An engine of mass 180 kg is to be supported on four helical springs. When the engine speed is 900 rpm, there is a primary vertical periodic disturbing force of maximum value 300 N due to the unbalanced reciprocating weights. The engine vibrates in the vertical direction with no horizontal or angular movement is assumed.
i) Find the stiffness of each spring to limit the maximum total periodic force on the foundations to 20 N.
ii) What will be the amplitude of vibration of the engine when its speed is 600 rpm ?