The Supply Department.
Page 19
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Silent Chains for Camshaft Driving.
In the past two or three years, motor vehicles have been brought to such a high pitch of perfection that noises which previously were unnoticed have now to be reckoned with. These noises are largely due to the use of straight-tooth or bevel gearing, both of which classes of gearing become noisy as the teeth show signs of wearing away. Motor manufacturers have been trying to find some form of transmission which will he as nearly silent as possible, and chain makers now assert that the silent typo of chain meets their requirements in every particular. Chain gearing is now used for the driving of magnetos, camshafts and gearshafts, in addition to its being extensively employed for the final drive to the road wheels. The principle of the silent type of chain is now well known to up-to-date engineers. When such a drive is employed, there is no sliding motion or hammering of the wheel teeth against the chain when the latter comes into contact with the gear teeth, and, further, a very-large number of teeth is in mesh with the chain at one time Hans Renold, Ltd., of Progress Works, Manchester, has just issued a little 8-page booklet on the subject of silent chains for camshafts, and, small though this book is, it contains much valuable information. Among the advantages that are claimed for the use of a silent-chain-driven camshaft are the following: (1) that the chain maintains its full efficiency, which, by the way, is very high. until it is quite worn (cut; (2) that there is no increase of noise from the chain as it wears, because its engagement with the wheels remains correct; (8) that, although c certain degree of accuracy is required for the distances between the wheel centres, the limits of error for machining may be stated in hundredths of an inch, whereas, with spur gears, they must be counted in thousandths : (4) that there wilt probably be an increase in the life of the valve gear as a whole.
On the other hand, a chain drive has certain disadvantages, and these must not. he disregarded. They are : CI) it. is more difficult to adjust the relative positions of the camshaft and crankshaft, as the pitch of the -wheels is so coarse that a movement of one tooth makes a comparatively-large difference in the relative angular positions of
the two shafts; (2) although the tooth form adopted for the wheels of silent-chain gears allows of the automatic adjustment for the increased pitch of a chain, due to stretch and the wear and tear, it cannot take up the increased pitch on the straight portions of the chain between the wheels. This latter causes the camshaft to lag slightly, and, for that reason, the allowable wear of a silent chain for a camshaft drive is much less than for most other applications; (3) the life of a chain may not be very great, but this has yet to be proved.
These three drawbacks may he minimized, or neutralized, by the use of an adjustable chainwheel, such as we illustrate herewith. An examination of this view will show that the wheel is provided with a loose hub, which huh is keyed to the end of the camshaft ; the wheel body is hmely mounted over the fixed huh, and is secured to the latter by three or more bolts which pass through diagonal slots in the flanges of both the wheel and the hub. It will be noted that the direction of the diagonal slots in the two parts ore opposed so as to permit of the angular positions of the two parts being varied. We strongly advise interested parties to write to Hans lienold, Ltd., for a copy of this small booklet.