can be adjusted by filing a little off the face of the caps securing the rod ends.
Brakes.
As remarked in our opening article, the adjustment of brakes should be • made little and often. The adjustment of the foot brake on the Thornycroft is by means of a thumb nut connecting the top ends of the foot brake blocks. That or the side brake is by right and left-hand screwed rods on both sides of the vehicle, close to the side brake blocks.
Don't Play with the Carburetter.
• As a general rule, the Thornycroft Co. deprecates any interference with the carburetter setting as turned out from the works. If the petrol consumption is found to be heavy, it is probable that the cause of the-trouble is elsewhere. In the event, however, of any driver being of opinion that he can improve the adjustment, he is recommended to obtain alternative jets and choke tubes, and not to alter the existing ones by reamering them out, or other methods. Occasionally a little grit may get under the needle, and cause flooding.