HAMMER DEVICE

Abstract: The present invention relates to a hammer device, preferably a down-the-hole hammer, including a casing, a piston (10), a drill bit (11) and means for activating the piston (10) to frequently strike the drill bit (10). In down-the-hole hammers the kinetic energy of the piston is transmitted by elastic waves through the drill bit and finally to the rock. However, said transmission is not carried out in an optimal way since the piston is not related to the drill bit in terms of length and mass. Also the drill bit does not cooperate with the rock in the best mode. The aim of the present invention is to improve the energy transmission from the piston to the rock via the drill bit. This is realized by paying attention also to the distribution of the impedance in the piston and the drill bit of a hammer device.

Description: Hammer device The present invention relates to a hammer device, preferably a down-the-hole hammer, including a casing, a piston, a drill bit and means for activating the piston to frequently strike the drill bit. The invention also relates to a piston and a drill bit per se. In down-the-hole hammers the kinetic energy of the piston is transmitted by elastic waves through the drill bit and finally to the rock. However, said transmission is not carried out in an optimal way since the piston is not related to the drill bit in terms of length and mass. Also the drill bit does not cooperate with the rock in the best mode. In prior art down-the-hole hammers very little attention has been paid to the adaption of the piston to the drill bit when said drill bit has a mass concentration at the end directed towards the rock. The aim of the present invention is to further improve the energy transmission from the piston to the rock via the drill bit. This is realized by paying attention also to the distribution of the impedance in the piston and the drill bit of a hammer device as defined in the appending claims. Below an embodiment of a down-the-hole hammer according to the present invention is described, reference being made to the accompanying drawings, where Fig.l schematically discloses the piston and the drill bit of a down-the-hole hammer according to the present invention; Fig.2 discloses the relationship between the applied force versus the penetration for a drill bit working a rock surface; Fig.3 discloses in a diagram the relationship between the degree of efficiency versus the relationship Z M /Z T ; Fig.4 discloses in a diagram the relationship between the degree of efficiency versus the relationship T M /T T ; Fig.5 discloses in a diagram the relationship between the degree of efficiency versus the parameter β; and Fig.6 discloses a diagram showing the compressive and tensile stresses in the piston and the drill bit. In Fig.l the piston 10 and the drill bit 11 are schematically shown. As is evident from Fig.l the piston 10 and the drill bit 11 have a reversed design relative each other. The piston 10 has two portions 10a and 10b. The portion 10a has the length I-tøi and the impedance while the portion 10b has the length L^i and the impedance Z-p^. The drill bit 11 has two portions 11a and lib. The portion 11a, i.e. the head of the drill bit, has the length M^ and the impedance ZM 2 while the portion lib, i.e. the shaft of the drill bit, has the length Ixj^ an< ^ the impedance Zr 2 . When stress wave energy is transmitted through pistons and drill bits it has been found that the influence by variations in the cross-sectional area A, the Young's modulus E and the density can be summarised in a parameter Z named impedance. The impedance Z = AE/c, where c = (E/ ) 1 / 2 , i.e. the elastic wave speed. Any combinations of A, E and that corresponds to a certain value of the impedance Z gives the same result in respect of stress wave energy transmission. It should be pointed out that the impedance Z is determined in a certain cross-section transverse to the axial direction of the piston 10 and the drill bit 11, i.e. the impedance Z is a function along the axial direction of the piston 10 and the drill bit 11. Therefore, within the scope of the present invention it is of course possible that the impedances Z for the different portions 10a, 10b, 11a and lib may vary slightly, i.e. Z M1 , Z T1 , Z>p 2 a nd Zji2 -~° n °t need to have a constant value within each portion but can vary in the axial direction of said portions 10a, 10b, 11a and lib. In the practical design of the piston 10 and the drill bit 11 the provision of e.g. circumferential grooves and/or splines are quite frequent. Also the provision of e.g. a circumferential shoulder may be necessary. It should also be pointed out that even if e.g. the portions 10a and 10b must have different impedances and Z T1 resp. it is possible to design the piston 10 with a generally constant cross-sectional area by using different materials in the portions 10a and 10b. It is also necessary to define a further parameter, namely a time parameter T. The definition is T = L/c, where L is the lengt of the portion in question and c is the elastic wave speed in the portion in question. Thus for the portion 10a T M1 = I/C M1 , for the portion 11a T M2 = IM2 °M2 for the portion 10b TJJI = lη_ ./ c ^ι and for the portions lib T T2 = ι 2 /cι> • The reason why it is necessary to have the time parameter T instead of the length L is that different portions may consist of different materials that have different values regarding the elastic wave speed c. Within the scope of the present invention it is also possible that e.g. the portion 10a can consist of several sub-portions having different elastic wave speed c. In such a case the time parameter T is calculated for each sub- portion and the total value of the time parameter T for the entire portion 10a is the sum of the time parameters T for each sub-portion. Fig.2 shows the relationship between the force F applied to the rock versus the penetration u into the rock . The line ! illustrates the relation between the force F and the penetration u when a force F is loaded to the rock. Thus ] _ = F/u during the loading sequence and k is a constant. The force Fj_ corresponds to the penetration u__ . The unloading of the force F is illustrated by the line k 