USE OF THE CLUTCH

New Zealand Times, Volume LII, Issue 12229, 29 August 1925, Page 9

USE OF THE CLUTCH

ATTENTION TO LUBRICATION CAUSE OF DEFICIENCIES.: References to stripped gears call to mind several occasions upon which I have had to fit new bevel pinions or crown wheels to various makes of vehicles that, have certainly not a reputation for suffering from deficiencies in bevel gear sizes and dimensions. These expenses (states a writer in “Motor Transport”! have been account-, ed for either by clutch fierceness or by misuse of the clutch. Even when the smoothest of clutches is engaged as carefully as possible, heavy stresses are imposed upon gear'teeth in moving a heavy vehicle, and its load; but when a clutch is fierce or is violently engaged, such stresses may be increased tenfold. •No small ■ proportion of drivers of Ford vans ana one-tonners are positively brutal in their use of the clutch when changing from low to high gear: Possibly the fact that there is a big step up in the gear ratio may encourage them to engage the clutch suddenly after the engine has been accelerated considerably bn low gear, but whatever the excuse, the violent forward jerk thus given to the whole vehicle is just as bad for the transmission gears in this type of, vehicle as for other transport units when the clutch holds suddenly and violently. In brief, it is simply asking for trouble to neglect a fierce clutch or to engage cny type of clutch “with a bang.” I have had to renew gearbox pinions stripped for different reasons. In several cases the underlying cause lay in failure to give proper attention to the lubrication of the clutch spigot bearing. Stiff working at that point prevented the rapid Variations in speed between flywheel and clutch shaft which are necesary in changing gear quietly and without damage. In two cases I can recall the second-speed gears were stripped owing to the innumerable “crash changes” from first to second, but these need not have occurred if the clutch bearing had been properly lubricated and working freely. Admittedly the change from first .to second speed, or vice versa, is generally the one which gives drivers most trouble, and which causes more noise than any other; bat although things in this direction may he normally Jbad, there is no reason why they should be worse than necessary, and most assuredly ’that state is encouraged when lubricaition of the clutch spigot is neglected or inefficient, CLUTCH BRAKE ADJUSTMENT Another cause of stripped gears in change-speed boxes is an improperly adjusted dutch-stop or clutch-brake. When changing from, a low speed' into a high one the r.p.in. of thO clutch shaft must be reduced, and, if the clutch is a heavy one, a stop or brake should be fitted and so adjusted that it tends to press upon and decelerate the clutch and its shaft when the pedal, is depressed. Moreover, it should have that effect before the pedal is completely depressed; so that there may be a margin for wear, and render frequent adjustments unnecessary. A clutch brake should not, however, come into action too soon after movement of the pedal is commenced, for the following reason: In changing gear from a high ratio to a iower one, the clutch shaft needs to be accelerated, exactly the reverse of the requirement when a change up is to he made; but if the clutch brake comes into action immediately the pedal movement begins, acceleration of the clutch cannot occur. In changing down it is difficult enough to ensure that the necessary acceleration shall take place, even when a clutch brake is omjtted; so, if there must be choice between a clutch brake adjustment which is too effective and one which is hot effective enough, the latter is to be preferred. After all, it is possible in most oases to fain the effect of a clutch brake by waiting for the gears when an upward change is made; in other words, by pausing for a while with foot off the accelerator, the gear level in neutral and the clutch depressed until the engine has slowed down. True, when a heavy vehicle is being started from rest, there is not much time to waste while moving the lever from first to second speed, or the road wheels will soon cease revolving but by a little judgment and management the change can generally be effected without much trouble. SLIPPING AND BURNT-OUT CLUTCHES - i References to clutches in a series of this nature would not he complete without mention of the causes of the slipping clutch and of the factors which tend to the burning-out of clutch leathers. Clutch slijf in the case of one of tho cone type is generally due to one or other, or a combination, of the following :—(l) Too light a spring pressure; (2) the formation of a ridge on the portion of the lining extending beyond the real edge of the flywheel; (3) too short a rearward travel of the clutch pedal: and M) faulty aligi ment of the clutch shaft. Other cauies I have observed from time to time In isolated cases are:—(s) oil on the leather; (6) an unsuitable cone angm; (') I insufficient clearance between clutch cone and flywheel bolts; and (8) a lazy driver. The last is probably the most frequent cause of burnt-out clutches, for his habit of leaning on the clutch pedal—in other words, ÜBing It ns a footstool and pressing heavily on it with his foot during almost the whole time his vehicle is runmng—is just the same in effect as running with the clutch springs too lightly adjusted ; the spring pressure is reduced by the extent of the pressure applied to the pedal • As to the first cause, the spring pressure is usually adjustable, and m„such oases tlie fault is easily corrected. When no adjustment is provided and leather has been used for the friction facing, there are two alternate treatments; the first is to fit a stronger spring, while the second—and preferable is to use a more efficient facing, such as Ferodo, in place of leather. Clutch friction fabrics of this nature require less spring pressure, other conditions being equal; moreover, they are far less liable to burn out when slipping does occur, a fact which needs no explanation beyond mentioning that asbestos generally enters largely into their construction. The formation of a ridge on the

surface of the frietiori facing of a clutch is due either to the facing material being too Thick or insufficiently compressed before it is fitted, so. that the complete width cone doe's. not: come into contact with the flywheel, and thus does not become compressed like the rest of the, leather. Whether it causes slipping or . not, it is certainly conducive ito that ' defect, and its removal with a sharp knife or discarded safety razor Wade is desirable if trouble is to tie avoided. RESTRICTED PEDAL MOVEMENT Too short a travel of the clutch pedal is often due to the' floorboard slot not being continued, far., enough back; limitation, of the pedal movement/ in this way, of course, limits the movement of the dutch cone, and the latter may thus be prevented from engaging the flywheel., as deeply and firmly as if the spring pressure were permitted full play. But faulty adjustment or a shortcoming of design may he theunderlying cause. The levers and connections between clutch pedal and striking fork may not have free play at some point. Occasionally an adjurtment is available to correct this, or it may be necessary to lengthen or shorten a rod. Faulty alignment of the clutch shaft, is rarely due to neglect or misuse by the driver, short of ' a ■collision .or chassis distortion caused by running over excessively uneven," surfaces; hue oil on the clutch leather can generally he avoided, or at least counteracted, by cleaning the clutch out with paraffin. The latter, however, as well as petrol, should he kept away. "from clutch facings if possible; it is prone to make for fierceness by removing the “nature” of the material. . An unsuitable cone angle is the designer’s fault. The face of the cone should be at about 14deg. with the shaft. . Another possible design fault is an insufficient clearance between clutch cone and flywheel bolts, though this may be due to the fitting of too thin a facing material when a replacement has been made; a thicker material .is needed of course, if the lining becomes compressed with use, and allowance should be made for this. As for No. B—the lazy driver—who sometimes is not really lazy Wit has acquired the had habit inquestion—-* that cause should be eradicated once * it is not onlf bad for the clutch facing, but it gives rise to excessive and rapid wear of tho clutch striking, mechanism, ball thrust races, etc. SLIPPING IN PLATE CLUTCHES The main causes of slipping in plate clutches are two, opart from faults m design or unsuitable oil in designs of the Helo-Shaw; type. They are: —insufficient or unequal spring pressure, and a limitation of tho movement of the poda! or the operating levers. Adjustments in most cases are avauable to correct both of these deficiencies. • A bigger clearance for pedal travel in the floorboard elot is more often required for plate clutches than for the cone type, because sometimes there is a good deal of in the operating details, for which allowance must be made.