Hydraulic linear actuator the tensile force of which varies as a function of the applied load

Abstract

 A hydraulic linear actuator the tensile force of which varies as a function of the applied load basically comprises two coaxial hydraulic actuators or cylinders (1, 6), which are set one inside the other and are provided with a stem (3) in common, in which the inner actuator, of smaller section, is designed to perform the fast movement, whilst the outer actuator is designed to generate, at the maximum pressure, the force necessary to carry out the work. The inner cylinder (6) has a cylinder head (5) the body of which constitutes the piston of the outer cylinder (1).
Housed on said inner cylinder head (5) is a one-way valve (4) with conical seat, coaxial to the stem (3), said valve (4) being designed to enable free passage of the oil from a first front chamber (10) of the outer cylinder (1) to a second front chamber (12) of the inner cylinder (6) but not vice versa.

Description

FIELD OF APPLICATION

[0001] Forming the subject of the present invention is a hydraulic-fluid linear actuator made up of two cylinders of different cross section, which are coaxial and act on the same stem and one of which is contained entirely within the other, said linear actuator finding advantageous use in applications in which the maximum tensile force is required only in a stretch of the total stroke of the actuator, whilst a high speed of approach to a point of start of operation and of return is required in conditions of reduced load.

[0002] Typical fields of application are wood-splitting machines or log-splitting machines, and similar machines.

STATE OF THE ART

[0003] Current wood-splitting machines, and similar machines, the working principle of which is based upon the force generated by fluid-dynamic pressure, are equipped, as power actuator, with a hydraulic jack. Numerous types of hydraulic-circuit configurations are adopted in order to achieve a high speed at reduced load and maximum force during the operating step:

[0004] One technique consists in adopting dual pump circuits: in the step of approach to the piece, both of the pumps deliver hydraulic fluid to the cylinder, whilst in the working stage a pressure valve excludes the pump with greater piston displacement, and the movement of the actuator is determined just by the capacity of the pump with lower piston displacement. This solution, in addition to implying the use of two pumps imposes the need to size the hydraulic circuit as a function of the maximum capacity.

[0005] Another solution consists in the use of pumps with variable displacement controlled by the pressure of the working fluid via a circuit that keeps the power absorbed by the pump or the torque required by the pump constant. This solution involves the use of costly equipment and finds application on high-category machines. Furthermore, as in the previous case, the sizing of the hydraulic circuit must be chosen according to the maximum capacity supplied by the pump.

[0006] A technique already in use and patented by the present applicant consists in using a cylinder similar to the one forming the subject of the present application, the characteristic of which consists in exerting a thrust that differs as a function of the applied load. Said cylinder possesses the characteristic of varying the section of thrust automatically and hence the force exerted only during the step of exit of the stem and thus finds valid application in machines that carry out cleaving of logs by pushing the splitting tool.

[0007] On the other hand, there exists a wide range of low-cost wood-splitting machines that operate by carrying out splitting when the cylinder is going back in, i.e., ones in which the cylinder basically exerts a pulling action of the tool. In this case, the invention according to the previous patent cannot be applied.

[0008] The present invention does not represent a variant of the previous one since, albeit remaining similar from a constructional standpoint, it is based upon different principles because the same physical laws are not applicable on account of the substantial difference in the geometries between the two types of product.

[0009] One of the purposes of the invention is to achieve the advantages obtained with the previous invention also with wood-splitting machines of the type operating by exerting a tensile force and to achieve further economic and functional advantages, which will be more clearly understandable from the ensuing description with reference to the attached plate of drawings, which illustrate, purely by way of non-limiting example, a preferred embodiment of the invention, as well as a variant thereof.

[0010] In the plate of drawings:

Figure 1 illustrates, in sectioned axonometric view, a first embodiment of the invention;

Figure 2 is an axial sectional view of the invention illustrated in Figure 1;

Figures 3a, 3b and 3c illustrate the operating cycle of a first embodiment of the invention and namely: Figure 3a shows the device according to the invention during the phase of approach to the piece characterized by the movement of just the piston of the inner cylinder; Figure 3b shows the step of closing of the one-way valve and start of the movement of the inner cylinder head; and Figure 3c shows the operating step, characterized by the movement of the inner cylinder head fixedly with the piston; and

Figure 4 illustrates a variant of the invention in an axial cross section similar to that of Figure 2.

