The invention relates to a method of performing a tightening operation by means of a handheld power wrench with a first and a second angular measuring device. The invention also relates to a power tool adapted to perform said method.

Background

Modern professional power wrenches are provided with a number of sensors and means for detecting and reporting parameters of on-going and performed tightening operations . Parameters that may be detected and reported include the applied torque, the installed angular increment of a bolt, screw or nut of a joint and in some instances the clamp force installed into the joint.

In US 5 476 014 Al a process and device are disclosed in which a gyro is utilised to combine with an angle meter of the output shaft with respect to tool housing in order to calculate an absolute angle of the output shaft. With the process and device known from US 5 476 014 the precision of a tightening operation of a hand held power tool may be improved.

In some operations, there is however a need for a further improved accuracy in order to more closely monitor the quality of a performed tightening operation.

WO 2015/139952 Al discloses a method of obtaining a high clamp force accuracy in a threaded joint mounting process. In this method a gyro or the like is used to monitor the clamp force installed into a joint. Specifically, the gyro is used to monitor the angle of the tool, which is recalculated to a clamp force.

The clamp force is a parameter that is relevant in many tightening operations, but in some operations other parameters may however be more interesting and more easily monitored. Hence, there is a need of a method that provides an improved accuracy for tightening operations, but which offers an alternative to a method in which the operation is governed based on the clamp force installed into a joint.

Summary of the invention

An object of the invention is to provide a method of performing an operation of tightening a joint with an increased accuracy.

Specifically, an object of the invention is to provide an accurate monitoring of the installed angle of a handheld torque delivering power tool without the use of torque arms or other means for

restricting movement of the tool.

In accordance with a first aspect of the invention this object is achieved by means of a method of tightening a joint by means of a handheld torque delivering power tool, the power tool comprising:

an output shaft arranged to rotate about an axis of the output shaft ,

a motor arranged to drive the output shaft,

a housing that houses the motor,

a first angular measuring device for monitoring the angle of rotation of the output shaft about the axis of the output shaft with respect to the housing,

a second angular measuring device arranged to monitor angle of rotation of the housing around the axis of the output shaft relative to the surrounding area, and

a torque monitoring device,

wherein the method comprises the following steps:

monitoring the torque during a tightening operation;

monitoring an angle of the output shaft relative to the

surrounding area by means of a combined monitoring of the angles of rotation of the first and the second angular measuring device;

calculating an instantaneous differential coefficient based on the monitored torque per monitored angle of the output shaft relative to the surrounding area, and interrupting the tightening operation in response to a predetermined decrease of the differential coefficient when the differential coefficient reaches below a predetermined stop gradient.

According to a second aspect the invention relates to a handheld torque delivering power tool comprising:

an output shaft arranged to rotate about an axis of the output shaft ,

a motor arranged to drive the output shaft,

a housing that houses the motor,

a first angular measuring device for monitoring the angle of rotation of the output shaft about the axis of the output shaft with respect to the housing,

a second angular measuring device arranged to monitor angle of rotation of the housing around the axis of the output shaft relative to the surrounding area,

a torque monitoring device, and

a control unit, wherein the control unit is arranged to perform the following steps:

monitoring the torque during a tightening operation;

monitoring an angle of the output shaft relative to the

surrounding area by means of a combined monitoring of the angles of rotation of the first and the second angular measuring devices;

calculating an instantaneous differential coefficient based on the monitored torque per monitored angle of the output shaft relative to the surrounding area, and

providing a command to interrupt the tightening operation in response to a predetermined decrease of the differential coefficient when the differential coefficient reaches below a predetermined stop gradient .

With the above method and power tool according to the first and secon aspect of the invention a yield tightening is performed, which indicates that a plastic deformation of the fastener in the joint is achieved. When performing a tightening operation that results in a controlled plastic deformation of the fastener of the joint the quality of the joint is ascertained.

In a specific embodiment the step of interrupting the tightening operation is preceded by a step of identifying the appearance of a substantially linear torque per angle coefficient and registering a linear gradient that corresponds to the substantially linear torque per angle of rotation, wherein the predetermined stop gradient is set in relation to said linear gradient. This step is in the power tool achieved by the control unit.

