Abstract:
A pneumatic percussion power tool has a percussion tool housing in which a drive piston and a percussion piston are movable in an axial direction. The motion of the drive piston, which is generated by a crank mechanism, is transmitted to the percussion piston through a pneumatic spring in a first chamber so that the percussion piston cyclically strikes a ram or a tool. The backward motion of the percussion piston rebounding from the ram is supported by increasing air pressure in a third chamber that is supplied with air by the drive piston through a second chamber and a communicating channel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to a pneumatic impact mechanism in which a drive piston and a percussion piston move axially within an impact mechanism housing. 
     2. Description of the Related Art 
     These types of impact mechanisms are commonly used in hammer drills and sledgehammers and in practice two types of impact mechanisms have proven useful, among others. The first type consists of an impact mechanism with a hollow drive piston moved inside the impact mechanism housing, in the cavity of which the percussion piston is moved. The other type consists of an impact mechanism with a hollow percussion piston moved inside the impact mechanism housing, in the cavity of which the drive piston is moved. The commonality of both impact mechanism types is that the drive piston is driven by a crankshaft, for example, and that a pneumatic spring is created between the drive is piston and the percussion piston that transfers the drive motion of the drive piston onto the percussion piston and forces it in a direction of impact where it finally meets a tool, for example a chisel, transferring its impact energy onto it. Afterward, the percussion piston rebounds and another impact motion starts, supported by the drive piston. 
     The advantages of the impact mechanism types described are in their minimal requirements with regard to sealing of the separating joints so that robust steel-steel glide pairs can be used in the high pressure range without using additional sealing elements. Moreover, the impact mechanisms exhibit good startup behavior at low temperatures. 
     Nevertheless, under certain operating conditions, the problem arises in that, after an impact, the return motion of the percussion piston is not sufficient to make a forceful impact, despite the recoil impulse and the suction effect of the drive piston. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The objective of the invention is to provide a pneumatic impact mechanism in which the recoil behavior of the percussion piston is improved. 
     According to the invention, the objective is met by means of a pneumatic impact mechanism with a impact mechanism housing, a drive piston that is driven axially back and forth, an axially moving percussion piston located in front of a drive surface of the drive piston coaxial to the drive piston as seen in an impact direction, a first chamber in front of the drive surface of the drive piston and located behind a rear surface of the percussion piston, a second chamber located behind the drive surface of the drive piston and with a third chamber located in front of the rear surface of the percussion piston, wherein the second chamber and the third chamber can be made to communicate with one another by means of a connection channel. 
     The design of the pneumatic impact mechanism enables the drive piston, when it makes a forward motion, to transfer its energy to the percussion piston through a pneumatic spring created in the first chamber, thus transferring its energy indirectly onto the tool. When the drive piston makes a backward motion, air pressure forms in the second chamber located behind the drive piston. This air pressure is passed through the connection channel to the third chamber in front of the percussion piston. In this way, when the drive piston is moved backward, the return motion of the percussion piston is supported independent of its recoil after the impact and independent of the suction effect of the drive piston transferred by the first chamber. A reliable return motion of the percussion piston is the result even under difficult operating conditions, so that when the drive piston repeats its forward motion, another forceful impact can be made. 
     The communication between the second chamber and the third chamber enables the pressure change in the second chamber caused by the motion of the drive piston to change the pressure in the third chamber by means of the connection channel. 
     In a first preferred embodiment form of the invention, the drive piston is moved inside the impact mechanism housing whereas the percussion piston is moved inside a cavity formed at one end of the drive piston. 
     Alternatively, in another very advantageous embodiment form, the percussion piston is moved inside the impact mechanism housing whereas the drive piston is moved inside a cavity formed in an end face. The solution according to the invention is suitable for both of the pneumatic impact mechanisms mentioned. 
     In a preferred development of the invention, the second chamber is located between a rear surface of the drive piston and a rear tubular base fastened to the impact mechanism housing, whereas the third chamber is located between a forward surface of the percussion piston and a rear tubular base fastened to the impact mechanism housing. This enables additional chambers, as compared to the state of the technology, to be created behind the drive piston and in front of the percussion piston without expensive additional design measures. 
     In the process, the drive piston is designed such that it has a piston head that constitutes both the drive surface and the rear surface, a bracket with which to fasten to a drive unit and a center member that connects the piston head to the bracket. This design makes it possible to locate the tubular base between the piston head and the bracket, which creates the second chamber in a simple manner. 
