In addition , there is a device which detects whether the elevator is descending faster than its maximum designed speed ; if this happens , the device causes bronze brake shoes to clamp down along the vertical rails in the shaft , stopping the elevator quickly , but not so abruptly as to cause injury . In addition , there is a device which detects whether the elevator is descending faster than its maximum designed speed ; if this happens , the device causes copper brake shoes to clamp down along the vertical rails in the shaft , stopping the elevator quickly , but not so abruptly as to cause injury .