WO1999050029A1

WO1999050029A1 - Hydraulically operated power wrench and method for controlling the same

Info

Publication number: WO1999050029A1
Application number: PCT/EP1999/002026
Authority: WO
Inventors: Jörg Hohmann; Frank Hohmann
Original assignee: Hohmann Joerg; Frank Hohmann
Priority date: 1998-03-28
Filing date: 1999-03-25
Publication date: 1999-10-07
Also published as: DE19813900A1

Abstract

The invention relates to a hydraulically operated power wrench and to a method for controlling the same. Said power wrench comprises a lever which can be turned by means of a piston-cylinder unit, with a ratchet arrangement for progressively turning a ratchet sleeve; a drive unit with a motor-driven volumetric pump, an input electronics unit for inputting a desired prestressing force, the size of the screw, the thread pitch and the grip of bolt, an output electronics unit for determining a desired screw gripping-angle from the inputted values, a detecting device for determining the actual screw-gripping angle based on the course of travel of the piston-cylinder unit starting from the joining point and a control and cut-off device for stopping the movement of the piston-cylinder unit when the desired screw-gripping angle is reached.

Description

"Hydraulically operated power wrench and method for its control"

The invention relates to a hydraulically operated power wrench and a method for its control with a lever pivotable by a piston-cylinder unit with a ratchet arrangement for gradually rotating a ratchet bushing and a drive unit with a motor-driven, volumetric pump

Such a power wrench is described, for example, in DE 34 16 879 C2. In this power wrench there is a predetermined ratio between the piston path of the piston-cylinder unit and thus the swivel angle of the ratchet bushing and the volume flow of the volumetric pump, which is derived from the formula

V _D

H ARD

where H is the stroke of the piston-cylinder unit, ND is the volume of the oil introduced into the cylinder and AKD is the piston area of the piston of the piston-cylinder unit

In the case of a drive unit consisting of a motor-driven, volumetric pump, in particular a piston pump, the volume flow of the pump results from the rotational speed of the drive motor of the pump and the pump volume delivered per revolution corresponds to a specific stroke distance H of the piston-cylinder unit

In the previously known power screwdrivers of the type mentioned, a control of the tightening torque, which results directly from the pressure acting on the piston-cylinder unit and the lever arm of the pivotable lever, was carried out in such a way that the pressure of the drive unit on one Target tightening torque corresponding target pressure was set so that an adjustable pressure valve opened when the target pressure and thus the tightening torque was reached and the stroke movement of the piston-cylinder unit came to a standstill With this known control, however, the known disadvantages of the Tightening connected via the torque of a power screwdriver, which is known to be strongly dependent on external circumstances, such as thread friction, friction with respect to a contact surface and deformation of the components to be joined by means of the screw connection, and the like, and therefore is not an exact measure for - 2 - is the actual bolt preload

On the basis of Hooke's law, the screw preload actually achieved is directly proportional to the screw rotation angle for a given thread pitch, since a specific screw rotation angle corresponds to a specific screw elongation and thus a specific screw preload

To determine this angle of rotation, it is known in the case of rotationally driven power screwdrivers, for example from DE 43 31 846 AI, to first tighten a screw connection until the components of the screw connection rest on one another only when the various components of this screw connection lie snugly on one another, ie when the joint point is reached, the knowledge is used in the known method that until the joint point is reached there is only a slight increase in the torque applied by the power screwdriver and thus the pressure acting on the power screwdriver, but this torque and thus the pressure from the joint point is steep increases and thus the kink in the torque or pressure curve can be used as the starting point for determining the actual screw clamping angle. In the method described in DE 43 31 846 AI for tightening a screw connection, the joint point is detected by forming the gradient of the tightening torque rd, which has a significant jump at the joint point. An indication of how the actual screw clamping angle is determined in comparison to a predetermined target screw clamping angle is not found in DE 43 38 846 AI

