EP2213423B1 - Pneumatic striking mechanism - Google Patents

Description

FIELD OF THE INVENTION

The present invention relates to a pneumatic impact device according to the preamble of claim 1, in particular an electrically driven, pneumatic impact mechanism, for a machine tool, in particular a hand tool, e.g. a chisel hammer.

Such a percussion goes for example from the DE 39 10 598 A1 out.

DESCRIPTION OF THE PRIOR ART

An electrically driven chisel hammer with a pneumatic impact mechanism is among others from the EP 1 779 980 A2 known, the schematic representation of the percussion 501 from Fig. 6 is in Fig. 1 accepted.

In a guide tube 530, a flying piston 569 is arranged between an excitation piston 520 and an end piece of a tool 599. The flying piston 569 and the excitation piston 520 close airtight with a wall of the guide tube, so that an airtight closed space 580 between the flying piston 569 and the excitation piston 520 is formed. The space 580 is hereinafter referred to as the pneumatic space 580.

The excitation piston 520, driven by an eccentric drive 522, 523, 531, periodically moves in the guide tube 530 back and forth. Due to its coupling to the excitation piston 520 by means of the pneumatic space 580, the flying mass 569 is likewise excited to a periodic movement between the excitation piston 520 and the end piece of the tool 599.

Fig. 2 shows schematically the course of the movement of exciter piston 520 and free piston 580 over the time t; the course is inter alia in Fig. 13A of EP 1 779 980 A2 shown. The location axis x indicates the distance to the end piece of the tool 599. As the exciter piston 520 moves toward the tool 599 at its highest speed (to low x values), the excitation piston 520 and the flying piston 569 come closest to each other. The pneumatic chamber 569 is strongly compressed and accelerates in succession the flying piston 569 in the direction of the tool 599. The flying piston 569 then strikes unattenuated on the tail of the tool 599 on. Part of the kinetic energy of the flying piston 569 is transferred to the tool. As with a partial elastic shock with a heavy shock partner, the flying mass 569 reverses its direction of motion and moves at reduced speed towards the excitation piston 520. The stroke H of the excitation piston 520, the angular velocity of the exciter piston 520, and the maximum length a of the pneumatic space 580 are matched to one another such that the movement of the flying piston 569, as shown, is excited resonantly by the excitation piston 520.

There is a need to further increase the impact of the chisel hammer without increasing the power consumption of the chisel hammer. The hammering effect of the chisel hammer essentially results from the energy released in a workpiece during a blow. The power consumption results from the product of the energy delivered per beat and the beat frequency of the beats. Consequently, the beat frequency of the beats must be lowered.

The energy released per impact depends on the kinetic energy that 569 receives up to the collision. The acceleration work is done by the exciter pistons 520, which increases with increasing velocity of the exciter piston 520 in the guide tube 530. The speed of the excitation piston 520 is determined by the angular velocity and the stroke H of the exciter piston 520. Although increasing the angular velocity is not suitable due to the thus increasing impact frequency of the impacts, the stroke H of the excitation piston 520 can be increased. However, this requires a greater maximum length a of the pneumatic space 580 and thus a longer impact mechanism to ensure a resonant excitation of the flying piston 569.

In order for the chisel hammer to be ergonomically held by a user during operation, however, the dimensions of the chisel hammer and thus also of the impact mechanism are limited.

The kinetic energy of the air piston 569 can also be achieved by increasing its mass, but then an operator experiences a higher kickback when accelerating the air piston 569 by the exciter piston 520.

DISCLOSURE OF THE INVENTION

One object is to provide a beating machine tool that allows for improved impact performance while taking ergonomic considerations into account. The object is achieved by a pneumatic striking mechanism according to claim 1. The pneumatic striking mechanism comprises: a flying piston which is movable along a striking axis; a striking surface which limits a movement of the flying piston along the striking axis a direction of impact; an excitation piston which limits movement of the flying piston along the striking axis opposite to the direction of impact; a pneumatic space between the air piston and the excitation piston; a drive for periodically moving the excitation piston with a stroke H along the striking axis, whereby the flying piston is excited to a periodic movement between the striking surface and a minimum approach to the exciting piston. An aspect ratio of the maximum length of the pneumatic space to the stroke H is set smaller than 1.55.

