Technical field The present invention relates to a tensioning device adapted to tension an endless motion-transmission means which is arranged with respect to a vehicle engine. The endless motion-transmission means may for example be a V-belt or chain which is driven from the crankshaft of the vehicle engine. The tensioning device incorporates a lever means which is supported for pivoting at a first end and which has a second end where a tensioning wheel adjustable with respect to the motion transmission means is supported for rotation. A power-developing means connected to the lever means is designed to act with tensioning force upon the lever means so that the tensioning wheel is thereby kept in an active tensioning position.

The object of the tensioning device is to keep a running wheel or a sliding shoe pressed against the belt or chain in order thereby to keep the belt or chain taut and hence to counteract the occurrence of oscillations in the belt or chain during operation.

State of the art V-belt tensioners and chain tensioners are already known, particularly in combination with belts and chains respectively which are used for transmitting driving power from a vehicle engine to various kinds of auxiliary or supplementary equipment fitted to the engine or the vehicle. In vehicles, V-belts are used inter alia for driving coolant pumps, generators, cooling fans and servo steering pumps. Chain tensioners are used for example for the tensioning of endless driving chains designed to transmit driving power from an engine's crankshaft to one or more of its camshafts.

Examples of such known belt and chain transmissions which may be referred to are American patent specifications 5.366. 415,6. 036.612 and PCT publication WO 00/00756 respectively. They refer to hydraulic tensioning devices with internal and external mechanical springs or to purely hydraulically acting devices.

Object of the invention Today's vehicle engines in particular incorporate belt tensioners with internal springs which achieve necessary damping by friction force applied by means of washers (often made of plastic) clamped in the belt tensioner guide bearing which has to be used to enable the belt tensioner to move (oscillate) to the necessary extent during operation.

However, these friction generating washers entail a clear disadvantage in that they often wear relatively quickly, which means that their damping effect decreases quickly and in a short time completely ceases. The result is that the belt itself also wears because the belt tensioner's tensioning action on the belt is greatly reduced, making it easy for undesirable oscillations in the belt to occur.

When for any reason it becomes appropriate to change a driving belt of a vehicle engine, e. g. on the occasion of servicing, it is necessary in the case of today's belt tensioners to tension the replacement belt by hand, which is usually done by means of a locking bar. What happens in practice is that the mechanic who has to carry out the tensioning operation has great difficulty in doing so because he has also simultaneously to hold the new belt in position by hand. This also entails significant risk of his being injured if he loses his grip and slips. The situation is not improved by the fact that the working space around the vehicle engine is now very limited and tending to become even more so with each new vehicle model.

The object of the present invention is to overcome the abovementioned problem which relates to today's belt tensioners and to provide a new type of tensioning device which works entirely by fluid power, i. e. it only uses pressurised liquid and gas for achieving the intended tensioning and damping action. The tensioning device is therefore not to be provided with any internal or external mechanical springs.

The belt tensioning is also not to be dependent on the vehicle engine being in operation, and the tensioning force generated is to remain constant even if a certain amount of belt elongation (stretching) occurs.

To sum up, the invention therefore aims to provide a fluid-powered tensioning device which enables belt changing to be performed without difficulty by only one person quickly, easily and safely and which results in belts lasting longer. This means that servicing intervals can be lengthened, which is highly advantageous from the cost point of view.

Summary of the invention The above objects are achieved according to the invention by the tensioning device indicated in the introduction (and in the preamble of claim 1) also exhibiting the features indicated in the characterising part of claim 1.

What is novel and unique about the invention is therefore the fact that the cylinder's fluid chamber contains not only hydraulic fluid under pressure but also at least one gas cushion which constitutes a motion-damping springing means in the fluid chamber.

The damping function is thus due to the at least one gas cushion in the liquid in the fluid chamber of the hydraulic cylinder; unlike the hydraulic fluid in the fluid chamber of the cylinder, the gas in the gas cushion is in fact compressible. The gas cushion has to be hermetically enclosed in the fluid chamber between the mass of hydraulic fluid and the inside of the fluid chamber.

