Abstract:
IN A PROTECTIVE DEVICE FOR A GRINDING MACHINE HAVING A MOTOR DRIVING A GRINDING WHEEL, THE SPEED OF ROTATION OF THE GRINDING WHEEL IS MAINTAINED SUBSTANTIALLY INVERSELY PROPORTIONAL BY A RELAY WHICH IS RESPONSIVE TO THE DIFFERENCE BETWEEN TWO SIGNALS WHICH VARY WITH THE DIAMETER AND SPEED RESPECTIVELY. A SECOND RELAY IS FURNISHED ADDITIONALLY WITH A CONSTANT SIGNAL AND OPENS A SWITCH IN THE CIRCUIT TO THE MOTOR WHEN THE DIFFERENCE BETWEEN THE FIRST TWO SIGNALS IS GREATER THAN THE THIRD SIGNAL. A TIME-DELAY RELAY IS PROVIDED WHICH, WHEN THE MOTOR STARTER BUTTON IS OPERATED, BYPASSES THIS SWITCH FOR A LONG ENOUGH TIME TO ALLOW THE MOTOR TO COME UP TO MINIMUM SPEED.