     In another advantageous configuration of the invention, an idle channel is provided that has at least one idle opening provided in a wall of the drive piston and which penetrates a wall of the impact mechanism housing. The idle channel is connected either to the connection channel or the outside. Through the idle channel, it is possible to short circuit the first and second chambers so that no pressure relationship can form in the pneumatic impact mechanism that acts on the percussion piston when the pneumatic impact mechanism is at idle. 
     In an especially advantageous embodiment form, a shifting control slide is provided that can switch between an impact position and an idle position. When it is in the impact position, it creates the connection between the second and third chambers by means of the connection channel while blocking the idle channel. In the idle position, it blocks off the connection channel and opens the idle channel, thus precisely bringing about the transfer between idle and impact positions. It is advantageous here to carry out the axial shifting of the control slide by coupling the control slide to the tool or to the die located between the percussion piston and the tool. When switching to idle, the tool or the die slides somewhat forward out of the housing when lifted away from the rock, with the control slide also following this motion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other advantages and features of the invention are explained in more detail below with the aid of the accompanying figures. Shown are: 
     FIG. 1 a schematic sectional illustration of a pneumatic impact mechanism according to the invention in the impact position; 
     FIG. 2 the impact mechanism of FIG. 1 in the idle position; 
     FIG. 3 a schematic sectional illustration of a second embodiment form of the pneumatic impact mechanism according to the invention in the impact position; 
     FIG. 4 the impact mechanism according to FIG. 3 in the idle position; 
     FIG. 5 a third embodiment form of a pneumatic impact mechanism according to the invention in the impact position; 
     FIG. 6 the impact mechanism according to FIG. 5 in the idle position; 
     FIG. 7 a fourth embodiment form of a pneumatic impact mechanism according to the invention in the impact position; 
     FIG. 8 the impact mechanism according to FIG. 7 in the idle position; 
     FIG. 9 another type of impact mechanism as the fifth embodiment form for a pneumatic impact mechanism according to the invention in the impact and idle positions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a pneumatic impact mechanism according to the invention in the impact and idle position, respectively. 
     In an impact mechanism housing  1 , a drive piston  2  is shifted in an oscillating axial motion by means of a connecting rod  3  belonging to a crankshaft drive of known design, which is not shown. 
     The connecting rod  3  is hinged to a bracket  4  of the drive piston  2 . The bracket  4  is connected to a piston head  6  in one piece through a center member  5 . The drive piston  2  thus consisting of the bracket  4 , the center member  5  and the piston head  6  can also be constructed out of a number of parts—different than what is shown in the figure—if it is sensible to do so for manufacturing or assembly reasons. 
     A cavity  8  is formed in a forward end  7  of the drive piston  2 . A percussion piston  9  that can move axially is inserted into this cavity. Between the drive piston  2  and the percussion piston  9  is a first chamber  10  that is enclosed by the drive piston  2  and that contains air at ambient pressure in the initial state. 
     At the beginning of an impact, the drive piston  2  moves forward, i.e.—with reference to FIGS.  1  and  2 —to the left. Due to inertia, the percussion piston  9  follows in a delayed manner, resulting in the increase in air pressure in the first chamber  10  so that a pneumatic spring results, which transfers its energy in delayed fashion to the percussion piston  9 . This is finally accelerated forward as well and impacts a die  11 , shown only schematically, wherein the motive energy of the impact piston  9  is transferred as impact energy. The die  11  conveys the impact energy to a tool, not shown, for example a chisel. In place of the die  11 , a stem of the tool can be employed directly as well. 
     At the point of impact shown in FIG. 1, an air equalization channel  12  in the wall of the drive piston  2  is opened, whereupon the first chamber  10  can be ventilated and air losses can be equalized in a known fashion. 
     After the impact, the percussion piston  9  rebounds back in the direction of the drive piston  2 , which is already in its return motion as well due to the crankshaft drive. Due to a negative pressure forming in the first chamber  10 , the return motion of the percussion piston  9  is aided until the drive piston  2  again makes its forward motion and begins a new impact cycle. 
     In impact mechanisms known from the state of the technology it turns out that for certain operating conditions, the return motion of the percussion piston proceeds unsatisfactorily and is not sufficiently supported by the suction effect in the first chamber. This results in the percussion piston not returning back far enough and the next impact not being made with the required energy. An unsatisfactory working result and irregular behavior of the hammer is the result for the user. 
