GB2027629A

GB2027629A - A drilling and chiselling hammer

Info

Publication number: GB2027629A
Application number: GB7922587A
Authority: GB
Original assignee: Hilti AG
Current assignee: Hilti AG
Priority date: 1978-08-14
Filing date: 1979-06-28
Publication date: 1980-02-27
Also published as: NO150027B; SU1071233A3; DE2835570C2; SE436989B; ATA523779A; IT7923950A0; PL117394B1; FR2433632B1; GB2027629B; IT1125390B; MX147430A; PL217730A1; YU182579A; FI71818B; ZA793299B; JPS5531590A; NL186530C; FR2433632A1; AU4923079A; AT372327B

Abstract

The invention aims to improve the power/weight ratio of a two-stroke engine employed as the prime mover in a drilling and chiselling hammer 1 (sometimes also known as a percussion tool) of the kind in which percussion impacts are transmitted to a tool holder 3 of the tool by way of a percussion piston 9 which is reciprocated by the engine, thereby to enable the tool to be lighter, so that it can more readily be manipulated in the case of a hand-held tool. To achieve this improved efficiency, crank 7 of the percussion piston 9 is accommodated in a chamber 1a into which air is drawn by way of a non- return valve 11 upon movement of the piston 9 towards the top dead-centre position and from which air is displaced, under compression, through a pipeline 13 to an entry slot 1e of the cylinder 1c of the engine. <IMAGE>

Description

SPECIFICATION A drilling and chiselling hammer This invention relates to a drilling and chiselling hammer having a percussion mechanism and an internal-combustion engine, a crankshaft of the engine being coupled to a crank which reciprocates a driving piston of the percussion mechanism by way of a connecting rod.

Drilling and chiselling hammers of this kind are used primarily where fairly high demolition or working performances are involved. The internalcombustion engines serving for the drive are, as a rule, so-called 2-stroke engines. As compared with a 4-stroke engine, this form of construction has the advantage of a simpler construction and, thus, also of lower weight with comparable performance. 2-stroke engines are, therefore, for reasons of weight and also of price, customary for hand-held tools and machines.

After each working stroke the 2-stroke engine must be scavenged with a benzine/air (or petrol/air) mixture, i.e. the fumes present in the cylinder are displaced by the mixture and a fresh ignitable charge is fed to the engine. So that the mixture shall flow into the cylinder, a pump is necessary. In virtually all 2-stroke small engines, the crankcase of the engine itself is used as a scavenging pump. However, this is bound up with a considerable disadvantage. The bearing points of the crankshaft and of the connecting rod, as well as the cylinder wall, must be adequately lubricated. Because of the flow of the petrol/air mixture, the lubricating oil is washed away in a very short time, so that, in the absence of countermeasures, the engine would be destroyed within a very short space of time. In order to remedy this, a petrol/oil mixture is used in 2-stroke engines.The task falling to the oil is to lubricate the mechanisms and moving parts of the engine. The petrol is the energy carrier for combustion.

Since the oil passing into the combustion chamber is only incompletely combusted during the combustion of the petrol/air mixture, it emerges as a bothersome blue exhaust smoke.

Furthermore, the efficiency of conventional 2stroke engines is generally lower than that of a comparable 4-stroke engine. This lower efficiency can be attributed principally to the scavenging losses arising in the scavenging of the combustion chamber. In the case of fairly large, stationary engines or engines incorporated in vehicles, a considerable expenditure is generally incurred in the scavenging of the combustion chamber. Thus, for example, turbofans, rotary slide fans or even piston pumps are known for this purpose. In the case of portable tools, such as drilling and chiselling hammers, such measures are, ho.wever, out of the question from the very start, for reasons of weight.

The problem underlying the invention is to provide a drilling and chiselling hammer having an internal-combustion engine, in which the engine, because of good scavenging, achieves a high efficiency, the hammer having, as a whole, a favourable power to weight rratio.

