a crosspiece 4 having a contoured area 9 for attachment thereof to a rod not shown between flanges 1 and 2 a central shaft 5 is mounted on which a spool 6 is supported for rotation a washer 7 is moulded in flange 1 and contains a hole provided with two flats the dimensions of which correspond with two matching flats 8 provided on the end of shaft 5 the shaft is thus immobilized from rotation and is also immobilized axially by a nut 10 screwed on a threaded terminal part 11 progressing from left to right on the drawing after washer 7 there is threaded on shaft 5 a first ball bearing generally indicated by the numeral 12 one race of which is integral with a first brake plate 13 a first compression spring 14 is positioned between brake plate 13 and a thrust washer 15 abutting spool 6 two bearings 16 support spool 6 on shaft 5 and a second thrust washer 17 abuts the opposite side of spool 6 for a second compression spring 18 the latter bears on one side of a pinion 19 integral with a second brake plate 20 brake plates 13 and 20 each include an inwardly facing friction gasket 24 and 25 arranged to engage the lateral faces 26 and 27 of spool 6 the first spring 14 constantly tends to separate face 26 axially from gasket 24 similarly the second spring 18 constantly tends to separate face 27 axially from gasket 25 pinion.