DESCRIPTION OF THE INVENTION

[0011] From the foregoing, it emerges clearly that the main purpose of the present invention is to obtain a device, i.e., a component, that is able to move at high speed during the steps of approach of the tool to the log, to exert the maximum force available during the step of initial penetration of the tool in the log, and to resume the movement at high speed when the log, once split, exerts only a moderate resistance to advance of the tool.

[0012] This component has been studied in particular for wood-splitting machines in which it is necessary to exert the maximum force only in the initial step of cleaving of the log whilst in the step of approach and in the final step of separation of the split pieces the force required is particularly small.

[0013] According to the invention, a linear actuator is provided that substantially consists of two hydraulic cylinders, one inside the other, which are provided with a stem in common: the inner actuator, of smaller section, represents the system of fast movement, whilst the outer actuator constitutes the component that is able to generate, at the maximum pressure, the force necessary to carry out the work of splitting of the log.

[0014] A first peculiar characteristic of the invention consists in the fact that the cylinder head 5 of the inner cylinder 6 constitutes at the same time the piston of the outer cylinder 1; said cylinder head 5 will be referred to in the sequel of the present description by the term "inner cylinder head".

[0015] Housed on said inner cylinder head 5 is a one-way valve 4, which has a conical seat, is coaxial to the stem 3, and enables free passage of the oil from the first front chamber 10 of the outer cylinder to the second front chamber 12 of the inner cylinder but not vice versa.

[0016] According to the present invention, during the step of return, in the event of contact between the "inner cylinder head" 5 and the outer cylinder head 2 of the outer cylinder, the one-way valve 4 is kept forcibly open: in these conditions the (incompressible) working fluid is free to pass through the inner front chamber to the outer front chamber.

[0017] A second peculiar characteristic of the invention is that the rear chamber of the inner cylinder 6 is without end plate and communicates directly with the rear chamber 13 of the outer cylinder 1.

[0018] A third peculiar characteristic of the invention is that it envisages a spring 9 having suitable characteristics, which is mounted within the outer cylinder 1 and is designed to exert a thrust of appropriate intensity on the inner cylinder head 5 in the direction of the stem 3, which tends to keep it in contact with the outer cylinder head 2 of the outer cylinder 1.

[0019] According to the invention, in the step of approach of the tool to the log of wood that is to be split, the pressure of the working fluid conveyed in the first front chamber 10 of the outer cylinder flows through the one-way valve 4 into the inner cylinder, bringing about fast advance of the corresponding piston 7.

[0020] In this step, the modest pressure necessary for moving the inner piston 7 is not able to overcome the force exerted by the spring 9, and hence the inner cylinder head 5 remains bearing upon the outer cylinder head 2, and the one-way valve 4 consequently remains open.

[0021] In the operating step (contact with the piece of wood to be split), the pressure of the working fluid, increased by the presence of a mechanical resistance to advance, overcomes the force of the spring 9 and consequently determines recession of the inner cylinder head 5 from the outer cylinder head 2 and closing of the one-way valve 4. Since the working fluid, for example oil, contained in the inner cylinder 6 is not able to flow away through the closed valve 4, it behaves like a rigid mechanical element. In these conditions, the pressure in the inner cylinder 6 rises to values determined by the ratio of the respective areas of the cylinders in the same way as a pressure intensifier.

[0022] Once splitting of the log is completed, the reduction in the pressure deriving from the lower resistance to advance, given that it is no longer able to overcome the force of the counteracting spring, determines movement of the inner cylinder head 5 towards the outer cylinder head 2 and consequent opening of the one-way valve 4.

