GB2143617A - Mechanical coupling which damps torsional vibrations - Google Patents

Abstract

The coupling has at least two weights (12), each of whose centre of gravity is at a distance from the axis (20) of the two main shafts (2, 5) serving as input and output. The weights are rotatably mounted in bearings (7, 8) which are rigidly connected to one of the main shafts (2, 5). Disposed between the other main shaft (2, 5) and the weights (12, 13) are transmission means (17, 18) tending to rotate the weights (12, 13) in relation to a zero position which the weights take up when the main shafts (2, 5) are running off-load and in synchronism with one another. The coupling has a torsional resistance which decreases with increasing load - i.e., the coupling is a mechanical coupling which has a very soft characteristic on full load. Also, the coupling helps to obviate the transmission of torsional vibrations from one of the main shafts (2, 5)- as a rule, the input shaft (2) - to the other main shaft - as a rule, the output shaft (5). <IMAGE>

Description

SPECIFICATION Mechanical coupling which damps torsional vibrations This invention relates to a mechanical coupling which damps torsional vibrations, has varying torsional resistance and serves to transmit a torque from a drive shaft to the shaft of a driven machine which absorbs power.

It is of course a particular phenomenon in plants where a reciprocating engine, such as a diesel engine, is used to provide the drive that torsional vibrations occur leading, for instance, to variations in the frequency of a generator constituting the driven machine; the result is variations in the electrical output which disturb the electricity network supplied by the generator.

The modern trend towards ever lower speeds for large engines means that the problem of vibration damping between the engine and the generator, more particularly in respect of low-frequency vibrations, is becominy more and more critical. If a coupling between the input drive and the output drive can provide at least substantial damping of the fundamental vibration frequency, the lowest critical speed of the system comprising the engine, coupling and generator, such speed being determined by the coupling, must be well below the operating speed i.e., the coupling must be very soft.

Since the materials, such as spring steel or rubber, used in mechanical couplings are not infinitely deformable no mechanical couplings meeting this critical speed requirement have yet been devised for slow-running engines. Consequently, couplings which do meet the requirement have usually been liquid couplings. Their disadvantage is their slip, so that output shaft speed does not coincide with input shaft speed; also, they cause losses of efficiency of from 5 to 7%. Finally, they are expensive to produce and maintain.

It is therefore an object of the invention to provide a mechanical coupling of sufficient resilience or softness which reduces the critical speed hereinbefore defined considerably below the operating speed - i.e., which enables the system to operate in overcritical conditions.

According to one aspect of the present invention, therefore, a mechanical coupling has an input member and an output member, the said members being rotatable about a common axis, one of the said members carrying at least two weights each of which is mounted for rotation about a rotational axis and has its centre of gravity displaced from the said rotational axis, and transmission means connecting each weight to the other of the said members such that rotation of the weight about its rotational axis without rotation of the said one of the said members produces rotation af the said other of the said members.

According to another aspect of the present invention, a mechanical coupling for damping torsional vibrations with a varying torsional resistance in transmitting a torque between two main shafts, a drive shaft and a concentric shaft of a driven machine, comprises: at least two weights peripherally distributed around the axis of the two main shafts and arranged with their centres of gravity spaced from the said axis, the weights being mounted by means of at least one additional shaft for rotation relatively to their bearings, the said bearings being rigidly connected to one ofthe main shafts; and transmission means between the other main shaft and the weights which produces a rotation of the weights away from an ideal zero position associated with offload rotation.

The term "weight" is intended to refer to the total mass or weight of a "rotor" which moves relatively to its bearings; this weight of the "rotors" can be assumed to act at a distance from the main shaft axis on the centre of gravity of the "individual weight".

The "ideal zero position" in relation to rotation of the weights therefore occurs when, in a theoretical hypothetical case, the coupling transmits no power with the input and output shafts rotating synchronously with one another and at a constant speed. In this ideal theoretical state of synchronism - i.e., with the two main shafts rotating completely unloaded and synchronously - the weights take up such a position due to the centrifugal forces acting on them that, in the absence of any deflection (a in Figure 1), they apply no torque to the axis of their bearings and/or, by way of the transmission means, to the main shaft not connected to the bearings.However, the unavoidable losses in the sub-system comprising the rotor of the associated driven machine and whichever main shaft is not connected to the bearings cause the weights to rotate away from this ideal zero position into the "no-effective-load" zero or neutral position in which the coupling transmits a reduced torque to the output shaft.

