lock washer 49 is used above nut 40 cooperating cylindrical cam elements 52 and 54 are fitted within the bore 56 of an enlarged portion 57 of strap 16 around pin 34 the outer cam element 52 is a hollow cylindrical sleeve which carries cam surface 58 the inner cam element 54 has a matching cam surface 60 on its upper edge and has a hexagonal recess 62 in its lower portion to receive the hexagonal portion 36 of pin 34 which passes through a central bore 64 in inner cam element 54 hexagonal recess 62 and hexagonal portion 36 of pin 34 keep inner cam element 54 from rotating when strap portion 16 is pivoted about pin 34 to open the door outer cam element 52 pivots with strap portion 16 and causes cam surface 58 to rotate over cam surface 60 cam surfaces 58 and 60 are shaped to cause outer cam element 52 to rise as strap portion 16 is swung toward its open position i e toward the open position of door 18 and to cause outer cam element 52 to move downwardly as strap portion 16 is swung toward its closed position inwardly projecting internal shoulders 66 above the top of outer cam element 52 and shoulder 67 transfer this lifting effect to strap portion 16 which thereby rises.