In another specific embodiment the tightening operation is interrupted at a point where the instantaneous differential coefficient of the monitored torque per monitored angle of the output shaft relative to the surrounding area has reached below 90% of the linear gradient, or 75% of the linear gradient. This step may also be achieved by the control unit.

Other features and advantages of the invention will be apparent from the figures and from the detailed description of the shown embodiment.

Short description of the drawings

In the following detailed description reference is made to the accompanying drawings, of which:

Fig. 1 is a diagram of the dependency of the torque T with respect to an angle a of an output shaft during a tightening operation;

Fig. 2 shows a power tool in an initial position;

Fig. 3 shows a power tool in an position where both the tool housing and output shaft have rotated with respect to the axis of the output shaft; and

Fig. 4 shows the steps of the inventive method. Detailed description of the shown embodiment of the invention

Fig. 1 shows a diagram of the dependency of the torque T that is installed into a joint with respect to an angle a by which a fastener of the joint is rotated.

For many tightening operations it is a desire to install a controlled clamp force into the joint. This may be achieved by controlling the applied torque, which is proportional to the clamp force. In a specific method, known as the yield method, the joint is tightened until the fastener is exposed to a plastic deformation. In such a method it is important to closely monitor the fastener such that the plastic deformation is produced in a controlled manner. A too

important plastic deformation will adventure the reliability of the joint and risk to rupture the fastener. Therefore, the tightening operation should be controlled so as to interrupt the operation as soon as a plastic deformation has been noted. This is something that demands a high accuracy and that hitherto has not been achievable with a handheld power tool .

Fig. 1 illustrates a curve of the monitored torque T as a function of the monitored angle a; T= F( ) . The curve represents a typical curve of a tightening operation in which the joint is tightened into the yield area, i.e. such that a plastic deformation of the fastener is achieved. As is shown in fig. 1 the torque is constant during a first period in which the fastener is rotated towards the joint, i.e.

without creating any clamp force in the joint. When the fastener reaches the joint a clamp force will be installed into the joint and, simultaneously, the torque will start to increase. During a given period in which the fastener is being elastically deformed the torque T increases linearly with respect to the rotation a of the fastener. This linearity is upheld for as long as the deformation of the fastener is strictly elastic.

As is illustrated in the curve of fig. 1 a linear gradient ki

corresponding to the gradient of the instantaneous differential coefficient dT/d of the torque T per angle a in the substantially linear portion may be calcaluted. The linear gradient ki corresponds to the inclination of the curve and hence ki equals the mean gradient of dT/da in the region from i to a.2, i.e ki = ΔΤ/Δα = (T ₂ -Ti) / ( 2~ai) . When the fastener starts to deform plastically the instantaneous differential coefficient dT/da will decrease. In accordance with the inventive method the tightening operation may be set to be interrupted when the differential coefficient reaches a predetermined stop

gradient k _st o _P . The predetermined stop gradient k _st o _P at which the tightening should be interrupted may be set in relation to a specific fraction of the linear gradient ki in the linear portion, i.e.

k _s top=x*ki, where 0<x<l . It may e.g. be set to correspond to half of said gradient (x=0.5), a quarter of said gradient (x=0.25), three quarters of said gradient (x=0.75) , 90% of said gradient (x=0.9) or any other gradient. The gradient must however be a compromise between the need to interrupt the tightening before the fastener risks to rupture and the need to be sure that the plastic deformation has actually started .

Hence, if the predetermined stop gradient k _s to _P at which the tightening should be interrupted differs too little from the linear gradient ki of the linear portion the tightening operation may be interrupted prematurely due to a natural variation or a tolerance fault. If, on the other hand, the predetermined stop gradient k _s to _P is set too high the risk of rupture of the fastener will increase. The predetermined stop gradient k _stop may be tested by a skilled person, and may

preferably be within 25-90% of the differential coefficient ki in the linear portion. The tightening operation may e.g. be set to be interrupted when the instantaneous differential coefficient dT/da has reached below 75% or 90% of the linear gradient ki . The angle at which the tightening operation is interrupted may be defined as a _s to _P and the corresponding torque at which the tightening operation is interrupted may be defined as T _s to _P , even though the operation is not controlled towards a specific angle nor a specific torque but to the

predetermined stop gradient k _s to _P of the differential coefficient dT/d .