Each load leads to a further rotation of the weights and, therefore, of the main shaft not connected to the bearings in relation to the main shaft which is connected thereto and to the weights. Consequently, the driving torque transmitted by the coupling is transmitted by it as a centrifugal torque. At a constant speed this torque is proportional to sin (2) in which i3 denotes the transmission ratio of the transmission means and W denotes the angle of rotation between the input and output.The torsional resistance, which is the first derivative dM/dW of the driving torque, is therefore proportional to cos (2eP)- Consequently as the angle bur increases, the torsional resistance of the coupling decreases to reach zero at an angle 0W 45 Theoretically, the angle ew can vary between 0 and 45" (Figure 1).Conveniently, however, the angle e does not exceed from 35 to 40 at the full service load in order to give some reserve in practice; a given coupling can be adapted to the maximum load to be transmitted by appropriate choice of the centre-ofgravity distances and/or the size of the weights and/or the transmission ratio e of the transmission means. Advantageously, these parameters can be variable in a given coupling design to enable a single "size" of coupling to cover a wide operating range.

The vibration damping provided by the coupling arises because the torsional vibrations emanating from the driving engine are mainly converted into oscillations A4i of the weights at the angle eq associated with the instantaneous load but do not pass through to the driven machine.

The novel coupling may be constructed to be reversible in all senses - i.e., the direction of rotation of the system and its input and output and, therefore, the "flow direction" of the torque to be transmitted can be changed over without loss of effectiveness. Advantageously, the additional shafts are received in a casing of the coupling; also, the additional shafts can be disposed in a plane perpen dicularto and intersecting the main shaft axis or in a plane normal to such axis or parallel to such axis.

Various transmission means may be used. Bevei gears and/or bevel segments and a worm gear with toothed segments make very satisfactory transmission means. Linkage systems or transmission means in the form of a planetary transmission can be used, the sunwheel of the planetary transmission being secured to one of the main shafts.

It may in some circumstances be convenient to damp the turning movements of the weights, for instance, by means of the displacement of a body of liquid in the form, for instance, of a liquid filling of the coupling casing which receives the weights.

Alternatively, damping can be provided by friction plates disposed between the weights and the bearings.

As previously stated, the foregoing descriptions of how the novel coupling operates relate to a state of the system associated with a predetermined constant synchronous speed; below such speed, however, the centrifugal torque produced by the weights is low. Consequently, unless special provision is made, the weights move abruptly into an end position at start-up. Conveniently, therefore, impactdamping abutments, for instance in the form of springs or rubber buffers, may be provided for this end position. To ensure that the weights are not in an undefined position when the coupling is stationary, means may be provided to move the weights into and retain them in a defined position; such means may be a spring. Another possibility is for the characteristic of the varying torsional resistance to be modified by resilient means such as a spring.

Optionally, a clutch adjacent to or integral with a coupling according to the invention can be provided in one of the main shafts, preferably in the input shaft.

The invention may be carried into practice in various ways but a number of couplings embodying the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view, partly in section on the line I-I of Figure 2, showing one construction of the coupling; Figure 2 is a plan view corresponding to Figure 1; and Figures 3 to 6 are respective views, also in diagrammatic form, of further possible constructions of the novel coupling.

Referring now to Figures 1 and 2, the coupling transmits torque between two concentric main shafts, a drive shaft 2 and a driven or output shaft 5.

The drive shaft 2 extends from a driving engine 1, which is on the right of Figure 1 and which is, for instance, a diesel engine, to a frame 3 to which a bearing housing 4 is so secured as to be concentric of and opposite to the shaft 2. The housing 4 receives the driven or output shaft 5 which is in alignment with the drive or input shaft 2 and which extends to a driven machine 6 such as an electricity generator; the drive and output shafts 2, 5 have a common axis 20.