In accordance with the invention, this problem is solved in that the crank of the percussion mechanism is arranged in a chamber which is separated from the remaining parts of the hammer and which has an inlet valve and which communicates, by way of a fiow-off aperture, with inlet slots which lead to the combustion chamber of the engine.

Since the crank chamber of the percussion mechanism is separated from the remaining parts of the hammer, this can be used as a scavenging pump for the engine. Because the stroke volume of the percussion mechanism for a given 2-stroke engine corresponds approximately to the stroke volume of the engine, the crankcase of the percussion mechanism is well suited to serve as a scavenging pump for the engine. The mounting of the crankshaft is mostly arranged in the engine crankcase and the driving piston is usually lubricated from the percussion mechanism. The connecting-rod bearings of the percussion mechanism can be designed, in problem-free manner, with tight grease-lubricated bearings.

Lubrication of the crankcase of the percussion mechanism is therefore not necessary. Since the crank chamber of the engine does not come into contact with the fuel/air mixture in the case of a carburettor engine or with the air sucked in in the case of an injection engine, the lubrication of the crankshaft bearings connecting-rod bearings and of the cylinder wall of the engine can be planned from optimum points of view. Thus, for example, splash lubrication or oil mist lubrication can be used. The fuel/air mixture drawn in thus does not have to fulfill any special lubricating functions. It is thus possible to operate the tool without using 2stroke mixture containing oil, which has a favourable effect on operating costs and exhaust gases.

Known 2-stroke engines additionally have a further disadvantage. During scavenging, part of the fuel/air mixture passes as scavenging losses through the outlet slots and by way of the exhaust muffler into the open air. It is virtually impossible to avoid this in 2-stroke engines. The consequences are high fuel consumption, since this lost fuel does not participate in the combustion process, and a high proportion of noncombusted toxic gases which escape through the exhaust. In the case of so-calied fuel-injection engines, this is prevented in that scavenging is effected with pure air. Then, as soon as the piston has masked the control slots and the combustion chamber is gas-tight, fuel is injected by means of a high-pressure injection nozzle.Three advantages result from this measure: As a result of the subsequent injection of the fuel, performance is distinctly increased, fuel consumption is decisively reduced and exhaust emission is considerably improved. The scavenging proposal in accordance with the invention, using the crankcase of the percussion mechanism, is thus suitable both for carburettor engines and for injection engines.

In operation, an ignitable mixture is drawn in by way of a carburettor, or air is drawn in by way of an air filter, through the inlet valve. Although this system is specifically suitable for an internal combustion engine working on the 2-stroke principle, it can also be used in relation to 4-stroke engines providing a so-called boost, so that an increase in performance can be acheived therewith. The solution in accordance with the invention is particularly simple and requires, as compared with a known drilling and chiselling hammer driven by an internal-combustion engine, merely an inlet valve as well as a connecting line from the flow-off opening of the separated chamber to the inlet slots of the engine. The measures required in accordance with the invention thus result in virtually no increase in weight.

For achieving a simple construction of the tool it is advantageous that an end face, facing the connecting rod, of the driving piston should form a wall part of the chamber separated from the remaining parts of the hammer. Where the percussion mechanism is a pneumatic percussion mechanism, no additional measures of any kind are necessary, since the driving piston reciprocating in a cylinder is already sealed off relative to the air cushion arranged between the driving piston and percussion piston providing such a mechanism. Compression of the draw-in air or of the mixture in the chamber can be determined in optimum manner by determining the crank stroke as well as the dead space, within specific limits, independently of the driving engine.

Because the arrangement of the invention has a chamber which is separated from the remaining parts of the hammer, fresh possibilities emerge as compared with known tools in which the crank of the percussion mechanism and the crank of the driving engine are arranged in the same chamber.