The maximum length of the pneumatic space is the distance of the excitation piston to the flying mass when the exciter piston is in its position remote from the tool holder and the flying mass is arranged adjacent to the striking surface. The maximum length serves as a size for laying out and characterizing the impact mechanism. During operation, the pneumatic space usually takes at no time the maximum length.

Circulation of the flying bucket in the striking mechanism is composed of a first phase with a movement from the minimum approach to the exciting piston to the beat and a second phase with a movement from the striking position to the next minimum approach to the exciting piston. The first phase and the second phase are completed together within a period of time dictated by the period of movement of the exciter piston. Due to the deceleration of the flying piston until the momentary standstill, the duration of the second phase increases at the expense of the duration of the first phase. The flying piston manages the distance between minimum approach and the stroke in a shorter time, ergo, as desired, with a higher speed.

The deceleration of the air piston during the second phase takes place when the dimensions of stroke and maximum length of the pneumatic space are suitably selected. At the beginning of the second phase, the pneumatic space is compressed because the excitation piston still moves in the direction of impact after the impact or the flying piston initially moved at a greater speed against the direction of impact than the exciter piston. This results in a pressure increase in the pneumatic space, which slows down the flying mass. The pressure increase is greater, the smaller the volume of the pneumatic space or the greater the remaining stroke movement of the excitation piston in the direction of clubface.

Supported by realized percussion and numerical simulations, it has been found that with typical parameters of mass of the air piston, diameter of the pneumatic space and impact frequency during operation, said ratio 1.55 increases the impact energy due to slow movement of the air piston in the second phase is reached.

In the dependent claims embodiments of the impact mechanism are described.

One embodiment provides that the stroke is selected in dependence on the maximum length of the pneumatic space such that the flying piston changes the direction of movement at least once during the movement between the striking surface and a following minimum approach to the exciter piston. For this purpose, a ratio of less than 1.50 may be advantageous. A change in the direction of movement during the second phase results in a longer path traveled by the flying piston during one revolution. The velocity of the flying piston during the first phase is higher, also taking into account the boundary condition of the given period of time for one revolution.

An embodiment provides that the stroke is selected in dependence on the maximum length of the pneumatic space such that the flying piston touches the striking surface at least twice between two successive minimum approaches to the excitation piston. For this, a ratio of less than 1.40 may be advantageous. The reversal of the direction of movement by the second impact leads to a high velocity of the flying mass at the end of the second phase. The flying piston can therefore approach the exciter piston strongly and experiences a higher acceleration in the direction of the striking surface due to the pneumatic space thereafter.

One embodiment provides that if the mass of the flying mass is greater than 400 g, the aspect ratio is selected smaller than 1.55 and if the mass of the flying piston is less than 400 g, the aspect ratio is selected less than 1.40.

An embodiment provides that when a ratio of the mass of the beatipel to the mass of the flying mass is less than 1.2, the aspect ratio is chosen smaller than 1.40.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1

einen Schnitt durch ein bekanntes Schlagwerk;

Fig. 2

eine Flugbahn eines Flugkolbens in dem bekannten Schlagwerk;

Fig. 3

einen Schnitt einer Ausführungsform einer schlagenden Handwerkzeugmaschine;

Fig. 4

einen Schnitt einer Ausführungsform eines Schlagwerks;

Fig. 5

eine Flugbahn eines Flugkolbens mit bekannten Parametern des Schlagwerks;

Fig. 6

eine Flugbahn des Flugkolbens einer Ausführungsform des Schlagwerks;

Fig. 7 bis 9

weitere Handwerkzeugmaschinen mit Schlagwerken.

The following description explains the invention with reference to exemplary embodiments and figures. In the figures show:

Fig. 1

a section through a known percussion;

Fig. 2

a trajectory of a flying piston in the known percussion;

Fig. 3

a section of an embodiment of a striking hand tool;

Fig. 4

a section of an embodiment of a striking mechanism;

Fig. 5

a trajectory of a flying piston with known parameters of the impact mechanism;

Fig. 6

a trajectory of the flying piston of an embodiment of the striking mechanism;

Fig. 7 to 9

other hand tool machines with striking mechanisms.