The invention is based on the problem of creating a hydraulically operated power wrench and a method for controlling a hydraulically operated power wrench of the type mentioned at the beginning, by means of which the operation is simplified and operating errors are eliminated as far as possible without special requirements being placed on the specialist knowledge of the operator further damage to the screws to be tightened and to the power wrench are to be avoided, the accuracy with which the screws are tightened is increased and reliable transmission of the measurement and control signals are ensured

Based on this problem, it is proposed in a hydraulically operated power screwdriver with a lever which can be pivoted by a piston-cylinder unit with a ratchet arrangement for the gradual rotation of a ratchet bushing and a drive unit with a motor-driven, volumetric pump, that according to the invention an input electronics for inputting a desired Preload, the screw size, the thread pitch and ^" the clamping length or the screw material, the material quality, the screw size, the thread pitch and the clamping length, one - 3 -

Evaluation electronics for determining a target screw clamping angle from the entered values, a scanning device for the actual screw clamping angle from the stroke of the piston-cylinder unit starting from the joint point and a control and switch-off device for switching off the movement of the piston-cylinder unit when the Target screw clamping angle are provided

The invention is based on the consideration that there is a geometric, unchangeable relationship between the volume flow of the pump and the stroke of the piston-cylinder unit, so that one revolution of the motor-driven, volumetric pump has a very specific stroke of the piston-cylinder unit Since this stroke distance in turn corresponds to a certain angle of rotation of the ratchet bushing, the actual screw clamping angle results directly from the revolutions of the pump by simple conversion. Accordingly, the actual screw clamping angle can be determined from the volume flow of the drive unit through the scanning device

Preferably, the scanning device for determining the actual screw angle can have an actual pressure sensor and, when the screw is tightened, the break points of the actual pressure when the joint point is reached, when the screw rotation is inserted in the intermediate steps and when the piston of the piston-cylinder unit stops Use the interval limits for the volume flow of the drive unit at the end of the stroke, determine the actual screw angle from the volume flow within these interval limits and switch off the movement of the piston-cylinder unit when the set screw angle is reached.

This design of the scanning device takes advantage of the fact that when the ratchet bushing is gradually turned at the pump outlet, a steep rise in pressure begins, which changes with a bend in the pressure curve, which occurs when the screw connection is tightened up to the joint point several steps take place, at the end of each step again a kink in the pressure curve occurs when the piston-cylinder unit hits a stop at the end of the stroke path Another kink point occurs before the end of the stroke path when the joint point is reached because of At this point, the pressure rise is proportional to the bolt preload

As long as the specified target bolt preload, i.e. the target bolt angle determined from it, has not yet been reached, the piston-cylinder unit covers the full stroke distance to the stop during each step, so that the interval between the kink point of the pressure curve at the beginning and at The end of the stroke is a measure for the respective screw clamping angle - 4 - a certain volume flow of the pump is assigned, which in turn results from its speed, there is a clear relationship between the screw clamping angle and the volume flow of the drive unit

If the desired screw clamping angle is reached during the last step when tightening the screw connection, which can be determined in the scanning device by adding up the stroke lengths in the individual steps since the joining point, the movement of the piston-cylinder unit is switched off, for example by switching off the drive unit or by opening a pressure valve in the line connection between the drive unit and the piston-cylinder unit

If an adjustable pressure valve is arranged between the piston-cylinder unit and the pump, which can be set to a set pressure corresponding to the set preload force, and the scanning device determines the actual screw clamping angle when the set pressure is reached from the volume flow of the drive unit , it is possible to have an increase in the pressure at the pressure valve set by the scanning device if the desired screw clamping angle is not reached and to cause the screw to continue to rotate by the piston-cylinder unit until the desired screw clamping angle is reached Before reaching the target screw clamping angle, simply connect it to the interval of the pressure curve, the break point of which results from opening the pressure valve when the set target pressure is reached. Of course, this target pressure must not be set so high that when it is reached Target pressure the target - preload is exceeded

