阅读说明：本技术 一种变功率控制装置及液压系统 (Variable power control device and hydraulic system ) 是由 孙光明 孔维涛 郭志敏 崔凯 于 2020-06-30 设计创作，主要内容包括：本发明涉及液压控制领域,公开了一种变功率控制装置及液压系统。其中,变功率控制装置包括变量泵、功率控制阀、变量控制油缸、液压油箱、反馈阀套、反馈阀芯、杠杆、功率调节阀和电比例调压阀。本发明提供的变功率控制装置,能够根据发动机的需求功率调节电比例调压阀的电流,从而调节电比例调压阀的出口油压,再通过功率控制阀、功率调节阀、变量控制油缸、反馈阀套、反馈阀芯和杠杆配合动作实现变功率调节,使发动机的需求功率与变量泵的功率匹配,减少发动机的功率损失,降低油耗,降低使用成本。(The invention relates to the field of hydraulic control and discloses a variable power control device and a hydraulic system. The variable power control device comprises a variable pump, a power control valve, a variable control oil cylinder, a hydraulic oil tank, a feedback valve sleeve, a feedback valve core, a lever, a power regulating valve and an electric proportional pressure regulating valve. The variable power control device provided by the invention can adjust the current of the electric proportional pressure regulating valve according to the required power of the engine, so as to adjust the outlet oil pressure of the electric proportional pressure regulating valve, and then realize variable power adjustment through the matching action of the power control valve, the power regulating valve, the variable control oil cylinder, the feedback valve sleeve, the feedback valve core and the lever, so that the required power of the engine is matched with the power of the variable pump, the power loss of the engine is reduced, the oil consumption is reduced, and the use cost is reduced.)

1. A variable power control device is characterized by comprising a variable pump (2), a power control valve (3), a variable control oil cylinder (1) and a hydraulic oil tank, wherein the power control valve (3) can enable a rodless servo cavity (12) of the variable control oil cylinder (1) to be selectively communicated with an oil outlet of the variable pump (2) or the hydraulic oil tank, a variable control piston (11) of the variable control oil cylinder (1) is connected with a swash plate of the variable pump (2), and a rod cavity of the variable control oil cylinder (1) is provided with an elastic reset piece; further comprising:

the feedback valve sleeve (30) is connected with the variable control piston (11) of the variable control oil cylinder (1), the inlet of the feedback valve sleeve (30) is connected with the oil outlet of the variable pump (2), and a feedback valve core (40) capable of moving relative to the feedback valve sleeve (30) is arranged in the feedback valve sleeve (30);

the power control valve comprises a lever and a power regulating valve (5), wherein one side of a first end of the lever is abutted with a power control valve core (31) of the power control valve (3), the other side of the first end of the lever is abutted with a valve core of the power regulating valve (5), and a second end of the lever is abutted with a feedback valve core (40);

and the outlet of the electric proportional pressure regulating valve (6) is connected with the inlet of the power regulating valve (5), and the current of the electric proportional pressure regulating valve (6) can be regulated according to the required power of an engine on the engineering vehicle.

2. A variable power control device according to claim 1, further comprising a power spring (50) connected to the lever, the power spring (50) being provided on the same side of the lever as the power regulating valve (5).

3. The variable power control device according to claim 1, wherein a high-pressure oil chamber (14) is provided on the variable control piston (11), and the high-pressure oil chamber (14) is communicated with an inlet of the feedback valve sleeve (30).

4. A variable power control device according to claim 1, characterized in that the feedback spool (40) is located below the lever, the second end of the lever being supported by the top of the feedback spool (40).

5. The variable power control device according to claim 1, wherein a pin is provided at a middle portion of the lever, and the lever is rotatable with respect to the pin.

6. The variable power control device according to claim 1, characterized in that the power control valve (3) comprises a first port P communicating with an oil outlet of the variable pump (2), a first port T communicating with the hydraulic tank, and a first working port communicating with the rodless servo chamber (12);

the first working oil port can be selectively communicated with the first P port or the first T port.

