GB2049801A - Hydraulic Drive for Machine Tools - Google Patents

Abstract

The drive comprises a piston (1) and cylinder (2), the piston (1) being stepped and having around it a flanged sleeve (12). Springs (15, 16) are set between the steps of the piston (1) and the flanges (13, 14). Two passages (17, 18) are drilled through the sleeve (12). When one passage is open the other is closed and vice versa. The sleeve (12) is held in position by a spring loaded detent (22) and can move between two positions, opening and closing the passages (17, 18) so that the piston (1) oscillates in the cylinder (2). A piston rod (11) connects the piston (1) to the tool so that it can be driven. <IMAGE>

Description

SPECIFICATION Hydraulic Drive For Machine Tools This invention relates to a hydraulic drive for a machine tool and to a tool incorporating such a drive. In particular, but not exclusively, the drive is for machine tools which operate with short quick high power strokes. Such machines include plate shears, nibblers and bevelling machines.

Many machine tools are known which operate by reciprocating a part relative to a workpiece on which the tool is to operate. Many of these tools especially those whose stroke is short and quick are powered by electric motors, since these can easily supply the necessary power over a short stroke and period of operation. However in some places where there is a fire risk, such as in coal mines, oil storage tanks and chemical plants, the use of electric motors is avoided wherever possible because of the danger of an electric spark causing a fire or explosion. It is possible to obviate the danger of a spark from an electric motor by adding a suitable casing and insulation material. However to fireproof an electric motor completely adds substantially to the weight and cost of the tool.It has therefore been proposed to replace electric motors with hydraulic drives so as to avoid the need for insulating casings if the tool is to be used in a dangerous atmosphere.

An example of a novel machine tool which is operated by a hydraulic drive is given in our copending application No. . It has been found that the hydraulic drive described therein is unsuitable for use in machine such as shears or nibblers because these machines are required to operate with a stroke of less than 5 cm, usually less than 2 cm, with a cycle time of about 1 second. The hydraulic drive described in the above application operates on machine tools having a stroke of at least 10 cm and with a cycle time of at least 5 seconds.

It is an object of the present invention to provide a hydraulic drive for a machine tool which can operate a tool with a short stroke of short period of operation, such as a nibbler or shears.

According to the present invention there is provided for a machine tool a hydraulic drive including a piston and cylinder, a switching member movable axially of the cylinder and comprising a first limb associated with the piston and the cylinder and a further limb at each end of the first limb extending radially of the cylinder, a pair of opposed bias means acting between the piston and the switching member in the axial direction and a disengageable retention means for retaining the switching member against the action of the bias means in either a first position, in which the piston is urged to move in one direction, or a second position, in which the piston is urged to move in the opposite direction, the retention means being disengaged by the abutment of the piston with one of the further limbs, whereby the switching member is moved from one position to the other, the direction of movement of the piston is reversed and the retention means is re-engaged with the switching member to retain it in the other position.

Preferably, the switching member is a cylindrical sleeve having an inwardly directed flange on each end, the flanges comprising the further limbs. The piston may fit inside the sleeve.

The sleeve need not extend the whole lengths of the piston, because the piston may be provided with one or more outer rings which will provide opposed steps for abutment with the switching member. The step may also be used to mount the bias means. The bias means may also be mounted in drillings or recesses formed in the piston.

The arrangement of the piston cylinder and switching means should be such that the piston is movable in the cylinder in the normal fashion and this will necessitate the use of standard seals known to those skilled in the art. The switching member may operate on one or more electric switches, which may control the operation of valves or pumps in the hydraulic circuit to cause pressure fluid to be supplied to the appropriate side of the piston.

However, it is preferred that the switching member has in it a passage located near each radial limb. In the first position one passage leads pressurised fluid into the cylinder on one side of the piston. The other passage may lead fluid out of the cylinder from the other side of the piston or may be blocked off by the cylinder wall. In the second position the roles of the two passages are reversed. In the first position the switching member blocks the pressure fluid inlet to the other side of the piston and in the second position it blocks the pressure fluid inlet to the one side of the piston. The advantage of this arrangement is that it is only necessary to have one pressure fluid supply, connected to both inlets, and one pump.