2 • Thus k 2 = F/u during the unloading sequence and k 2 is a constant. When complete unloading has taken place there is a remaining penetration u 2 which means that a certain work has been carried out upon the rock, said work being illustrated by the triangular dotted area. The amount of work that said area represents is defined as W. The kinetic energy of the piston 10 when moving towards the drill bit 11 is defined as Wk. As stated above the aim of the present invention is to maximize the degree of efficiency, which is defined as the relationship W/Wj ς . The present invention is based on the idea that the mass distribution of the piston 10 is such that initially a smaller mass, i.e. the portion 10b is contacting the drill bit 11. Subsequently, a larger mass, i.e. the portion 10a, follows. It has turned out that by such an arrangement almost all of the kinetic energy of the piston is transmitted into the rock via the drill bit. The most important parameter is the impedance ratios røι/Z τl and Z M2 /Z T2 - Said parameter should be in a certain interval. In order to have an optimum degree of efficiency it is also important that the time parameter ratios T M1 /T T1 and M / T T2 are in a certain interval. In Fig.3 a diagram shows the relationship between the degree of efficiency W/Wk versus the impedance ratio ZM/ZT, said ratio being valid for both the piston 10 and the drill bit 11. When setting up the diagram in Fig.3 T M /T T = 0,5 and β = 1, see below concerning definition of β. As can be learnt from Fig.3 the peak of W/W k is within the interval 3,0 - 5,5, preferably 3,5 - 4,5 of Z M /Z T . in said preferred interval the degree of efficiency W/W k is higher than 95 %. The highest degree of efficiency W/W k is achieved when Since the degree of efficiency W/W^ has its peak when Zrø/Zip = 4 it can be concluded that the theoretically preferred design is when the different portions 10a, 10b, 11a, lib of the piston 10 and drill bit 11 each have a constant impedance Z in their axial directions. Also the portions 10a and 11a should have the same impedance and the portions 10b and lib should have the same impedance. However, this is not likely to happen in the practical embodiments, see above. Therefore, it should again be emphasized that the impedances %ι___ , Z T1 , I 2 and Zrø 2 need not have constant values but can vary in axial direction of the corresponding portions 10a, 10b, 11a and lib resp.. The only restriction is that the ratios j /Zrp ! and M2 /Zτ 2 are in the intervals specified in the appending claims. In Fig.4 a diagram shows the relationship between the degree of efficiency W/W k versus the time ratio T M /T T , said ratio being valid for both the piston 10 and the drill bit 11. When setting up the diagram in Fig.4 Z M /Z T = 4 and β = 1, see below for definition of β. As can be learnt from Fig.4 the peak of W/ ^ is within the interval 0,35 - 0,75, preferably 0,4 - 0,6, of T M /T T . In said preferred interval the degree of efficiency W/W jς is well over 90 %. The highest degree of efficiency is achieved when T M /T T = 0,5. Thus the optimum design according to the present invention is when rø. is equal to Trø 2 and Tιj>__ is equal to T« 2. When using the findings according to this invention as regards the impedance ratio Z M /Z T and the time ratio T M /T T in dimensioning work it is also necessary to introduce a parameter named β. Said parameter β = 2L H k x / A T2 E T2r where I-H - Lτ2 + L 2 ; k l i s t e constant illustrated in Fig.2; A T2 is the cross-sectional area of the portion lib; and E T2 is the Youngs' modulus for the portion lib. In Fig.5 the relationship of the degree of efficiency / j . versus the parameter β is shown. When setting up the diagram of Fig.5 Z M /Z T = 4 and T M /T T = 0,5. From Fig.5 it can be learnt that the degree of efficiency W/W^ decreases for an increasing value of β. Therefore it is important that proper matching values for L H and Arp 2 are chosen and also that a material having a proper Youngs' modulus E-p 2 is chosen. For practical reasons it is not possible to give β a too small value although the degree of efficiency W/W jς increases for a decreasing value of β. A very important favourable feature of the present invention is that the piston and the drill bit of a hammer device according to the present invention are not subjected to any tensile stresses worth mentioning during the rock crushing work period of the stress wave. Thus the original stress wave can be reflected several times within the system without generating any tensile stress waves worth mentioning. In Fig.6 the highest positive (tensile) stress and the highest negative (compressive) stress in every cross-section of the piston 10 and drill bit 11 are shown. In the diagram the shown stresses are are dimensionless since they are related to a reference stress. From Fig.6 it can be seen that generally only the piston 10 is subjected to any tensile stresses and that the value of said stresses is negligeable. It should be pointed out that since tensile stresses are almost absent in the piston and drill bit according to the present invention said details will have a longer life than corresponding details in a conventional down-the-hole hammer. It is the tensile stresses that give rise to fatigue of details of that kind. The diagrams according to Figs.3, 4 ,5 and 6 have been set up by using a computer program simulating percussive rock drilling. However, the computer program has only been used to verify the theories of the present invention, namely to have a reversed design of the piston 10 and the drill bit 11. It should be pointed out that the present invention is in no way restricted to a down-the-hole hammer but is also applicable in e.g. so called impact breakers and hard rock excavating machines. Generally speaking the invention can be used in a piston-drill bit system where the piston is acting directly upon the drill bit. Also there is no limitation concerning the activation of the piston. This means that such activation can be effected by e.g. a hydraulic medium, by air or by any other suitable means. Also the invention is in no way restricted to the embodiment described above but can be varied freely within the scope of the appending claims.