A cross-shaft 9 is so mounted in two bearings 7, 8, visible in Figure 2, in the frame 3 as to be perpendicular to and to intersect the axis 20 and two flyweight rotors 10, 11 are so mounted on the cross-shaft 9 as to be rotatable independently of one another. Each of the rotors 10, 11 comprises diametrically opposed flyweights 12, 13 whose respective centres of gravity S are at a distance rfrom the axis of the cross-shaft 9 and which are secured by way of lever arms 14, 15 to one end of one of a pair of hollow additional shafts 16. Each additional shaft 16 is rotatably mounted on the cross-shaft 9 and carries at one end one part 17, in the form of a bevel gear, of a transmission means 17, 18.The other part 18 thereof takes the form of a second bevel gear which is connected by way of a yoke 19 to that end of the output shaft 5 which extends from the bearing housing or sleeve 4 into the frame 3.

As a result of centrifugal forces F which act on the respective centres of gravity S, the rotors 10, 11 take up the ideal zero position, shown in solid lines in Figure 1, when both the shafts 2, 5 are running unloaded at a constant synchronous speed. In practice the rotors 10, 11 take up a different position due to the power needed to overcome friction in the driven system even when no useful power is being transmitted, the difference between the actual position and the ideal zero position being indicated by an angle ewO. In this position - i.e., the position previously referred to as the "no-load" zero position the coupling transmits a low torque to the driven shaft 5, since the angle ewO, shown greatly exaggerated in Figure 1, and therefore sin (6ro), are relatively small.The torsional resistance is relatively high in this no-load zero position since the cosine of the angle 4ic is near its maximum value.

When the driven machine 6 demands a particular power, the coupling reacts by the rotors 10,11 moving further away from the ideal zero positon i.e., by increasing the deflection or deviation a - and forming with the ideal zero position an angle zero such that the coupling transmits exactly the required power. Since the cosine of the angle eMB decreases inversely to its sine, the on-load torsional resistance of the coupling is reduced considerably and continues to decrease as the load increases.

The coupling shown in Figure 3 is a diagrammatic view of a system having features very similar to the basic construction shown in Figures 1 and 2. Like elements therefore have the same references as in Figures 1 and 2. The frame 3 of Figures 1 and 2 becomes a closed rotating parallelepipedic casing 23 which possibly contains a liquid for damping the movement of the weights 12, 13. The casing 23 also contains, in addition to the elements of Figures 1 and 2, impact-damping abutments, for instance, rubber buffers 21 or springs or oil dash pots, for limiting the movement of the weights 12, 13. As already stated, these abutments serve more particularly to damp and limit the abruptness with which the weights 12, 13 move into the end position when the shafts 2,5 start to turn.

The drive shaft 2 of the coupling shown in Figure 4 is rotatably mounted in a rotating casing 33 by means of bearings 24,25, the casing 33 being rigidly connected to the output shaft (not shown). Weights 32 take the form in this case of four separate quadrants arranged so that one pair is in a vertical plane when the other pair is in a horizontal plane, the quadrants being rigidly secured to spindles 36, one such spindle 36 being provided for each weight 32.

Each spindle 36 is rotatably mounted in bearings 27 carried by beams 29 secured between the ends of the casing 33, for instance, by welding. Transmission means 37 for transmitting the torque is constituted by toothed rings or toothed segments which are integral with the weights 32 and which engage a worm gear 38 rigidly secured to the input or drive shaft 2.

The weights 32 of the construction shown in Figure 5 are similar to the weights 32 of the construction shown in Figure 4; they are rigidly secured to additional shafts 46, but these extend in radial planes parallel to the axis of the drive shaft 20 and are rotatably mounted in bearings 47,48 in the ends of a casing 43 which is rigidly connected to the output shaft (not shown).

The transmission means of this construction takes the form of links or levers 40, one end of each of which is pivotally connected to a collar 41 rigidly connected to the input shaft 2, the other ends of the levers 40 being pivotally mounted on projections 44 on the weights 32.

At least one of the additional shafts 46 has a friction damper 22; this includes a disc 30 which is rigidly secured to the shaft 46 and a ring 31 which is keyed to the shaft so as to rotate with it but to be free to move axially of the shaft. Springs 35 which bear on the disc 30 press the ring 31 axially of the shaft 46 on to an end wall of the casing 43. The ring 31 has a friction plate 34 opposite a corresponding plate 26 on the casing end wall. Like the oil filling mentioned in connection with Figure 3, the damper 22 serves to damp the movements of the weights 32.