[0023] As a result, the oil contained in the first front chamber 10, which is pressurized by the action of the spring 9, is free to pass through the one-way valve 4, bringing about fast movement of the inner piston 7 in a direction opposite to the movement of' the inner cylinder head 5 that has brought about opening of the valve 4 itself.

[0024] A second possibility may moreover arise: if the resistant force, and consequently the pressure, does not drop below a threshold value corresponding to the resistance of the counteracting spring 9, the inner cylinder 6 terminates its useful stroke and comes into contact with the end plate 16 of the outer cylinder 1. In this case, the pressure present in the first front chamber 10 brings about opening of the one-way valve 4 set on the inner cylinder head 5, causing fast advance of the inner piston 7.

[0025] In this condition, the tensile force of the device is represented by the product of the pressure and the section of the inner piston 7 minus the section of the stem.

[0026] Once the operating cycle is completed, it is sufficient to reverse the hydraulic flow so that the fluid pumped into the rear chamber 13 will cause recession of the inner cylinder head 5 until it comes into contact with the outer cylinder head 2. Once said position is reached, the one-way valve will automatically be opened, enabling the oil contained in the inner front chamber 12 of the inner cylinder 6 to flow freely away, so enabling total exit of the stem 3.

[0027] According to a variant of the invention (Figure 4) that is particularly designed for increasing the value of mechanical resistance that determines recession of the inner cylinder head 5 and consequent closing of the one-way valve 4, there is provided in the outer cylinder head 2 a seat of appropriate section, which is closed by lip seals 15 and inserted within which is a projection of the inner cylinder head 5.

[0028] As a result of the above solution, the pressure present in the first front chamber 10 during the stage of approach to the piece of wood to be split does not act on the entire cross section of the inner cylinder head 5, but only in the circular sector comprised between the lip seal 15 and the one-way valve 4. Once the surface of thrust has thus been reduced, it is possible to achieve higher pressure values before overcoming the counteracting force of the pre-loaded spring 9. As has been anticipated, this enables development of tensile forces that are higher during approach, i.e., it enables the use of springs of smaller size given the same force of approach. The lip seal 15 likewise performs the function of one-way valve during the steps of return, thus enabling exit of the oil from the chamber comprised between the inner cylinder head 5 and the outer cylinder head 2 towards the exhaust pipe.

[0029] With reference to Figure 1, which illustrates the cylinder according to the invention as it appears once assembly is completed in a first embodiment provided purely by way of example, it may be noted that the cylinder is in the operating step, i.e., in the condition in which it develops the maximum force.

[0030] Emerging clearly from the sectioned view are: the tube 1, the outer cylinder head 2, and the end plate 16 of the outer cylinder, as well as the corresponding seals. The piston 5 of the outer cylinder in turn constitutes the inner cylinder head and is rigidly joined in a fluid-tight way to the corresponding tube 6. As has already been said, in the present description said piston 5 is also referred to as "inner cylinder head".

[0031] Coaxial to the stem is the conical valve 4, which is withheld in place by a disk 4a provided with grooves for passage of oil.

[0032] A compression spring 9 set between the inner cylinder head 5 and the end plate 16 exerts a thrust in the direction of the outer cylinder head 2.

[0033] The front inlet of the hydraulic circuit is represented by the fitting 8, which communicates with the first front chamber 10 of the outer cylinder. The area of contact 11 between the conical surface of the valve 4 and the inner cylinder head 5 totally closes the second chamber 12 of the inner cylinder, whilst the rear chamber 13 is common to both of the cylinders.

[0034] In Figure 2, the cylinder according to the invention is in the resting stage, i.e., in the condition in which the stem 3 is totally out.

[0035] Appearing from the sectioned view are: the tube 1, the cylinder head 2, and the end plate 16 of the outer cylinder, as well as the corresponding seals, the inner cylinder head 5 and the internal tube 6, as well as the conical valve 4, withheld in position by the disk 4a, and the compression spring 9.

