GB2073322A - Single-acting fluid cylinders with means for damping oscillations of the applied load - Google Patents

Abstract

Single-acting pressurized fluid cylinders, especially hydraulic cylinders of the single cylinder and plunger type include between the piston surface (A) of the tubular plunger or piston (3) and the opposite end (2) of the cylinder (1), a partition which divides the cylinder space into two chambers (V1, V2). The partition (6) has at least one throttle opening (7), by means of which relative oscillatory motion of the piston (3) and the cylinder (1) is damped. <IMAGE>

Description

SPECIFICATION Damping device for single-acting pressurized fluid cylinders The invention relates to a damping device for single-acting pressurized fluid cylinders, especially hydraulic cylinders of the s.c. plunger type.

Single-acting hydraulic cylinders are frequently used in applications, where heavy masses are to be lifted and gravity is used for the return of the cylinder, e.g. for the operation of ship gates, tilting of melting furnaces in iron works, etc. Single-acting cylinders are often given a s.c. plunger design with a closed or open plunger, as illustrated in Fig. 1 and Fig. 2, respectively. The mass m is carried by the hydraulic pressure P in the oil volume V. The pressure acts upon the piston surface A, and the oil volume V is composed of the cylinder volume and the volume of the conduits. The friction in the seal between the piston, i.e. the plunger, and the cylinder can be represented by a friction coefficient B, and a possible leak through the seal by a leak coefficient C.

The system mass-cylinder is analogous to a mechanical mass-spring system with a damper as shown in Fig. 3. The spring force F corresponds to the compressibility in the oil volume V and the damper D to the friction and leakage in the cylinder seal. Of course, one tries to minimize friction and leakage in the cylinder, since they involve losses. This means that the inherent damping is very small, and the system is therefore inclined to perform considerable oscillations. The relative inherent damping is generally of the order of 0.01-0.2. However, in applications where a short transient period without many overshoots is desired, the relative damping should be at least about 0.5.

Previously known methods to increase the damping are to increase the friction or leak of the cylinder, to change the relation between the mass m and the volume V, or to introduce some kind of damping device into the system, such as an accumulator provided with a throttling or a more sofisticated damping network.

An increase of the friction will result in energy losses, and moreover it is very difficult to control the friction, i.e. there are problems in obtaining the desired damping. An increase of the leak flow is certainly not as difficult to control, but even this measure will lead to energy losses, both when the cylinder is in operation and when it is idle.

If the inherent damping is composed mainly of either friction or leakage, the damping can be increased by changing the relation between mass and volume, m/V. Damping caused by leakage, consequently, increases if m/V increases, whereas damping caused by friction increases if the relation m/V decreases.

By means of a correctly dimensioned gas charged accumulator ACK and an associated throttling S, as shown in Fig. 4, it is possible to achieve additional damping. However, by such a measure, a delay is introduced, resulting in that the cylinder cannot respond quickly enough to changes in the desired speed or direction. Moreover, when starting the cylinder, it is hard to avoid jerks. Gas-loaded accumulators require service, since the biasing pressure must be checked regularly.

Starting jerks can be reduced by inserting a valve which is more complicated than the throttle, e.g. a valve sensing the rate of pressure increase and being adapted to open at a certain increase rate.

Furthermore, the Swedish Laid-Open Print 364 1 67 discloses how to achieve damping at double-acting hydraulic cylinders by connecting the two cylinder chambers to each other by means of a throttle situated either in an external channel, namely in a channel outside the cylinder chambers, or in the piston itself.

Certainly, a damping can hereby be achieved, but at the same time it gives only a relatively low efficiency, i.e. minor forces for a given hydraulic pressure and given cylinder dimensions.

The object of the invention is to achieve the desired damping for single-acting pressurized fluid cylinders while securing a good efficiency and without complicated external auxiliary devices. This object is achieved by means of a damping device having the features defined principally in the appended claim 1.

Suitable embodiments appear from the subclaims.

The invention will be described further below with reference to the drawings.

Figures 1-4 illustrate schematically, as discussed above, previously known s.c. plunger cylinders having a closed and an open plunger (Fig. 1 and 2, respectively), an analogue mechanical system (Fig. 3) and a previously known damping device (Fig. 4).

Figure 5 shows schematically a first embodiment of a damping device according to the invention; Figure 6 shows similarly a second embodiment; and Figs. 7 and 8 show two diagrams illustrating the damping effect obtained by the invention.

The single-acting hydraulic cylinder shown in Fig. 5 comprises a cylinder tube 1 having a lower, closed end wall 2 and a plunger or piston 3 displaceable therein. The plunger or piston is sealed against the cylinder tube 1 by means of a sealing 4 and carries a mass m.

This mass m can be moved relative to the support (on which the cylinder tube 1 rests) by the influence of hydraulic oil which is supplied via a feed conduit 5.

The plunger or piston 3 is tubular and has at its top a piston surface A facing the end wall 2 of the cylinder tube 1. Between the piston surface A and the end wall 2 a cylinder room is defined, in which the hydraulic oil pressure acts when the cylinder is operated.