Another feature concerned with a yield tightening is to prepare for the interruption in advance such that the targeted the predetermined stop gradient k _st o _P is met. Specifically, as the invention relates to a hand held tool, special care has to be taken to the movement of the tool .

Figures 2 and 3 show a power tool in two different positions. In figure 2 the power tool is illustrated in an initial position and in figure 3 the power tool is shown in an off-set position in which the output shaft has rotated about the axis of the output shaft with respect to the housing with an angle _a , and in which the housing has rotated around the axis of the output shaft relative to the

surrounding area with an angle c<b with respect to the initial position.

The invention is based on the idea of monitoring both the rotation _a of the output shaft about the axis of the output shaft with respect to the housing, and the rotation oib of the housing around the axis of the output shaft relative to the surrounding area and to deduce a combined angle a = _a + ¾ of the axis of the output shaft with respect to the surrounding area. The combined angle a is hence the sum of _a and (¾ and corresponds to the rotation of a fastener with respect to a joint, the joint being fixed with respect to the surrounding area.

In figure 4, a method in accordance with the invention is illustrated. The method comprises steps 101-108, which correspond to the following actions :

101 -Tightening a joint with a handheld power tool;

102 -Monitoring the torque T delivered by power tool;

103 - Monitoring rotation of output shaft with respect to the tool housing; 104 - Monitoring rotation of tool housing around the axis of the output shaft ;

105 - Calculating a differential coefficient of the monitored torque per monitored angle; 106 -Identifying an installed clamp force, e.g. by identifying the appearance of a substantially linear torque per angle coefficient;

107 - Identifying a predetermined decrease of the torque per angle coefficient ;

Step 106 may be omitted and in such a case step 107 will be defined in an absolute number, i.e. as a specific predetermined decrease of the torque per angle coefficient and not in relation to the substantially linear torque per angle coefficient.

108 - Interrupting the tightening operation in response to the

identification of the predetermined decrease. In order to perform these steps the power tool needs to be furnished with sensors and the like. Specifically, the power tool 10 is provided with an output shaft 11, which is to be connected to and drive the rotation of a fastener, and a handle 12 at which the power tool is to be held by an operator. The power tool may also be provided with a second handle for positioning of the operator's other hand. A motor 13 and preferably a gear (not shown) is arranged to drive the output shaft .

A housing 14 is arranged to house the motor 13, gears and other features, such as a first angular measuring device 15, a second angular measuring 16 device torque monitoring device 17 and a control unit 18.

The first angular measuring device 15 is arranged to monitor the angle of rotation _a of the output shaft 11 about the axis of the output shaft with respect to the housing 14. The second angular measuring device 16 is arranged to monitor the angle of rotation (¾ of the housing 14 around the axis of the output shaft 11 relative to the surrounding area. The torque monitoring device 17 is arranged to monitor the torque T during a tightening operation .

Further, a control unit 18 is arranged in the power tool to control the operation of the tool based on the monitored torque T and angles of rotation _a and

In the control unit 18 an instantaneous differential coefficient dT/da of the monitored torque T per monitored angle a of the output shaft 11 relative to the surrounding area is calculated. The control unit 18 is arranged to instruct the motor 13 to interrupt the tightening

operation in response to a predetermined decreased gradient k _s to _P of the differential coefficient dT/da of the monitored torque T per monitored angle a of the output shaft 11 relative to the surrounding area. The decrease may be defined in absolute numbers or as fraction of the differential coefficient ki in the linear portion of the curve shown in figure 1.

Returning now to figure 1. The angles a _n defined in the diagram are all the sum of the angles a _a and ab provided by the first and second angular measuring device 15 and 16, respectively. Hence ai corresponds to the sum of the individually monitored angles a _a -i and oib-i ·

Accordingly; a ₂ = a _a -2 + a _b -2, and a _s to _P = a _a - _s to _P + a _b -sto _P .

Above, the invention has been described with reference to specific embodiments. The invention is however not limited to these

embodiments. It is obvious to a person skilled in the art that the invention comprises further embodiments within its scope of

protection, which is defined by the following claims.