330 THE HUMAN MOTOR 



CO-EFFICIENTS / OF SLIDING. 



VALUE OF / 



NATURE OF BODIES IN SLIDING CONTACT. DURING THE 



MOVEMENT. 



Sandstone on sandstone (smooth and dry) 0-71 (Rennie) 



Limestone on limestone (hard and polished) ... 0-58 (Roadelet) 



Granite (smooth) on granite (rough dressed)... 0-66 (Rennie) 



Wooden chest on pavement 0-58 (Regnier) 



beaten earth 0-33 (Hubert) 



Iron on iron or brass, dry 0-18 (Various) 



Brass on brass, dry 0-15 



Iron on ice (skates) 0-02 (Cheuller) 



Metal on metal (with lubrication) 0-10 (Various) 



Leather straps on metals (dry) 0-56 



(greased) 0-23 



The values of these co-efficients are modified by the use of lub- 

 ricants, notably oils and tallow. They increase with the speed. f 1 ) 

 It is so difficult to assure constancy in the state of rubbing 

 surfaces that the concordance of the measurements cannot be 

 satisfactory. Friction is greater at the start than in the course 

 of motion. 



256. In the rolling of cylindrical bodies (wheels, rollers, castors) 

 the value of the friction is much less than in sliding. To overcome 

 it, there must be an effort proportional to the pressure between 

 the bodies and inversely proportional to the radius. It is pre- 

 sumed that this effort acts at the centre (in the case of wheels) 

 i.e., tangentially to the surface. A few values of the co-efficient 

 9, for ordinary wheels have been given ( 39) . For brass rollers 

 on brass or wrought iron, it has been found that ( 2 ) : 



0-0039 0-0044 



9 = ~~7=~ and 7=~i respectively, 



r being the radius in centimetres. 



With steel rollers on brass or on steel : 



0-0045 0-0039 



9 = T=- and ,_ , respectively. 

 Vr Vr 



For wrought iron i oilers on brass : 



0-0075. 



9= vr 



( 1 ) Rennie (Phil. Trans., 1829) ; Morin (Mem. Sav. Etr., vols. v., vi) ; Jenkin 

 (Journ. de Physique 1st Sept. vol. vi.) ; Miiller (Pogg. Ann. vol. 'cxxxix, 

 1870) ; Kimball (Amer. Journ.,1817) ; Bulletin, Technol. des Anc. el Arts 

 et Metiers, 1895, p. 657 ; 1897, p. 1228. 



( 2 ) John Perry : Applied Mechanics. 