According to the invention, the cylinder room is divided into two chambers V1 and V2 by means of a partition 6, which is provided with at least one throttle opening 7. In the embodiment according to Fig. 5, the partition 6 is constituted by an end plate which is attached to the lower end of the piston tube 3, and the throttle opening 7 consists of a central, circular hole. It is understood that the partition 6 and the throttle opening 7 or openings can be formed in different ways, as long as the cylinder room is divided into at least two chambers communicating with each other via one or more throttles. Hereby, an effective damping sf relative oscillatory motion of the piston 3 and the cylinder 1 is obtained.This will be understood by assuming that the feed conduit 5 is closed, and the piston 3 with the mass m carried thereby is brought to oscillate relative to the cylinder tube 1 and the support. Upon such an oscillatory motion, a flow must pass through the throttle in either direction, since the pressurized fluid will always try to equalize the pressures in the two chambers V1, V2, and one of these chambers (namely V2 in Fig. 5) changes its volume during the oscillatory motion. This pulsating flow of pressurized fluid results in an energy loss, i.e. energy is required for forcing the flow through the throttle. This energy is taken from the oscillatory motion, which is therefore damped correspondingly.

Normally, the size of the throttling is approximately 1-30 mm in diameter and is determined in view of the intended use of the cylinder, while considering the size of the mass m, the maximum cylinder volume V1 + V2 and the proportional relation of V1 and V2, the working pressure and compressibility of the pressurized fluid, the desired operational speed; the required damping effect and other parameters. If the throttle opening 7 is made very large, one approaches the case of an entirely open plunger (according to Fig. 2), whereas, if the throttle opening 7 is very small, one approaches the case of a totally closed plunger (acc. to Fig. 1).

As discussed above, these two extremes result only in the inherent damping caused by friction and leak in the piston sealing. By means of a correctly dimensioned throttle opening 7, a desired relative damping can, however, beeachieved, normally in the order of 0.5-1.0.

In Fig. 6 a second embodiment of the inventive damping device is shown, and the same reference numerals indicate corresponding parts. This embodiment differs from the previous one in that the tubular plunger or piston 3 is displaceable externally of the cylinder tube 1. Moreover, the partition 6' with the throttle opening 7 is fixed to the upper end of the cylinder tube 1. Thus, in a corresponding way, a division into two chambers V1, V2 is accomplished, and in this case the volume of the upper chamber V1 is variable.

An advantage of this embodiment is that the damping device, apart from its primary function of damping oscillations, also works as a damper of emergency lowering, i.e. in case of breakage of a tube or a hose in the feed system 5. In such a lowering movement, the flow is forced to pass through the throttle 7, whereby the lowering speed is reduced. A further advantage of the embodiment according to Fig. 6 is an improved damping (as compared with the embodiment according to Fig. 5) in case of very small oscillations during stationary movements. This effect relates to the non-linear characteristics of a simple throttling in the form of a circular hole in a plate or the like. On the other hand, there is an energy loss of appr. 510%, which is of course a draw-back and a result of the flow passing through the throttle during a stationary lifting movement.

The diagram in Figs. 7 and 8 illustrate the advantages of the invention; Fig. 7 shows the variation of the oil pressure during a stationary lifting movement, on the one hand with (fully drawn curve), and on the other hand without (dashed curve) the inventive damping device. Fig. 8 shows in a similar manner the piston or plunger movements as a function of time.

The diagrams in Figs. 7 and 8 are the result of a simuiation in a computer, assuming that the damping device is designed in accordance with Fig. 5 and that the plunger (piston) at the starting moment, i.e. at the time O's, is locatedi in its bottom position, and that the hydraulic pressure is appr. 3 MPa. At the starting moment, the hydraulic oil is supplied through the feed conduit at a constant flow rate (q), so that the pressure is built up. In order to clearly illustrate the appearance of an oscillatory motion, a great difference between motion friction and rest friction is further assumed. The difference in fictional force corresponds to a pressure of 5 MPa in the cylinder. When sufficient pressure has been built up in the cylinder to overcome the sum of the frictional force and the weight of the carried mass, the movement of the piston will start. At this moment, the large rest friction disappears, whereupon the pressure in the cylinder is quickly reduced and the oscillatory motion starts. The following values have been used for the particular parameters: Mass m=108.103kg Area A= 7,6.10-2 m2 (corresponding to a diameter of 0,31 m) Volume V1 = 124 dm3 Volume V2 = 64 dm3 (at the starting moment) Flow rate q = 0,9 dm3/s Diameter of throttling opening d = 8 mm Thus, in the case an optimal damping has been obtained for a throttle opening diameter of 8 mm. However, acceptable damping has been achieved for values in the interval 6-10 mm.

Finally, it should be pointed out that the device can be modified in many ways within the scope of the inventive idea. Thus, as mentioned above, more than one partition, each having a throttle (in the form of one or several throttle openings), can be disposed in the piston and/or the cylinder tube. Moreover, the partition can be divided into two or more portions defining between themselves the throttle openings. The configuration of the latter can likewise be varied at will, such as slot-shaped, polygonal, elliptical, etc. It is also possible to insert, in parallel to the throttle opening, a non-return valve in the partition so as to reduce the energy losses at a stationary lifting motion.

Claims (4)

1. A damping device for single-acting pressurized fluid cylinders, especially hydraulic cylinders of the s.c. plunger tube, comprising a cylinder closed at at least one end and a piston slidable relative to the cylinder and having a piston surface facing said closed end, the piston being displaceable by the influence of a pressurized fluid in the cylinder space between said piston surface and said end, characterized in that at least one partition with at least one throttle opening is disposed between said piston surface and said end for dividing said cylinder space into at least two chambers communicating with each other via said throttle opening or openings, whereby damping of relative oscillatory motion of the piston and the cylinder is obtained.

2. A damping device as set fourth in Claim 1, characterized in that the piston is slidable inside the cylinder and that the partition is attached to the piston.

3. A damping device as set fourth in Claim 1, characterized in that the piston is tubular and slidable outside the cylinder and that the partition is attached to the cylinder.

4. A damping device substantially as here it before described with reference to Fig. 5 or Fig. 6 of the accompanying drawings.