CH. VII ACCELERATION 159 



motor shaft must therefore be 0'742, or the equivalent of 

 95*5 pounds acting at a distance of 6 inches from the 

 centre of the shaft. 



The conditions of the preceding example are to some 

 extent realised in the case of thoce lifts which are driven 

 by belting. The motor is always running, and when 

 the lift has to be started, the belt is thrown over from 

 a loose onto a fast pulley. In practice, the belt always 

 slips and stretches ; if it did not do so it would be impos- 

 sible to work a lift on this principle, on account of the 

 shock occasioned to the gearing. 



The rotating parts of a motor shaft generally have a 

 moment of inertia sufficiently great to assist materially in 

 starting, without any special alteration in the design. In 

 order to make use of the principle to the extent that the 

 theory indicates as possible, an elastic coupling would 

 have to be contrived that would take up the shock and 

 limit the acceleration of the car. 



The slipping of the belt might be utilised to effect a 

 smooth start without using an elastic coupling. When a 

 belt or coupling slips, the torque transmitted is limited to 

 that at which slipping takes place. If the slipping torque 

 is constant, the acceleration of the main shaft is constant, 

 and so is the retardation of the motor shaft. The condi- 

 tions then are as if a definite load the slipping torque 

 were thrown on the motor. The motor shaft will be 

 retarded, and the main shaft accelerated until the two 

 speeds are equal, and slipping will then cease. The 

 amount of retardation and acceleration will depend 011 the 

 moment of inertia of the two shafts. The slipping of the 

 belt in the belt-driven lift thus assists the start by 