The hydraulic cylinder has preferably a pressure-medium supply line via which hydraulic fluid under pressure can be led into the fluid chamber. This line incorporates advantageously a check valve which prevents hydraulic fluid from flowing out from the fluid chamber. To render the fluid chamber quick and easy to empty, e. g. when the tensioning force of the tensioning device has to be slackened to facilitate belt changing, it is preferable that the check valve be provided with some suitable type of deactivating means whereby the check valve's blocking action which prevents outflow can be neutralised.

The lever means may for example be a straight lever bar or a lever element of some other design, which is supported for pivotable guidance at one end and carries the tensioning wheel at another end. The piston movable in the fluid chamber has protruding from the hydraulic cylinder a piston rod whose free end is connected articulatedly to the lever bar/lever element at a point at a distance from said ends. In practice it may perhaps be preferable that the hydraulic fluid under pressure in the fluid chamber takes the form of oil from the pressure oil system of the vehicle engine.

The fact that the oil pressure from the engine ceases when the engine stops does not represent any complication, owing to the presence of the check valve which is situated in the pressure medium supply line and which effectively prevents hydraulic fluid from flowing back out of the fluid chamber and thereby causing reduced pressure in the chamber. The fact that the tensioning force generated by the tensioning device can thus be kept constant is a substantial advantage compared with today's known tensioning devices, which lose tensioning force as the belt lengthens (stretches), which is a great disadvantage.

The invention is explained and made clearer below with reference to a very simple schematic embodiment of a tensioning device of simple design depicted in the attached drawings, and how this tensioning device can be arranged to keep a driving belt tensioned.

Brief description of the drawing The single drawing depicts in side view and partly in longitudinal section a tensioning device according to the invention.

Description of an embodiment The diagram depicts a tensioning device 2 intended to tension an endless motion- transmission means which in this case takes the form of a V-belt 4 driven from a vehicle engine M not depicted in more detail. The tensioning device incorporates a

lever means in the form of a straight lever bar 6. This lever bar has a first end 8 hinged for pivoting with respect to the vehicle engine, and a second end 10 which supports for rotation a V-belt tensioning wheel 12 mounted on it. The device 2 also incorporates a power-developing means in the form of a hydraulic cylinder 14 connected to the lever bar 6 via a piston rod 16 which has its free end articulatedly connected to the lever bar 6 at a connection point 18 between the lever bar ends 8 and 10. The hydraulic cylinder 14 is designed to exert upon the lever bar 6 a strong enough tensioning force to keep the tensioning wheel 12 in an active tensioning position whereby sufficient tensioning force is exerted upon the V-belt 4.

The hydraulic cylinder 14, which has its righthand end supported for pivoting with respect to a bracket 20 fastened to the vehicle engine M, has a cylindrical housing 22 enclosing a fluid chamber which is bounded by the housing walls and by a piston 24 which is movable axially in the housing. The piston rod 16, which is fastened to the piston 24, protrudes from the hydraulic cylinder 14 via a central aperture in the lefthand endwall of the cylinder housing 22.

The fluid chamber of the hydraulic cylinder contains not only hydraulic fluid 26 under pressure but also a gas cushion 28 which constitutes a motion-damping springing means inside the fluid chamber. Hydraulic fluid 26 under pressure from the pressure oil system of the vehicle engine (see arrow OM) can be led into the fluid chamber via a pressure-medium supply line 30 which is connected to the cylinder housing 22 and which incorporates a check valve 32 which prevents hydraulic fluid from flowing out from the fluid chamber, e. g. when the vehicle engine has stopped. The check valve 32 is advantageously provided with some kind of deactivating means, e. g. an openable drain valve, whereby the check valve's blocking action for preventing backflow can be neutralised, e. g. when the fluid chamber has to be emptied of hydraulic fluid.