Due to the inaccuracies in the control mentioned above, it is therefore advantageous to set this target pressure somewhat lower than the target pretensioning force in order to actually achieve the specified target screw pretensioning angle and thus the target pretensioning force by a subsequent step the screw is turned by the piston-cylinder unit through a corresponding angle and the movement of the power screwdriver is then switched off

Since the input electronics, the evaluation electronics and the scanning device can preferably consist of a uniform electronic module, it is advantageous to provide an electronic pressure gauge in the line connection between the piston-cylinder unit and the drive unit as the actual pressure transducer, the measurement data of which as electrical measurement variables directly Can be used by the electronic component An electronic pressure meter of this type is required if the drive motor for the volumetric pump is not an electric motor but, for example, an air motor - 5 -

If the drive motor for the volumetric pump is an electric motor, the actual pressure sensor can be designed as a current consumption measuring device, since the current consumption of an electric motor that drives a volumetric pump is directly proportional to the pump pressure

If the drive motor is an AC-powered drive motor, the phase position of which changes with the torque to be applied and thus in proportion to the pressure generated by the volumetric pump, the actual pressure sensor can also be designed as a phase angle measuring device

The scanning device for the actual pressure can preferably have a differentiating device, since the break points of the pressure curve to be recorded result from the differentiation of the actual pressure as an abrupt change in the slope and enable the break points to be determined precisely

To determine the actual screw clamping angle, the relevant volume flow of the drive unit can be calculated from the number of revolutions of the volumetric pump in the interval limits in the scanning device, preferably by the speed of revolution of the volumetric pump and the duration of the intervals of the pressure curve between the break points for calculating the Volume flow can be used

The method according to the invention for controlling a hydraulically operated power wrench of the type mentioned at the outset has the steps of entering a nominal preload force, the screw size, the thread pitch and the clamping length or the screw material, the material quality, the screw size, the thread pitch and the clamping length into an input electronics, Determination of a desired screw clamping angle from the entered values in an electronic evaluation unit, determination of the reaching of the joint point in a scanning device, determination of the actual screw clamping angle from the stroke of the piston-cylinder unit starting from the joint point in the scanning device and switching off the movement of the piston cylinder -Unit when the desired screw clamping angle is reached by means of a control and shutdown device

The scanning device can easily determine the actual screw clamping angle from the volume flow of the drive unit, the scanning device preferably having an actual pressure sensor for determining the actual screw angle and when the screw is tightened the kink points of the actual pressure when the joint point is reached Insert the screw rotation in the intermediate steps and when the piston of the piston-cylinder unit stops at the stroke end as interval limits for the volume flow - 6 -

Drive unit, determines the actual screw clamping angle from the volume flow within these interval limits and switches off the movement of the piston-cylinder unit when the set screw angle is reached. The approach to the set screw clamping angle can advantageously be checked in such a way that between the piston An adjustable pressure valve is arranged in the cylinder unit, which is set to a somewhat lower target pressure than that corresponding to the target pretensioning force. If the scanning device then determines the actual screw tension angle from the volume flow of the drive unit when the set target pressure is reached, the If the target pressure can be exactly determined by differentiating the pressure curve at the break point, the scanning device can calculate a required further rotation angle, increase the pressure setting on the pressure valve accordingly and continue to turn the screws by the piston-cylinder unit until the target Effect the screw clamping angle and then switch off the movement of the piston-cylinder unit

The invention is explained below with reference to an embodiment shown in the drawing

Fig. 1 is a schematic representation of an inventive

Power wrench with drive unit and electronic control unit and

Fig. 2 is a schematic representation of the pressure curve when

Screw clamping

A hydraulically operated power wrench 1 comprises a piston-cylinder unit 2, a pivotable lever 3, which can be acted upon by the piston-cylinder unit 2, with a ratchet arrangement 5 for gradually turning a ratchet bushing 4 Between stops in the piston-cylinder unit 2 a swivel angle α of the lever 3 This corresponds to the full stroke of the piston-cylinder unit. Accordingly, there is a fixed geometric relationship between the stroke H and the swivel angle

