GB2024945A - Piston-cylinder mechanisms with a braking function - Google Patents

Abstract

In a piston-cylinder actuator the movement of the piston 2 towards its cylinder end wall 3 is braked by throttling the discharge of working medium from the chamber 4 defined between the piston and the end wall 3, the discharge being forced to flow through an opening or openings (16) which extend axially in the cylinder side wall 15. By this means, the available cross-sectional area of the opening or openings is progressively reduced by the passage of the piston 2 thereacross. The or each opening 18 may be in the form of a groove of tapering depth (Figures 3 and 5) or a row of apertures (Figure 6). <IMAGE>

Description

SPECIFICATION Piston-cylinder mechanisms with a braking function The invention relates to piston-cylinder mechanisms in which movement of the piston towards a cylinder end wall is braked by throttling the discharge of a working medium from a chamber defined between the piston and said end wall.

A typical mechanism of this kind is a hydraulically actuated servo motor the piston of which is connected to a mass which is to be moved by means of a piston rod. For example, such a mechanism is used in a machine for producing blow-moulded hollow plastics articles, in which the problem is to move a heavy blowmoulding unit from one working position to another and back again. This displacement has td be effected rapidly, but the movement must be gently braked before the particular end position is reached so that the mass to be moved, e.g. a blow-moulding unit, comes to abut gently on a stop. The transporting rate is desirably reduced to less than 5 cm/s shortly before the stop is reached, the deceieration being as steady as possible.

In the past, such braking action has been obtained, with a standard commercial pistoncylinder mechanism for example, by providing, in the end wall of the cylinder, an annular slot coaxial with the piston for the discharge of the working medium for the cylinder chamber. Attached to the piston was a conical bushing tapering gradually to, at most, the diameter of the piston, and passing into the annular slot as the piston moved towards the end wall, and thus gradually reducing the flooded cross-section of the annular slot. It is desired to achieve a braking function with a speed reduction down to a final speed of less than 5 cm/s with these known working cylinders, the cross-section of the annular gap is so small, i.e.

there is so little clearance between the parts which constitute the annular gap, i.e. between the bushing and the bore in the end wall, that the practical limits of manufacturing technology are reached. Consequently, the reproducability is poor and even slight mechanical wear results in a much altered braking function. Moreover, the drop in pressure in the long, narrow annular slot and hence the braking function depend greatly on the influences caused by the temperature of the working medium.

The aim of the invention is to provide a pistoncylinder with a braking function with which a reduction in speed down to a final speed of, in particular, less than 5 cm/s can be reliably achieved. The mechanism should be relatively simple to manufacture and the braking function should not depend greatly on the temperature of the working medium.

To this end, the invention provides a pistoncylinder mechanism in which movement of the piston towards a cylinder end wall is braked by throttling the discharge of a working medium from a chamber defined between the piston and said end wall, wherein the discharge duct for such working fluid opens into said chamber through the cylinder side wall along an axial length thereof whereby the cross-sectional area of said opening is reduced by passage of the piston thereover.

As it moves towards the end wall, the piston progressively covers the discharge duct from the cylinder so that the flooded cross-section of the discharge for the working medium becomes smaller, the discharge is thereby progressively throttled and themovement of the piston is progressively braked. The channel, with its opening into the cylinder chamber, is simple to manufacture and reproducible. The construction of the opening in the cylinder wall of the cylinder chamber is not inherently prone to wear and the mechanism can operate for years on end with substantially the same braking effect, satisfactorily giving a speed reduction down to a final speed of less than 5 cm/s. The temperature of the working medium has only a slight effect on the desired braking action.

In one preferred embodiment, the discharge opening from the cylinder is a groove extending in the axial direction of the cylinder A groove of this kind is particularly easy to manufacture, with a very high degree of reproducibility.

According to another embodiment, the discharge opening is a groove or notch extending in the cylinder wall at an inclined angle relative to the cylinder axis. The braking effect obtained thereby can be represented by a curve.

In yet another embodiment, the discharge opening comprises a series of apertures spaced along an axial length of the cylinder side wall.

Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings wherein: Figure 1 shows in longitudinal section a first embodiment of the invention; Figure 2 is a section on the line Il-Il in Figure 1; Figure 3 is a section on the line Ill-Ill in Figure 2; Figure 4 shows a second embodiment of the invention; and Figures 5 and 6 show alternative designs for the discharge opening which may be used in mechanisms of the invention.

The mechanism 1 with a braking function comprises, between its piston 2 and end wall 3, a cylinder chamber 4 which can be acted upon by a working medium. The pressure medium in this case is oil which is conveyed from a tank 5 via a pump 6, a valve 7 and a duct 8 into the cylinder chamber 4. When the piston 2 moves towards the end wall 3, the oil flows out of the cylinder chamber 4 via a duct 9 arti the correspondingly set valve 7, back into the tank 5.

The cylinder also defines a second cylinder chamber 10 between the piston 1 and a second end wall 1 and this second chamber 10 can Plso be acted upon by the pressure oil. The piston 1 is connected, by means of a piston rod 12, to a mass 13, e.g. a blow-moulding unit in a machine for producing blow-moulded hollow articles, this mass 1 3 being intended to travel along a path 1 4.

In a first operating cycle, the mass should be moved from a first working position in which the piston 2 abuts on the end wall 3, to a second working position in which the piston 2 abuts on the second end wall 11. In a subsequent operating cycle the blow-moulding unit is moved back to its first working position, and so on.

During displacement of the blow-moulding unit, the piston 2 travels at a certain transporting speed towards the end wall 3, for example, and before it reaches this end position the transporting speed is reduced to a final speed at which the piston 2 comes to abut on the end wall 3. This final speed should be so low, for example 5 cm/s, that neither the working cylinder nor the blow-moulding unit being moved is damaged by any great impact.

