GB2030901A - Portable power tool - Google Patents

Abstract

To enable a portable power tool (such as a drilling and chiselling tool) of the kind which is powered by an internal-combustion engine, to be constructed with a relatively small flywheel 15, and to ensure that the flywheel has adequate kinetic energy at slow speeds of the engine, the flywheel 15 is mounted upon the engine crankshaft 4 by way of a step- up gearing. In the illustrated use, such gearing comprises a planetary system. <IMAGE>

Description

SPECIFICATION A hand tool, such as a drilling and chiselling hammer The invention relates to a hand tool, such as a drilling and chiselling hammer, driven by an internal-combustion engine having a crankshaft and a flywheel which is connected to the crankshaft.

Hand tools of this kind are used for instance on fairly large demolition sites and on remote building sites which are not provided with an electrical power supply. As compared with electricallyoperated tools, these tools have the advantage of greater mobility. However, known internal combustion engine powered tools are considerably heavier than conventional electrically-operated tools; further, they develop considerably more sound or noise, and are vibrate more than electric tools.

Accordingly, it is very tiresome to work with the previously-known tools for any considerable time.

In order on the one hand to compensate for nonuniformity or imbalance of the drive motor and on the other hand to compensate for the time lag between power delivery of the internalcombustion engine, without involving a large reduction in speed. So that, when the tool is lifted away from substrate being worked, aceleration of the engine speed, and consequential undesired excessive noise development, can be prevented, known tools usually have a throttle mechanism with which the speed of the engine can be reduced to an idling speed. Actuation of this throttling mechanism is effected manually or automatically. Now, in order to prevent, during these intervals or when the tool is offered to the substrate again, "fading out" of the engine from its low idling speed, the flywheel must have a specific rotary moment or inertia. This is possible, for example, with a flywheel of large dimensions.

However, in the hand tools, with which'this invention is concerned, for reasons of weight and space, the size of the flywheel is necessarily limited.

The currently-customary hand tools which have internai-combustion engine drive are mostly socalled high-speed engines, in which the necessary power is achieved only at a relatively high running speed. However, this results in a very high noise level and severe vibrations, which is why the tools can in time cause damage to health and are generally no longer usable. Also, socalled slowmoving engines require a large and heavy flywheel, and these are not suitable for portable tools.

The problem underlying the invention is to provide a hand tool which can be run at a low engine speed and which is light in weight.

In accordance with the invention, this is achieved by providing, between the crankshaft and the flywheel, a transmission gearing which increases the speed of the flywheel as compared with the speed of the crankshaft. Since the energy content of a flywheel increases to the fourth power with increase in speed, despite the additional transmission gearing, a saving in weight of the entire tool and reduction in engine speed is possible.

In the case of small tools, the flywheel usually also performs additional functions. Thus, it is customary to design the flywheel to serve also as a fan to produce an air stream which cools the engine. With such an arrangement, the increase in the speed of the flywheel arising from the use of the transmission gearing results also in a considerably larger throughput of air, so that even in the event of fairly long continuous use of the tool, despite a low flywheel diameter, sufficient cooling of the engine is achieved. Since the flywheel often serves additionally as generator to produce ignition sparks, the ignition voltage can be kept constant, to some extent, even with low engine speed.The transmission gearing which increases the speed of the flywheel as compared with the speed of the crankshaft brings, moreover, the additional advantage of a positional stabilisation of the entire tool as a result of the gyroscopic effect of the flywheel. Unintentionai tilting movements of the entire tool during use, as a result of eccentrically-acting forces, are thereby largely eliminated. The tool also vibrates to a lesser extent during operation.

For reasons of space and weight, it is advantageous to construct the transmission gearing to be as compact as possible. In order to achieve this, it is advantageous for the transmission gearing to be designed as a planetary gearing. Such a planetary gearing can, moreover, be arranged coaxially to the crankshaft, so that the external shape of the hand tool need hardly be disadvantageously affected in any way.

To achieve favourable force transmission and also for dimensioning reasons, it is advantageous that a web or planet carrier of the planetary gearing should be driven by the crankshaft, that a toothed annulus of the gearing should be connected stationarily to a housing of the tool and that a sun wheel of the gearing should be coupled to the flywheel. With suitable design of the toothing, two or more planet pin ions can be provided on the web or planet carrier, so that uniform force distribution is obtained.