[0036] In conditions of absence of load, the oil coming in through the fitting 8 penetrates into the first front chamber 10 and from there - through the passage 11 - penetrates into the second front or inner chamber 12, causing the inner piston 7 to drop. The passage 11 between the conical surface of the valve 4 and the inner cylinder head 5 is kept open by the thrust of the spring 9 on the inner cylinder head 5, which causes the latter to bear upon the outer cylinder head 2.

[0037] The pressure present within the front chamber 10 does not act only upon the inner piston 7 but also upon the top face of the inner cylinder head 5: if said pressure does not generate a force sufficient to compress the spring 9, the inner cylinder head 5 remains bearing upon the outer cylinder head 2, thus keeping the valve open, and the inner piston 7 continues to drop as long as the tool does not encounter resistance.

[0038] Figure 3a highlights the motion of descent of the inner piston 7. At the instant when the tool meets with a considerable resistance to advance, the pressure inside the first front chamber 10 increases to values that are sufficient to overcome the force of the counteracting spring 9, bringing about compression thereof and consequent descent of the inner cylinder head 5, so causing closing of the passage or gap 11 between the valve 4 and the inner cylinder head 5 itself.

[0039] Figure 3b highlights the instant when the inner cylinder head 5, by moving away from the outer cylinder head 2, brings about closing of the gap 11. The oil confined in the second front chamber 12, since it behaves notoriously as an incompressible fluid, transmits the entire thrust exerted on the surface of the cylinder head to the inner piston 7. The oil contained in the second front chamber 12 reaches pressure values that are particularly high, determined by the ratio between the surface of the inner cylinder head 5 and the top surface of the inner piston 7.

[0040] Figure 3c highlights the motion of descent of the inner cylinder head 5 fixedly to the inner piston 7.

[0041] At the instant in which the tool no longer encounters resistance to advance and the pressure within the first front chamber 10 decreases, the thrust exerted by the spring 9 tends to cause the inner cylinder head 5 to rise, so causing re-opening of the gap 11 and transfer of the oil contained in the first front chamber 10 to the second front chamber 12, bringing about fast descent of the inner piston 7.

[0042] Instead, if the resistance to advance does not drop below a given threshold, the motion of the inner piston 7 fixed to the inner cylinder will bring the internal tube 6 into contact with the end plate 16 of the outer cylinder, which prevents any further descent thereof.

[0043] In said conditions, if the resistance to advance is not higher than a given value, the pressure present in the first front chamber 10 is able to cause opening of the valve 4 and consequent further descent of the inner piston 7.

[0044] If, instead, the resistance to advance remains very high, the operator will have to reverse the direction of the motion, bringing about a partial rise of the cylinder and consequent dropping of the pressure in the top chamber to zero. This enables the cylinder to return into its initial conditions.

[0045] Figure 4 is a cross section of a variant of the embodiment so far described purely by way of example.

[0046] In said figure, the cylinder is in the resting stage, i.e., in the condition where the stem is totally out.

[0047] Emerging clearly from the sectioned view are: the tube 1, the outer cylinder head 2 and the end plate 16 of the outer cylinder, as well as the corresponding seals, the inner cylinder head 5 and internal tube 6, as well as the withheld conical valve 4 and the compression spring 9.

[0048] Provided in this variant is a lip seal 15 of appropriate shape, which is designed to prevent the pressure of the working fluid present in the first front chamber 10 from exerting its own action of thrust on the surface of the peripheral circular ring 14 of the inner cylinder head 5.

[0049] The pressure present within the first front chamber 10 does not act only on the inner piston 7, but also on the top part of the inner cylinder head 5 comprised between the lip seal 15 and the internal section of the internal tube 6. As emerges from Figure 4, said surface is much smaller than the total surface of the top face of the inner cylinder head 5. On account of said reduction of the surface, the pressure necessary for overcoming the force of the counteracting spring 9 is much greater, since said pressure multiplied by the surface of the inner piston 7 determines the tensile force of the cylinder in the stage of approach. The result is an increase in the switching threshold between the speed of approach and the working speed. This enables any mechanical resistance present on the machine to be overcome or else it enables use of counteracting springs of lower rigidity and smaller dimensions.

