US 3210837 A
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Description (OCR text may contain errors)
Oct. 12, 1965 A. HASSELLOF 3,210,837
METHOD OF EFFEC'IING PRE-STRESS Filed Dec. 2, 1963 1N ENTOR 44% 62 55543;
United States Patent 3,210,837 H METHOD OF EFFECTING PRE-STRESS Alke Hassellof, Surahammar, Sweden, assignor to Allmanna Svenska Elcktriska Aktie'bolaget, Vasteras, Sweden, :1 Swedish corporation Filed Dec. 2, 1963, Ser. No. 327,236 Claims priority, application Sweden, Dec. 5, 1962, 13,081/ 62 2 Claims. (Cl. 29404) The present invention relates to a method of effecting pre-stress or increasing the pre-stress in composed constructions and to pre-streissed constructions manufactured or adapted to increased pre-stress in accordance with this method.
In order to effect pre-stressed constructions a pressure means is necessary, which with support against the construction in question is charged with great force, which is thereby transmitted to the construction and compresses its parts so that the desired pre-stress is obtained. The pre-stress is effected thus by tightening or stretching the pressure means at erection until the parts of the construction are compressed with sufiiciently great force. This procedure is suitable in such cases where use is made of simple pressure means such as tension rods, draw bolts or the like. The method is on the other hand detailed and difficult to execute in such cases where other pressure means are used, for example strip or thread casings wound around the construction.
The method according to the present invention is especially suitable for such cases, since the construction may be at first built up without pre-stress or with very little pre-stress. Further, the method should be used to pre-stress or to obtain an increased pre-stress in composed constructions, which are normally subject to an intermittent working load, such as for example press frames, rolling stands and motor frames, whereby according to the invention the construction is loaded with a force being at least equal to the desired pre-stress and adapted to charge the construction in a way statically imitating said normal working load, so that a clearance occurs between at least two of the parts of the composed construction, after which filling pieces are inserted in said clearance.
Especially for such pre-stressed frames which include two substantially semi-circular yokes separated by two pillars and held together by a prestressed thread or strip casing wound around the yokes and pillars, the method according to the invention leads to a considerable simplification of the manufacture. Such frames have namely been manufactured previously by winding a steel strip or the like under heavy drawing pressure in several layers around pillars and yokes but this known procedure necessitates an expensive apparatus and is both detailed and costly. After the winding of the strip casing has been carried out the pre-stress can no longer be regulated, but according to the method according to the invention the pre-stress in such a frame may easily be increased to a higher value if for some reason it is too low.
The invention will be described in more detail in the following with reference to the accompanying figures which show schematically how the method is adapted to a frame of the type described above.
FIGURE 1 shows a frame including two semi-circular yokes 1 and 2 separated by two pillars 3 and 4 and held 3,210,837 Patented Oct. 12, 1965 together by a strip casing 5 wound around yokes and pillars. The frame in FIGURE 1 is erected without any real pre-stress and in order to obtain pre-stress in the frame, which is necessary if the frame shall be able to take up forces without clearance arising between its parts, the frame is charged with two forces P and P parallel with the pillars and working towards the yokes. These forces which are equal and directed in opposite directions are symbolised in FIGURE 2 by two arrows. The forces P and P should be at least equal to the desired pre-stress in the frame, but preferably considerably greater in order that the desired pre-stress can with certainty be attained. When the non-stressed frame according to FIGURE 1 is charged by the above-named forces the strip casing 5 is stretched and a clearance 6 occurs between the upper yoke 1 and the pillars 3 and 4. In these clearances one or several filling pieces 7 are placed, which retain the frame in the extended position according to FIGURE 2 even after the forces P and P have been removed. The pressure in the strip casing 5 then also remains substantially unchanged and the desired pre-stress has thereby been obtained.
The method according to the invention may be adapted to all kinds of prestressed, composed constructions irrespective of which form the pressure means has. The adaptation of the method is thus not limited to the shown type with pressure means in the form of a thread or strip casing but several variations are feasible within the scope of the following claims.
1. Method of effecting a desired degree of pre-stress in a construction comprising a plurality of pillars separating a pair of yokes at opposite ends of a surrounding strip casing, said method including the steps of applying equal forces in opposite directions to said yokes to stretch said casing and give it a stress at least equal to the desidered pre-stress, thereby providing clearances between said yokes and the end of said pillars, then inserting fillers to substantially fill each of said clearances and removing said forces, said fillers thereby retaining the construction in extended pre-stressed condition.
2. Method of producing a pre-stressed frame construction comprising a pair of yokes separated by a plurality of intermediate pillars and a clamping means surrounding said yokes and pillars, said method including the steps of fitting said yokes and pillars loosely in said surrounding clamping means, applying equal forces in opposite directions to said yokes to stretch said clamping means and give it a stress at least equal to the desired final pre-stress, thereby providing clearance between said yokes and the end of said pillars, then inserting fillers to substantially fill each of said clearances, and removing said forces, said fillers thereby retaining the construction in extended pre-stressed condition.
References Cited by the Examiner UNITED STATES PATENTS 1,872,115 8/32 Byerlein 29452 XR 2,539,056 1/51 Brown 29-447 2,938,260 5/60 Holben et al. 29l56.4 3,115,696 12/63 Evans 29149.5
WHITMORE A. WILTZ, Primary Examiner.
THOMAS H. EAGER, Examiner.