The pivoting of the pivot lever 3 for tightening a screw, not shown, is carried out by applying pressure oil to the piston-cylinder unit 2 from a drive unit 6, while the return stroke of the lever 3 is effected by a return spring, not shown. The return stroke takes place as an idle stroke, since the ratchet arrangement 5 releases the rotary connection between the lever 3 and the ratchet bush 4 on the return stroke

The drive unit 6 has two pumps 7, 8 connected in parallel, which act as piston pumps - 7 - are designed with different delivery volumes The pumps 7, 8 are driven by a common drive motor 9, which in the exemplary embodiment shown is designed as an electric motor. A combined switchover and pressure control valve 11 is connected to the pressure side of the pumps 7, 8, the latter Operation will be explained furthermore. A pressure sensor 12 is also arranged in the pressure line to the power wrench 1. Instead of a pressure sensor 12 arranged in the pressure line, the pressure in the pressure line can also be measured by a current consumption measuring device 13 connected to the drive motor 9 or by a drive motor 9 connected phase angle measuring device 14 take place

A tachometer generator 15 can also be connected to the drive motor 9

An electronic control unit 16 controlling the drive unit 6 is connected to an input electronics 17, preferably an input keyboard, and has inputs for the pressure sensor 12 and / or the current consumption measuring device 13 and / or the phase angle measuring device 14 and / or the tachometer generator 15. An output of the electronic ones Control unit 16 is connected to the changeover and pressure control valve 11. The electronic control unit 16 comprises evaluation electronics 18 for determining a desired screw tension angle from the screw values entered into the input electronics 17, namely a desired preload force, the screw size, the thread pitch and the clamping length The electronic control unit 16 comprises a scanning device 19 for the actual screw clamping angle and a control and switch-off device 20 for switching off the movement of the piston-cylinder unit 2 when the desired screw clamping angle is reached. A differentiating device 21 in the elect Ronic control unit 16 serves to represent the break points Ai to As, B] to B3 and C] to C from the pressure curve shown in FIG. 2 when tightening a screw connection as a sudden change in the pressure rise and for evaluation in the scanning device 19 and the control and To provide shutdown device 20

The tightening of a screw connection by means of the screw device shown in FIG. 1 takes place in the following manner with reference to FIG. 2

First of all, the nominal preload force, the screw size, the thread pitch and the clamping length are entered into the input electronics 17 and from there to the evaluation electronics 18 in the electronic control unit 16. The evaluation electronics 18 determines a nominal bolt preload angle from these screwing data, since the SoH preload force corresponds to a certain screw elongation, which in turn is directly related to the thread pitch - 8 - there is a proportional relationship to the nominal screw clamping angle

The target preload is determined by the modulus of elasticity of the screw material, the permissible tensile stress, i.e. the material quality, the shaft cross-section and the screw length and can be specified, for example, on the screw itself or in a table. The evaluation electronics can also determine the target preload, however the screw material and the screw quality are entered in addition to the screw size, the thread pitch and the clamping length

This desired screw tension angle is only measured from the joint point B3, that is to say from the point at which the various components of the screw connection lie snugly. This joint point is designated B3 in FIG