Identical or functionally identical elements are indicated by the same reference numerals in the figures, unless stated otherwise.

EMBODIMENTS OF THE INVENTION

Fig. 3 shows schematically as an example of a striking hand tool an electro-pneumatic chisel hammer 1, other examples not shown are, inter alia, rotary hammers, combi hammers.

In a machine housing 2, a drive train with a primary drive 3, a drive shaft 4 and a striking mechanism 5 is arranged. Between the primary drive 3 and the drive shaft 4, a transmission 7 may be connected. The primary drive 3 is preferably an electric motor, for example a universal motor or a brushless motor. The drive shaft 4 is rotated at speeds in the range between 1 Hz and 100 Hz, for example at 10 Hz to 60 Hz. The rotational movement of the drive shaft 4 is through the Schlagwerk 5 transferred in a periodic impact movement along a striking axis 8. A held in a tool holder 9 tool is driven out of the chisel hammer 1 by the periodic beats along the striking axis 8 in the direction of impact 99 out. A return of the tool in the chisel hammer 1 against the direction of impact 99 is effected by pressing the chisel hammer 1 to a workpiece.

Fig. 4 shows an exemplary construction of the striking mechanism. 5

The striking mechanism 5 has an excitation piston 12 and a flying piston 13 which are movable along the striking axis 8. In the illustrated embodiment, the excitation piston 12 and the flying piston are guided through a wall 11 of a guide tube 10.

At a tool-side end of the guide tube 10, a striker 20 is mounted in an anvil guide 21. A tool-facing end 22 is in contact with a tool 8 which is held in the tool holder 9. A tool-facing end 23 of the striker 20 protrudes from the anvil guide 21 in the interior of the guide tube 10. In the beating operation of the striker 20 abuts a tool facing away from the end 24 of the striker 21. In this position, the tool facing away from end 23 of the striker 20 defines the position of the striking surface 27 of the impact mechanism. 5

As indicated, the striker 20 may be provided as an intermediary between the flying mass 13 and a tool 8 in the impact mechanism 5. This allows in particular a design of the impact mechanism 5, which is independent of a mass of the tool 8 used. The striker 20 can be chosen much heavier than the typical mass of the tool 8 for this purpose.

In another embodiment, no striker 20 is provided. The flying mass 13 strikes directly on an end face of the tool 8. The end face forms the striking surface 27 in this case. The tool 8 is engaged in the tool holder 9 as far as possible in the direction of the impact mechanism 5. In this position, the tool 8 defines the clubface.

The exciter piston 12 is forced by the drive shaft 4 to a periodic movement along the striking axis 14. The drive shaft 4 is rotated about its axis of rotation 30 and thereby moves an axis of rotation 30 eccentrically arranged wobble finger 31. The wobble finger 31 is connected via a linkage 32 with the exciter piston 12. A stroke H of the excitation piston 12 is defined as the distance between the two positions, in which the exciter piston 12 of the striking surface 27 is closest to or furthest away. The stroke H of the excitation piston 12 is predetermined by the distance 33 of the wobble finger 31 from the rotation axis 30 and corresponds approximately to twice the crank radius 33 of the wobble finger 31. The movement of the exciter piston 12 is periodic and depending on the design of the eccentric drive 4, the movement is sinusoidal or, to a good approximation, sinusoidal.

The excitation piston 12 and the flying mass 13 define an air-tightly sealed space between them, the pneumatic space 19. A cross-sectional area A of the pneumatic space 19 corresponds approximately to a cross-sectional area of the flying piston 13 and the exciter piston 12. An airtight termination may e.g. be achieved by sealing rings 15, 16. The pneumatic space 19 has a maximum length L when the excitation piston 12 is at the maximum distance to the impact surface 27 and the flying mass 13 is adjacent to the impact surface 27.

A simple model of the trajectory of the flying piston 13 is explained below with reference to a conventional impact mechanism and a striking mechanism 5 according to one embodiment. The model is used to find parameters of the percussion mechanism 5, in which the flying mass 13 is decelerated at least to a standstill between a blow to the striking surface 27 and a next minimum distance to the excitation piston 12 or even changes its direction of movement.