     The problem is solved according to the invention in that a connection channel  13  is provided that causes a second chamber  14  to communicate with a third chamber  15 . 
     The second chamber  14  is located behind a drive surface  16  of the drive piston  2 —as seen in the direction of impact. As seen in FIGS. 1 and 2, the second chamber  14  is formed by a rear surface  17  provided at the piston head  6 , a rear tubular base  18  belonging to the impact mechanism housing  1 , the center member  5  and the actual impact housing  1 . 
     The third chamber  15  is located in front of a rear surface  19  of the percussion piston  9  and is formed by an impact surface  20  of the percussion piston  9  serving as a front surface, a front tubular base  21  belonging to the impact mechanism housing  1 , the actual impact mechanism housing  1  and the drive piston  2 . 
     It is not usually necessary to seal the various separating joints between the moving parts. Here, steel-steel glide pairs are commonly used. 
     When the drive piston  2  is shifted backward after an impact, a negative pressure is produced not just in the first chamber  10 —as is known in the state of the technology—to pull the percussion piston  9  back. In the second chamber  14 , a positive pressure arises that leads to the third chamber  15  through the connection channel  13  and acts on the impact surface  20  of the percussion piston  9  at that location. Support of the rearward motion of the percussion piston  9  is the result. In reverse, the forward motion of the percussion piston  9  is also boosted when the drive piston  2  makes its forward motion since the negative pressure arising in the second chamber  14  is also passed on to the third chamber  15 . 
     The function of the first chamber  10  formed between the drive surface  16  of the drive piston  2  and the rear surface  19  of the percussion piston  9  is thus not affected. 
     In a wall  22  of the drive piston  2 , there are a number of idle openings  23  that move back and forth in front of an idle notch  24  designed into the impact mechanism housing  1  when the drive piston  2  is moved axially. The idle notch  24  is connected through an air channel  25  to the connection channel  13 . The idle openings  23 , the idle notch  24  and the air channel  25  together form an idle channel. 
     The impact mechanism goes into idle when the user of the hammer drill or sledgehammer employing the impact mechanism lifts the tool from the rock he is working on. In doing so, the tool and the die  11  slide by a certain amount out of the impact mechanism housing  1 . The percussion piston  9  follows and comes to rest in the position shown in FIG.  2 . In so doing, the percussion piston  9  passes by an area of an edge  26  in the housing, and produces a connection between the first chamber  10  through the idle openings  23 , the idle notch  24  and the air channel  25  to the connection channel  13 . 
     By producing the connection between the first chamber  10  and the second chamber  14  or the third chamber  15 , the air system is short-circuited. This means that when the drive piston  2  continues to move, the air is pumped out of the second chamber  14  into not only the third chamber  15 —as in impact operation—but also into the first chamber  10  where it provides air equalization and thus, for the most part, even air pressure in all three chambers. The percussion piston  9  is thus not forced from its forward-most position. 
     For the purposes of completeness, another ventilation opening  27  is mentioned, by means of which a possible air cushion that can arise between the rear tubular base  18  and the bracket  4  can be discharged. 
     FIGS. 3 and 4 show schematically a sectional illustration of a second embodiment form of the invention. The same parts as those in the first embodiment form are identified with the same labels, and their description is not repeated here. 
     In comparison to the first embodiment form of the invention shown in FIGS. 1 and 2, the second embodiment form is provided with an axially shifting control slide  28  that is tensioned on one side by a spring  29  supported against the impact mechanism housing  1 . The control slide  28  can be shifted between an impact position shown in FIG.  3  and an idle position shown in FIG. 4, depending on the position of the die  11 . 
     A connection opening  30  and an idle opening  31  are provided in the control slide  28 . In the impact position, the control slide  28  is located in a position at which the connection opening  30  allows a connection between the connection channel  13  and the third chamber  15 , whereas the control slide  28  prevents a connection of the first chamber  10  to the outside by the fact that the idle opening  31  is not sitting over the air channel  25 . 
     When switching between impact and idle operation, the die  11 , the percussion piston  9  and the control slide  28  slide by a specific amount in the direction of the tool, whereupon the connection opening  30  blocks off the connection channel  13  while the idle opening  31  is shifted over the air channel  25 . This allows the first chamber  10  to be connected to the outside, allowing idle behavior to take place. 