When the desired screw tensioning angle has been determined, which may possibly be displayed together with the entered screw data on a screen (not shown), the motor 9 is started to drive the pumps 7, 8 or a pressure relief valve is switched on when the drive motor 9 is running. and pressure control valve 1 1 closed, so that the pump pressure builds up in the pressure line to the power wrench 1. This pressure initially only rises up to point A] in FIG. 2 and corresponds to the resistances in the lines, the power wrench 1 and in the screw connection during tightening up to joint point B3. FIG. 2 shows that two have been reached until joint point B3 Empty stroke A] -Bι and A2-B2 are required These empty strokes correspond to the full stroke of the piston-cylinder unit 2 up to the stop at the end of the stroke, at which the pressure in the pressure line is steep except for that set in the shutdown and pressure control valve Pressure increases As described below, this pressure can be either a constant pressure pmax or a first pressure pj and then the pressure pmax. Between each stroke, the piston-cylinder unit, controlled by the electronic control unit 16, returns. back to their starting position

If, during the third step in tensioning the screw connection, the joint point B3 is reached, the screw is tightened according to the screw data, whereby, according to the increasing screw preload, the required screw torque and thus the pressure in the pressure line to the piston-cylinder unit 2 increase evenly a step in the end of stroke length H is reached, the pressure rises from the point C] again strongly to pi or ^to pmax

The screwing process is repeated in several steps in the same way, the intervals I] to 16 corresponding to the actual screw clamping angle - 9 -

As already mentioned, the pressure curve can be determined either by means of an electronic pressure sensor 12 arranged in the pressure line or by a current consumption measuring device 13 connected to the drive motor 9 or by a phase angle measuring device 14 connected to the drive motor 9 by differentiating the pressure curve using the differentiating device 21 in In the electronic control unit 16, the break points A] to A ^ Bl to B3 and C] to Cß can be determined very precisely, so that the actual screw clamping angle can be compared at any time with the target screw clamping angle

The actual screw clamping angle results from the intervals I] to Iß in such a way that the pumps 7, 8 designed as piston pumps deliver a very specific hydraulic oil volume for each revolution of the drive motor 9, to which a certain stroke distance H of the piston-cylinder unit 2 depending on the effective piston area

For example, if the drive motor 9 runs at constant speed, it is sufficient to add up the time for the intervals I] to! (, Between the break points B3-C1, A4-C2, A5-C3, A6-C4, Aγ-C5 ₅ A8-C6 , since at constant speed of the drive motor 9 the pumps 7, 8 deliver a constant volume inflow which corresponds exactly to a specific stroke path H to be summed up of the piston-cylinder unit 2 and thus the screw clamping angle. This requires a time switch that only the intervals I] until I added up

If a tachogenerator 13 is connected to the drive motor 9 and pays the revolutions of the drive motor, neither a time switch is necessary, nor does the drive motor have to run at a constant speed, since, as already stated, each revolution of the drive motor 9 has a very specific demand volume of the pumps 7 , 8, which has a corresponding stroke of the piston-cylinder unit 2

The switching off of the movement of the piston-cylinder unit 2 when the desired screw clamping angle is reached can be controlled in different ways. For example, the changeover and pressure control valve 11 is set to a maximum pressure pmax, which is always higher than the pressure which is reached when it is reached of the desired screw clamping angle in the pressure line, it is necessary to send a signal from the control and shutdown device 20 in the electronic control unit 16 to the drive motor 9 or to the changeover and pressure control valve 11, which drives the drive motor 9 when the setpoint is reached - Turns off the screw clamping angle or by opening a pressure relief valve in the changeover and pressure control valve 11 causes a rapid pressure relief in the pressure line to the piston-cylinder unit 2 and thus leads to the lifting movement coming to a standstill that the - 10 -

Intervals I] to I5 correspond to the actual screw clamping angle

Another possibility of bringing the actual screw clamping angle into agreement with the target screw clamping angle is to set a pressure pi in the changeover and pressure control valve 11 which is lower than the pressure which arises when the target screw clamping angle is reached In this case, the stroke movement of the piston-cylinder unit 2 initially ends at the break point C5, since the set pressure p 1 is reached at this point and the changeover and pressure control valve 11 opens to the hydraulic container 10

