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THE INDIA RUBBER WORLD 



[June 1, 1914. 



THE MAGNETIC CLUTCH AS A SAFETY DEVICE. 



The hazard assumed by workmen engaged at and about .rub- 

 ber mixing rolls, washers, crackers, calenders, etc., has long 

 been recognized, and many devices have been developed and 

 installed for the purpose of making it possible to shut down 

 the machinery promptly in case of accident. The most recent 

 safety appliance to be used on rubber machinery is the magnetic 

 clutch, but even these devices were introduced several years 

 ago. The latest type of magnetic clutch embodies not only a 

 means of coupling the driving shaft to the driven member, but, 

 in addition, includes an automatic brake, which is applied when 

 the current is interrupted either intentionally or by accident. 

 Magnetic couplings or clutches require but little space along the 

 shaft, are made up of few parts and have perfect running bal- 

 ance. They have no links or other parts which are liable to 

 breakage or sticking, and do not throw grease or dirt. In fact 

 only two criticisms had ever been made on early installations. 



One objection was that the time lag between the opening of 

 the operating coil circuit and the releasing of the coupling 

 varied, being so small as to be practically negligible when the 

 couplings were fully loaded, but large when the couplings were 

 operating under light load conditions. 



The second objection was that they were not adapted to reac- 

 celerate the load after the machinery had been shut down by 

 their disengagement, for the reason that they were not made to 

 engage gradually, but took hold almost as suddenly as a jaw 

 coupling. 



The first objection appears to have first been recognized by 

 Mr. Myers, chief engineer of the Firestone Tire & Rubber Co., 

 who suggested the installation of an automatic band brake on the 

 mill side of the cut-off couplings. The second has been over- 

 come by the design of a new form of magnetic clutch which is 

 adapted for gradual acceleration. 



This type of safety device has been developed by the Cutler- 

 Hammer Clutch Co., of Milwaukee, which concern has been 

 very active during the past two years, co-operating with the 

 superintendents of various rubber factories, in installing mag- 

 netic clutch safety devices, and in making exhaustive tests to 

 prove their contentions. In this article we will first describe the 

 clutch and magnetic brake referred to above, and then give the 

 results of some actual tests made with the apparatus. 



Fig. 1 shows a cross section and a side elevation of the Cutler- 



Fic. 1, Magnetic Clutch with Autom.\tic Band Brake. 



Hammer magnetic clutch equipped with an automatic band 

 brake. The clutch comprises two members, the field or driving 

 member A, which carries the magnetizing winding B, and the 

 armature or driven member C. The driving member has a 

 hub D, which is bored to fit the driving shaft. The driven mem- 

 ber has a similar hub E. which is also bored to fit the driven 

 shaft. Near the outer edge of the member A is an annular 

 groove containing the magnetizing coil B. Attached to the hub 

 £ is a flexible spring steel plate G^ which carries the armature 



C. Between the two parts of the clutch at the outer edge is a 

 friction facing H, which prevents the armature from coming di- 

 rectly against the face of the coil, and also provides high fric- 

 tional contact for driving. This friction facing is made from 

 woven asbestos and brass wire, similar to the brake lining ordi- 

 narily used in automobiles. On the hub D are two contact 

 rings /, which are insulated from each other, and which are at- 

 tached to the ends of the magnet winding B. The winding is 

 tlius supplied with current by contact with a pair of brushes 

 A.' connected with the source of power. Attached to the driven 

 member is a brake drum L, around which the brake band M 

 fits. This band is of the same material as that used in the fric- 

 tion facing between the clutch members. The ends of the brake 

 band are pivoted at A' and O to a lever P, which carries an 

 adjustable weight Q at its outer end. Near the center of this 

 lever is pivoted a vertical rod R, which extends up into the cyl- 

 inder 5. This cylinder encloses a solenoid, by means of which 

 the lever P may be raised when the solenoid circuit is closed, 

 thus loosening the brake band around the drum L. 



The operation of the device is as follows : Assuming that the 

 clutch is mounted on the ends of the driving and driven shafts 

 between the electric motor and the mill, and that the mill is at 

 rest, the current is gradually applied to the magnet coil by 

 means of a rheostat. As the current increases in the coil the 

 flexible steel plate containing the armature is pulled toward the 

 coil. The friction gradually increases between the armature and 

 the friction facing until the current is strong enough to rotate 

 the driven member at the same speed as the driving member. 

 When the current is applied to the magnetic coil, the same 

 source of current is employed to energize the solenoid and to 

 lift the weighted lever P, thus loosening the brake band M, and 

 allowing the brake drum L to run free with the driven shaft 

 The rod T is attached to some permanent support, and only 

 serves to hold one side of the brake band at a point which 

 acts as a pivot when the brake is applied or loose. 



In case of accident to the operator or to the machine the 

 switch is thrown by means of a rod or lever conveniently- 

 placed, thus breaking the circuit supplying current to the clutch 

 coil. This also interrupts the solenoid circuit, allowing the lever 

 P to drop. When this lever drops the weight applies consid- 

 erable leverage to the brake band, tightening the band around the 

 drum. Thus it will be seen that in addition to shutting off the 

 power, the brake is applied the instant that 

 the two shafts are uncoupled, bringing the 

 mill to a quick stop. 



The efficiency of this apparatus was re- 

 cently demonstrated by a series of tests car- 

 ried out in the plant of the Racine Rubber 

 Co., of Racine, Wisconsin. There is installed 

 in this plant a 200 horse power, 590 r. p. m. 

 motor geared to a line shaft, from which are 

 operated three rubber mills having 20 and 22- 

 inch rolls, 60 inches long. The clutch is in- 

 stalled between the motor and the mill gears. 

 It is 30 inches in diameter, built in combina- 

 tion with a 33-inch solenoid operated band 

 brake. The herringbone pinion mounted on 

 the shaft to which the motor may be coupled 

 or uncoupled by means of the magnetic 

 clutch, has 21 teeth, lj4 d. p., and 14- 

 inch face. The gear driven by this pinion has 126 teeth, and 

 weighs approximately 3,500 pounds. From the gear ratios speci- 

 fied it will be seen that the speed of the mill shaft is approxi- 

 mately 100 r. p. m. The three mills driven from this shaft 

 are each operated through machine molded cast gears, the pinion 

 having 16 teeth, and the gear on the drive roll 7S teeth. The 

 speed of the drive roll is, therefore, 21 r. p. m. The gear ratio 

 between the drive roll and front roll is 17 to 27 — the friction 

 ratio being, consequently, 1.45 to 1. From the foregoing data 



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