The construction shown in Figure 6 differs from that shown in Figure 5 only in details; for instance, the transmission means of Figure 6 takes the form of a planetary transmission 50 whose sunwheel 51 is rigidly connected to the input shaft 2. Satellites 52 meshing with the sunwheel 51 are secured to the additional shafts 46. The weights 32 are disposed on the shafts 46 adjacent the satellites 52. The construction of Figure 6 has no friction damper 22 but it does comprise a coil spring 49 which is disposed between the casing 43 and the sunwheel 51. The function of the spring 49 is to vary the "characteristic" of the torsional resistance of the coupling by its resilience being superimposed as a fixed amount, for instance 10% of coupling resilience on full load, upon the varying resilience of the coupling.The effectiveness of the spring 49 is particularly apparent in torque transmission at start-up of the main shafts 2,5 - i.e., at a period when in the absence of centrifugal forces the coupling itself is not transmitting torque to the output shaft 5.

The spring 49 can also serve to locate the weights 32 positively in the end positon when they are stationary, for instance in the zero position shown in Figure 1 in which, as stated, the lack of deflection means that no torque is being transmitted. Another possibility is so to act on the weights, for instance again by springs, that in the stationary state they engage the abutments 21 of Figure 3 to obviate impacts at start-up.

While damping means, buffers, means for modifying the torsional resistance characteristic and means for locating the weights when the coupling is stationary have been described only in relation to certain of the embodiments, it will be apparent that each embodiment could have any or all of these elements.

In each of the embodiments means could be provided for varying the distance between the centre of gravity and the axis of rotation of each weight and/or the size (i.e. the mass) of each weight and/or the transmission ratio of the transmission means.

Claims (16)

1. A mechanical coupling having an input member and an output member, the said members being rotatable about a common axis, one of the said members carrying at least two weights each of which is mounted for rotation about a rotational axis and has its centre of gravity displaced from the said rotational axis, and transmission means connecting each weight to the other of the said members such that rotation of the weight about its rotational axis without rotation of the said one of the said members produces rotation of the said other of the said members.

2. A mechanical coupling for damping torsional vibrations with a varying torsional resistance in transmitting a torque between two main shafts, a drive shaft and a concentric shaft of a driven machine, the coupling comprising: at least two weights peripherally distributed around the axis of the two main shafts and arranged with their centres of gravity spaced from the said axis, the weights being mounted by means of at least one additional shaft for rotation relatively to their bearings, the said bearings being rigidly connected to one of the main shafts; and transmission means between the other main shaft and the weights which produces a rotation of the weights away from an ideal zero position associated with off-load rotation.

3. A coupling as claimed in Claim 2 in which the bearings are carried by a casing.

4. A coupling as claimed in Claim 2 or Claim 3 in which the distance from the centre of gravity and/or the size of the weights and/or the transmission ratio of the transmission means is variable.

5. A coupling as claimed in Claim 2 or Claim 3 or Claim 4 in which the axis of the or each additional shaft is disposed in a plane perpendicular to and intersecting the main shaft axis.

6. A coupling as claimed in Claim 2 or Claim 3 or Claim 4 in which the axis of the ooh teach additional shaft is disposed in a plane normal to the main shaft axis and is spaced from the main shaft axis.

7. A coupling as claimed in Claim 2 or Claim 3 or Claim 4 in which there is a plurality of additional shafts, each having an axis which is parallel to the main shaft axis.

8. A coupling as claimed in any of Claims 1 to 5 in which the transmission comprises bevel gearing.

9. A coupling as claimed in any of Claims 1 to 4 or Claim 6 in which the transmissiion means comprises a worm gear and toothed elements meshing therewith.

10. A coupling as claimed in any of Claims 1 to 4 or Claim 7 in which the transmission means comprises a linkage system.

11. A coupling as claimed in any of Claims 1 to 4 or Claim 8 in which the transmission means comprises a planetary transmission having a sunwheel secured to one of the main shafts.

12. A coupling as claimed in any of the preceding claims in which rotation of the weights is limited by impact-damping abutments.

13. A coupling as claimed in any of the preceding claims which includes means for damping the movement associated with a rotation of the weights.

14. A coupling as claimed in any ofthe preceding claims which includes means for retaining the weights in a defined position when the coupling is stationary.

15. A coupling as claimed in any ofthe preceding claims which includes resilient means for modifying the varying torsional resistance characteristic.

16. A mechanical coupling substantially as described herein with reference to Figures 1 and 2 or Figure 3 or Figure 4 or Figure 5 or Figure 6 of the accompanying drawings.