Fig. 5 shows a trajectory 100 of the flying piston 13 for a conventional, long impact mechanism, plotted over the time t. The trajectory 100 is determined by means of an ad-initio simulation. The parameters of the impact mechanism are: beat frequency f = 14.5 Hz; Mass of the striker m ₁ = 2,119 kg; Mass of the flying mass m ₂ = 1,248 kg; Stroke H = 0.094 m; maximum length of the pneumatic space L = 0.204 m; Cross sectional area of the pneumatic space A = 0.0034 m ² ; Beat number q = 0.25. The trajectory 101 of the excitation piston 12 is also shown. Fig. 6 shows a trajectory 200 of the flying piston 13 for a short striking mechanism 5 according to one embodiment. The only one opposite Fig. 5 changed parameter is the maximum length L of the pneumatic space: L = 0.139 m.

The trajectory 100 of the long impact mechanism can be subdivided into two phases 102, 103 limited by reversal points 104, 105 of the trajectory 100. The first turning point 104 results at the minimum distance of the flying piston 13 to the exciting piston 12 second reversal point 105 results from the impact of the flying mass 13 on the striking surface 27.

The trajectory in the region of the first reversing point 104 can be described by a collision of the flying piston 13 on the moving exciter piston 12. The effective mass of the excitation piston 12 is assumed to be infinite, because the exciter piston 12 is rigidly coupled to the drive. Typically for a resonant excitation, the first reversal point 104 coincides with the maximum velocity of the exciter piston 12. The velocity v _{1 of} the flying mass 13 after the first turning point 104 is thus approximately v ₁ = 2π * H * f + v ₃ , where v ₂ denotes the velocity before the first turning point 104.

Der Formfaktor e weist Werte von 0 bis 1 auf; für kurze gedrungene Stosspartner in der Nähe von 1 und für eher länglich aufgebaute Stosspartner in der Nähe von 0. Beispielhafte Werte für den Schlagzahl k liegen im Bereich von 0,05 bis 0,35. Beispielhaft kann die Stosszahl (q) zu 0,22 gewählt sein, wenn ein Verhältnis m ₁/m ₂ der Masse (m₁) des Döppers zu der Masse (m₂) des Flugkolbens (13) grösser als 1,2 ist und andernfalls die Stosszahl (q) zu 0,12 gewählt sein.In the impact of the flying piston 13 with the striker 20 and the tool, the amount of speed v _{2 of} the flying piston 13 after the impact is less than the speed v ₁ before the shock, since a part of the kinetic energy of the flying piston 12 in the striker 20th is transmitted. The ratio (impact number q) of the speeds v ₂ / v ₁ is given by the mass m _{2 of} the flying mass 13, the mass m _{1 of} the striker 20 and a form factor e of the mating partners: $k=e\cdot \frac{m_2-m_1}{m_2+{\mathrm{<figure-callout\; id="1"\; label="m"\; filenames="imgf0003.png,imgf0006.png"\; state="\{\{state\}\}">m</figure-callout>}}_1}\mathrm{,}$

The form factor e has values from 0 to 1; for short squat mating partners near 1 and for more elongated mating partners near 0. Exemplary values for the strike number k are in the range of 0.05 to 0.35. By way of example, the impact number (q) may be selected to be 0.22 if a ratio m ₁ / m _{2 of} the mass (m ₁ ) of the striker to the mass (m ₂ ) of the flying mass (13) is greater than 1.2 and otherwise the number of impacts (q) should be 0.12.

During the first phase 102 and the second phase 103, the volume V of the pneumatic space 19 changes. As a result, the pressure p within the pneumatic space 19 also changes. A force on the flying mass 13 results due to the pressure difference of the surroundings (approx bar) and the pressure p within the pneumatic space 19. The flying mass 13 thus also experiences an acceleration between the two reversal points 104, 105, which increases or decreases its velocity v ₁ , v ₂ .

