Patent Application
20210154807
This nonprovisional application claims priority under 35 U.S.C. § 119(a) to German Patent Application No. 20 2019 106 525.7, which was filed in Germany on Nov. 22, 2019, and which is herein incorporated by reference.
The present invention relates to an electrical portable power tool, in particular a cordless screwdriver, with a preferably electric drive accommodated in a housing, which drives a drive shaft, with an output unit which can be positively connected to the drive by means of a mechanical shut-off clutch in order to transfer torque from the drive to an insertable tool connected with the output unit, wherein the mechanical shut-off clutch comprises a control ring which is axially supported against the force of a return spring and at least one switch element guided on a control cam, the control cam being limited at one end by a first switching contour for providing a release torque of the shut-off clutch, and at the other end by a boundary contour.
In screwdrivers, in particular cordless screwdrivers, which are used for industrial series screw fastenings, for example in the production of automobiles, there is a need to reliably form screw fastenings, i.e. at a defined tightening torque, with a very high repeatability and the highest possible screw-in speed. Such a cordless screwdriver is known, for example, from DE 101 24 569 A1. In this case, a mechanical shut-off clutch is often used, which enables a separation of the power transmission between the drive and the output unit when exceeding a predefined tightening torque. This shut-off clutch comprises a control ring which, when the predefined tightening torque is exceeded, is adjusted axially against the force of a return spring, thereby enabling the shut-off clutch, i.e. opening or releasing it. To open the shut-off clutch, therefore, the torque of the drive train must ultimately overcome the force of the return spring. With increasing torque, which is introduced in the shut-off clutch, the introduced torque is thus transferred to the control ring until the force of the return spring, which counteracts the control ring in the axial direction, is less than the force component that results in the axial direction due to the applied torque. The control ring is then axially deflected against the force of the return spring and there is a relative rotation between the first switching contour and the switch element.
This axial displacement of the control ring and, ultimately, the opening of the shut-off clutch is commonly used as a sensor signal to switch off the drive and, if necessary, to brake it and thus bring it to a halt. Various techniques are used here. In particular, a microswitch can be used here for switching, or a magnetic field can also be changed by the axial deflection. In the context of the present invention, it is also provided that during the axial displacement of the control ring, a slider control with a magnet received therein is axially deflected. Using a Hall sensor, the change in the magnetic field induced by the axial deflection of the magnet is able to detect what can be used as a sensor signal for switching off the drive.
After releasing the shut-off clutch, the control ring is axially pushed back in the direction of the drive by the force of the return spring and forces the switch element back to the control cam. The part of the shut-off clutch connected to the drive in a rotationally fixed manner is then rotated further by the existing rotational energy of the drive until the motor comes to a standstill. At a low speed of the drive, the drive is switched off and stopped so rapidly that a renewed release of the shut-off clutch is avoided. To ensure a defined screwing process, after a short time, the drive is also frequently further operated at a very low speed until the first switching contour again bears on the switch element. The shut-off clutch is now in its starting position for the next screw fastening.
However, when the screwing process is carried out at too high a screw-in speed, there is a risk that the rotational energy is so great that, after releasing the shut-off clutch, the braking time is no longer sufficient to bring the motor to a standstill in a timely manner such that the shut-off clutch is actually released only once. In these cases, rather, the shut-off clutch is released at least once more, before the drive is braked and comes to a standstill. The repeated release of the shut-off clutch, however, leads to an increased tightening torque and thus to process instability within the screwing process. In addition, the release of the shut-off clutch is associated with an acoustic effect, which can usually be referred to as a clacking noise. If the shut-off clutch is released repeatedly, this is often irritating for the user since by hearing the clicking sound multiple times, the user assumes that the screw fastening has been performed incorrectly.
It is therefore an object of the present invention to reduce the aforementioned disadvantages, in particular to provide a portable power tool which provides improved process reliability at high screwing speeds.
This object is achieved according to the invention by a portable power tool of the type mentioned above in that the control cam is assigned a second switching contour for providing a pre-torque, which is less than the release torque.
By means of the second switching contour, ultimately a pre-torque is provided, which also leads to axial deflection of the control ring mounted axially against the force of the return spring. This additional axial deflection can now be used to generate a pre-switching signal well before the actual release torque is reached, which ultimately corresponds to the tightening torque, in order to switch off or brake the drive at this early point in time or at least prepare for these steps. In the context of the invention, however, it is alternatively or additionally provided, in addition to the detection of the axial adjustment of the control ring, to also detect a relative rotation of the control ring with respect to the part of the shut-off clutch that is non-rotatably connected to the drive shaft. This relative rotation ultimately occurs when the pre-torque is exceeded and the switch element slides over the second switching contour and then comes to rest on the first switching contour. By means of the second switching contour, therefore, increasing torque can be detected even before the shut-off clutch is actually released, in order to initiate or prepare for the shutdown or braking of the drive.
It has also proven to be advantageous if the first switching contour has a switching point, the tangential slope of which defines the release torque of the shut-off clutch, and if the control cam is assigned a second switching contour with a switching point, the tangential slope of which defines the pre-torque, which is less than the release torque. Since the slope of the tangent at the pre-switching point can determine the release torque or the pre-torque, a simple option is provided for defining the release torque and the pre-torque. In this connection, it has then proven advantageous if the tangential slope in the switching point is greater than the tangential slope in the pre-switching point. If the stroke of the second switching contour is the same as or less than that of the first switching contour, this ensures that when the second switching contour is released, the pre-torque can be clearly distinguished from the release torque of the first switching contour.
