Field of the invention

The present invention relates to hydraulic actuation of a sliding carriage guided for movement along a linear path.

Background of the invention

A backhoe is an item of excavating equipment comprising a digging bucket on the end of a two-part articulated arm, the two parts being termed a main boom and a dipper. Backhoes are typically mounted on the back of a vehicle, such as a tractor or front loader, and can be used for example to dig trenches.

It is known from EP 0 692 579 to connect the lower end of the boom arm of the backhoe pivotally to a king post assembly which allows pivotal slewing movement of the main boom about a vertical axis, so that the main boom can be rotated from one side to the other of the centre line of the vehicle. The king post assembly is itself mounted upon a guide and is movable along the guide in a direction transverse to the vehicle centreline. A fluid operated ram is connected between the king post assembly and an anchor point on the vehicle, so that extension or contraction of the ram causes movement of the king post assembly along the guide.

The present invention is concerned with a hydraulic system which may serve the purpose of the fluid operated ram in EP 0 692 579 but may alternatively be used to move any carriage that is guided for movement along a linear path.

There are several criteria that need to be taken into consideration in the design of such a hydraulic actuator. A first consideration is the size of the actuator, in that it must be capable of being accommodated within the path of travel of the carriage yet it must not limit the movement of the carriage. Another consideration is that the actuator needs itself to be supported in a suitable manner to ensure that its piston rod and cylinder axis always remain in alignment, even when under load. A further consideration is that it should be possible to offer protection to any extended piston rod as it risks being damaged in the dirty environment in which heavy equipment, such as a backhoe, operates. Two solutions proposed in EP 0 692 579 involve repositioning the cylinder of the ram, either relative to the track or relative to the carriage. In this way, the maximum displacement of the carriage is not determined by the stroke of the ram. However, when operating in a dirty environment, neither of the embodiments described in the latter patent specification offers a solution satisfying the above criteria.

Figures 1 and 2 of the accompanying drawings show different positions of a hydraulic system described in EP 1 264 938 which is believed to represent the closest prior art to the present invention and is incorporated herein by reference. This system uses only rams to move the king post assembly of a backhoe from one end position to the other. The system uses two hydraulic cylinders arranged "in series". By this, it is meant that the total possible displacement of the carriage is equal to the sum of the strokes of the individual cylinders. The illustrated arrangement is preferred to the other embodiment described in EP 1 264 938 which uses a cylinder with telescopically collapsible piston rods because it allows the cylinder body to be guided so as to maintain the pistons rods and cylinders in axial alignment when under load.

In Figures 1 and 2, the sliding carriage is designated 10 and it is constituted by a king post assembly on which the boom arm of a backhoe is pivoted. The carriage 10 is guided to slide along a stationary support frame 12 mounted transversely on the vehicle body, so that the carriage can move from an extreme left position, shown in Figure 1 , to an extreme right position, shown in Figure 2. The carriage 10 is moved by a two hydraulic jacks or rams 14 and 16 arranged in series with one another. The cylinders 14a and 16a of the two jacks are secured to one another and to a slotted guide plate 18 that receives a pin 20 secured to the carriage 10 by a fixed bracket (not shown). The end of the piston rod 14b of the upper jack 14 is connected by a pivot 22 to the support frame 22 while the free end of the rod 16b of the jack 16 is connected by a pin pivot 24 to a bracket 26 projecting from the right hand side of the carriage 10.

The present invention provides an improved hydraulic system that avoids several shortcomings of the system illustrated in Figures 1 and 2, which will now be explained.

First, it will be noted from Figure 2 that in its right hand side position, the carriage 10 stops short of the end right hand end of the frame 12 on account of the protruding bracket 26. Shortening the bracket 26 does not solve the problem because the length of the two jacks 14 and 16 would then need to be reduced to allow the carriage to reach the left hand end position and that would reduce the strokes of the two jacks.

Second, the separation between the pivot pins 22 and 24 in the position of Figure 2 is equal to the full width of the guide frame 12. Consequently, when pressure is applied to move the carriage to this position, the force tending to bend the piston rods out of augment with the cylinders is a maximum. Furthermore, the pin 20 in this right hand position of the carriage 10 is at the very end of the cylinder of the upper jack 14 so that the pin 20 has little effect to prevent the piston rod 14b from being bent out of alignment with its cylinder 14a.

A still further problem arises when pressure is applied to move the carriage 10 away from its right hand end position shown in Figure 2. The jacks 14 and 16 are not equally effective in both directions because each of their pistons has a large surface area on one side and a smaller surface area on the side connected to the piston rod. The piston surfaces used to return the carriage to the position shown in Figure 1 are the smaller annular surfaces. When moving the carriage to the left, as viewed, the jacks can apply enough force to overcome the dynamic friction on the carriage 10, but at times they cannot overcome the static friction. In other words, the jacks can keep the carriage moving to the left but on occasions they cannot initiate the movement when the carriage is in the position shown in Figure 2.