There is no need for external valves and so the operation of the switching member is not dependent on such valves which may be slow in operation.

The bias means may be any suitable spring, such as a leaf or pneumatic spring. Preferably, the bias means are coil compression springs. The inclusion of the bias means is necessary to ensure that the switching member moves quickly between the two positions when the retention means is disengaged. As the piston moves in one direction, one or both of the bias means acts against the movement and therefore store energy.

The energy is instantaneously expended when the retention means is disengaged, thereby moving the switching member rapidly to the other position. If the switching member were only moved by the action of the piston bearing on it, the movement would be slow.

The retention means is preferably a spring loaded detent whose spring is strong enough to retain the detent engaged with either one of two abutments in the switching member against the action of the bias means. However as soon as the piston abuts a radial limb of the switching member it should exert a pressure thereon sufficient to enable the action of the detent spring to be overcome, thereby disengaging the detent from the abutment. Each abutment may preferably be a groove in a cylindrical switching member.

The piston will carry in conventional fashion a piston rod which will extend out of the cylinder and will be adapted to receive a machine tool, such as a nibbler or plate shears. Such tools are commercially available.

The present drive has the advantage over other hydraulic drives that it can be used for machines requiring a small stroke with a short cycle time.

Moreover the power to be applied may be greater than that supplied by an electric motor of equivalent weight. The hydraulic drive has the advantage over electric motors that it can be used in dangerous enviroments where the danger of a spark causing a conflagration is large.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which; Figure 1 is a cross sectional side view of a drive according to the present invention; Figure 2 is a plan view of the drive of Figure 1; and Figure 3 is an end view of the drive of Figure 1.

Referring to the drawings, the drive comprises a piston 1 and cylinder 2. A pressure connecting block 3 is fixed onto the cylinder 2 and comprises a pressure fluid inlet 4 which connects to inlet chamber 5, pressure fluid outlet 6 which connects to outlet chamber 7, and an on/off control 8. With the control 8 in the position shown in Figure 2 in full lines the inlet and outlet chambers are directly connected so that pressure fluid led into the inlet 4 is passed immediately to the outlet 6. With the control 8 in the position shown in chain lines in Figure 2, the inlet and outlet chambers are sealed from one another.

The piston 1 comprises a cylindrical member having on its outside a cylindrical step 9 located equidistantly from the ends of the piston 1. The step 9 has on it a series of lubrication grooves 1 0.

A piston rod 11 is formed on one end of the piston 1.

A switching member in the form of a cylindrical sleeve 1 2 is located between the piston 1 and the cylinder 2. The sleeve 12 has its internal diameter the same as the external diameter of the step 9 and has its external diameter the same as the internal diameter of the cylinder 2. The cylindrical sleeve 12 is longer than the piston 1 and has a flange 13, 14 bolted onto each end. Each flange 13, 14 extends radially inwardly in the cylinder and extends past the piston 1. Coil springs 1 5, 16 are located on either side of the step 9 and abut the flanges 13, 14 respectively. Passages 17, 1 8 are drilled through the sleeve 12 close to the flanges 13, 14 respectively. The sleeve 12 has on its outside a series of lubrication grooves 19, and a pair of abutment grooves 20, 21.

A spring loaded detent 22 is fixed to the cylinder 2, and has its detent member 23 extending through the cylinder 2 so that it may engage with one of the grooves 20,21. The detent member 23 is urged into abutment with one of the grooves 20, 21 by coil spring 24.

Inlet grooves 25, 26 are formed on the inside of the cylinder 2, and are connected by conduit 27 to inlet chamber 5. Outlet grooves 28, 29 are also formed on the inside of the cylinder 2, and are connected by conduit 30 to outlet chamber 7.

The abutment grooves 20, 21, the inlet groove 25 and outlet groove 28, the inlet groove 26 and outlet groove 29, and each end of the piston 1 and each flange 13 or 14, are spaced from one another by the same distance. This distance is equal to the stroke of the piston 1.

