A. S. Kimball— Laws of Fric 



Art. XL.— J. new Investigation of one of the Laws of Friction : 

 by A. S. Kimball, Professor of Physics in the Worcester 

 Institute of Industrial Science. 



Reuleaux, in the appendix to bis recently published "Cin- 

 ematics of Machinery," says that ''many engineering schemes 

 have failed because they were designed in accordance with the 

 its given in our text-books as the laws of friction." 

 He furthermore adds, "that it is time that the experiments of 

 Bochet and Hirn should be raised from their place as foot-notes 

 to a position in the text." 



During the last year, I have conducted experiments, on as 

 extensive a scale as our laboratory would allow, for the purpose 

 of settling, if possible, certain contested points in the doctrine 

 of friction. 



Our manuals of mechanics, following Morin and Coulomb, 

 say the coefficient of friction does not. vary with the velocity. 

 Bochet says that it decreases as the velocity in, r, as, s. Hii n says 

 that it increases as the velocity increases. Contradictory as these 

 statements are, it is probable that each contains a partial truth. 

 They need to be combined to make a romp!, t* st itemeut 



The results of my experiments, which this paper is to de- 

 scribe, would indicate that the following is the true law, within 

 the range of my experience. The coefficient of friction at very 

 low vel .<-iti - is stu . it ii reuses u> idly at first, then more 

 gradually as the velocity increases, until at a certain rate, which 

 depends upon the nature of the surfaces in contact and the 

 intensity of the pressure, a maximum coefficient is reached. 

 As the" velocity continues to increase bevond this point, the 

 coefficient decreases. An increase in the intensity of the press- 

 ure (the number of pounds on a square inch), changes the posi- 

 tion of the maximum coefficient, and makes it convsp md to a 

 smaller velocity. The more yielding the materials between 



which the friction -urs tl • hiirher is tin vel -cm ,t which 



the maximum coefficient is found. Heating the rubbing bodies 

 ■mi j < tl •■ .sition of the maximum coefficient to a higher 

 velocity, since l,v heat the h. ii s are ma 1c soft. r. and are caused 

 to yield to pressure-with greater ease F- >r a n .nMderable range 



coefficient is sensibly constant. 



sified as follows . 



-) Sliding friction down an inclined pi; 



Sliding frictio - a a horizontal plai 



of belts on the surface of cast iron pulleys. 