Summary of the invention

With a view to overcoming at least some of the above shortcomings, the present invention provides a hydraulic system for moving a carriage guided for movement along a linear path by a support frame, the system comprising two hydraulic jacks, each having a cylinder body and a piston rod, wherein the cylinder bodies of the two jacks are joined to one another, the end of the piston rod of one jack is connected to the support frame and the end of the piston rod of the other jack is connected to the carriage, characterised in that the two piston rods project from the same side of the joined cylinder bodies and in each end position of the carriage, the piston rod of a respective one of the jacks is fully extended while that of the other jack is fully retracted. - A -

In the preferred embodiment of the invention, the two jacks have strokes of unequal length and the second piston rod is connected to the side of the carriage that is nearer to the side of the frame connected to the first piston rod.

Advantageously, the joined cylinders are formed with a projecting pin engaged in a slot in a guide movable with the carriage in order to prevent misalignment between the axes of the piston rods and the cylinders when under load.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which :

Figures 1 and 2, as earlier described, show a hydraulic system as taught by EP 0 692 579, and

Figures 3, 4 and 5 show a hydraulic system of the present invention in different positions of the carriage.

Detailed description of the preferred embodiment(s)

To avoid unnecessary repetition, in Figures 3 to 5 components serving the same function as already described with reference to Figures 1 and 2 have been allocated similar reference numerals in the 100's series and will not be described a second time.

Instead of using two hydraulic jacks of equal stroke length and with the piston rods projecting from their respective cylinders in opposite directions, the hydraulic circuit of Figures 3 to 5 used two jacks 1 14, 116 of unequal stroke length and with their respective piston rods 114b and 116b projecting in the same direction from the cylinders 1 14a and 1 16a. The end of the piston rod 114b of the longer jack 116 is connected to the frame 1 12 by a pivot pin 122 while the end of the piston rod 116b of the shorter jack 116 is connected by a pivot pin 124 to a bracket 126 secured to the carriage 110. It will noted that the pin 124 is located on the left side of the carriage 110, that is to say the side nearer the side of the frame 112 connected to the piston rod 114b. A guide plate 118 secured to the carriage 1 10 has a slot which receives a pin 120 fixed to joined cylinders 114a and 116a near to the end of the cylinder 1 16a from which the piston rod 1 16b projects. It would be possible to secure the slotted guide plate 1 18 to the cylinders 1 14a, 116a and the pin to the carriage 110 as was the case in the hydraulic circuit of Figures 1 and 2.

In Figure 3, the carriage 110 lies in its left hand position in the frame 1 12 with the piston rod 1 14b fully retracted and the piston rod 116b fully extended.

To move the carriage to the intermediate position shown in Figure 4, hydraulic fluid is applied to the larger face of the piston of the hydraulic jack. If necessary, the force can be supplemented by applying fluid under pressure to the annular smaller face of the piston of the jack 1 16. The combined force is sufficient to initiate movement of the carriage 1 10 to the right and this can continue until the piston rod 114b is fully extended as shown in Figure 4. Continued application of fluid under pressure to the smaller face of the piston of the jack 1 16 can apply sufficient force to overcome dynamic friction and keep the carriage moving until it reaches the right hand end position shown in Figure 5. In this second end position, the piston rod 1 14b is fully extended but the piston rod 116b is fully retracted.

To return the carriage 1 10 to its left hand end position of Figure 3, the above process is reversed. First pressure is applied to the larger face of the piston of the jack 1 16 and, if necessary to the smaller face of the piston of the jack 1 14, to overcome static friction and initiate movement of the carriage 1 10. On reaching the position in Figure 4 in which the piston rod 1 16b is fully extended, the smaller face of the piston of the jack 1 14 is used to overcome the dynamic friction and maintain the movement of the carriage 110 to the left.

It will be seen from the above description that the hydraulic circuit of Figures 3 to 5 overcomes all of the shortcomings of the prior art as described above.

First, is it noted that, because the bracket 126 does not project beyond the side of the frame, it does not interfere with the carriage reaching either of its end position.

Second, the maximum separation between the ends of the piston rods, as shown in Figure 5, is less than the full width of the support frame by the width of the carriage

1 10. Hence, the tendency for the cylinders 1 14a, 116a to move up and down in the plane of the figures or to move in direction normal to the plane of the figures is reduced. Furthermore, in all positions of the carriage, the cylinders 114a, 116a are supported in the same central position, corresponding to the position of the pin 120.

Third, to initiate movement of the carriage in either direction, hydraulic fluid always acts on the large face of the piston of one of the two hydraulic jacks. There is therefore always available sufficient force to overcome static friction and to start the carriage moving. Furthermore, in contrast to the prior art, the same force is available to move the carriage in both of its end positions.

Last, it will be seen that the hydraulic system can be mounted above the carriage where it is less exposed to dirt. Furthermore, it is readily possible to mount a fixed shield on the frame 22 to protect the piston rod 1 14b when it is extended. Another shield may be fixed to the underside of the part of the hydraulic cylinder 114a that projects beyond the cylinder 1 16a to protect the piston rod 116b when it is extended.