While the piston-cylinder unit 2 is moving back to the starting position, the pressure control valve 1 1 set to the pressure p], controlled by the electronic control unit 16, is closed, so that a further increase in pressure during the subsequent working stroke of the piston-cylinder Unit is possible. Thus, the screw connection is further rotated by the power wrench 1 until the interval Iß has passed and the actual screw clamping angle now matches the target screw clamping angle, after which, as already described, either the drive motor 9 is switched off or a pressure relief valve in the switchover - And pressure control valve 11 is opened so that the pressure oil required by the pumps 7, 8 flows back into the oil container 10

In the embodiment shown in FIG. 1, two pumps 7, 8 with different delivery volumes are connected in parallel. During the empty stroke Ai-B], A2-B2, both pumps 7, 8 collectively reach the pressure line to the piston-cylinder until the joining point B3 is reached. Unit 2 When the joint point B3 is reached, a pressure relief valve in the changeover and pressure control valve 1 1 opens, causing the volume flow of the pump 8 to flow back into the oil container 10 and only the pump 7 in the pressure line to the piston-cylinder unit 2 prompts In this way, the joint point B3 is reached very quickly without the need to align the entire drive unit 6 with regard to its performance to the high pressure required when tightening the screw connection

By opening the pressure relief valve, the power consumption of the drive motor 9 drops suddenly and rises again when the screw is further tightened, so that this switching point can be taken into account by the current consumption measuring device 13 or by the phase angle measuring device in the scanning device 19 when the actual screw clamping angle is recorded

Of course, the pressure relief valve for the pump 8 can also be set so that a part of the intervals 11 to I5 is still detected by both pumps 7, 8 when one - 1 1 - particularly fast screwing is desired

The invention has the particular advantage that a highly precise tightening of a screw connection to a predeterminable target preload force can be achieved via a target bolt tension angle that can be calculated therefrom, without the actual bolt tension angle having to be measured directly by means of a special angle measuring device, since the The actual screw clamping angle can be determined from the volume flow of the hydraulic drive unit 6 for the power wrench 1 and this volume flow is geometrically predetermined from the dimensions of the pumps 7, 8 and their speed

Claims

- 12 -

P a t e n t a n s r u c h e

Hydraulically operated power wrench (1) with

a lever (3) which can be pivoted by a piston-cylinder unit (2) and has a ratchet arrangement (5) for gradually rotating a ratchet bushing (4),

- a drive unit (6) with a motor-driven volumetric pump (7, 8),

- An input electronics (17) for entering a target preload, the screw size, the thread pitch and the clamping length or the screw material, the material quality, the screw size, the thread pitch and the clamping length,

- evaluation electronics (18) for determining a desired screw clamping angle from the entered values,

- A scanning device (19) for the actual screw angle from the stroke of the piston-cylinder unit (1), starting from the joint point (B3) and

- A control and switch-off device (20) for switching off the movement of the piston-cylinder unit (2) when the desired screw clamping angle is reached

Power screwdriver according to Claim 1, in which the scanning device (19) determines the actual screw clamping angle from the volume flow of the drive unit (6)

Power screwdriver according to Claim 2, in which the scanning device (19) for determining the actual screw angle has an actual pressure sensor (12, 13, 14) and when the screw is tightened the break points (A1-A3, Bi-Bs, C i- Cό) of the actual pressure when reaching the joint point (B3) when inserting the screw rotation in the intermediate steps and when the piston-cylinder unit (1) stops at the stroke end as interval limits (Ii to Iß) for the volume flow of the drive unit (6) , the actual screw angle is determined from the volume flow within these interval limits (Ii to Iß) and the movement of the piston-cylinder unit (1) is switched off when the desired screw clamping angle is reached

Power wrench according to claim 1, 2 or 3, in which an adjustable pressure valve (11) is arranged between the piston-cylinder unit (1) and the pump (7, 8), which can be set to a desired pressure corresponding to the desired biasing force is, the scanning device (19) the actual screw clamping angle when reaching the target pressure from the volume flow of the drive unit (6). determined at - 13 -