7. The variable power control device according to claim 6, further comprising a pressure cut-off valve (9) including a second P port communicating with an oil outlet of the variable pump (2), a second T port communicating with the hydraulic oil tank, and a second working oil port communicating with the first T port; a pilot control oil port of the pressure cut-off valve (9) is communicated with an oil outlet of the variable pump (2);

the second working oil port can be selectively communicated with the second P port or the second T port.

8. The variable power control device according to claim 7, characterized by further comprising:

the load sensitive valve (10) comprises a third P port, a third T port, a third working oil port, a first pilot control oil port and a second pilot control oil port, the third P port is communicated with an oil outlet of the variable pump (2), the third T port is communicated with the hydraulic oil tank, and the third working oil port is communicated with the second T port; the third working oil port can be selectively communicated with the third P port or the third T port; the first pilot control oil port is communicated with an oil outlet of the variable pump (2);

the second pilot control oil port and a return spring of the load sensitive valve (10) are located on the same side, an oil outlet of the variable pump (2) is communicated with an oil inlet of the flow control valve (20), and an oil outlet of the flow control valve (20) is simultaneously communicated with the second pilot control oil port and the load.

9. A variable power control device according to claim 1, characterized in that the electro proportional pressure regulating valve (6) is a pressure reducing valve.

10. A hydraulic system comprising a variable power control apparatus according to any one of claims 1 to 9.

Technical Field

The invention relates to the field of hydraulic control, in particular to a variable power control device and a hydraulic system.

Background

In the open type hydraulic system, the output power of the variable pump is the product of the output flow Q of the pump and the pressure P of the oil outlet of the variable pump, and since the pressure P of the oil outlet of the variable pump changes along with the change of the load, in order to stabilize the output power, the output flow Q of the variable pump needs to be controlled by a constant power control device, so that the product of Q and P is kept unchanged, and the output power is in a constant state.

The current power control device usually adopts a "double-fold line" type constant power control, which is a control mode similar to constant power. The power control method cannot feed back the power requirement of the electric control engine to the power setting requirement of the variable pump according to the actual working condition, so that certain energy waste inevitably exists in the double-fold line type constant power control, the requirement of saving is not met, and the use cost of customers is increased.

Disclosure of Invention

The invention aims to provide a variable power control device and a hydraulic system, which can save energy and reduce use cost.

In order to achieve the purpose, the invention adopts the following technical scheme:

a variable power control device comprises a variable pump, a power control valve, a variable control oil cylinder and a hydraulic oil tank, wherein the power control valve can enable a rodless servo cavity of the variable control oil cylinder to be selectively communicated with an oil outlet of the variable pump or the hydraulic oil tank; further comprising:

the feedback valve sleeve is connected with the variable control piston of the variable control oil cylinder, the inlet of the feedback valve sleeve is connected with the oil outlet of the variable pump, and a feedback valve core capable of moving relative to the feedback valve sleeve is arranged in the feedback valve sleeve;

the power control valve comprises a lever and a power regulating valve, wherein one side of a first end of the lever is abutted with a power control valve core of the power control valve, the other side of the first end of the lever is abutted with a valve core of the power regulating valve, and a second end of the lever is abutted with a feedback valve core;

and the outlet of the electric proportional pressure regulating valve is connected with the inlet of the power regulating valve, and the current of the electric proportional pressure regulating valve can be regulated according to the required power of an engine on the engineering vehicle.

As a preferable technical solution of the above variable power control device, the variable power control device further includes a power elastic member connected to the lever, and the power elastic member and the power adjusting valve are disposed on the same side of the lever.

As a preferable technical scheme of the variable power control device, a high-pressure oil chamber is arranged on the variable control piston and is communicated with an inlet of the feedback valve sleeve.

As a preferable technical solution of the variable power control device, the feedback valve core is located below the lever, and the second end of the lever is supported by the top of the feedback valve core.

As a preferable technical scheme of the variable power control device, a pin shaft is arranged in the middle of the lever, and the lever can rotate relative to the pin shaft.