The drive is provided with handles 31 and 32 for ease of carrying and for holding the drive in position in use. In use the inlet 4 and outlet 6 are connected into a hydraulic circuit including a pump (not shown). The piston rod 11 is connected by conventional means to a machine tool, such as a nibbler 33 as shown in chain lines in Figure 1.

The drive is used as follows. The pump is switched on and the pressure fluid is supplied to the inlet 4. In the position shown in Figure 2 the fluid is immediately returned to the hydraulic circuit and the drive is inactive. If the control 8 is moved to the on position shown in chain lines in Figure 2, the pressure fluid is supplied from the inlet chamber 5 via conduit 27, inlet groove 25 and passage 17 to the top (as shown in Figure 1) of the piston 1. The sleeve 12 and flange 14 cover the inlet groove 25 and so prevent pressure fluid being supplied to the other side of the piston.

Similarly outlet groove 28 is covered by the sleeve 12 and flange 13, but outlet groove 29 is uncovered so that pressure fluid is led out of the cylinder via outlet groove 29 and conduit 30 to the outlet chamber 7, whence it is returned to the hydraulic circuit via the outlet 6.

With the control 8 in the on position the sleeve 12 is held in place by the detent member 23 being in engagement with the abutment groove 21. As the cylinder is pressurised, the piston 1 moves downwardly in the sleeve 12, compressing the spring 1 6. This action is not sufficient to overcome the action of the detent spring 24. As soon as the piston has moved through its stroke it comes into abutment with the flange 14 and exerts pressure thereon. The pressure overcomes the action of the detent spring 24 and disengages the detent member 23, allowing the sleeve 12 to move freely in the cylinder 2. The sleeve 1 2 moves rapidly downwardly under the influence of the coil spring 1 6. As it does so the inlet groove 25 is covered by the flange 13 and sleeve 2 but the outlet groove 28 is uncovered. Similarly the passage 1 8 is now aligned with the inlet groove 26 and the outlet groove 29 is covered by the flange 1 4 and sleeve 12. The piston 1 is therefore pressurised to move the other way. In this new position the detent member 23 is now engaged in abutment groove 20, where it is held by the detent spring 24 against the action of coil spring 15, which is now being compressed. When the piston 1 abuts the flange 1 3 the detent member 23 is again disengaged and the sleeve 12 returns to its original position and the cycle recommences. The piston rod may then be reciprocated rapidly, activating the machine tools.

The drive illustrated above is made in bolt together form to enable easy maintenance, and may be used to drive other tools which may easily be attached to the piston rod at 34, as illustrated for the nibbler 33.

Claims (7)

Claims

1. A hydraulic drive, for a machine tool, including a piston and cylinder, a switching member movable axially of the cylinder and comprising a first limb associated with the piston and the cylinder and a further limb at each end of the first limb extending radially of the cylinder, a pair of opposed bias means acting between the piston and the switching member in the axial direction, and a disengageable retention means for retaining the switching member against the action of the bias means in either a first position, in which the piston is urged to move in one direction, or a second position, in which the piston is urged to move in the opposite direction, the retention means being disengaged by the abutment of the piston with one of the further limb, whereby the switching member is moved from one position to the other, the direction of movement of the piston is reversed and the retention means is re-engaged with the switching member to retain it in the other position.

2. A hydraulic drive according to claim 1, wherein the switching member is a cylindrical sleeve having an inwardly directed flange on each end.

3. A hydraulic drive according to claim 1 or 2, wherein the piston is provided with a pair of opposed steps.

4. A hydraulic drive according to any one of claims 1 to 3, wherein there is a passage near each radial limb of the switching member for directing pressure fluid to the appropriate side of the piston.

5. A hydraulic drive according to any one of claims 1 to 4, wherein the bias means are compression coil springs.

6. A hydraulic drive according to any one of claims 1 to 5, wherein the retention means is a spring loaded detent.

7. A hydraulic drive, for a machine tool, substantially as hereinbefore described with reference to the accompanying drawings.