METHOD FOR A THREADED JOINT MOUNTING PROCESS

Abstract

A method of obtaining high clamp force accuracy at a threaded joint mounting process by a power tool includes: registering, during a certain interval of the mounting process, tightening and loosening torques applied on the joint, rotational movement of an output shaft relative to a housing, and housing angular displacements about a rotation axis and in relation to an immobile point; calculating output shaft true rotational movements by comparing its rotational movements relative to the housing with the registered housing angular displacements; comparing the applied tightening and loosening torques during the certain interval to determine a clamp force related tightening torque; relating the determined tightening torque to the calculated output shaft rotational movements to determine a clamp force coefficient of the threaded joint; and completing the mounting process by tightening the joint to a target clamp force level by applying a determined clamp force coefficient based torque.

Claims

1-3. (canceled)

4. A method of obtaining high clamp force accuracy at a threaded joint mounting process performed by a hand-held torque delivering power tool comprising a housing and a motor driven output shaft, the output shaft rotatably supported in the housing about a rotation axis (A), the method comprising: registering, during a certain interval (.sub.1.sub.2) of the threaded joint mounting process, a tightening torque applied on the joint, a rotational movement of the output shaft relative to the housing, and occurring angular displacements (X) of the housing about the rotation axis (A) and in relation to an immobile point; registering, during the certain interval (.sub.1.sub.2) of the threaded joint mounting process, a loosening torque applied on the joint, the rotational movement of the output shaft relative to the housing, and occurring angular displacements (X) of the housing about the rotation axis (A) and in relation to an immobile point; calculating true rotational movement of the output shaft relative to an immobile point by comparing the rotational movement of the output shaft relative to the housing with registered occurring angular displacements (X) of the housing about the rotation axis (A) and in relation to an immobile point; comparing the applied tightening torque during the certain interval (.sub.1.sub.2) with the applied loosening torque during the certain interval (.sub.1.sub.2) to thereby determine a clamp force related tightening torque; relating the determined clamp force related tightening torque to the calculated rotational movement of the output shaft relative to an immobile point to determine a clamp force coefficient of the threaded joint; and completing the threaded joint mounting process by tightening the threaded joint to a target clamp force level (F.sub.T) by applying a torque based on the determined clamp force coefficient.

5. The method according to claim 4, wherein the occurring angular displacements (X) of the housing in relation to the immobile point are registered via signals delivered by at least one gyro unit attached to the housing.

6. The method according to claim 4, wherein the occurring angular displacements (X) of the housing in relation to the immobile point are registered via signals delivered by at least one accelerometer unit attached to the housing.

7. The method according to claim 5, wherein the occurring angular displacements (X) of the housing in relation to the immobile point are registered via signals delivered by at least one accelerometer unit attached to the housing.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0017] A preferred embodiment of the invention is described in detail below with reference to the accompanying drawings.

[0018] FIG. 1 shows a diagram illustrating an initial phase of the threaded joint tightening method according to the invention.

[0019] FIG. 2 shows a diagram illustrating a second phase of the method according to the invention.

[0020] FIG. 3 shows a side view of a torque delivering power tool suitable for performing the method according to the invention.

[0021] FIG. 4 shows a top view of the power tool in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The method according to the invention is based on a previously described method, like the one in U.S. Pat. No. 5,571,971, wherein an improved correlation is attempted to be obtained between the measured applied tightening torque and the obtained clamp force in a threaded joint. Basically the method comprises a sequence of threaded joint tightening and loosening movements of the joint wherein the applied torque is measured. The determined difference between the torque applied in the tightening direction T.sub.tight and the torque applied in the loosening direction T.sub.loosen gives information on the magnitude of the friction forces in the joint. This is illustrated by the following formulas:

T.sub.fight=T.sub.friction+T.sub.Clamp

T.sub.loosen=T.sub.frictionT.sub.Clamp

The resultant

T.sub.clamp=(T.sub.tightT.sub.loosen)

[0023] As illustrated by the linear curve 1 in FIG. 1, the threaded joint is initially tightened by the application of a tightening torque up to a certain point 2 defined by an angular position 2, thereby reaching a torque magnitude well below the supposed target torque level T.sub.T in point 8. The applied torque is registered by a torque sensor in the power tool being used. Then a loosening torque is applied on the joint in point 4 resulting in a reverse rotation of the joint during a certain angular interval extending between .sub.2 and .sub.1, illustrated by numeral 3 in the diagram. As illustrated in FIG. 1 and the formulas above the loosening torque is substantially lower than the tightening torque depending on the fact that the clamp force installed in the joint will act in the loosening direction. The friction will be presumed to be the same during a loosening operation and tightening operation but directed opposed to the applied torque during both operations. As indicated in the formulas above the clamp generating torque T.sub.Clamp will act to loosen the joint in both the loosening operation and tightening operation and will hence be added to the friction torque T.sub.friction during a tightening operation, but subtracted from the friction torque T.sub.friction during a loosening operation.