Claims: Claims 1. Hammer device, preferably a down-the-hole hammer, including a casing, a piston (10) , a drill bit (11) and means for activating the piston (10) to frequently strike the drill bit (10) , c h a r a c t e r i z e d in that the piston (10) and the drill bit (11) have a reversed design relative each other in respect of impedance (Z) , i.e. the piston (10) has a portion (10a) at its rear end having the impedance Z^i, said portion (10a) corresponding to a portion (11a) at the front end of the drill bit (11) , said portion (11a) having the impedance Z M2 , and a portion (10b) at the front end of the piston (10) having the impedance Z T1 , said portion (10b) corresponding to a portion (lib) at the rear end of the drill bit (11) , said portion (lib) having the impedance Z T2 , and that the ratios Z M1 /Z T1 and Z M /Z T2 are in the interval 3,0 - 5,5. 2. Hammer device according to claim 1, c h a r a c t e r i z e d in that the ratios Z M1 /Z T1 and Z M2 / Z T 2 are ~- n the interval 3,5 - 4,5, preferably in the magnitude of 4. 3. Hammer device according to claim 1 or 2, c h a r a c t e r i z e d in that the piston (10) and the drill bit (11) have a reversed design relative each other in respect of a time parameter (T) , i.e. the portion (10a) at the rear end of the piston (10) having the time parameter Tjii said portion (10a) corresponding to the portion (11a) at the front end of the drill bit (11) , said portion (11a) having the time parameter T M2 and the portion (10b) at the front end of the piston (10) having the time parameter T T1 , said portion (10b) corresponding to the portion (lib) at the rear end of the drill bit (11) , said portion (lib) having the time parameter T T2 , and that the ratios T M ι/T τl and T M2 /T T are in the interval 0,35 - 0,75. 4. Hammer device according to claim 3, c h a r a c t e r i z e d in that the ratios Trø i /T-π and j4 /Trp 2 are in the interval 0,4 - 0,6, preferably in the magnitude of 0,5. 5. Piston (10) intended to be used in a hammer device, preferably a down-the-hole hammer, further including a casing, a drill bit (11) and means for activating the piston (10) to frequently strike the drill bit (10) , c h a r a c t e r i z e d in that the piston (10) has a portion (10a) at its rear end having the impedance Z M1 , that the piston (10) has a portion (10b) at its front end having the impedance Zη___, that the ratio he interval 3,0 - 5,5. 6. Piston (10) according to claim 5, c h a r a c t e r i z e d in that the ratio Z M1 /Zrpι in the interval 3,5 - 4,5, preferably in the magnitude of 4. 7. Piston (10) according to claims 5 or 6, c h a r a c t e r i z e d in that the portion (10a) at the rear end of the piston (10) has a time parameter T M1 , that the portion (10b) at the front end of the piston (10) has a time parameter η_ _, and that the ratio j ^i/T- i is in the interval 0,35 - 0,75. 8. Piston (10) according to claim 7, c h a r a c t e r i z e d in that the ratio T M1 /T T1 is in the interval 0,4 - 0,6, preferably in the magnitude of 0,5. 9. Drill bit (11) intended to be used in a hammer device, preferably a down-the-hole hammer, further including a casing, a piston (10) and means for activating the piston (10) to frequently strike the drill bit (11) , c h a r a c t e r i z e d in that the drill bit (11) has a portion (11a) at its front end having the impedance Z M2 that the drill bit (11) has a portion (lib) at its rear end having the impedance Z T2 , and that the ratio Z M2 /Z T2 is in the interval 3,0 - 5,5. 10. Drill bit according to claim 9, c h a r a c t e r i z e d in that the ratio Z M2 /Z T2 i in the interval 3,5 - 4,5, preferably in the magnitude of 4. 11. Drill bit (11) according to any of claims 9 or 10, c h a r a c t e r i z e d in that the portion (11a) at the rear end of the drill bit (11) has the time parameter T j$ 2, that the portion (lib) at the front end of the drill bit (11) has the time parameter T T2 , that the ratio TM2 T T2 s in the interval 0,35 - 0,75, preferably in the interval 0,4 - 0,6, and most preferably in the magnitude of 0,5.

Inventors: BECCU RAINER, BECCU, RAINER
Applicants: SANDVIK AB
Application year: 1991
Main classification: E21B4
IPC codes: B25D17, E21B4