

APPLIED MECHANICS. 



|_KKH TION OF STRAI-S. 



,- table should be employed without any allowance 

 (or lubrication. 



TABLE OF 



The best experiments made to fix these numbers are 

 those by Renue, who tested the materials under a pres- 

 sure of 36 Ibs. per square inch of rubbing surface. For 

 greater pressures the friction is rather less in proportion, 

 but the numbers given are sufficiently near for practical 

 use. In general, it appears that when both surfaces are 

 of the same material, the friction is greater than when 

 they are of different materials. This is believed to be 

 owing to the presence of a certain amount of that cohe- 

 sive force which holds the molecules of any material 

 together ; and it is found to be greater the smoother the 

 surfaces, and therefore the more intimate their contact. 

 Independently of this circumstance, however, it is gene- 

 rally found inexpedient to make the rubbing surfaces in 

 machinery of the same material, especially when there is 

 any risk of their becoming heated by great pressure or 

 rapidity of motion. In such cases, the particles of the 

 oue appear to blend with those of the other ; the surfaces 

 become cut into ridges and hollows, and sometimes co- 

 hesion takes place with such force that the materials 

 themselves give way rather than separate from each other 

 at their surfaces. This result is particularly observe! in 

 case* where iron and iron rub upon each other, especially 

 when the iron is soft. When it is desired that the rub- 

 bing surfaces should be both of iron, it is better to case- 

 harden the rubbing surface*, as is usually done in the 

 case of carriage axled. ' The process of case-hardening is 

 effected by exposing the smooth surface to a red heat 

 for several hours in a furnace, in contact witty substances 

 capable of furnishing it with carbon, and nitrogen,, such 

 as pruniate of potash, leather shavings, and the like. 

 The outer skin of the iron U penetrated by the carbon 

 and nitrogen, and becomes a species of steel, which is 

 rendered very hard by plunging it while still red-hot into, 

 cold water. After hardening, the surfaces have to be 

 carefully ground smooth and true with oil and emery, or 

 kuch-hke polishing substances ; and, before use, the 

 emery must be carefully cleaned off, aa its presence would 

 otherwise cause the surfaces to cut, to become hot, and 

 to cohere. In the case of shafts revolving in bearings, 

 the shafts being generally of iron, are made to rest in 

 bushc* made of brass, gun-metal, tin, or some soft alloy. 

 The rubbing surfaces are made to lit each other accu- 

 rately, being turned, bored, filed or scraped where neces- 

 sary, until the contact is made nearly perfect When the 

 contact is very imperfect, or only takes place at a few 

 points of the surfaoe, these become rapidly worn and cut, 

 and tha heat produced by their wear expands unequally 



other portions of the surfaces, which become abraded in 

 their turn. In cases where great pressure is sustained 

 on a bearing, the surfaces of contact are extended as 

 much as possible, so that the intensity of pressure on any 

 portion of the surface may be aa small as possible. For 

 shaft) lying in cylindrical bearings, the extension of sur- 

 face is best effected by lengthening the bearing, not )>y 

 increasing its diameter. By increasing the length, the 

 surface is proportionally increased ; the pressure per 

 square inch is proportionally diminished ; the amount of 

 friction is not altered, but the tendency to cutting or 

 wear is reduced. But were the diameter increased, 

 although the surface would be also increased, and the 

 wear reduced, yet the friction would act at an increased 

 leverage, and have greater effect as a resisting force. 

 \\ hen the pressure is directed longitudinally along the 

 shaft, the bearing surface is increased by forming on the 

 shaft numerous collars or projecting rings, which are 

 made to press against fixed ring-bearings fitted betw< < n 

 them. The total pressure of the shaft is thus suhdividi d 

 over the combined surface of all the collars, and the ring- 

 bearings being made capable of adjustment by regulating 

 screws, no one of them is unduly pressed upon. Such a 

 form of bearing is particularly useful in steam-vessels 

 fitted with screw-propellers. The propeller, in revolving, 

 tends to throw the water backwards from the vessel, and 

 the reaction of the water is the force which propels the 

 vessel. But this force, sometimes amounting to several 

 tons, is communicated through the shaft of the propeller 

 to some fixed point in the vessel j and at this point is 

 situated the bearing such as we have described, techni- 

 nically called the pushiny or thrust btarimi. 



FRICTION OF STKAl'S. The friction of straps 

 upon pulleys depends upon the extent to which they are 

 tightened, the extent of circumference with which they 

 are in contact, and their breadth. It is commonly 

 believed, that the greater the diameter of pulley, the 

 more surely does the strap cause it to revolve without 

 slipping. Theoretically, however, and we believe prac 

 tioally, it will be found that, with equal degrees of tight- 

 ness, equal breadths of strap, and equal circumstances 

 as to perfection of contact, the friction of a strap on the 

 circumference of a pulley is the same, whatever be its 

 diameter. The only circumstance that can affect the 

 constancy of the result is, that straps not being perfectly 

 flexible, lie more closely to surfaces curved to a large 

 radius than to those of smaller radius. When a certain 

 amount of power has to be communicated through a 

 strap, the speed at which the strap moves has to be 

 taken into account, because power bein.^ pressure multi- 

 plied by velocity, the greater the velocity with which the 

 power is transmitted, the less the pressure that has to be 

 communicated at that speed. In this sense, then, it 

 appears that the larger the pulley the less is the slip of 

 the strap, because the greater the circumference of the 

 pulley revolving at a given angular velocity, the greater 

 U its absolute velocity through space, and therefore the 

 less the pressure required to communicate a given power. 

 It is found, practically, that a leather strap 8 inches wide, 

 embracing tyalf the circumference of a smoothly-turned 

 iron pulley, and travelling at the rate of 100 feet per 

 minute, can, communicate 1 horse-power. For commu- 

 nicating any given power at any given velocity, the 

 breadth of the strap may be found thus : 



Rule. Multiply the power (horse) by 800, and divide 

 by the speed (in feet per minute); the quotient is the 

 breadth of strap in inches. 



Example 1. Required the breadth of strap, travelling 

 600 feet per minute, to communicate 1'2 horse ; 



12x800 

 \N iilth, (j/wj 16 inches. 



Note. When the diameter of pulley (in feet) and the 

 number of revolutions per minute are given, the speed 

 of the strap is found by multiplying the given mini her 

 Dilutions by the diameter (in feet), and by 3f. 

 nnpU2. Required the breadth of strap tor com- 

 municating 10 horse-power to a pulley 3ft. 6 ins. dia- 

 meter, revolving 150 times per minute. 



