Screwed joint-connections by HYTORC in The Windfair Newsletter

High-strength screwed connections (8.8; 10.9 and 12.9) larger than M16 are the most frequently used connection type on wind energy turbines. Axially stressed bolt connections are used in many cases in designs of wind energy plants. Typical examples are the ring flange joints for steel tube or concrete towers, the gears, the machine-house etc. Due to the high dynamic stress a special significance is assigned to the pre-stressing force for the fatigue strength of the bolts. Using a sufficient preload, these screwed connections should behave like one unit under any operating conditions.

It is imperative that from screw to screw the same preload exists with repeated accuracy. This however, due to a large variety of conditions, can either disperse or not be achieved at all. The result: regular inspections or retightening of the screwed connections. This is particularly hard to achieve on windpower application.
The goal of the design was to reduce the dispersing of the preloads to a minimum level, in order to make the drastic diminution more economical with respect to DIBt guidelines.
Furthermore, a cost-benefit optimization in the entire process of screwed connections on windturbines was aimed for, in order to guarantee their sustainability. At the same time, improvements in job safety were considered for installations with cramped conditions. HYTORC founds different ways to achieve nearly maintenance-free screwed joint connections by using Standard hydraulic or pneumatic field proven Torque-Tools.

1. Load-controlled tightening with a hexagon washer DISC:
The Hytorc DISCTM is a hexagon washer which is arranged under the bolt nut in place of the conventional washer. The DISCTM consists of two components. The external component is a high washer. The internal component is a thread segment. The interior surface of the first component is constructed with longitudinal ribbing. The thread segment is pressed inward from below into the 1st component, so that a plane bearing surface of the DISCTM results.
The ribbing of the thread segment in the process engages in the washer. This guarantees that the thread segment and the bolt do not also turn during the tightening operation. During the tightening a counter-force arises which is preserved/maintained also after completion of the tightening operation.

How does the DISC work ?
The lower seal of the thread segment is formed by a retaining ring. This retaining ring prevents the thread segment from slipping into the first component. In delivery condition both components are pressed together.

For bolt connections in WEA the bolt elongations on the basis of the pre-stressing forces are great, so that DISCTM with large installation heights are predominantly used. The tall embodiment of the DISCTM will be used exclusively within the scope of the development project.

The tightening devices conventionally used require a reaction arm for the application of the torque. Said reaction arm is supported during the tightening operation on the adjacent bolt. The static equilibrium is fulfilled by a horizontal force pair

During the tightening operation the bolt connections receive a planned horizontal lateral load in addition to the torque MA, said planned horizontal lateral load being able to be determined in dependency of the support length and of the torque.

The tightening operation of bolt connections with DISC takes place without a reaction arm. As a result a nearly lateral-load free pre-stressing of the connection should be made possible.

A special drive is used for this purpose. The socket known from conventional tightening operations is encased in the drive and can turn freely within.

The tightening device is connected via the drive to the DISCTM During the tightening operation the bolt nut is turned by the socket. The drive supports itself during the tightening operation on the DISCTM and holds the DISCTM in its position, so that it cannot turn with the bolt nut. Until the snug torque is achieved there is a lateral reversing between the nut and DISCTM. The retaining ring of the thread segment is loosened and the thread segment travels in purely axial direction until it achieves the required tightening torque MA.

The reaction forces for the tightening torque MA are conducted via the DISCTM into the parts to be stressed. The surface condition of the DISCTM leads to a shear engagement between the hexagon washer and the flange

It can be summarized that the use of the DISCTM reduced the dispersions of the achieved pre-stressing forces compared to conventionally used bolt sets. An increased measure of security of construction for bolt sets can be achieved for bolt connections with DISCTM which were tightened at the control pre-stressing force.

The benefits of the DISC are:

- Reaction Point free torque application
- Back-Up Wrench free torque application
- side load free torque application
- high load accuracy
- hand free operation

The Disc:
- operates with Standard Hex Nut
- hard finished nut seating surface
- notched hex corners for inverted operation

Nearly maintenance-free screwed joint connections can be achieved by using the DISC in conjunction with hydraulic or pneumatic torque-tools.


2. Yield-controlled tightening with a SmartPump (VDI-2230):
In the yield-controlled tightening technique, the yield point of the bolt serves as a controlled variable for the assembly preload. Irrespective of the friction at the bearing surface, the bolt is tightened until the yield point or proof stress of the bolt is approximately reached as a result of the combined tensile and torsional stresses. As with angle-controlled tightening, the joint is first of all to be preloaded with a snug torque.
In the yield-controlled tightening, the yield-point of the bolt is recognized by measuring the torque and the angle of rotation during tightening and by determining their difference quotient, which is equivalent to the slope of tangent on torque/angle curve. As soon as plastic deformations occur, the difference quotient drops. This drop to a certain fraction of the maximum value determined beforehand in the linear part of the torque/angle-curve activates a cut-off signal. If the assembly preload increases as a result of lower thread friction, the torsional proportion is correspondingly reduced. A separate design of the bolt for the highest possible assembly preload is therefore not necessary here. The tightening factor αa > 1, which is always present, is therefore ignored when designing the bolt.

Practical Solution
A mobile SmartPump in conjunction with mobile HYTORC hydraulic torque tools
With a newly designed portable hydraulic torque-tool system it is possible to do different types of bolting methods including documentation, like tightening by torque-; angle controlled tightening (also controlled tightening into the plastic range ) and yield-controlled tightening for big screw-connections from M16 to M90. The basic are HYTORC hydraulic torque tools, which have a torque-performance from 150 Nm up to 85.000 Nm. HYTORC hydraulic Torque-Tools have a torque-accuracy of +3%. This type of tool has enough force to tight large screws into the yield-point or into the plastic range. But to use these powerful and strong hydraulic torque tools for yield controlled tightening, a newly designed intelligent hydraulic pump is necessary, for the controlling of this process.

In the yield controlled tightening, the hydraulic-pump recognizes the yield-point. The assembly preload for yield controlled tightening or controlled tightening into the plastic range is always greater / bigger than for torque-controlled tightening. The greater / bigger the assembly preload is, the closer to maintenance-free screwed connections. Why? In the case of yield controlled tightening beyond the elastic limit of a screw, the bolt are utilized up to at least 100% of their respective yield point. The Windpower Industry is using high-strength screw-connections. The plastic elongation, which the bolt undergoes in the yield-controlled process is very small, so that the re-usability of bolts tightened by yield control is scarcely affected. The hardness of the bolting of the threshold torque and the cut-off criterion should be adapted to the joint in question. Large screw-connections or large bolts can be tightened by yield control now. That means also big screw-connections or bolts can be nearly maintenance-free. The Tool System, which can do yield-controlled tightening is portable and can be used in the field.

Summary:
With the hexagon washer DISC or with the yield controlled tightening process with the help of the SmartPump nearly maintenance-free screwed joint-connections can be achieved in the
Windpower-Industry. With both systems, the user is working with Standard hydraulic Torque
Wrenches.