As a preferred technical solution of the above variable power control device, the power control valve includes a first P port, a first T port, and a first working oil port, the first P port is communicated with an oil outlet of the variable displacement pump, the first T port is communicated with the hydraulic oil tank, and the first working oil port is communicated with the rodless servo chamber;

the first working oil port can be selectively communicated with the first P port or the first T port.

As a preferred technical scheme of the variable power control device, the variable power control device further comprises a pressure cut-off valve, wherein the pressure cut-off valve comprises a second P port, a second T port and a second working oil port, the second P port is communicated with an oil outlet of the variable pump, the second T port is communicated with the hydraulic oil tank, and the second working oil port is communicated with the first T port; a pilot control oil port of the pressure cut-off valve is communicated with an oil outlet of the variable pump;

the second working oil port can be selectively communicated with the second P port or the second T port.

As a preferable aspect of the variable power control device, the variable power control device further includes:

the load sensitive valve comprises a third P port, a third T port, a third working oil port, a first pilot control oil port and a second pilot control oil port, the third P port is communicated with an oil outlet of the variable pump, the third T port is communicated with the hydraulic oil tank, and the third working oil port is communicated with the second T port; the third working oil port can be selectively communicated with the third P port or the third T port; the first pilot control oil port is communicated with an oil outlet of the variable pump;

and the second pilot control oil port and a return spring of the load sensitive valve are positioned on the same side, an oil outlet of the variable pump is communicated with an oil inlet of the flow control valve, and an oil outlet of the flow control valve is simultaneously communicated with the second pilot control oil port and the load.

As a preferable embodiment of the variable power control device, the electric proportional pressure regulating valve is a pressure reducing valve.

The invention also provides a hydraulic system which comprises the variable power control device.

The invention has the beneficial effects that: the variable power control device provided by the invention can adjust the current of the electric proportional pressure regulating valve according to the required power of the engine, so as to adjust the outlet oil pressure of the electric proportional pressure regulating valve, and then realize variable power adjustment through the matching action of the power control valve, the power regulating valve, the variable control oil cylinder, the feedback valve sleeve, the feedback valve core and the lever, so that the required power of the engine is matched with the power of the variable pump, the power loss of the engine is reduced, the oil consumption is reduced, and the use cost is reduced.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments of the present invention will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the contents of the embodiments of the present invention and the drawings without creative efforts.

Fig. 1 is a hydraulic schematic diagram of a variable power control device according to an embodiment of the present invention.

In the figure:

1. a variable control oil cylinder; 11. a variable control piston; 12. a rodless servo cavity; 13. plugging by screwing; 14. a high pressure oil chamber; 2. a variable displacement pump; 3. a power control valve; 31. a power control valve core; 41. a cross bar; 42. a vertical rod; 5. a power regulating valve; 6. an electric proportional pressure regulating valve; 7. a first throttling unit; 8. a second throttling unit; 9. a pressure shut-off valve; 10. a load sensitive valve; 20. a flow control valve; 30. a feedback valve housing; 40. a feedback spool; 50. a power spring.

Detailed Description

In order to make the technical problems solved, the technical solutions adopted and the technical effects achieved by the present invention clearer, the technical solutions of the present invention are further described below by way of specific embodiments with reference to the accompanying drawings. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some but not all of the elements associated with the present invention are shown in the drawings.

As shown in fig. 1, the present embodiment provides a variable power control device, which includes a variable control cylinder 1, a variable pump 2, a power control valve 3 and a hydraulic oil tank, wherein the power control valve 3 can make a rodless servo chamber 12 of the variable control cylinder 1 selectively communicate with an oil outlet of the variable pump 2 or with the hydraulic oil tank.