In a conventional 2-stroke engine, the air/fuel mixture is drawn into the crankcase of the engine and is pre-compressed during the expansion phase of the combustion chamber in the crankcase. Shortly before reaching the lower dead centre, overflow channels are freed by the piston and the pre-compressed mixture can flow from the crankcase into the combustion chamber. Upon further rotation of the crank, however, the pressure in the crankcase and also in the overflow channels decreases once more, so that the flow into the combustion is slowed down. For this reson, only incomplete scavenging of the combustion chamber is possible.If, on the other hand, scavenging pressure is increased by reducing the crank chamber, then the danger exists that too large a part of the ignitable mixture escapes uncombusted, as scavenging loss, through the exhaust slots, which in turn leads to increased fuel consumption. Now, in order to be able to adapt the scavenging pressure to the scavenging process in optimum manner, it is advantageous that the crank of the percussion mechanism is offset relative the crankshaft of the engine in such a way that the percussion mechanism lags relative to the engine with a crank displacement (or angle between cranks) of 10 to 60 . Thus, it is possible that, starting from the lower dead-centre position of the engine piston, upon further rotation of the crankshaft, an adequate scavenging pressure is present until the inlet or overflow slots are once more masked by the piston. Since the percussion mechanism is coupled to the crankshaft this crank displacement always remains the same.

In practice it has proved to be advantageous for the crank displacement (or angle between cranks) to amount to 40 . Then, in a given position of the inlet or overflow slots, the result is achieved that a sufficient scavenging pressure is present up to the closing of the inlet or overflow slots, which prevents any return flow of the scavenging air and makes possible a far better scavenging than is achievable with crankcase scavenging. On the other hand, upon opening of the inlet slots, the pressure drop or gradient between the scavenging pressure and the pressure in the combustion chamber is small, so that a flow steam can form in the combustion chamber, and the newly inflowing mixture displaces the combustion gases for the greater part through the exhaust slots.

The invention will be described further, byway of example, with reference to the accompanying drawings in which: Fig. 1 is a part-sectional side view of a drilling and chiselling hammer in accordance with the invention; Fig. 2 is a diagrammatic sectional plan illustrating parts of the percussion mechanism and of the engine in the upper dead-centre position of the driving piston, the section having been taken on the line A-A of Fig. 1; Fig. 3 is a view similar to Fig. 2 but with the relevant parts in the lower dead-centre position of the driving piston.

The embodiment of the drilling and chiselling hammer of the invention, illustrated in Fig. 1, comprises a housing designated as a whole by the reference numeral 1. Fastened to the rearward end of the housing 1 is a handle 2. A tool holder 3 is arranged at that end of the housing 1 which is remote from the handle 2. Mounted for rotation in the housing 1 is a crankshaft, designated as a whole by the reference numeral 4, of an internalcombustion engine, a connecting rod 5 being connected to the crank shaft 4 by way of a crank pin 4a. The connecting rod is connected, in turn, to a piston 6. This piston 6 is guided in a cylinder 1 c.

At the upper end of the crankshaft 4, an eccentric or crank, designated as a whole by the reference numeral 7, is connected by way of a thread 4b to the crankshaft 4. Fastened to a crankpin 7a of the eccentric or crank 7 is a connecting rod 8 of a percussion mechanism of the hammer. The connecting rod 8 is in turn connected to a driving piston 9, for example of a pneumatic percussion mechanism. This driving piston 9 is guided in a sleeve 10. The percussion mechanism is thus coupled to the internal-combustion engine.

Arranged on the housing 1, in the region of the crank 7, is an inlet valve which is designated as a whole by the reference numeral 11. In the inlet valve 1 1, a ball 1 1 a is loaded, by a spring 1 1 b, against a valve seat 11 c. The inlet valve 11 functions as a non-return or one-way valve. When the driving piston 9 moves in the direction of the tool holder 3, air or an fuel/air mixture is drawn through the inlet valve 11 into a chamber 1 a in which the crank 7 and the connecting rod 8 move.