This braking action of the working cylinder is obtained by throttling the pressure oil which is to be conveyed out of the cylinder chamber 4 ofthe working cylinder. For this, at least one channel 1 6 passing through the cylinder wall 1 5 is provided, which opens relative to the cylinder chamber 4 along a portion 1 7 of the path of the piston 2 towards the end wall, the braking action occurring in this portion. This opening is designated 18.

The opening of the channel 1 6 into the pressure chamber 4 is, as shown in Figures 1, 2 and 3, in particular, a groove or notch 18 extending at an inclined angle relative to the cylinder axis, the cross-section of which decreases towards the end wall 3. In most of the illustrated embodiments, three channels 1 6 or openings 18 are provided, uniformly distributed round the periphery of the cylinder chamber 4. In Figure 5, there are four. However, in principle, only one channel is required. As the piston 2 moves towards the end wall 3, the pressure oil in the cylinder chamber 4 flows through the channels 16 provided. The piston travels at a transporting speed until it reaches the channel, or the opening thereof, which is provided above the portion 17.

As the piston moves on towards the end wall 3, it successively starts to cover the opening 18 so that a successively smaller flooded cross-section is available for the discharge of the oil, thus progressively throttling this discharge. The final speed of the piston is reached at the point where the piston 2 is covering the entire opening 18 of the channel 1 6 and the piston becomes stationary at this point.

In the embodiment shown, the opening 1 8 of the channel 1 6 ends slightly in front of the end wall 3, however. An additional discharge duct 19 is provided in the end wall and is provided with a fine throttle 20. This finely adjustable throttle 20 serves to throttle the discharge of the residual oil from the cylinder chamber 4 and thus set up a uniformly braked movement of the piston 2 up to the end wall 3, at the final speed which was reached at the end of the portion 1 7.

The opening 18 of the channel 16, shown in Figures 1, 2 and 3, in particular, in the form of a groove inclinded relative to the cylinder axis or extending towards the end wall 3, can easily be produced by means of a face-milling cutter inclined relative to the cylinder axis. Figures 5 and 6 show additional designs which may be used for the discharge opening or openings.

Figure 5 shows an axially extending groove and Figure 6 a series of holes. The channel 1 6 is a collecting channel from which the connections lead through the cylinder wall into the cylinder chamber.

Figure 4 shows a double-action working cylinder with a braking action in front of the two end positions of the piston 2 and before it reaches the end wall 3 and end wall 11. The right hand end of the working cylinder with the cylinder chamber 4 is constructed as described above with reference to Figure 1. The left hand end of the working cylinder 1, with the cylinder chamber 10, between the piston 2 and the end wall 11, has at least one channel 21 for discharging the working medium from the cylinder chamber 10, this channel 21 opening along a portion 22 of the path of the piston 2 towards the end wall 11, relative to the cylinder chamber 10. Thus, in a similar way to that described above for the cylinder chamber 4 in Figure 1, the movement of the piston, in this case towards the end wall 11, is successively braked from a transporting speed to a final speed.

As shown in Figure 4, the pressure oil is conveyed by the pump 6 via a valve 24 and a duct 23 into the cylinder chamber 10. At the same time, the valve 24 blocks a discharge duct 25 which leads from the channel 21 and a fine throttle 26 connected to the cylinder chamber 10, into the oil tank 5. At the same time, the discharge of oil from the cylinder chamber 4 through the duct 9 and through the valve 7, which is coupled to the valve 24, into the oil tank 5 is opened up.

Thus the piston 2 mvoes towards the end wall 3.

For a subsequent operating cycle, the coupled valves would be reversed so that the pressure oil flowed into the cylinder chamber 4, whilst at the same time the discharge through discharge duct 9 would be blocked and the discharge duct 25 for the oil from cylinder chamber 10 would be opened. In this second operating cycle, the piston 2 would travel towards the end wall 11 and would there come to a gentle stop at a low final speed.

Claims (11)

1. A piston-cylinder mechanism in which movement of the piston towards a cylinder end wall is braked by throttling the discharge of a working medium from a chamber defined between the piston and said end wall, wherein the discharge duct for such working fluid opens into said chamber through the cylinder side wall along an axial length thereof whereby the crosssectional area of said opening is reduced by passage of the piston thereover.

2. A mechanism according to Claim 1 wherein said opening comprises an elongate aperture extending in an axial direction along the cylinder sidewall.

3. A mechanism according to Claim 2 wherein the depth of the groove reduces linearly along its length.

4. A mechanism according to Claim 3 wherein the minimum depth of the groove is at its end nearest said end wall.

5. A mechanism according to Claim 1 wherein said opening comprises a series of apertures spaced along an axial length of the cylinder side wall.

6. A mechanism according to any preceding Claim wherein the discharge duct opens into said chamber at a plurality of peripherally spaced locations in the cylinder side wall.

7. A mechanism according to any preceding Claim including an auxiliary duct opening into said chamber through said end wall, the auxiliary duct being provided with a fine throttle for controlling the speed of the piston in the immediate proximity of said end wali.

8. A mechanism according to Claim 7 wherein the fine throttle is adjustable.

9. A mechanism according to any preceding Claim wherein the piston is double acting and wherein a said discharge duct arrangement is provided at both ends of the cylinder.

1 0. A mechanism according to Claim 7 and Claim 9 wherein a said auxiliary duct is formed in both cylinder end walls.

11. A piston cylinder mechanism substantially as described herein with reference to and as illustrated by Figures 1 to 3 or Figure 4 of the accompanying drawings.