412 



THE HUMAN MOTOR 



that of the return stroke 



h = -55 kg. 



The work done, per complete stroke, is therefore : 

 26 (4-7 -f -55) = 1-365 kgm. 



The rate of working being 54 strokes per minute, the hourly pro- 

 duction of work is : 



1-365 x 54 x 60 = 442-6 kilogrammetres. 

 The weight of metal removed per hour was 45-57 grammes. 



If the muscular efforts of the 

 two arms are calculated, we find 

 that they considerably exceed the 

 useful efforts, owing to the fact 

 that they are exerted at an angle 

 to the plane of the file. Fig. 283 

 shows this in the case of the right 

 arm, and fig. 284 for the left arm. 

 From these figures we can readily 

 see that 



v 



3.2 



= V (3-2) 2 + (2-2) 8:= 3-88 kg. 

 F' := Y/76-8) 2 + (2-5) 2 = 7-24 kg. 



H' 

 FIG. 284. 



The effort required for the return stroke of the file is, as above 

 stated, -55 kg. The total effort is therefore - 



3-88 -f 7-24 + -55 = 11-67 kilogrammes. 



The useful effort H -f H' + h is 4-7 + -55 = 5-25 kg. as shown 

 above. Hence the efficiency is : 



In other words, 55% of the total muscular effort is expended 

 in maintaining a pressure of the file on the work. 



It should also be noted that the efforts of the muscles are con- 

 secutive and not concurrent. This can be demonstrated by 

 using three similar tambours whose styles operate on one cylinder 

 revolving at a very high speed, and arranging that the lengths 

 of the connecting tubes, etc., for each tambour are, as far as 

 possible, of equal dimensions. The results of such an experiment 

 are given in fig. 285. 



It will be seen from these curves that the combined pressure 

 of the hands is the first effort to come into play. The horizontal 

 effort of the left hand commences T n seconds later, and that of 

 the right hand at a further interval of ^ seconds. It follows, 

 therefore that the workman does not commence his stroke for a 

 short time after he has placed the file on the work. 