The chamber 1 a is sealed off relative to the remaining parts of the housing 1 by a seal 12. This chamber 1 a furthermore has a flow-off opening 1 d. A pipeline 1 3 leads from the flow-off opening 1 dto an inlet slot 1 e in the wall of the cylinder 1 c. When the driving piston 9 changes its direction and moves towards the handle 2, the chamber 1 a is reduced in size and the air or fuelair mixture drawn therein is ejected through the pipeline 13. When the piston 6 frees the inlet slot 1 e, the air or mixture can flow into the combustion chamber 1 b. Then, combustion waste gases, already contained in the combustion chamber 1 b after previous ignition, are displaced through an exhaust slot If. This is designated in technical terminology as "scavenging".

In the case of a carburettor engine, this scavenging is effected with an ignitable fuel/air mixture. This results in itself (or potentially) in the disadvantage that part of the mixture can also escape through the exhaust slots 1 fand the efficency of the internal-combustion engine is correspondingly reduced. In the case of a fuelinjection engine, on the other hand, scavenging is effected with pure air. The fuel is injected separately through an injector (not shown) into the combustion chamber 1 b only when both the inlet slot 1 e and the exhaust slot If are closed. This, of course, has a favourable effect on efficiency.

Arranged at the lower end of the crankshaft 4 is a flywheel 14 which is customary in an internalcombustion engine. This flywheel 14 serves, on the one hand, to provide balance between the power output of the internal-combustion engine and the power consumption of the engine during the compression phase as well as the percussion work of the percussion mechanism. Moreover, the flywheel additionally serves as generator to produce the ignition voltage and as fan wheel for providing a flow of cooling air which is drawn in through a cover 1 5 and is blown past the cylinder Ic.

Evident from the section, which can be seen in Fig. 2, through the tool in accordance with the line A-A of Fig. 1 are the principal parts of the engine of the tool. The driving piston 9 is shown in its top dead-centre position. With respect to the direction of rotation D, on the other hand, the piston 6 has already passed its respective top dead-centre position. The crankpin 7a of the percussion mechanism is thus angularly displaced relative to the crank pin 4a of the internal-combustion engine by an angle a, such that the percussion mechanism lags behind the internal-combustion engine by this angle a.

Upon further rotation of the crank 7, the chamber 1a is again reduced in size and the air or mixture contained therin is ejected through the flow-off opening 1 d. In comparison with a conventional 2-stroke engine in which the crankcase of the engine acts as scavenging pump, the lagging of the percussion mechanism relative to the internal-combustion engine has the advantage that during the entire scavenging operation, the scavenging pressure lies above that in the combustion chamber, so that return flow is prevented.

Fig. 3 shows the same engine parts as Fig. 2, but in the bottom dead-centre position of the driving piston 9. The piston 6 of the percussion mechanism is already moving away from its crank 7. Accordingly, the air or fuel/air mixture which has passed from the chamber la through the flow-off opening Idand the inlet slot le into the combustion chamber is compressed. In this position, the power requirement of the percussion mechanism is low. The energy stored in the flywheel 1 4 can thus be used virtually fully for the compression of the fuel/air mixture contained in the combustion chamber 1b. Thus the phase displacement (or shift) between the percussion mechanism and the internal-combustion engine also has a favourable effect.

Claims

1. A drilling and chiselling hammer having a percussion mechanism and an internalcombustion engine, a crankshaft of the engine being coupled to a crank which reciprocates a driving piston of the percussion mechanism by way of a connecting rod, characterised in that the crank of the percussion mechanism is arranged in a chamber which is separated from the remaining parts of the hammer and which has an inlet valve and which communicates, by way of a flow-off opening, with inlet slots which lead to the combustion chamberofthe engine.

2. A drilling and chiselling hammer as claimed in claim 1, characterised in that an end face, facing the connecting rod of the driving piston forms a wall part of the chamber which is separated from the remaining parts of the hammer.

3. A drilling and chiselling hammer as claimed in claim 1 or 2, characterised in that the crank of the percussion mechanism is angularly displaced relative to the crank of the engine in such a way that the percussion mechanism lags behind relative to the engine by a displacement angle of lotto 600.

4. A drilling and chiselling hammer as claimed in claim 3, characterised in that the crank displacement amounts to 400.

5. A drilling and chiselling hammer substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.