Torque-Dependant, Releasable Clutch for a Hand-Held Power Tool

Abstract

A torque-dependent, releasable clutch for a hand-held power tool is specified, which includes a first clutch element, which interacts with a second clutch element to transmit a torque, at least one of the two clutch elements being movable with respect to the other clutch element and being pretensioned by a spring element, the spring element having a nonlinear load-displacement characteristic.

Claims

1. A torque-dependent, releasable clutch for a handheld power tool comprising: a first clutch element; a second clutch element that interacts with the first clutch element to transmit a torque, the first clutch element being movable with respect to the second clutch element; and a spring element adapted to pretension the first and the second clutch element, the spring element having a nonlinear load-displacement characteristic.

2. The clutch according to claim 1, wherein the spring element has a progressive load-displacement characteristic.

3. The clutch according to claim 1, wherein the spring element is a pressure spring or a helical spring, in which a diameter of the turns is varied over a length and/or a wire diameter is varied over the length and/or a pitch of turns of the spring element is varied over the length.

4. The clutch according to claim 3, wherein the spring element is a rotationally symmetrical pressure spring having a variable pitch and/or a variable turn diameter.

5. The clutch according to claim 1, wherein the clutch is a friction clutch, a disk clutch, or a cone clutch.

6. The clutch according to claim 1, wherein the clutch is a rolling element friction clutch, in which the first and second clutch elements are coupled to each other via at least one rolling element, which interacts with at least one guide curve on the first and second clutch elements.

7. The clutch according to claim 6, wherein the at least one rolling element is a ball or roller.

8. The clutch according to claim 6, wherein the guide curve is closed in a circumferential direction and has at least one cam element, which interacts with at least one rolling element.

9. The clutch according to claim 1, further comprising a tensioner to set a pretensioning force that acts upon the spring element.

10. A handheld power tool that comprises a clutch according to claim 1.

11. The handheld power tool according to claim 10, wherein the handheld power tool is an electrical handheld power tool or a screwdriver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] 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:

[0041] FIG. 1 shows a greatly simplified side view of a handheld power tool according to the invention in the form of a screwdriver, which comprises a clutch according to the invention;

[0042] FIG. 2 shows an enlarged side view of a torque-dependent, releasable clutch in the form of a ball friction clutch, which has a spring element in the form of a helical spring;

[0043] FIGS. 3-5 show schematic representations, which illustrate the two interacting clutch elements, including a ball which ascends on a cam element, in different phases, FIG. 3 illustrating a closed clutch, FIG. 4 illustrating the start of the opening of the clutch, and FIG. 5 showing the complete release of the clutch;

[0044] FIG. 6 shows a cylindrical spring element having a progressive characteristic with a variable turn pitch;

[0045] FIG. 7 shows a tapered spring element having a progressive characteristic with a invariable turn pitch;

[0046] FIG. 8 shows a tapered spring element having a variable turn pitch and a greatly progressive spring characteristic;

[0047] FIG. 9 shows a diagram which illustrates the increase in torque, expressed as a percentage, for each displacement step in a spring element having a linear displacement characteristic and in a spring element having a nonlinear characteristic;

[0048] FIG. 10 shows the schematic representation of a torque profile in the clutch according to FIG. 2 over the opening displacement at a high spring rate and at a low spring rate;

[0049] FIG. 11 shows a schematic representation of the release torque of the clutch according to FIG. 2 as a function of the spring displacement in the case of a nonlinear spring element having a steadily increasing (progressive) characteristic, as well for a spring element having a steep, linear load-displacement characteristic and for a spring element having a flat, linear load-displacement characteristic; and

[0050] FIG. 12 shows a torque-dependent, releasable clutch.

DETAILED DESCRIPTION

[0051] FIG. 1 shows a greatly simplified side view of a handheld power tool according to the invention in the form of a screwdriver, which is designated as a whole by reference numeral 10.

[0052] Handheld power tool 10 includes a housing 12, which has a pistol-shaped handle 14, on the lower end of which a batter pack 16 is removably provided. A switch 18 in the form of a pistol trigger is provided on handle 14 for turning on the screwdriver. In housing 12, a drive comprising an electric motor and a gearset, for example in the form of a planetary gearset, is indicated by reference numeral 22. Drive 22 activates a torque-dependent, releasable clutch 24, whose output drives a tool holder 20 in the form of a bit holder. The release torque of clutch 24 is settable with the aid of a setting device 27.

[0053] FIG. 2 shows the general structure of a torque-dependent, releasable clutch 24, which may be used in handheld power tool 10.

[0054] Clutch 24 is designed as a ball friction clutch and has a first clutch element 30, which interacts with a second clutch element 32. First clutch element 30 is driven by a drive shaft 28. A flange 39 is provided on output shaft 26, on which a spring element 40 in the form of a helical spring is supported, with the aid of which second clutch element 32 is pretensioned in the direction of first clutch element 30. Second clutch element 32 is accommodated on a shank 29 of output shaft 26 in a rotatably fixed manner yet displaceable in the axial direction against the action of the tension of spring element 40.

