CHARACTERISTICS OF FRICTION CLUTCHES.
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Inadequacy of Technical Data Supplied by Clutch Manufacturers.
IT IS NOT the intention of the writer to discuss in this article the relative merits of any of the different types of friction clutches now on: the market, nor is this article written as an exposition of the theory of the friction clatch, The intention is to attract attention to the importance of the clutch as a necessary mechanical part of the commercial vehicle, and also to call attention to -the lack of adequate technical information /covering the principles that must underlie the design of a successful friction clutch.
Take any friction clutch catalogue, as at present issued by a manufacturer of clutches, and on turning the pages one will probably find a table giving what purports to be the horse-power capacity of the various sized clutches for a given number of revolutions per minute. Such a table will probably show that the manufacturer believes the horse-power capacity of each clutch varies directly with the number of revolutions per minute. In no case, within the writer's knowledge, is any information given covering the "pick-up " capacity of a clutch at any given speed, nor has he yet come across a trade catalogue giving the starting-torque of any clutch.
The remarkable development and increasing use of motor vehicles. that have taken place in the last few years has thrown a great deal of light upon the qualities that are essential for a successful clutch. It is somewhat of a shock to the mill or shop man to realize that some of the motorcar manufacturers equip their machines with clutches that, in spite of the exceptionally severe services they are subjected to, actually outlast the vehicle, of which they are a part, without requiring any special attention or repairs. The purchaser of a motor vehicle does as a rule pay particular attention to the design of clutch utilized before buying, but this is not usually the case with buyers of clutches for ether uses. Would any.rnill or shop man, using a frittion clutch to start up and run heavy machinery, expect to buy one at a, reasonable price that will not require a new disc, collar, brake band or such-like replacement at fairly frequent intervals?
Part of the trouble is caused by the fact that the buyer of a clutch, even if he secures one big enough to transmit the power wanted at the running speed of the machine in question, does not make certain that it has eapacity enough to start up from rest when the machine is heavily loaded. It is seldom realized by the buyer that the "pick-up " capacity of a clutch is inversely proportional to the speed, while its transmitting power is directly proportional to the speed. This subject is full of interest from both a practical and theoretical point of view. The following statements, for the truth of which there can probably be found no confirmation in present trade catalogues, may probably be endorsed by many readers of TEE COMMERCIAL MOTOR, who have had much experience in the use of friction clutches and can thus support the writer's belief in their accuracy :—
(1) The starting torque of the common friction clutch is constant up to a certain speed of the driving shaft, and beyond this point it rapidly drops. (2) The " pick-up " capacity of a clutch varies inversely with the speed. (3) The work done by a clutch in" pickingup" a load is directly proportional to the weight of the load. (4) The work done by a clutch in " piekingup " a load is directly proportional to the square of the velocity of the mechanism to which it is attached, That is to say, if we double the speed of the driving shaft and then throw in the clutch on the same load, the work the clutch will have to do to start up the same load from rest to double the normal speed will be four times as great as the work required of the clutch to start the same load at the normal speed. We may state this algebraically as follows:—
F=ix—XV2
F = the amount of work done.
W = the weight moved. where V = velocity of moving parts in feet per second. This is the well-known equation expressing the work of acceleration.
Now let us refer to the types of clutches used by motor vehicle manufacturers. It seems to the writer that the most successful clutches which are used in the construction of motor vehicles are those that are made to slip as well as to grip. The heavy motorbuses in the streets of Paris are run in such a manner that .their speed is controlled by the clutch, i.e., at the slower speed of the buses the clutch is allowed to slip. This is probably ‘a new way of looking at the use of a clutch to many of us. It is evident that a clutch that cannot be thrown in too quickly, and that will slip as it gradually picksrup the load, is spared the destructive treatment that, must be the lot of a clutch that is thrown in too quickly and made to destroy itself starting up a heavy load from rest to a high speed. For instance, suppose a clutch is used to start up a load of 10,000 lb. from rest to an average of one foot per second, then the work required to pick-up is found from +lie preceding equation to be : 10,000
F – X 12 ss 156 footlb.
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Now let us .speed up the machine to say, 3 ft. per second, and throw our clutch in. The "pick-up" power required for the same machine is expressed thus: 10,000X32 F = – 1410 foot lb. o •
This means that a clutch nine times as large is required to "pick-up" the load in the same period of tithe. Yet the clutch manufacturer would probably offer a smaller clutch in the second instance than in the first because the speed or revolution per min,ute is three tunes ,as large and, as per his catalogue, only reouires a clutch of about one-third the size. From some data jotted down at intervals the writer finds that quite a number of friction clutches now on the market have been given a rated horsepower by multiplying the number of square inches of frictional contact by the mean radius, by the number of revolutions, by some constant, and dividing the product by 33,000. Thesconstant is generally 20, and it represents the number of lb. of pressure per square inch of frictional contact. This figure multiplied by the coefficient of friction for the material used, and then by the number of souare inches of frictional con-, tact, gives approximately the power required from the leverage and motive power provided to throw in the clutch,
The writer does not profess to be an expert on the clutch question. although it has caused some thought and some trouble in the past. All the trouble experienced could not always be traced to the want of judgment on the part of the operator in giving the clutch too much work to do. The average clutch manufacturer is not as yet sufficiently' imbued with the spirit of the times.to make public the engineering informs:. tion covering this subject, either in his catalogue or by the ready and courteous tongues of his salesman.