The pressure p can be estimated by an adiabatic approximation, ^κ at the (p · V) is constant, κ (kappa) the isentropic exponent (approximately 1.4 for air in the prevailing pressure range of 0.5 bar to 10 bar) and V the volume of referred to pneumatic space 19. It is assumed that a neutral volume V ₀ at which a pressure p in the pneumatic space 19 corresponds approximately to the normal pressure p _{0 of} the environment (about 1 bar) corresponds to half the maximum length of the pneumatic space 19, ie if the distance x of the flying piston 13 to the excitation piston 12 x = L / 2.

In the long percussion mechanism, the volume of the pneumatic space 2 in the first and second phases 102, 103 changes only slightly compared to the neutral volume V ₀ . This is partly due to the, compared to the maximum length L, low stroke H. Correspondingly, there are only minimal deviations from the ambient pressure p ₀ and low forces on the flying mass 13. The influence of the pneumatic space 19 on the movement of the flying mass 13 in the long impact mechanism is negligible. The velocity v ₁ remains approximately constant during the first phase 102 and the velocity v ₂ during the second phase 103.

Approximately it is assumed that the flying mass 13 and the exciter piston 12 touch each other at the first turning point 14, at a distance x = L- ½ H from the striking surface 27. Under the boundary condition that within a period, ie the time span f ^-1 , the distance L -½ H is traveled by the flying mass 13 once at the first speed v ₁ and once at the second speed v ₂ , the following results for the first speed: $${\mathrm{<figure-callout\; id="1"\; label="v"\; filenames="imgf0003.png,imgf0006.png"\; state="\{\{state\}\}">v</figure-callout>}}_1=\frac{2\pi \cdot f\cdot H}{1-q}\mathrm{,}$$

In the case of the short percussion mechanism 5, the trajectory 200 likewise has the two reversal points 204, 205, which result from a minimal approach to the exciter piston 13 and a subsequent impact on the striking surface 27.

During the first phase 202, the flying mass 13 moves from the first turning point 204 to the second turning point 205, similarly to a long striking mechanism. The velocity v ₁ is approximately constant and is approximately v ₁ = 2π * H * f + v ₃ , where v _{3 is} the velocity short of the first reversal point 204. For an estimate of the velocity v ₃ = 2 f · ( a -½ H ), it can be assumed that the movement from the face 27 to the first turning point 203 occurs approximately during half a period (½ f ^-1 ).

The second phase 203 of the short impact mechanism 5 differs from the second phase 103 of the long impact mechanism. The velocity of the flying mass 13 is reduced to zero, in the illustrated example the movement of the flying mass 13 even reverses. The driving force for the braking results from the strong coupling of the flying piston 13 to the exciting piston 12 by means of the pneumatic space 19.

Subsequently, parameters of the percussion mechanism 5 are estimated, in which the speed v _{2 of} the flying piston 13 is braked to at least zero after the second reversal point 205.

The braking force results from the overpressure ( pp ₀ ) of the pneumatic space 19 with respect to the environment, which acts on the cross-sectional area A of the pneumatic space 19. Due to the movement of the flying piston 13 in the direction of the excitation piston 12 also reduces the volume V of the pneumatic chamber 19 and correspondingly increases the pressure ( pp ₀ ). The pressure change can be determined based on the adiabatic approximation p * V ^K = p ₀ * V ₀ ^κ .

The deceleration is typically carried out at the latest within a quarter of a period (T = ¼ f ^-1) to the second reversal point 205. During this period of time T, the driving piston 12 moves slowly. A change in the pressure p in the pneumatic chamber 19 is dominated during the period T by the movement of the flying piston 13. After the time period T, the excitation piston 12 reaches a speed which is significantly greater than the speed v _{2 of} the flying piston 13. The relative distance increases rapidly and is soon greater than ½ L , which is why the flying mass 13 is accelerated again in the direction of the exciter piston 12.

During the time period T, the position x1 of the excitation piston 12 is assumed to be approximately constant equal to the minimum possible distance to the striking surface 27 ( x ₁ = LH ). The volume of the pneumatic space V during the period T is given by: V = A ( LHv ₂ * t ) , where the velocity v ₂ for calculating the volume V is assumed to be constant.