[0024] Hence, by measuring the difference between the tightening torque T.sub.tight and the loosening torque T.sub.loosen the magnitude of the clamp generating torque T.sub.Clamp and the friction force T.sub.friction may be calculated. Typically, the clamp generating torque T.sub.Clamp is as low as about 10% of the friction related torque T.sub.friction. To get redundant measurement of the friction force acting in the threaded joint a tightening torque may be reapplied on the joint in the angular position .sub.1, illustrated by numeral 5. In the exemplary representation in FIG. 1 the linear torque growth during re-tightening between angular positions 5 and 6 takes place on a somewhat lower level than the initial tightening up to point 2. This may happen as result of a certain settling of the joint and in some cases due to a mechanical wear and smoothening of the friction generating surfaces during the first tightening and the following loosening. However, the difference between the upper tightening curve from .sub.1 to .sub.2 between points 1 and 2, and the lower loosening curve from .sub.2 to .sub.1, between points 4 and 3, represents two times the clamp force related torque, and an analysis of the re-tightening between angular positions 5 and 6 is not necessary for the assortment of the clamp force related torque.

[0025] By means of a bolt specific constant C.sub.Clamp dependent of the physical properties of the threaded joint, such as diameter, and thread pitch the clamp force related torque T.sub.clamp can be calculated within the angular interval .sub.1.sub.2. The clamp force F.sub.clamp is proportional to the clamp force related torque T.sub.clamp according to the following relation:

F.sub.clamp=C.sub.clamp*T.sub.Clamp

[0026] As illustrated in the diagram in FIG. 2 the growing clamp force F obtained during tightening and loosening operations between .sub.1 and .sub.2 is represented by a linear curve extending by a certain derivative, and by extrapolating this curve up to the desired clamp force target level F.sub.T a target angular position .sub.T corresponding to the clamp force target level F.sub.T may be determined. The tightening operation may hence be performed to the specific target angular position .sub.T instead of a target torque T.sub.T.

[0027] It is important for the obtained accuracy of the angular position .sub.T that the angular positions .sub.1 and .sub.2 are determined with great precision. As the method is intended to be used for hand held power tools the normal measurement of the rotational movement of the output shaft in relation to the tool housing is not enough, because the reaction torque exerted on the tool housing is manually counteracted it is not possible to foresee the angular displacement of the tool housing with respect to the rotation axis of the threaded joint. Therefore, in order to obtain an as accurate as possible registration of the lapsed rotation angle of the threaded joint during the tightening and loosening operations of this method occurring angular displacements of the tool housing in relation to an immobile point have to be measured and compensated for in the calculating process.

[0028] In FIG. 3 a hand held power tool adapted to perform the method according to the invention is illustrated. The tool comprises a housing 10 with a handle portion 11 for manual support of the tool and a motor driven output shaft 13 rotatable about a rotation axis A and arranged to be connected to a non-illustrated threaded joint to be tightened. The power tool further comprises both a torque meter to register the delivered output torque and an angle sensor for registering the rotational movement of the output shaft 13 relative to the tool housing 10. These devices are of a type common to this type of power tools and are not illustrated in detail. The power tool is connected to a power source via a cable 14.

[0029] In order to measure and register occurring angular displacements of the tool housing 10 about the rotation axis of the output shaft 13 and in relation to an immobile point there is employed a gyro unit 15 attached to the housing 10. By this gyro unit 15 it is possible to register any angular displacement X of the tool housing 10 about the rotation axis A of the output shaft 13 and in relation to an immobile point. The latter could be any fixed point adjacent the threaded joint or in the surrounding environment. The registered angular displacements of the tool housing 10 is calculated from the signals delivered by the gyro unit 15 and subtracted from the rotation angle registered by the angle sensor in the tool housing 10 to get the true lapsed angular movements of the threaded joint in relation to an immobile point.

[0030] A non-illustrated programmable calculating unit may be support on board the tool itself or be a separate unit connected to the tool via the cable 14 or any type of wireless communication.

[0031] It is to be understood that the invention is not limited to the described example but could be varied within the scope of the claims. For instance, the occurring angular displacements X of the tool housing during tightening operations could be measured and registered by other types of sensors like accelerometers perhaps in combination with gyro units.