In this embodiment, the power control valve 3 is a two-position three-way reversing valve, and includes a first P port, a first T port, and a first working oil port, where the first P port is communicated with the oil outlet of the variable pump 2, the first T port is communicated with the hydraulic oil tank, and the first working oil port is communicated with the rodless servo cavity 12 of the variable control oil cylinder 1. The power control valve 3 has two states, a left position and a right position, respectively. When the power control valve 3 is in the right position, the first P port is communicated with the first working oil port; when the power control valve 3 is in the left position, the first T port is communicated with the first working oil port. The power control valve 3 further has a balance critical point, and when the power control valve core 31 of the power control valve 3 is located at the balance critical point, the first P port, the first T port and the first working oil port are not communicated with each other.

The variable control piston 11 of the variable control oil cylinder 1 is connected with the swash plate of the variable pump 2, and an elastic reset piece is arranged in the rod cavity of the variable control oil cylinder 1. In this embodiment, the elastic restoring member is a spring. The variable control piston 11 of the variable control cylinder 1 acts to control the rotation of the swash plate of the variable pump 2, thereby realizing the displacement adjustment of the variable pump 2.

The variable power control device further comprises a feedback valve sleeve 30 and a lever, wherein the feedback valve sleeve 30 is connected with the variable control piston 11 of the variable control oil cylinder 1, an inlet of the feedback valve sleeve 30 is connected with an oil outlet of the variable pump 2, and a feedback valve core 40 capable of moving relative to the feedback valve sleeve 30 is arranged in the feedback valve sleeve 30. When the oil pressure at the oil outlet of the variable displacement pump 2 changes, the oil pressure at the inlet of the feedback valve sleeve 30 changes, so that the feedback valve core 40 moves in the feedback valve sleeve 30, and the oil pressure change at the oil outlet of the variable displacement pump 2 is fed back by the feedback valve core 40 in real time.

One side of the first end of the lever abuts against the power control valve body 31 of the power control valve 3, and the second end of the lever abuts against the feedback valve body 40. The action of the feedback valve core 40 is converted into the action of the power control valve core 31 through a lever, and then the dynamic balance of the system is realized by using the elastic reset piece and the oil pressure change in the rodless servo cavity 12.

In this embodiment, the variable control piston 11 is provided with a high-pressure oil chamber 14, the high-pressure oil chamber 14 is communicated with an inlet of the feedback valve sleeve 30, the feedback valve sleeve 30 is vertically arranged, and the feedback valve core 40 is vertically arranged in the feedback valve sleeve 30. The high-pressure oil cavity 14 is an oil channel with two open ends, the opening at one end of the high-pressure oil cavity 14 is communicated with the oil outlet of the variable displacement pump 2, and the other end of the high-pressure oil cavity is provided with a plug 13 which plays a role in separating the high-pressure oil cavity 14 from the rodless servo cavity 12. The feedback valve sleeve 30 can drive the feedback valve core 40 to move horizontally along with the variable control piston 11, and the feedback valve core 40 can also vertically lift relative to the feedback valve sleeve 30 when the system pressure changes.

The middle part of the lever is provided with a pin shaft, and the lever can rotate relative to the pin shaft. Specifically, the lever includes a cross rod 41 and a vertical rod 42 forming an L-shaped structure, a pin is disposed at a connection position of the cross rod 41 and the vertical rod 42, and one side of the vertical rod 42 is abutted to the power control valve core 31.

The feedback spool 40 is located below the lever, with the second end of the lever supported by the top of the feedback spool 40. Specifically, the top of the feedback valve spool 40 abuts the lower sidewall of the crossbar 41, and the crossbar 41 is supported by the top of the feedback valve spool 40.

Preferably, the top of the feedback valve core 40 is a hemisphere structure to reduce the friction force when the feedback valve core 40 and the lever move relatively. In other embodiments, a roller may be provided on the top of the feedback valve core 40 to allow the feedback valve core 40 to be in rolling engagement with the lever crossbar 41.

Further, the variable power control apparatus further includes a power elastic member 50 connected to the lever, and the power elastic member 50 is related to the magnitude of the force by which the power control valve body 31 of the power control valve 3 starts to operate. In this embodiment, the power elastic member 50 is a spring.

The process of implementing dynamic balance by the variable power control device is briefly described below by taking the increase of system pressure as an example.