Unter Einsetzen der obig beschriebenen Beziehungen und einer Reihenentwicklung nach der Zeit bis zur ersten Ordnung ergibt sich mit T = (N f)^-1: $$\frac{L^{\mathrm{\kappa }}}{2{\left(L-H\right)}^{\mathrm{\kappa }}}\cdot \frac{\mathrm{\kappa }}{L-H}+\left(\frac{L^{\mathrm{\kappa }}}{2{\left(L-H\right)}^{\mathrm{\kappa }}}-1\right)\cdot \frac{1-q}{q}\frac{N}{2\mathrm{\pi }\mathit{H}}\stackrel{!}{\ge }\frac{m_2}{A\cdot p_0}\cdot N^2{f}^2$$

Aus der Ungleichung wird ersichtlich, dass ein Erhöhen der Querschnittsfläche A, des Hubs H und/oder ein Verringern der Masse m₂ des Flugkolbens 13, der maximalen Länge L des pneumatischen Raums 19, der Schlagfrequenz f tendenziell zu einem Schlagwerk 5 führen, bei dem die Bewegung des Flugkolbens 13 bis zum Stillstand abgebremst wird.The flying mass 13 stops when the integral of the decelerating force over the time period T corresponds to the momentum of the flying mass 13, ie v ₂ · m ₂ , after the second turning point 204: $$v_2\cdot m_2\stackrel{!}{<}{\displaystyle {\int }_0^T}A\cdot p_0\cdot \left[{\left(V_0/V\right)}^{\mathrm{\kappa }}-1\right]\mathit{dt}\mathrm{,}$$

Substituting the relationships described above and a series expansion to time to first order, T = ( N f ) ^-1 : $$\frac{L^{\mathrm{\kappa }}}{2{\left(L-H\right)}^{\mathrm{\kappa }}}\cdot \frac{\mathrm{\kappa }}{L-H}+\left(\frac{L^{\mathrm{\kappa }}}{2{\left(L-H\right)}^{\mathrm{\kappa }}}-1\right)\cdot \frac{1-q}{q}\frac{N}{2\mathrm{\pi }\mathit{H}}\stackrel{!}{\ge }\frac{m_2}{A\cdot p_0}\cdot N^2{f}^2$$

It can be seen from the inequality that increasing the cross-sectional area A, stroke H and / or decreasing the mass m _{2 of} the flying mass 13, the maximum length L of the pneumatic space 19, the beat frequency f tends to result in a percussion mechanism 5 in which the movement of the flying piston 13 is braked to a standstill.

The parameter N is preferably greater than 4, based on the described assumption that deceleration occurs within a quarter period T = ¼ f ^-1 .

In the introduction it is stated that a choice of the beat frequency f and the mass m _{2 of} the flying piston 13 are imposed by narrow limits. The cross-sectional area A of the pneumatic space 19 is closely coupled with the shape and impact characteristics of the flying piston 13. However, the outer boundary conditions may allow a largely free choice of the maximum length L of the pneumatic chamber 19 and the stroke H of the exciter piston 13.

For heavy impact mechanisms 5 with a flying mass 13 of mass m ₂ greater than 400 g of their otherwise typical parameters, such as a large number of impacts (q> 0.2), for example, a choice of the ratio of the maximum length L to the stroke H of: L / H <1.55; and for light impactors 5 of mass m ₂ less than 400 g, a choice of the ratio of: L / H <1.40.

The striking mechanism 5 is preferably operated in such a resonant manner that the first turning point 204 and the highest speed of the exciting piston 12 coincide, ie a difference between the respective times is less than 2% of the period ( T = f ^-1 ).

In resonant mode, based on studies on simulations and prototypes, it is assumed that a complete deceleration within a period of time T ₀ = ⅜ f ^-1 after the first reversal point 204 takes place. After the time period T ₀ , the speed of the exciter piston increases to 70% of its maximum value, whereby a rapid reduction of the braking overpressure to an accelerating negative pressure takes place.

The flying mass 12 takes about a period of time from ⅛ f ^-1 to ¼ f ^-1 for its movement to the striking surface 27. The braking can take place within a period of time from ⅛ f ^-1 to ¼ f ^-1 , for which reason N is at least 4, preferably 6 or 8. For a resonant operation, the parameters of the impact mechanism 5 can be determined according to the above inequality with the selected N.