Claims

1. A hydraulic linear actuator the tensile force of which varies as a function of the applied load, characterized in that it substantially comprises two coaxial hydraulic actuators or cylinders (1, 6) set one inside the other and provided with a stem (3) in common, in which the inner actuator, of smaller section, is designed to perform the fast movement, whilst the outer actuator is designed to generate, at the maximum pressure, the force necessary to carry out the work; the inner cylinder (6) having a cylinder head (5), the body of which constitutes the piston of the outer cylinder (1).

2. The actuator according to the preceding claim, characterized in that housed on said inner cylinder head (5) is a one-way valve (4) with conical seat, coaxial to the stem (3), said valve (4) being designed to enable free passage of the oil from a first front chamber (10) of the outer cylinder (1) to a second front chamber (12) of the inner cylinder (6) but not vice versa.

3. The actuator according to the preceding claim, characterized in that said inner cylinder (6) has a rear chamber that is without end plate and communicates directly with a rear chamber (13) of the outer cylinder (1).

4. The actuator according to the preceding claim, characterized in that it envisages a spring (9) mounted within the outer cylinder (1), designed to exert a thrust of appropriate intensity on the inner cylinder head (5) in the direction of the stem (3), which tends to keep it in contact with the outer cylinder head (2) of the outer cylinder (1).

5. The actuator according to the preceding claim, characterized in that during the stroke of approach to a point of start of operation, the working fluid conveyed under pressure in the first front chamber (10) of the outer cylinder (1) is designed to flow through the one-way valve (4) into the inner cylinder (6), bringing about fast advance of the corresponding inner piston (7); the modest pressure necessary for moving the inner piston (7) not being sufficient to overcome the force exerted by the spring (9), the result being that the inner cylinder head (5) bears upon the outer cylinder head (2) and that the one-way valve (4) consequently remains open.

6. The actuator according to Claim 4 or Claim 5, characterized in that during the operating stage the pressure of the working fluid is designed to increase on account of the presence of a mechanical resistance to advance, overcoming the force of the spring (9) and bringing about recession of the inner cylinder head (5) from the outer cylinder head (2), as well as closing of the one-way valve (4); in these conditions, the pressure in the inner cylinder (6) rising to values determined by the ratio of the respective areas of the cylinders, in a way similar to a pressure intensifier.

7. The actuator according to the preceding claim, characterized in that the reduction of the pressure in the first front chamber (10) deriving from a reduction of the resistance to advance, since it is no longer able to overcome the force of the counteracting spring (9), is such as to determine the movement of the inner cylinder head (5) towards the outer cylinder head (2) and the consequent opening of the one-way valve (4).

8. The actuator according to the preceding claim, characterized in that the oil contained within the first front chamber (10), pressurized by the action of the spring (9), is designed to pass through the one-way valve (4), bringing about rapid movement of the inner piston (7) in a direction opposite to the movement of the inner cylinder head (5) that has determined opening of the valve (4) itself.

9. The actuator according to Claim 6, characterized in that to increase the value of mechanical resistance to advance that determines recession of the inner cylinder head (5) and the consequent closing of the one-way valve (4), provided in the outer cylinder head (2) is a seat of appropriate cross section, which is closed by at least one seal (15) with cross section having the form of a lip, and inserted within which is a projection of the inner cylinder head (5); the result being that the pressure present in the first front chamber (10) during the stroke of approach does not act on the entire cross section of the inner cylinder head (5), but only on a circular sector comprised between the lip seal (15) and the one-way valve (4).

10. The actuator according to Claim 4, characterized in that said spring (9) is a compression spring and is housed in the rear chamber (13) common to both of the cylinders, and that said spring is designed to determine the value of pressure at which the inner cylinder head (5) moves away from the outer cylinder head (2), bringing about closing of the one-way valve (4) .

11. The actuator according to Claim 2, characterized in that said one-way valve (4) has a conical sealing surface and is coaxial to the stem (3), there being set between said valve (4) and said stem (3) an appropriate seal.

Drawing