3456 lbs. x 40 radius of third wheel F 10 inches radius of barrel at the centre of the chain the chain, or =13824 lbs. strain on 13824 2240 double the power, about 12 tons would be lifted by the 4 men. Frictional Gearing.-The pitch of frictional gearing varies from to 1 inch; the driving power is one-sixth of the interpressure between the wheels. Fig. 28 is a full size section of teeth 1 inch pitch-which is the 6126 tons, and as the snatch block, or running pulley, will inch pitch generally used for hoists-and represents the exact form of tooth found to answer best in practice for this purpose. Thickness of tooth =ths the pitch: width of space between the teeth = 3ths the pitch: depth of tooth ths the pitch: angle of point of tooth = 70°. Rope-Gearing.-Rope driving-gear-consisting of rope-belts working on grooved pulleys-is extensively used for transmitting power from the fly-wheel of engines, &c. ; it is best adapted for driving shafts which run at high and uniform speeds, such as the main shafts of factories. For such purposes its cost is about one-third that of belt-gearing. The ropes are generally made of hemp of 4 inches circumference for small powers, and 5 and 6 inches circumference for large powers. The slack or return side of the rope should be at the top, and the tight or driving side at the bottom of the pulleys. The ropes are never tightened to run straight over the tops of the pulleys, but hang with a good sag between the pulleys. Fig. 29 shows the form of pulley used for rope gearing. The sides of the grooves below the centre of the rope are inclined at an angle of 45°; the distance between the centres of the grooves is equal to from 45 to one-half the circumference of the rope; the distance of the centre of the rope from the top of the pulley, and also from the bottom of the groove is ths of the diameter of the rope. The circumference of the smallest pulley should not be less than thirty times the diameter of the rope. The circumferential velocity of the flywheel may be from 3000 to 5000 feet per minute-but a speed of 4500 feet gives, probably, the best results in practice. The shafts should be from 30 feet to 80 feet apart. The splice of the rope should be about 10 feet long. Weight of Pulleys for Rope-Gearing. The weight of cast-iron rope pulleys-turned and finished-made to the above proportions, may be calculated approximately by the following rule: Multiply the square of the pitch of the grooves in inches by the number of grooves, and by the diameter of the pulley in feet, and divide the last product by one of the following constant numbers, viz., by 13 if the pulley is cast whole; or by 10 if it is split-that is, in halves and bolted together;-and the quotient will be the weight in cwts. WEIGHT OF CAST-IRON ROPE-PULLEYS-TURNED AND FINISHED FOR ROPES 2 INCHES DIAMETER.-Pitch of grooves, 23 inches; the sizes above 8 feet diameter being in halves and bolted together. Horse-power of Rope-Gearing.-To find the number of indicated horse-power transmitted by rope-gearing. Rule: Multiply 8 times the square of the circumference of one rope by the number of ropes, and by the circumferential velocity of the driving pulley in feet per minute; and divide the product by 33,000. Strength of Ropes for Rope-Gearing. The breaking strength of the untarred or white hemp-ropes used for rope-belts, varies from 6,400 lbs. to 1,100 lbs. per square inch; the average breaking strength being 8,700 lbs. per square inch. The working strength is one-sixth of the breaking strength or 1,450 lbs. per square inch. Weight of Rope-Belts.-Rope-belts, 1 inches diameter, weigh about 3 lbs. per yard, and rope-belts, 2 inches diameter, weigh about 4 lbs. per yard, the weight of the rope in lbs. per yard being approximately equal to the square of the diameter of the rope in inches. Friction of Rope-Belts.-The friction of a rope working in a taper groove on a cast-iron pulley is three times greater than that of a rope working on a cast-iron pulley without a groove. The co-efficient of friction for a rope on a cast-iron pulley without a groove being 28; that of a rope working in a taper groove on a cast-iron pulley is 28 × 3 = .84, when the groove is not greased. If the groove be greased the co-efficient of friction is reduced to the extent of one-half. WEIGHT AND HORSE-POWER OF TOOTHED WHEEL GEARING. The weight of cast-iron toothed-wheel gearing may be found. approximately by the following rule. Multiply the number of teeth by the square of the pitch in inches, and by the width of the face in inches; and divide the product by one of the following constant numbers, which will give the weight of the wheel in lbs. 2.2 for spur mortice wheels complete with wood teeth. 2'4 for iron toothed spur wheels. 2.6 for bevel and mitre wheels complete with wood teeth. 2'9 for bevel and mitre iron toothed wheels. The weight of cast-iron spur wheels, cast from a good set of patterns, is given in table 19; the weight of cast iron bevel and mitre wheels, is onesixth less than the weight of cast iron spur wheels. The weight of spur and bevel mortice wheels complete with wood teeth, is one-tenth more than similar wheels of cast iron. The horse-power of an ordinary spur wheel may be found by multiplying the horse-power given in Table 19 by the number of revolutions the wheel will make per minute. The weight of small spur wheels, commonly called change wheels, is given at page 299. Machine-Moulded Wheels vary much in weight, and are usually made unnecessarily heavy,-wheels being sold by weight, many ironfounders make them as heavy as they can—their weight being generally from 25 to 50 per cent. heavier than pattern-moulded wheels. The weight of machinemoulded toothed-wheels may be found approximately by adding 30 per cent. to the weight of the pattern-moulded wheels given in Table 19, this percentage being the average overweight of a large number of machinemoulded wheels. The Weight of Cast-Steel Wheel-Castings may be found by adding 1 lb. for every 12 lbs. weight of similar wheels of cast-iron. The Breaking strain per square inch of section of good ordinary mild-steel castings varies from 28 to 34 tons, with from 10 to 15 per cent. elongation. |