[0055] A guide curve 35, running in the circumferential direction and having a plurality of recesses 36 for accommodating rolling elements 34 in the form of balls, is provided on second clutch element 32. In the present case, four rolling elements 34, and consequently also four assigned recesses 36, are provided on guide curve 35. A guide curve 37 running in the circumferential direction is provided on first clutch element 30, a cam element 38 being assigned to each rolling element 34.

[0056] A form-fitting transmission of torque from drive shaft 28 via first clutch element 30 to second clutch element 32 and thus to output shaft 26 normally results with the aid of recesses 36 on second clutch element 32, rolling elements 34 and assigned cam elements 38.

[0057] Second clutch element 32 is tensioned with respect to first clutch element 30 by the pretensioning force of spring element 40.

[0058] A torque-dependent release of clutch 24 is explained in greater detail below on the basis of FIGS. 3 through 5.

[0059] FIG. 3 shows the form-fitting transmission of torque from first clutch element 30 to second clutch element 32. Rolling element 34 in the form of the ball is accommodated within an assigned recessed 36 on guide curve 35 and abuts the edge of an assigned cam element 38 on guide curve 37 of first clutch element 30. A form-fitting torque transmission thus results.

[0060] FIG. 4 shows clutch 24 in a half-open state. This means that the torque has increased in such a way that the pretensioning force of spring element 40 has been overcome, and the rolling element has moved into illustrated location 34. In this case, a rolling friction and a sliding friction are present.

[0061] FIG. 5 shows the fully open clutch. The torque has increased to the extent that rolling element 34 has reached the apex of cam element 38, and the clutch is now open all the way. Rolling element 34 has rolled back into recess 36, since the vertical force component is becoming greater than the horizontal one. W1 shows the maximum opening displacement in the axial direction resulting for clutch 24.

[0062] According to the invention, a spring element 40 having a nonlinear load-displacement characteristic is used, preferably a spring element 40 having a progressive load-displacement characteristic.

[0063] FIGS. 6 through 8 show three variants of such a spring element having a progressive characteristic.

[0064] FIG. 6 shows a spring element 40a in the form of a cylindrical spring having a variable turn pitch.

[0065] FIG. 7 shows a spring element 40b in the form of a tapered spring having an invariable turn pitch.

[0066] FIG. 8 shows a spring element 40c in the form of a tapered spring having a variable turn pitch. A highly progressive characteristic results due to a structural interference between the tapering and variable pitches.

[0067] FIG. 9 shows torque increase M, expressed in percentage, as a function of spring displacement I.

[0068] Characteristic 42 shows the relative change in torque per displacement step for a linear spring element. Curve 44 shows the relative change in torque for each displacement step for a nonlinear spring element having a progressive characteristic.

[0069] I indicates the torque setting range.

[0070] The assigned relative change in torque per displacement step is very large with linear characteristic 42 and a low torque, but becomes small at higher torques.

[0071] With the nonlinear, progressive characteristic, assigned characteristic 44 demonstrates a relative change in torque which is nearly constant for each displacement step over entire torque setting range I.

[0072] FIG. 10 shows the sought-after release behavior of the clutch with the torque profile of the clutch over the opening displacement.

[0073] When using a spring element having a high spring constant and too low a pretensioning force, maximum torque M.sub.max results with an opening displacement >0. Conversely, if a spring element having a low spring constant or a sufficient pretensioning force is used, curve 48 results, release torque M.sub.s being at its maximum with a minimum opening displacement and then continuously decreasing up to the maximum opening displacement of 100%. This case represents the optimum state. The maximum transmitted torque is always reached precisely when the clutch opens, regardless of the pretensioning force selected. With a torque profile as represented in characteristic 46, different coefficients of friction of clutch elements 30, 32 and rolling element 34 may influence the maximum torque.

[0074] Finally, FIG. 11 shows a representation of torque M over opening displacement s of the clutch. Characteristic 50 shows the correlation which results when a spring element according to the invention having a progressively increasing load-displacement characteristic is used. Minimum torque M.sub.min, which results at spring displacement s.sub.1, increases approximately proportionately up to maximum torque M.sub.max, which results at maximum spring displacement s.sub.2. If a spring element having a linear spring constant with a steep characteristic is used, on the other hand, curve 52 would result. If a spring element having a linear spring characteristic is used, which is correspondingly flatter, a significantly lower maximum torque M.sub.max would result at spring displacement s.sub.2 with the same minimum torque M.sub.min.

[0075] Conversely, an inadequate minimum torque would result at s.sub.1, compared to characteristic 52 having a higher linear spring rate.

[0076] FIG. 12 shows an alternative embodiment of a torque-dependent, releasable clutch, which is designated as a whole by 24a. Instead of a ball friction clutch, this clutch 24a is designed as a friction clutch having two interacting, plate-shaped coupling elements 30, 32.

[0077] First coupling element 30, which is driven by drive shaft 26, is pretensioned against second coupling element 32 via a spring element 40a having a progressive spring characteristic. A setting device is used to adjust the pretension, which is indicated only by a disk 27, adjustable in the axial direction, as illustrated by double arrow 56.

[0078] 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