The present invention refers to a method for controlling vibration imparted on different types of rotary systems following imbalance in said systems. For minimizing such im¬ balance tendencies it has since long been provided mechanisms, which automatically outbalance the vibrations and allow the system to operate at a substantially reduced vi¬ bration level. This is achieved by means of rolling bodies freely movable in paths con¬ centric with the axis of the rotary system, which rolling bodies at rotation of the system automatically will find their appropriate angular positions along the path to compensate for the inherent imbalance of the system. Such mechanisms are commonly referred to as auto-balancing units, and the method itself is called auto-balancing.

Autobalancing mechanisms used e.g. for hand-held angle grinders have proven thenv selves to operate very well, whereas trials with corresponding mechanisms provided at big, floor-mounted grinding machines have given a very poor function. It furthermore has been found that at comparatively low rotational speeds and/or in combination with only slight out-of-balance in the system to be balanced, the balancing bodies will have difficulties to move to their optimum balancing positions, i.e. they will have a tendency to "float around" in the system without being able to find and stay in positions where they give the best possible balancing effect.

The purpose of the present invention is to provide a solution of this problem and to sug¬ gest a method for controlling vibration amplitude in rotary systems, whereby is obtained a rapid and appropriate automatic positioning of the balancing rolling bodies in their path, and this is achieved with the features defined in the accompanying claim 1.

Hereinafter the background to and the solution of the present invention will be described with reference to the accompanying drawings, showing schematically in

Fig. 1 a diagrammatical illustration over a mechanical system appropriate for being equipped with an autobalancing unit.

Fig. 2 is a diagram showing the vibrational characteristics for a discrete system with ini¬ tial eccentricity ε .

_c Fig. 3 illustrates in a schematical side view an autobalancing unit in out-balanced position.

Fig.s 4a and 4b illustrate schematically the forces acting upon a rolling body in an auto¬ balancing unit at different amplitudes, and

10

Fig. 5 illustrates schematically an autobalancing unit provided with an artificial imbal¬ ance in accordance with the present invention.

Most mechanical systems which are considered appropriate as applications for auto- - r balancing can be schematically described as in the accompanying drawing Fig. 1 , which very schematically shows a housing 1, being resiliently connected to a base 2, as illus¬ trated with a symbolic spring 3, and being provided with a shaft 4 having a rotating mass 5.

_Q Such a system has a vibration characteristic in accordance with the graph shown in Fig.

2, wherein is indicated the stationary vibration level, i.e. the vibration level resulting af¬ ter a long time and at different rotational speeds, for the rotating mass, (continuous line), and for the machinery housing (dash-line), when the rotating mass has an eccentricity ε relative to its shafting point.

5

In Fig. 3 is schematically illustrated in a side view an autobalancing unit, intended to be connected concentric to the rotary shaft or the like, which shall be outbalanced, and comprising an inner ring 6, an outer race ring 7 positioned about the inner ring 6 and concentric thereabout and a number of rolling bodies 8, preferably balls, but being of such a number that they occupy only a part of the volume contained between the two 0 rings 6 and 7. Although not shown in this figure the space between the rings 6, 7 is closed off externally by means of not shown end plates, and the free volume between

rings and endplates, not occupied by the rolling bodies 8 is usually filled with a me¬ dium, e.g. oil exerting a dampening and pulling-along effect on the rolling bodies. The unit in this figure is shown in out-balanced condition, whereby an out-off-balance effect is illustrated in the upper right hand quadrant of the outer race ring at OOB. The rolling bodies 8 here have taken up spaced apart positions along the outer race ring distributed about the position opposite to the position for the OOB, and thereby the disturbing out- off-balance OOB is compensated, and the centrifugal force F _c acting on the rolling body and the normal force F _N acting perpendicularly towards the race track are equal and directed in opposite directions and through the centre of rotation RC and the geometrical centre GC, which thereby coincide. Therefore there is no resulting force causing imbal¬ ance when the system is running. In this figure the out-off-balance symbol OOB is illus¬ trated as a rather small area, representing a comparatively small mass, and the rolling bodies 8 thereby are distributed over quite a long portion of the race track. If the size of the OOB and/or the distance between the position for OOB and the geometrical centre GC should increase, then the rolling bodies of course should become more concentrated opposite to the position for OOB, until they finally should be positioned close to each other.