If the desired screw clamping angle is not reached, the pressure setting on the pressure valve (1 1) is increased and the screw is further turned by the piston-cylinder unit (1) until the desired screw clamping angle is reached

5 power wrench according to claim 3 or 4, wherein the actual pressure sensor is designed as an electronic pressure gauge (12) in the connection between the piston-cylinder unit (1) and the drive unit (6)

6 power wrench according to claim 3 or 4, wherein the actual pressure sensor is designed as a current consumption measuring device (19) of an electromotive drive unit (6)

7 power wrench according to claim 3 or 4, wherein the actual pressure transducer is designed as a phase angle measuring device (14) of an electromotive drive unit (6) operated with alternating current

8 power wrench according to one of claims 3 to 7, in which the scanning device (19) has a differentiating device (21) for the actual pressure and the break points (A1-A3, B] -B8, Cι-C6) as a sudden change in Slope of the actual pressure curve result from the differentiation of the actual pressure

9. Power screwdriver according to one of claims 3 to 8, in which the volume flow of the drive unit (6) determining the actual screw clamping angle from the number of revolutions of the volumetric pump (7, 8) in the interval limits (II to Iß) in the scanner (19) is calculated

0 power wrench according to claim 9, in which the volume flow in the scanning device (19) is calculated from the rotational speed of the volumetric pump (7, 8) and the duration of the intervals (Ii to Iß)

1 Method for controlling a hydraulically operated power wrench with a lever which can be pivoted by a piston-cylinder unit, with a ratchet arrangement for gradually rotating a ratchet bushing and a drive unit with a motor-driven, volumetric pump with the steps

Entering a nominal preload force, the screw size, the material quality, the thread pitch and the clamping length or the screw material, the material quality, the screw size, the thread pitch and the clamping length in an input electronics,

- Determination of a target screw clamping angle from the entered values in one - 14 -

Evaluation electronics,

- determining the reaching of the joint point in a scanner,

- Determine the actual screw clamping angle from the stroke of the piston-cylinder unit, starting from the joint point in the scanning device and

- Switching off the movement of the piston-cylinder unit (2) when the desired screw clamping angle is reached by means of a control and switch-off device

The method of claim 1 1, wherein the scanning device determines the actual screw angle from the volume flow of the drive unit

Method according to Claim 12, in which the scanning device for determining the actual screw angle has an actual pressure sensor and, when the screw is tightened, the kink points of the actual pressure when the joint point is reached, when the screw rotation is inserted in the intermediate steps and when the piston stops Use the piston-cylinder unit at the stroke end as interval limits for the volume flow of the drive unit, determine the actual screw clamping angle from the volume flow within these interval limits and switch off the movement of the piston-cylinder unit (1) when the desired screw clamping angle is reached

The method of claim 1 1, 12 or 13, in which an adjustable pressure valve is arranged between the piston-cylinder unit and the pump, which is adjustable to a desired pressure corresponding to the desired biasing force, the scanning device for determining the actual -Screw angle determines the actual screw angle from the volume flow of the drive unit when the target pressure is reached; if the target screw angle is not reached, the pressure setting on the pressure valve is increased and the screw continues to be turned by the piston-cylinder unit until the target screw angle is reached

Method according to Claim 13 or 14, in which the scanning device has a differentiating device for the actual pressure and the breakpoints result from the differentiation of the actual pressure as a sudden change in the gradient of the actual pressure curve

Method according to one of claims 13 to 15, in which is calculated from the number of revolutions of the volumetric pump in the interval limits in the scanning device - 15 -

17. The method according to claim 16, wherein the for determining the

Actual screw clamping angle, the decisive volume flow of the drive unit is calculated from the rotational speed of the volumetric pump and the duration of the intervals in the scanning device.