The system pressure is increased, the oil pressure in the high-pressure oil cavity 14 is increased, the feedback valve core 40 moves upwards, the lever rotates anticlockwise, the power control valve core 31 moves leftwards, the power elastic part 50 is compressed, the power control valve 3 is switched to the right position to enable the rodless servo cavity 12 to be communicated with the oil outlet of the variable pump 2, the oil pressure in the rodless servo cavity 12 is increased, the variable control piston 11 moves leftwards, the swash plate rotates anticlockwise, and the displacement of the variable pump 2 is reduced; meanwhile, the left movement of the variable control piston 11 will drive the feedback valve sleeve 30 and the feedback valve core 40 to synchronously move left, so that the lever keeps a balanced state, and the power elastic element 50 will make the power control valve core 31 return to the balanced state again, thereby realizing dynamic balance.

Further, the variable power control device further comprises a power regulating valve 5 and an electric proportional pressure regulating valve 6, wherein a valve core of the power regulating valve 5 is abutted against the other side of the first end of the lever, an outlet of the electric proportional pressure regulating valve 6 is connected with an inlet of the power regulating valve 5, and the current of the electric proportional pressure regulating valve 6 can be regulated according to the required power of an engine on the engineering vehicle. In this embodiment, the power elastic element 50 and the power regulating valve 5 are disposed on the same side of the lever, and the power regulating valve 5 and the power control valve core 31 are disposed on two sides of the vertical rod 42 and both abut against the vertical rod 42.

The variable power control can be realized by using the variable power control device, and the process of performing the variable power control by the variable power control device will be briefly described below by taking the increase of the outlet pressure of the electric proportional pressure regulating valve 6 as an example.

And adjusting the current of the electric proportional pressure regulating valve 6 according to the required power of the engine on the engineering vehicle so that the outlet pressure of the electric proportional pressure regulating valve 6 is equal to the pressure corresponding to the required power of the engine. When the outlet pressure of the electric proportional pressure regulating valve 6 is increased, the valve core of the power regulating valve 5 pushes the lever to move right, the lever rotates clockwise, the power control valve core 31 is pushed to move right, the power control valve 3 is switched to the left position to enable the rodless servo cavity 12 to be communicated with a hydraulic oil tank, the oil pressure in the rodless servo cavity 12 is reduced, the elastic resetting piece enables the variable control piston 11 to move right, the swash plate rotates clockwise, and the displacement of the variable pump 2 is increased; meanwhile, the feedback valve core 40 and the feedback valve sleeve 30 will move right along with the variable control piston 11, so that the lever keeps a balanced state, and thus the power elastic part 50 will make the power control valve core 31 return to the balanced state again, thereby realizing dynamic balance and completing power reduction adjustment.

Further, the variable power control device further comprises a pressure cut-off valve 9, and a pilot control oil port of the pressure cut-off valve 9 is communicated with an oil outlet of the variable pump 2. In this embodiment, the pressure cut-off valve 9 is a two-position three-way reversing valve, and includes a second P port, a second T port, and a second working oil port, the second P port is communicated with the oil outlet of the variable displacement pump 2, the second T port is communicated with the hydraulic oil tank, and the second working oil port is communicated with the first T port; the second working oil port is selectively communicated with the second P port or the second T port.

Specifically, pressure trip valve 9 has two states, be left position and right position respectively, and when pressure trip valve 9 was in the left position, second P mouth and second work oil port intercommunication, when pressure trip valve 9 was in the right position, second T mouth and second work oil port intercommunication. Normally, the pressure cut-off valve 9 is in the right position under the action of the oil pressure of the pilot control port and the self-return spring.