In a further embodiment, the parameters of the percussion mechanism 5 are selected such that the flying mass 13 in the percussion mechanism 5 after the second reversal point 205 again touches the striking surface 27 (point 206) before the flying mass 13 flies up to the first reversal point 204. The extension of the trajectory of the flying piston 13 allows a higher speed while maintaining the beat frequency f.

In order for the flying mass 13 to return to the striking surface 27, the braking must take place to a standstill early. Thereafter, a positive pressure in the pneumatic chamber 19 must prevail for a sufficiently long period of time in order to accelerate the flying piston in the direction of the impact surface 27. Investigations have shown that this is achieved at a time T ₀ less than 2/6 f ^-1 . The speed of the exciter piston 12 reaches within the time period T ₀ only 50% of its maximum speed. The impact mechanism 5 can be designed according to the above inequality, wherein N is greater than 5, preferably greater than 8 or 10 is selected. The parameter N may be chosen to be greater than 8 for the two times hitting during one revolution of the flying piston.

The arrangement of the elements of a striking mechanism can be done in many ways. The FIGS. 7 to 9 show further embodiments. The for the interpretation of the percussion of Fig. 4 The above rules can also be applied to these percussion types.

Claims (6)

1. Pneumatic striking mechanism comprising:

   a free piston (13) movable along a striking axis (8),

   an impact surface (27) limiting the movement of the free piston (13) along the striking axis (8) in the impact direction (99),

   an exciter piston (12) limiting the movement of the free piston (13) along the striking axis (8) in the opposite direction to the impact direction (99),

   a pneumatic chamber (19) between the free piston (13) and the exciter piston (12),

   and

   a drive (3) for moving the exciter piston (12) periodically along the striking axis (8) with a stroke (H), the free piston (13) being excited into a periodic movement between the impact surface (27) and minimum proximity to the exciter piston (12),

   characterised in that

   the length ratio of the maximum length (11) of the pneumatic chamber (19) to the stroke (H) is selected to be less than 1.55.
2. Pneumatic striking mechanism according to claim 1, characterised in that, when the mass (m₂) of the free piston (13) is greater than 400 g, the length ratio is selected to be less than 1.55 and when the mass (m₂) of the free piston (13) is less than 400 g, the length ratio is selected to be less than 1.40.
3. Pneumatic striking mechanism according to claim 1, characterised in that, when the ratio m ₁/m ₂ of the mass (m₁) of the striker to the mass (m₂) of the free piston (13) is less than 1.2, the length ratio is selected to be less than 1.40.
4. Pneumatic striking mechanism according to one of the preceding claims, characterised in that the mass (m₂) of the free piston (13), the cross-sectional area (A) of the pneumatic chamber, the maximum length (L) of the pneumatic chamber, the stroke (H) of the exciter piston (12) and the impact coefficient (q) satisfy the following inequality when the striking mechanism has an impact frequency (f) during percussive operation: $$\frac{L^{\mathrm{\kappa }}}{2{\left(L-H\right)}^{\mathrm{\kappa }}}\cdot \frac{\mathrm{\kappa }}{L-H}+\left(\frac{L^{\mathrm{\kappa }}}{2{\left(L-H\right)}^{\mathrm{\kappa }}}-1\right)\cdot \frac{1-q}{q}\frac{N}{2\mathrm{\pi }\mathit{H}}\stackrel{!}{\ge }\frac{m_2}{A\cdot p_0}\cdot N^2{f}^2,$$

   

   where the parameter N is at least 4, p₀ designates the ambient pressure and κ designates the isentropic coefficient of the gas in the pneumatic chamber (19).
5. Pneumatic striking mechanism according to claim 4, wherein the impact coefficient (q) is selected to be 0.22 when the ratio m ₁/m ₂ of the mass (m₁) of the striker to the mass (m₂) of the free piston (13) is greater than 1.2 and otherwise the impact coefficient (q) is selected to be 0.12.
6. Pneumatic striking mechanism according to either of claims 4 or 5, wherein the parameter N is selected to be greater than 5.

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