It has also proven useful if the ratio between the tangential slope of the switching point and the tangential slope of the pre-switching point is preferably 1.2:1 or greater, preferably 1.5:1 or greater, and particularly preferably 1.8:1 or greater and more preferably 4:1 or less, preferably 3:1 or less and most preferably 2:1. This ensures that the value of the pre-torque is significantly lower than the value of the release torque so as to be able to reliably differentiate between the pre-torque and the release torque.
In addition, it has also proven to be advantageous if the angle between the tangential slope of the switching point and a plane which is oriented perpendicular to the drive shaft is preferably 50° or greater, preferably 55° or greater and particularly preferably 60° or greater and further preferably 80° or less, preferably 75° or less, and particularly preferably 70° or less. If this angle becomes too great, there is the risk that the switch element can only slide with difficulty over the first switching contour when the shut-off clutch is released, so that in particular the release torque increases too much. In this connection, it has then proven to be particularly advantageous when the angle between the tangential slope of the pre-switching point and a plane which is oriented perpendicular to the drive shaft is preferably 20° or greater, preferably 25° or greater, and particular preferably 30° or greater and more preferably 45° or less, preferably 40° or less, and particularly preferably 35° or less. This also ensures that a suitable distinction can be made between the pre-torque and the release torque.
In terms of production technology, it has also been shown to be advantageous if the control cam in the area of the second switching contour has a constant incline. In particular, this can be implemented particularly easily in terms of production technology. In addition, the characteristic increase in torque that occurs as a result of this makes it possible to detect that the switch element is located in the area of the second switching contour, as a result of which the shutdown and/or braking of the drive can then be initiated or executed. Moreover, by detecting and evaluating the temporal duration of the axial deflection of the control ring at a known screw-in speed, conclusions in terms of the torque rate can be drawn. The faster the axial deflection, the harder the screw joint. This ultimately allows for the shutdown or braking behavior of the drive to be controlled as a function of the hardness of the screw joint. Thus, in a hard joint—i.e. with a rise in a short amount of time—early and strong braking can be initiated, while in a soft screw joint—i.e. with a rise in a long amount of time—the braking operation can take place later or be weaker or can be omitted completely. The shutdown or braking of the drive is controlled by electronics in the portable power tool.
However, it has also proven to be favorable if the control cam comprises a first maximum, which defines the release torque, and a second maximum, which defines the pre-torque. By using two maxima, it is ultimately achieved that the deflection of the control ring, which is axially adjustable against the spring force of the return spring by means of the second switching contour, can be significantly shifted in time by the first switching contour up to a point before the deflection. In this context, however, it has also proven particularly useful when the first maximum is greater than the second maximum. This reflects the fact that the switching signal is generated by the deflection of the axially displaceable control ring, i.e. ultimately, the axial deflection of the control ring is to be detected. Due to the different height of the maxima, a distinction can then be made between the pre-torque and the release torque. In addition, the maxima can more easily detect a relative rotation between the adjusting ring and the part of the shut-off clutch that is connected to the drive shaft in a rotationally fixed manner.
It has also proven to be advantageous if the ratio between the height of the first maximum and the height of the second maximum is preferably 1.5:1 or greater, preferably 3:1 or greater and particularly preferably 4:1 or greater and more preferably 10:1 or less, preferably 8:1 or less, and more preferably 6:1 or less and most preferably 5:1. This also ensures that a clear distinction can be made between the pre-torque and the release torque, which ultimately reduces the risk that the axial deflection of the control ring induced by the pre-torque is maintained for the movement induced by the release torque.
In the context of the invention, it has also proven to be particularly advantageous if the amount of the derivation of the profile of the control cam in the area of the first switching contour and the second switching contour is greater than 0. This ultimately provides a continuously increasing control cam between the two switching contours in which, however, both the pre-torque as well as the release torque are in each case delimited by a clearly defined switching contour. In addition, this ensures that after the shut-off clutch is released and the drive is shut down and braked, the switch element cannot permanently move into an intermediate position between the first switching contour and the second switching contour. This would then lead to the fact that due to the intermediate position of the switch element, the second pre-torque could not be used in a further screwing process, which would result in the risk that the subsequent screwing process not being completed in time and the shut-off clutch being released multiple times.
It has also proven useful if the second derivation of the control cam profile in the area of the first switching contour and the second switching contour is greater than or equal to 0. This ultimately leads to a continuous increase in the incline—and thus the torque acting on the switch element.
It has also proven to be advantageous if the switch element and the control cam are provided multiple times, in particular three times. In this way, besides a symmetrical construction of the shut-off clutch, in particular also an improved transfer of torques is achieved.
As it has also been found advantageous if the boundary contour of an adjacent second cam is formed on the first switching contour of a first control cam. This further reduces the production effort in particular. In this context, it has also been shown to be advantageous if the edge of the boundary contour is steeper than the first switching contour. This achieves that, when reversing the direction of the drive—i.e. for dissolving an existing screw fastening—the release torque is greater than the tightening torque of the screw fastening.
It has also been shown to be particularly favorable if the shut-off clutch comprises a cam ring which is connected to the drive shaft in a rotationally fixed manner and on which the control cam is formed. The preferably axially non-displaceable cam ring in particular creates a functional division that prevents mechanical overloading of the individual components. In this context, it has also been shown to be advantageous if the switch element is designed as a switching ball. In the context of the invention, however, it is also provided that the switch element is designed, for example, as a switching cylinder that is guided on the control cam.
It has also proven useful if the return spring is designed as a compression spring whose spring force is adjustable. In particular, by means of the adjustability of the spring force, it can be achieved that various screw fastenings can be performed with the portable power tool.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:
The detail of a longitudinal section through the angle screwdriver 3 displayed in
As can also be seen in particular from
It can be seen from
In a partially sectioned view,
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20 2019 106 525.7 | Nov 2019 | DE | national |