Normally, when an overload or a failure occurs, the pressure cut-off valve 9 may be switched to the left position to safely protect the variable displacement pump 2. When the outlet pressure of the variable pump 2 is increased to the preset cut-off oil pressure, the pressure cut-off valve 9 is switched from the right position to the left position, and if the power control valve 3 normally works at the right position, the oil outlet of the variable pump 2 is communicated with the rodless servo cavity 12 through the power control valve 3; if the power control valve 3 is in the left position due to a fault, the oil outlet of the variable displacement pump 2 is communicated with the rodless servo cavity 12 through the pressure cut-off valve 9 and the power control valve 3. Therefore, when pressure cut-off valve 9 is in the left position, no matter power control valve 3 is in the left position or the right position, the oil outlet of variable pump 2 is communicated with rodless servo chamber 12 to push variable control piston 11 to move left, so that the swash plate rotates counterclockwise to gradually reduce the displacement of variable pump 2. After the oil pressure at the oil outlet of variable pump 2 is lower than the preset cut-off oil pressure, pressure cut-off valve 9 will automatically switch to the right position.

Further, the variable power control device further comprises a load sensitive valve 10 and a flow control valve 20, wherein the load sensitive valve 10 comprises two pilot control oil ports which are a first pilot control oil port and a second pilot control oil port respectively, and the second pilot control oil port and a return spring of the load sensitive valve 10 are located on the same side; the oil outlet of the variable pump 2 is directly communicated with the first pilot control oil port, the oil outlet of the variable pump 2 is communicated with the oil inlet of the flow control valve 20, and the oil outlet of the flow control valve 20 is communicated with the load and is simultaneously communicated with the second pilot control oil port.

In this embodiment, the load-sensitive valve 10 is a two-position three-way reversing valve, and includes a third P port, a third T port, and a third working oil port, where the third P port is communicated with the oil outlet of the variable pump 2, the third T port is communicated with the hydraulic oil tank, and the third working oil port is communicated with the second T port; the third working oil port is selectively communicated with the third P port or the third T port.

Specifically, the load sensitive valve 10 has a left position and a right position, when the load sensitive valve 10 is located at the left position, the third P port is communicated with the third working oil port, and when the load sensitive valve 10 is located at the right position, the third T port is communicated with the third working oil port. Preferably, the load sensitive valve 10 is a proportional valve.

The variable power control device further comprises a first throttling unit 7 and a second throttling unit 8, a third working oil port is communicated with the hydraulic oil tank through the first throttling unit 7 and is also communicated with a first T port through the second throttling unit 8, and the first T port is communicated with the hydraulic oil tank sequentially through the second throttling unit 8 and the first throttling unit 7. In this embodiment, both the first throttle unit 7 and the second throttle unit 8 are orifices, and in other embodiments, throttle valves may be used for the first throttle unit 7 and the second throttle unit 8.

When the system power is equal to the maximum power of the variable power control device corresponding to the current opening of the electric proportional pressure regulating valve 6, the power control valve core 31 of the power control valve 3 is at the balance critical point; when the system power is less than the maximum power of the variable power control device corresponding to the current opening of the electric proportional pressure regulating valve 6, the power control valve 3 is in the left position, and the displacement of the variable pump 2 is regulated by the load sensitive valve 10 and the flow control valve 20. The adjustment of the displacement of the variable displacement pump 2 by the load sensitive valve 10 and the flow control valve 20 will be briefly described below by taking the decrease in the opening degree of the flow control valve 20 as an example.

Since the power control valve 3 is in the left position and the pressure cut-off valve 9 is in the right position when the system normally works, the third working oil port is communicated with the rodless servo cavity 12.

When the opening degree of the flow control valve 20 is reduced, the pressure difference between the inlet and the outlet of the flow control valve 20 is increased, the valve core of the load sensitive valve 10 moves to the right, the load sensitive valve 10 is switched to the left position, the oil outlet of the variable pump 2 is communicated with the rodless servo cavity 12, the variable control piston 11 moves to the left, the swash plate rotates anticlockwise, and the displacement of the variable pump 2 is reduced.

Further, the oil outlet of the flow control valve 20 is communicated with the second pilot control oil port through a third throttling unit. The oil pressure directed to the second pilot control port is reduced by the third throttling unit to protect the load sensitive valve 10.

The embodiment also provides a hydraulic system which comprises the variable power control device.

It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.

In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance. Wherein the terms "first position" and "second position" are two different positions.

In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.