The present invention refers to a method for preloading a bolt, which extends with clearance through a through-hole in a first element, adapted to be clamped against a second element situated therebehind, by means of a nut arranged on a threaded end portion of the bolt projecting outside the first element.

More in detail this invention refers to such a method wherein a hydraulic power generating means is detachably connected to said end portion of the bolt, the power generating means is activated for applying a tensile force on said end portion of the bolt and generating in the bolt a tensile stress proportional to this tensile force, the nut is tightened for securing the tensile stress generated in the bolt and the hydraulic power generating means is thereupon deactivated and removed.

A method of this kind is earlier known through SE-B 470 127. At this known method the hydraulic power generating means during activation thereof is supported against a bridging member, which in turn is supported resting on the end surfaces of at least two other bolts situated on opposed sides of the bolt to be preloaded.

In spite of the advantages provided by this known method in comparison to other earlier known methods for preloading of bolts, it however has certain drawbacks. Primarily the above mentioned, known method can not be utilized for preloading bolts, which are positioned at the ends of rows of bolts, as in such cases it is not possible to provide a sufficient support for the bridging member. Furthermore the above mentioned method can neither be used in these cases, where the lateral space about the projecting end portion of the bolt to be preloaded is heavily limited.

The purpose of the invention is to provide an improved method of the type first mentioned, which can be used also in the case mentioned above. In this context it can be mentioned that the method according to the invention primarily is intended to be used for preloading so called casing bolts at large steam turbines and gas turbines. The invention however is not limited to this specific field of use.

The method proposed by the invention is characterized primarily in that the hydraulic power generating means during activation thereof is supported resting against said first element under intermediation of a peripheral portion of an annular pressure washer positioned between the nut and this element and extending laterally outside the nut.

The utilization of a pressure washer positioned between the nut and the first element proposed in accordance with this invention for supporting the hydraulic power generating means entails that the bridging member used at the known method, resting on adjacent bolts, may be eliminated and that the lateral dimensions for the equipment required for accomplishing the method can be substantially reduced.

In order to reduce the lateral space required for said equipment to a minimum it is appropriate to use a hydraulic power generating means, the outer lateral measure of which at least not significantly exceeds the outer diameter of the pressure washer.

In this context it can be mentioned that the peripheral supporting portion of the pressure washer serving for supporting the hydraulic power generating means, may have a small extension in radial direction without this causing a risk that the force applied on the pressure washer by means of the hydraulic power generating means shall give raise to inadmissably high stresses on the first element situated

behind the pressure washer, and which generally consists of casting. Said force will namely be distributed by the pressure washer along the entire surface engaging said element.

The pressure washer is furthermore preferably positioned resting against the bottom of a recess made about the through hole of the first element, which recess advantageously can have a planar bottom surface produced by machining.

In order to make possible a short axial length of the threaded end portion of the bolt projecting outside the first element, the hydraulic power generating means can suitably be detachably connected to said end portion of the bolt by means of a connecting member, which can be screwed into a threaded axial hole in the bolt.

The invention also refers to a device for preloading of a bolt. Different characterizing features for this device are defined in claims 5 - 10.

Hereinafter the invention will be further described with reference to the accompanying drawings, in which

Fig. 1 shows a partial view in longitudinal section through a bolt and an element which can be clamped thereto by means of a nut provided on a threaded end portion thereof and a device for preloading the bolt according to a first embodiment of the invention,

Fig. 2 shows a corresponding view of the bolt, the nut and said element after completed preloading of the bolt, and

Fig.s 3 and 4 show views corresponding to Fig.s 1 and 2, illustrating a tensile preloading device according to a second embodiment of the invention.

In Fig. 1 reference numeral 10 refers to a bolt, which extends with clerance through a through hole 11 in a first element 12, which can be clamped against a second, not shown, element situated therebehind, by means of a nut 14 provided on a threaded end portion 13 of the bolt 10, which projects outside the element 12. At its opposite end, not shown in the drawing, the bolt 10 may be anchored to said second element in an arbitrary manner. It might for instance also at this end have a projecting threaded end portion with a nut provided thereon. The element 12 can, e.g. consist of a casing of a gas or steam turbine, which is intended to be clamped against an element situated therebehind by means of a large number of parallel bolts positioned at short distance from each other.