The autobalancing principle requires that there is about 180° phase shift between the centre of gravity of the rotating mass and its deflection, i.e. when the centre of gravity of the rotating body 5 is pointing upwards on the body, then its shaft centre is in its lower¬ most position. This phase difference can be seen in the Fig. 2 diagram, as a "negative amplitude", i.e. the continuous line is in the appropriate area situated below the axis rep¬ resenting the rotational speed.

For most applications of autobalancing , e.g. for angle grinders, the rotational speed is generally in the intermediate area, i.e. on the plateau of the continuous line in Fig. 2.

Experiments have shown that the balancing function is dependent on a forced vibration amplitude U _f (intentional or caused by the design of the system), which can be expressed with the formula u _f = m _ro /m _tot «ε • Φ (ω/ω ₀ )

wherein m _m = the rotating mass of the system m _tot = the total mass of the system ε = the eccentricity ω = the rotational frequency ω ₀ = the resonance frequency 5 Φ (ω/ω ₀ ) = ^* goes against 1 the higher above the rotational frequency the system gets. According to the present invention it is now suggested that the above problem is solved in that the balancing bodies are influenced in a manner so as to help them initially to start finding their correct balancing positions, and this is done by intentionally increas- 10 ing the forced vibration amplitude U _f .

In accordance with the formula given hereabove this can be achieved by increasing the ratio between the rotating imbalance m.-,, ^* ^ and the total mass m _Iot , which last men¬ tioned is m _rot + m _h , i.e. the rotating mass together with the mass of the housing support¬ ing the rotating mass.

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This theory is further confirmed by the above mentioned finding that small hand held rotary implements are much more sensible to autobalancing functions than large ma¬ chinery which are furthermore often anchored in a large and heavy fundament, thereby having a substantially lower ratio m _ro /m _to( than the small hand-held tool.

20

With reference to Fig.s 4a and 4b it is schematically shown that at a system with imbal¬ ance there is a distance between the geometrical centre GC and the rotating centre RC, and this distance causes a resulting force F _R . In Fig. 4a is shown how a small distance al _- causes a small resulting force F _R , whereas a larger distance a2 shown in Fig. 4b causes a bigger force F _R .

The difference between heavy and light systems can be found in the size of the vibration amplitude. The resulting force F _R causing the balancing bodies (balls) in the balancing — unit to change their positions can be considered to be proportional to the vibration am¬ plitude. This is shown to be lower for heavy machines, such as floor-anchored grinding machines, as compared to light machines, e.g. hand-held angular grinders. The balls

thus will experience a lower resulting force at lower vibration amplitude, and if this is too low the balls will not be able to overcome the internal resistance of the system, such as moment of inertia, resistance from the oil provided in the housing of the autobalan¬ cing unit as a medium dampening the motion of the balls and carrying them along, and this means that the balls will not move to try to find their optimum positions from a bal¬ ancing point of view.

It now consequently is suggested to increase the imbalance in the system by adding an artificial imbalance to the system.

Experiments have proven that by imparting such an imbalance upon the system it is achieved also at comparatively low rotational speeds and/or at low "normal" or inherent imbalance in the system, that the balancing bodies rapidly and safely will find their proper positions for giving a satisfactory balancing effect to the system. It thus must be seen as a un-expected technical effect that by imparting upon a rotary system an in¬ creased artificial imbalance, an autobalancing unit associated with the rotary system will give a more satisfactory, and, above all, a more rapid balancing result than without such additional artificial imbalance.

For achieving this it is suggested, e.g. to provide the autobalancing unit with an addi¬ tional artificial imbalance body, as shown schematically in Fig. 5, where the outer race ring 7 is provided with a weight 9 increasing the "normal" imbalance in the system, and thereby initially increasing the resulting force F _R , whereby this will safely exceed the re¬ sistance in form of rolling resistance etcetera, and make die rolling bodies rapidly find- ing their appropriate positions.

Of course it is also possible to increase the "normal" imbalance within the system not by adding an extra weight, but also by removing weight at a proper position. Such a method for increasing the imbalance can have further advantages, as thereby no extra weight is added to the system as such.

The invention is not limited to the embodiments schematically shown in the drawings and described with reference thereto but modifications and variants are conceivable within the scope of the appended claims.