     Indeed, the use of the control slide  28  requires more mechanical hardware, but has the advantage in that the idle path, i.e. the path by which the tool must slide out of the impact mechanism housing  1 , can be shortened. This reflects in a shorter design. 
     As seen in FIG. 4, air pressure forms in the second chamber  14  when the drive piston  2  makes its return motion. This air pressure cannot be discharged through the connection channel  13 . To prevent extreme pressures, therefore, a dual-acting pressure relief valve  13   a  is provided in the connection channel  13 . 
     A variation to this is shown as a third embodiment form in FIGS. 5 and 6, again in the impact and idle positions. This third embodiment form differs from the second embodiment form according to FIGS. 3 and 4 in that the control slide  28  has a larger axial length and extends across the area of the second chamber  14 . 
     In the control slide  28 , in addition to the connection opening  30  and the idle opening  31 , there is a connection opening  32  and a ventilation opening  33 . 
     As seen in FIGS. 5 and 6, this allows the control slide  28  to control all openings to the first chamber  10 , the second chamber  14 , and the third chamber  15 . If the control slide  28  is in the impact position shown in FIG. 5, it causes the second chamber  14  and the third chamber  15  to communicate with one another by means of the connection channel  13  using the connection openings  30  and  32 . 
     In the idle position, the control slide  28  is moved forward, whereupon the idle opening  31  moves over the air channel  25  and produces a connection between the first chamber  10  and the outside in order to prevent a pressure build-up in the first chamber  10 . Further, a connection between the second chamber  14  and the outside is produced by means of the air opening  33  so that the second chamber  14  can ventilate without air having to be discharged through the connection channel  13  or an increased air pressure arising in the connection channel  13 . 
     Alternatively, another, fourth embodiment form is suggested as shown in FIGS. 7 and 8, which differs from that in FIGS. 3 through 6 in that the connection channel  13  is tied by means of a connection section  34  to the idle opening  31  in the control slide  28 . 
     This makes it possible—similar to FIGS.  1  and  2 —to short-circuit the first and second chambers  10 ,  14  so that when the drive piston  2  makes a pumping motion no pressure increase in the first or second chamber  10 ,  14  occurs. 
     The third chamber  15  is separated from the connection channel  13  by means of the control slide  28  and thus experiences no pressure increase. The percussion piston  9  remains fixed in the position shown in FIG. 8 without being able to be lifted up by the drive piston  2 . 
     Only when the user resets the tool onto the rock and thus shifts the die  11  backward are the percussion piston  9  and the control slide  28  also shifted backward, whereupon the impact operation returns. 
     FIG. 9 shows another type of pneumatic impact mechanism according to the invention as a fifth embodiment form in which a percussion piston  40  is moved inside an impact mechanism housing  1  axially. In the upper half of FIG. 9, the impact mechanism is shown in the impact position, whereas the lower half of FIG. 9 shows the impact mechanism in the idle position. 
     At a rear end  41  of the percussion piston  40 , a cavity  42  is formed in which a drive piston  43  is moved. 
     The drive piston  43  is constructed in a similar manner as in the previous embodiment forms and consists essentially of a bracket  44 , a center member  45  and a piston head  46 . 
     Between a drive surface  47  of the drive piston  43  and a rear surface  48  of the percussion piston  40 , a first chamber  49  is formed. 
     Analogous to the embodiment forms already described, a second chamber  50  is formed behind a rear surface  51  of the drive piston  43  as is a third chamber  52  in front of a front surface  53  of the percussion piston  40 . The second chamber  50  and the third chamber  52  are connected through a connection channel  54 . 
     The percussion piston  40  has an extension  55  that impacts a die, which is not shown, or a tool, which is also not shown. 
     An idle channel  56  branches off of the connection channel  54  that enables a connection between the first chamber  49  and the second chamber  50  in the idle position of the impact mechanism. An opening  57  to the connection channel  54  is covered by the percussion piston  40  in this case so that the communication between the second chamber  50  and the third chamber  52  is blocked off. 
     The rest of the design of the impact mechanism corresponds to the embodiment forms already described so that this does not have to be described again. Of course, the various configuration possibilities with regard to the connection channel and the control slide can also be transferred to this type of impact mechanism. 
     In other embodiment forms of the invention, the second chamber can also be a space with low volume dimensions that can be made to communicate with the connection channel, and sealed from the outside. This space can be located behind the drive piston and can contain at least a part of the drive unit for the drive piston.