Patent GB-1196491-A (GB)
Title: Concrete Prestressing apparatus and method
Inventor: HOWLETT JAMES W
IPC: E04C5/12
Filed: 1967-09-22
Abstract: 1,196,491. Tensioning rods. HOWLETT MACHINE WORKS. 22 Sept., 1967, No. 43166/67. Heading F2G. [Also in Division E2] Lengths of rod, cable or bar 11, 12, which may be end portions of each of a series of encircling rods adapted to prestress a circular concrete tank (107, Fig. 1, not shown), are connected by tensioning apparatus which includes at least one coupler 17 or 17a which incorporates a one-way gripping device in the form of a housing containing at least one sleeve 34 which is in the form of an incomplete cylinder of springy metal, slit at 37 and whose bore 36 is of an unstressed diameter slightly less than that of the rod &c. which it receives, teeth 35 in the bore 36 thereby gripping the rod at all times with a force which can be overcome readily by a jack 49 acting on a pair of jacking levers 41, 42 engaged with the lengths of rod &c., the periphery of the or each sleeve 34 having a succession of cam faces 39 provided by a helical cam formation on it, corresponding cam surfaces 38 being provided within the housing whereby tension forces in the rod &c. engaged by the sleeve 34 increase the gripping force thereon to provide a one-way gripping action. Preferably, as shown in Figs. 2 and 5, two couplers 17, 26, each provided with a pair of sleeves 34 whose helical cam formations are of opposite hand, are provided in conjunction with rod lengths 11, 12 and a separate bar 16, while the jacking levers 41, 42, consisting of a pair of arms 72, 73 and 74, 76 respectively, are formed with arcuate portions 67 by which they engage collars 23, 24 which are slotted to receive the rod lengths and which bear against the respective coupler housings. When the jack 49, which is preferably hydraulic, is actuated, it operates via a link rod 55 on the lever 42 and directly on the lever 41 to pivot the levers towards one another about pivots 84, 83 on a floating link 43, which has interchangeable end portions 81, 82 to enable its length to be changed, causing the couplers 17, 26 to grip the rod lengths 11, 12 and to slide over the bar 16 until the desired tension has been imparted. Release of the jack will then cause the bar 16 to be securely gripped but the slight movements of the sleeves 34 within the coupler housings will reduce the tension slightly. If the tension is thus too low, it can be increased by re-applying the jack 49 and rotating the bar in the manner of a turnbuckle. One-way gripping nuts 58, 61 are provided on the ends of the link rod 55 so that adjustment of the length of the link rod can be effected, each gripping nut being similar to the coupler 17 or 26 excepting that the housing is comprised of two half shells secured by screws and it contains only one gripping sleeve. In the modification of Fig. 8 one such gripping nut 21a replaces the coupler 26 since the separate bar 16 is replaced by an extension 16a of the rod length 11a. The rod length 12a is also extended and provided with a gripping nut 19a, the collars 23, 24 being adapted to engage the two gripping nuts for application of tension to the rod lengths. In a further simplification (Fig. 9, not shown) a single coupler having a housing which is welded or otherwise secured to one of the rod lengths and which contains only one gripping sleeve is engaged by one of the collars 23, 24, while a gripping nut similar to the nut 19a is engaged by the other collar, this arrangement being usable where operation in the manner of a turnbuckle is not necessary.