Fig. 1 furthermore shows a device for effecting a remaining tensile preload in the bolt 10. This device incorporates a hydraulic power generating menas, generally designated 15, and which comprises a generally cylindrical outer member 16, consisting of two parts 16' and 16", and a generally cylindrical inner member 17, arranged to be restrictedly displaceable in axial direction in the outer element 16, and consisting of two parts 17' and 17". The hydraulic power generating means 15 has two pressure chambers 18' and 18", each one being limited on one hand by an annular surface 19 ' and 19" resp. on the outer element 16, and on the other hand by an axially opposed annular surface 20' and 20" resp. on the inner element 17.

Each pressure chamber 18' and 18" resp. is sealed off by means of two sealing rings 21 provided in annular grooves in the elements 16 and 17. Each pressure chamber 18' and 18" resp., has a duct 22' and 22" opening therein, and through which pressurized hydraulic fluid can be fed into the pressure chamber.

The inner element 17 is detachably connected to the bolt 10 by means of a rod-shaped connecting piece 23, which at one of its ends has a tapering, threaded portion 24, which is screwed into a threaded tapering hole 25 in the end portion 13 of the bolt 10, and which at its other end has a cylindrical, threaded portion 26, on which the two internally threaded parts 17' and 17" of the element 17 are mounted. The outer element 16 rests against one end of a sleeve 27, which at its other end rests against a peripheral supporting portion 28' of a thrust washer 28 projecting laterally outside the nut 14 and being positioned between the nut 14 and the element 12.

The above described design and assembly of the hydraulic power means 15 implies that this, when it is activated by a simultaneous supply of hydraulic fluid under pressure to the two pressure chambers 18' and 18", will apply a tensile force on the end portion 13 of the bolt 10. In the bolt 10 this tensile force gives raise to a tensile stress proportional to the tensile force. By tightening the nut 14 this tensile stress can be secured as a remaining tensil preload in the bolt 10. The power generating means thereafter can be deactivated and removed. After removal also of the the sleeve 27 a cover 29 can be attached to the free end of the nut 14 and the bolt 10 by means of a set screw 30 in a manner shown in Fig. 2.

Such as can be seen from Fig.s 1 and 2 the element 12 is equipped with a recess 31 provided around the hole 11, and against the bottom of which the thrust washer 28 can rest. From Fig. 1 is seen that the hydraulic power generating means 15 and the sleeve 27 have the same outer diameter as the thrust washer 28. This means that the device utilized for preloading the bolt 10 in its entirety does not require any space laterally outside the recess 31.

In ordet to make possible a tightening of the nut 14 during the time the sleeve 27 is enclosing it, the nut near its outer end is provided with a plurality of radial holes 33 distributed along its circumference, whereas the sleeve 27 in its wall is provided with a through slot 34 extending over a substantial angle, e.g. 45° in the circumferential direction of the sleeve. Hereby the nut 14 may be operated by means of a proper tool inserted through the slot 34 and engaging in a hole 33 situated there inside.

The embodiment according to Fig.s 3 and 4 differs from that shown in Fig. 1 and 2 substantially therein that the thrust washer 28 is provided with a cylindrical sleeve 35 made in one piece therewith and projecting from the circumferential support portion 28', which substitutes the sleeve 27 of the first embodiment. There are furthermore an additional element 36 positioned between the sleeve 35 and the outer element 17 of the power generating means 15, and which is provided with a slot 37, which substitutes the slot 34 at the embodiment according to Fig. 1. The nut 14 is furthermore provided with an extension member 39 detachably connected thereto by means of a number of pins 38, and which has a series of holes 40, which substitutes the holes 33 at the embodiment according to Fig. 1.

The invention is not limited to the embodiments described and shown in the drawings. Several other embodiments are instead possible within the scope of the inventive idea. As an example it can be mentioned that the connecting piece 23 and the bolt 10 must not be designed for mutual thread engagement but can have connecting members of other types, e.g. bayonet coupling members, by means of which they may be detachably interconnected. Furthermore the closer design of the hydraulic power generating means can be modified in a number of different ways.