For practical reasons, there are limits to the transmission ratio between the crankshaft and the flywheel. Because additional transmission gearing is present, part of the potential saving in flywheel weight itself is cancelled out The speed of the flywheel should, accordingly, lie considerably above that of the crankshaft. On the other hand, the speed of rotation of the flywheel at the rated or normal speed of the engine should, also not be too great, because if it is too great the gyroscopic effect of the flywheel becomes so strong that proper handling of the tool is extremely difficult To achieve a favourable medium value it is advantageous for the transmission ratio of the transmission gearing to be of the order of 3:1. The transmission ratio should, of course, be a whole multiple, when the flywheel carries the magnet which induces current in the ignition coil.

The invention will be described further, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a part-sectional side elevation illustrating an embodiment of a drilling and chiselling hammer constructed in accordance with the invention; and Fig. 2 is a section, to an enlarged scale, taken along the line Il-Il of Fig. 1.

The drilling and chiselling hammer illustrated in the drawings comprises a housing which is designated as a whole by the reference numeral 1.

Arranged at the rearward end of the housing 1 is a handle 2. Evident on the end of the housing 1 remote from the handle 2 is a tool holder 3.

Mounted for rotation in the housing 1 is a crankshaft, designated as a whole by the numeral 4, of a driving engine. Fastened to a crankpin 4a of the crankshaft 4 is a connecting rod 5. The connecting rod 5 is, in turn, connected to a piston 6 of the driving engine. At the upper end of the crankshaft 4, a crank, which is designated as a whole by the numeral 7, is connected by a thread 4b to the crankshaft 4. This crank 7 bas a crankpin 7a to which a connecting rod 8 of a percussion mechanism of the tool is fastened for rotation. The connecting rod 8 is connected in turn to a driving piston 9 of the percussion mechanism. The driving piston 9 is guided in a sleeve 10.

That end of the crankshaft 4 which is remote from the thread 4b is surrounded by a planetary gearing. A web or planet carrier 11 of the planetary gearing is connected in torsionally-fast manner to the crankshaft 4. Carried by the web 11 are planet wheels 13 each of which is arranged on a respective axle 12. Each planet wheel 1 3 meshes, on the one hand, with a stationary toothed annulus 14 connected to the housing 1 and, on the other hand, with a sun wheel 1 spa which is arranged on a flywheel which is designated as a whole by the numeral 1 5. The flywheel~15, as well as the sun wheel 1 5a connected thereto, is mounted on the crankshaft 4, for rotation relative thereto, by way of a bearing bush 16.As a result of the planetary gearing consisting of the web or planet carrier 1 , the planet wheel 13, the toothed annulus 14, and the sun wheel 1 5a, the speed of the flywheel 1 5 is increased as compared with that of the crankshaft 4.

As can be seen from the section shown in Fig.

2, the web or planet 11 has three axles 1 2 and three planet wheels 1 3 mounted thereon. As can be seen from this figure, the planet wheels 13 mesh on the one hand with the toothed annulus 14 and on the other hand with the centrallydisposed sun wheel 1 spa. This arrangement, comprising three planet wheels, is to be understood merely as an example. Instead of three planet wheels there could be only one, or two, or more than three planet wheels. Moreover, although the drawings illustrate a hammer drill, the invention can be applied to other forms of internal combustion engine-driven hand-held tools, such as power drills and saws.

Claims (6)

1. A hand tool driven by an internal-combustion engine having a crankshaft and a flywheel connected to the crankshaft, characterised in that a transmission gearing, which increases the speed of the flywheel as compared with the speed of the crankshaft, is arranged between the crankshaft and the flywheel.

2. A hand tool as claimed in claim 1, characterised in that the transmission gearing is a planetary gearing.

3. A hand tool as claimed in claim 2, characterised in that a web or planet carrier of the planetary gearing is driven from the crankshaft, a toothed anulus of the gearing is connected stationarily to a housing of the tool and a sum wheel of the gearing is coupled with the flywheel.

4. A hand tool as claimed in claim 1, 2 or 3 characterised in that the transmission ratio of the transmission gearing is 1 :3.

5. A hand tool as claimed in any preceding claim which is a drilling and chiselling hammer.

6. A hand tool substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.