According to one aspect of {HEAD ~ the invention}, the coiled central section of {TAIL ~ the spring} is made harder than its ends 
According to one aspect of { HEAD ~ the invention } , the coiled central section of { TAIL ~ the spring } is made harder than its ends
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH