Patent Description:
Conventionally, there has been known a magnetic chuck in which a permanent magnet is coupled to a piston inside a cylinder and the permanent magnet is displaced together with the piston. With such a magnetic chuck, following the displacement of a piston that has received fluid pressure, a permanent magnet is brought into close proximity to a workpiece, along with the workpiece being attracted and retained thereby. Further, when the piston is displaced in a direction to separate away from the workpiece, the workpiece is released.

When a stacked thin workpiece is to be retained by such a magnetic chuck, there is a problem in that the magnetic force is also transmitted to the lower workpiece and a plurality of workpieces are retained at a time. In order to retain only a single workpiece, it is necessary to adjust the magnetic force to be suitable for the thickness of the workpiece.

Therefore, <CIT> proposes a magnetic chuck capable of adjusting a moving end position of a piston assembly including a permanent magnet. According to this magnetic chuck, the position of the permanent magnet is adjusted by an adjuster such that the magnetic force of the permanent magnet retains exactly a single workpiece. Therefore, only a single workpiece among the plurality of stacked workpieces is retained.

<CIT>, <CIT>, <CIT>, and <CIT> also disclose magnetic chucks according to the prior art.

However, since the magnetic force of the permanent magnet exerted on the workpiece is adjusted to a magnetic force suitable for retaining just a single workpiece, when the workpiece is conveyed to another place, the workpiece may fall due to acceleration of conveyance or generation of vibration.

An object of the present invention is to solve the aforementioned problems.

This problem is solved by the magnetic chuck according to claim <NUM>. Preferred embodiments of the invention are evident from the dependent claims.

According to the magnetic chuck of the present invention, the first piston can be controlled to be in the three positions, i.e., a position where no magnetic force acts on the workpiece, a position where the workpiece is retained with the maximum magnetic force, and a position where the workpiece is attracted with a predetermined magnetic force smaller than the maximum magnetic force. Therefore, the only one workpiece among the plurality of stacked workpieces can be attracted with the predetermined magnetic force, and the workpiece can be retained and transported with the maximum magnetic force. The workpiece is safely transported without falling even if acceleration at the time of transportation is large.

A description will be given with reference to <FIG> concerning a magnetic chuck <NUM> according to the first embodiment of the present invention.

As shown in <FIG> and <FIG>, the magnetic chuck <NUM> has a structure in which a first cylinder <NUM> and a second cylinder <NUM> are arranged in series. The first cylinder <NUM> includes a first cylinder tube <NUM>, a first piston <NUM>, a bottom cover <NUM> and an intermediate cover <NUM>. The second cylinder <NUM> includes a second cylinder tube <NUM>, a second piston <NUM>, and a top cover <NUM>. The magnetic chuck <NUM>, for example, is attached to a non-illustrated robot arm.

First, the configuration of the first cylinder <NUM> will be mainly described. The cylindrical first cylinder tube <NUM> is made of a paramagnetic material such as an aluminum alloy. The outer shape of the horizontal cross-section of the first cylinder tube <NUM> is rectangular. The first cylinder tube <NUM> has a circular first cylinder hole <NUM>. A side surface of the first cylinder tube <NUM> includes at a lower portion thereof a first port <NUM> for supplying and exhausting air and a second port <NUM> at an upper portion thereof for supplying and exhausting air. Air is supplied from an air supply source (not shown) to the first port <NUM> and the second port <NUM>. The first port <NUM> is connected to an upper end of an air passage <NUM> extending vertically in a side wall of the first cylinder tube <NUM>. The first cylinder tube <NUM> includes a circular fitting portion 14a protruding downward.

The first piston <NUM> includes a seal holder <NUM>, a core yoke <NUM>, a permanent magnet <NUM>, a cover yoke <NUM>, and a ring plate <NUM>. The disk-shaped seal holder <NUM> is made of a paramagnetic material such as an aluminum alloy. A piston seal <NUM> is mounted on the outer periphery of the seal holder <NUM> and is in sliding contact with the wall surface of the first cylinder hole <NUM>. The piston seal <NUM> of the seal holder <NUM> maintains airtightness between a first air chamber <NUM> and a second air chamber <NUM> which are described later. The seal holder <NUM> has a through hole at the center thereof, and includes a flange 30a protruding radially inward from an upper portion of the through hole.

The cylindrical core yoke <NUM> is made of iron and/or steel materials which are ferromagnetic materials. The core yoke <NUM> includes a protruding end portion 34a that protrudes upward, and a bottomed screw hole 34b that opens at a distal end of the protruding end portion 34a. A lower portion of the core yoke <NUM> has a circular recess 34c that opens downward. The protruding end portion 34a of the core yoke <NUM> is inserted into the through hole of the seal holder <NUM> from below and abuts against the flange 30a of the seal holder <NUM>.

A piston rod <NUM> is made of a paramagnetic material such as an aluminum alloy. The piston rod <NUM> is inserted into the through hole of the seal holder <NUM> from above and is screw-engaged with the screw hole 34b of the core yoke <NUM>. Thus, the seal holder <NUM> and the core yoke <NUM> are connected to a lower end of the piston rod <NUM>. A first seal member 94a is mounted to a root of the protruding end portion 34a of the core yoke <NUM>. The first seal member 94a maintains airtightness between the seal holder <NUM> and the core yoke <NUM>.

The cylindrical permanent magnet <NUM> is disposed on an outer periphery of the core yoke <NUM> and is attached so as to be surrounded by the seal holder <NUM>, the core yoke <NUM>, the cover yoke <NUM>, and the ring plate <NUM>. The permanent magnet <NUM> is magnetized in a radial direction.

The cylindrical cover yoke <NUM> is made of iron and/or steel materials which are ferromagnetic material, and is disposed on an outer periphery of the permanent magnet <NUM>. An outer periphery of the cover yoke <NUM> has a stepped portion 38a, and an outer diameter of an upper portion of the cover yoke <NUM> is larger than an outer diameter of a lower portion of the cover yoke <NUM>. A pair of wear rings <NUM> are mounted on the outer periphery of the cover yoke <NUM>. The first piston <NUM> is guided and supported by the first cylinder hole <NUM> via the pair of wear rings <NUM>.

The bottom cover <NUM> includes a bottom yoke <NUM>, an outer yoke <NUM>, a first housing <NUM> and a second housing <NUM>. The columnar bottom yoke <NUM> is made of iron and/or steel materials which are ferromagnetic materials. The bottom yoke <NUM> enters the recess 34c of the core yoke <NUM> when the first piston <NUM> descends. A lower portion of the bottom yoke <NUM> includes a flange 48a that protrudes radially outward.

The cylindrical outer yoke <NUM> is made of iron and/or steel materials which are ferromagnetic materials, and is disposed outside the bottom yoke <NUM>. An upper portion of the outer yoke <NUM> includes a flange 50a protruding radially outward. A lower portion of the outer yoke <NUM> has a recess 50b on its outer periphery and a stepped portion 50c on its inner periphery. An annular connecting plate <NUM> is disposed between the flange 48a of the bottom yoke <NUM> and the stepped portion 50c of the outer yoke <NUM>. The connecting plate <NUM> is made of a paramagnetic material such as an aluminum alloy. The outer yoke <NUM> is fixed to the bottom yoke <NUM> via the connecting plate <NUM>. The outer yoke <NUM> and the connecting plate <NUM> are airtightly joined to each other, and the connecting plate <NUM> and the bottom yoke <NUM> are joined to each other in an airtight manner.

The cylindrical first housing <NUM> is made of a paramagnetic material such as an aluminum alloy. The outer shape of the horizontal cross-section of the first housing <NUM> is a rectangular shape similar to the first cylinder tube <NUM>. The fitting portion 14a of the first cylinder tube <NUM> is fitted inside the first housing <NUM>. A lower end portion of the first housing <NUM> includes a flange 54a that protrudes radially inward.

The cylindrical second housing <NUM> is made of a resin material or a rubber material. The second housing <NUM> is fitted and fixed to the recess 50b of the outer yoke <NUM>. A second seal member 94b is mounted in a gap formed between the lower end of the fitting portion 14a of the first cylinder tube <NUM> and an upper surface of the outer yoke <NUM>. The second seal member 94b maintains airtightness between the first cylinder tube <NUM> and the outer yoke <NUM>.

An annular first damper 96a is disposed on the upper surface of the outer yoke <NUM>. When the first piston <NUM> descends, the first damper 96a comes into contact with the stepped portion 38a of the cover yoke <NUM> to alleviate shocks. The upper surface of the first damper 96a has a plurality of slits (not shown) extending in the radial direction. The lower end of the air passage <NUM> of the first cylinder tube <NUM> communicates with the first cylinder hole <NUM> via the slits of the first damper 96a.

The disk-shaped intermediate cover <NUM> is disposed above the first cylinder hole <NUM> and is fixed to the first cylinder tube <NUM> via a latching ring <NUM>. The intermediate cover <NUM> is made of, for example, iron and/or steel materials which are ferromagnetic materials, but is not limited thereto. The piston rod <NUM> is inserted through the intermediate cover <NUM> and extends upward from the intermediate cover <NUM>. The piston rod <NUM> is in sliding contact with a first rod packing <NUM> mounted on the intermediate cover <NUM>.

A second damper 96b is attached to the lower end of the intermediate cover <NUM>. When the first piston <NUM> ascends, the second damper 96b comes into contact with the seal holder <NUM> of the first piston <NUM> to alleviate shocks. A third seal member 94c that abuts against the wall surface of the first cylinder hole <NUM> is mounted on an outer periphery of the intermediate cover <NUM>. The third seal member 94c maintains airtightness between the first cylinder tube <NUM> and the intermediate cover <NUM>.

The first air chamber <NUM> is disposed under the first piston <NUM>, and the second air chamber <NUM> is disposed above the first piston <NUM>. The first air chamber <NUM> is a space surrounded by the first piston <NUM>, the first cylinder tube <NUM>, and the bottom cover <NUM>. The second air chamber <NUM> is a space surrounded by the first piston <NUM>, the first cylinder tube <NUM>, and the intermediate cover <NUM>.

The first port <NUM> communicates with the first air chamber <NUM> via the air passage <NUM> of the first cylinder tube <NUM> and the slits of the first damper 96a. The second port <NUM> communicates with the second air chamber <NUM>. When air is supplied to the first air chamber <NUM>, the first piston <NUM> is biased upward. When air is supplied to the second air chamber <NUM>, the first piston <NUM> is biased downward. The configuration of the first cylinder <NUM> is substantially as described above.

Next, the configuration of the second cylinder <NUM> will be mainly described. The cylindrical second cylinder tube <NUM> is made of a paramagnetic material such as an aluminum alloy. The outer shape of the horizontal cross-section of the second cylinder tube <NUM> is a rectangular shape similar to the first cylinder tube <NUM>. The second cylinder tube <NUM> has a bottomed second cylinder hole <NUM>. An inner diameter of the second cylinder hole <NUM> is larger than the inner diameter of the first cylinder hole <NUM>.

A side surface of the second cylinder tube <NUM> is provided with a third port <NUM> for supplying and exhausting air. Air having the same pressure as the air supplied to the second port <NUM> is supplied to the third port <NUM> from an air supply source (not shown). The piston rod <NUM> is inserted through a bottom portion 66a of the second cylinder tube <NUM>, and is in sliding contact with a second rod packing <NUM> mounted on the bottom portion 66a of the second cylinder tube <NUM>.

The second piston <NUM> is made of a paramagnetic material such as an aluminum alloy. The second piston <NUM> includes a piston portion 76a fitted in the second cylinder hole <NUM> and a shaft portion 76b projecting upward from the piston portion 76a. The piston rod <NUM> is inserted through the second piston <NUM> and is in sliding contact with a third rod packing <NUM> mounted on the second piston <NUM>. The second piston <NUM> is movable up and down relative to the piston rod <NUM>.

A piston seal <NUM> is mounted on the outer periphery of the piston portion 76a of the second piston <NUM> and is in sliding contact with the wall surface of the second cylinder hole <NUM>. The piston seal <NUM> of the second piston <NUM> maintains airtightness of a later-described third air chamber <NUM> from the outside. A wear ring <NUM> is mounted on the outer periphery of the piston portion 76a of the second piston <NUM>. The second piston <NUM> is guided and supported by the second cylinder hole <NUM> via the wear ring <NUM>.

A third damper 96c is attached to a lower end of the piston portion 76a of the second piston <NUM>. When the second piston <NUM> descends, the third damper 96c comes into contact with the bottom portion 66a of the second cylinder tube <NUM> to alleviate shocks. A fourth damper 96d is attached to an upper end of the piston portion 76a of the second piston <NUM>. When the second piston <NUM> ascends, the fourth damper 96d comes into contact with the top cover <NUM> to alleviate shocks.

The disc-shaped top cover <NUM> is fixed to an upper portion of the second cylinder tube <NUM> via a latching ring <NUM>. The shaft portion 76b of the second piston <NUM> is inserted through the top cover <NUM>. The piston rod <NUM> extending upward from the intermediate cover <NUM> passes through the bottom portion 66a of the second cylinder tube <NUM> and the second piston <NUM>, and extends upward from the second piston <NUM>.

The third air chamber <NUM> is disposed under the second piston <NUM>. The third air chamber <NUM> is a space surrounded by the second piston <NUM> and the second cylinder tube <NUM>. The third port <NUM> communicates with the third air chamber <NUM>. When air is supplied to the third air chamber <NUM>, the second piston <NUM> is biased upward. The configuration of the second cylinder <NUM> is substantially as described above.

A plurality of tie rods <NUM> are inserted through the second cylinder tube <NUM>, and an end portion of each of the tie rods <NUM> is screw-engaged with the first cylinder tube <NUM>. The first housing <NUM> is fixed to the first cylinder tube <NUM> by screw parts (not shown). As a result, the three constituent elements of the second cylinder tube <NUM>, the first cylinder tube <NUM>, and the first housing <NUM> are integrally connected. At this time, since the flange 50a of the outer yoke <NUM> is sandwiched and held between the fitting portion 14a of the first cylinder tube <NUM> and the flange 54a of the first housing <NUM>, the outer yoke <NUM> is also connected and fixed.

An end of the piston rod <NUM> extending upwardly from the second piston <NUM> has an externally threaded portion 44a. A stopper <NUM> and a lock nut <NUM> are screw-engaged with the externally threaded portion 44a of the piston rod <NUM>. The stopper <NUM> can abut against an upper end of the shaft portion 76b of the second piston <NUM>. When the lock nut <NUM> is loosened, the position of the stopper <NUM> relative to the piston rod <NUM> can be changed. When the lock nut <NUM> is tightened again after the position of the stopper <NUM> is changed, the stopper <NUM> is fixed at a desired position.

A piston area (pressure-receiving area) S1 of the first piston <NUM> by the air in the second air chamber <NUM> is equal to a difference between a cross-sectional area of the first cylinder hole <NUM> and a cross-sectional area of the piston rod <NUM>. A piston area (pressure-receiving area) S2 of the second piston <NUM> by the air in the third air chamber <NUM> is equal to a difference between a cross-sectional area of the second cylinder hole <NUM> and the cross-sectional area of the piston rod <NUM>. Since the cross-sectional area of the second cylinder hole <NUM> is larger than the cross-sectional area of the first cylinder hole <NUM>, the piston area S2 of the second piston <NUM> is larger than the piston area S1 of the first piston <NUM>.

Next, basic functions of the magnetic chuck <NUM> will be described. The first piston <NUM> of the magnetic chuck <NUM> is controlled to be in three positions of an "upper end position", a "lower end position" and an "adjustment position" by supplying and exhausting air to and from the first air chamber <NUM>, the second air chamber <NUM>, and the third air chamber <NUM>.

As shown in <FIG>, the "upper end position" refers to a position at which the first piston <NUM> is displaced to the uppermost position in the first cylinder hole <NUM> and the seal holder <NUM> abuts against the second damper 96b. The "upper end position" is achieved by supplying air to the first air chamber <NUM> and exhausting air from the second air chamber <NUM> and the third air chamber <NUM>. When the first piston <NUM> is controlled to be in the "upper end position", even if a workpiece W such as a ferromagnetic iron plate (hereinafter simply referred to as a "workpiece") is present at a lower end of the bottom cover <NUM>, the magnetic force of the permanent magnet <NUM> does not act on the workpiece W.

As shown in <FIG>, the "lower end position" refers to a position (a stroke end) when the first piston <NUM> is displaced to the lowermost position in the first cylinder hole <NUM> and the cover yoke <NUM> abuts against the first damper 96a. The "lower end position" is achieved by supplying air to the second air chamber <NUM> and exhausting air from the first air chamber <NUM> and the third air chamber <NUM>. When the first piston <NUM> is controlled to be in the "lower end position", if the workpiece W is present at the lower end of the bottom cover <NUM>, the workpiece W is attracted by the maximum magnetic force.

As shown in <FIG>, the "adjustment position" refers to a position where the first piston <NUM> is separated upward from the "lower end position" by a predetermined distance, in relation to the displacement of the second piston <NUM> to the uppermost position in the second cylinder hole <NUM>. The "adjustment position" is achieved by supplying air to the second air chamber <NUM> and the third air chamber <NUM> and exhausting air from the first air chamber <NUM>. When the first piston <NUM> is controlled to be in the "adjustment position", if the workpiece W is present at the lower end of the bottom cover <NUM>, the workpiece W is attracted by a predetermined magnetic force (hereinafter referred to as "adjusted magnetic force") smaller than the maximum magnetic force.

When the first piston <NUM> is controlled to be in the "lower end position" and the "adjustment position", most of the magnetic flux lines emitted from the permanent magnet <NUM> return to the permanent magnet <NUM> via the core yoke <NUM>, the bottom yoke <NUM>, the workpiece W, the outer yoke <NUM>, and the cover yoke <NUM>.

When a plurality of workpieces are stacked on the lower end of the bottom cover <NUM>, the magnitude of the adjusted magnetic force is adjusted in order that only the uppermost workpiece W is attracted. The magnitude of the adjusted magnetic force varies depending on the material, weight, and the like of the workpiece W. The magnitude of the adjusted magnetic force can be adjusted by changing the position of the stopper <NUM> relative to the piston rod <NUM>. The basic functions of the magnetic chuck <NUM> are as described above.

Next, operations will be described, in attracting and transporting only the uppermost workpiece W among a plurality of workpieces stacked on a floor surface, using the magnetic chuck <NUM> attached to the robot arm. A state in which the first piston <NUM> is controlled to be in the "upper end position" is referred to as an initial state.

From the initial state, the robot arm is driven, and the magnetic chuck <NUM> approaches, from above, a plurality of workpieces stacked on the floor surface. At the same time, the air in the first air chamber <NUM> is exhausted and air is supplied to the second air chamber <NUM> and the third air chamber <NUM>.

When air is supplied to the second air chamber <NUM>, the first piston <NUM> and the piston rod <NUM> are biased downward. When air is supplied to the third air chamber <NUM>, the second piston <NUM> is biased upward. The stopper <NUM> integral with the downwardly biased piston rod <NUM> abuts against the upwardly biased second piston <NUM>.

In this case, the piston area S2 of the second piston <NUM> is larger than the piston area S1 of the first piston <NUM>, and the air pressure acting on the second piston <NUM> is the same as the air pressure acting on the first piston <NUM>. Therefore, the force with which the upwardly biased second piston <NUM> pushes up the piston rod <NUM> via the stopper <NUM> exceeds the force with which the piston rod <NUM> is biased downward. The second piston <NUM> is displaced to the uppermost position in the second cylinder hole <NUM>, and is held at a position where the fourth damper 96d abuts against the top cover <NUM>. Further, the piston rod <NUM> is held in a state in which the stopper <NUM> integrated with the piston rod <NUM> abuts against the second piston <NUM>. In other words, the first piston <NUM> is controlled to be in the "adjustment position".

After the first piston <NUM> is controlled to be in the "adjustment position", the magnetic chuck <NUM> further approaches the plurality of workpieces stacked on the floor surface. When the bottom cover <NUM> comes into contact with the uppermost workpiece W, only the workpiece W is attracted (see <FIG>). Thereafter, when the magnetic chuck <NUM> moves upward and is sufficiently separated from the workpieces remaining on the floor surface, the air in the third air chamber <NUM> is exhausted.

Since air is continuously supplied to the second air chamber <NUM>, the first piston <NUM> and the piston rod <NUM> are biased downward. On the other hand, when the air in the third air chamber <NUM> is exhausted, the force biasing the second piston <NUM> upward disappears. Therefore, the first piston <NUM> is displaced to the lowest position in the first cylinder hole <NUM>, and the cover yoke <NUM> of the first piston <NUM> comes into contact with the first damper 96a. In other words, the first piston <NUM> is controlled to be in the "lower end position". At this time, the second piston <NUM> is pushed by the stopper <NUM> and displaced downward, and the third damper 96c comes into contact with the bottom portion 66a of the second cylinder tube <NUM>.

When the first piston <NUM> is controlled to be in the "lower end position", the workpiece W is retained by the maximum magnetic force (see <FIG>). Thereafter, the magnetic chuck <NUM> retaining only one workpiece W is moved to a predetermined location. Since the workpiece W is retained by the maximum magnetic force, the workpiece W does not drop even if the acceleration at the time of transporting is large, and is safely transported to a predetermined place.

Thereafter, the air in the second air chamber <NUM> is exhausted, and air is supplied to the first air chamber <NUM>. When air is supplied to the first air chamber <NUM>, the first piston <NUM> is biased upward. On the other hand, when the air in the second air chamber <NUM> is exhausted, the force biasing the first piston <NUM> downward disappears.

In this case, the force with which the first piston <NUM> is biased upward is set so as to exceed the magnetic force of attraction between the first piston <NUM> and the workpiece W. That is, the product of the pressure of the air supplied to the first air chamber <NUM> and the piston area (the cross-sectional area of the first cylinder hole <NUM>) of the first piston <NUM> that receives the pressure of the air in the first air chamber <NUM> is set so as to be equal to or greater than a predetermined value. The first piston <NUM> is displaced to the uppermost position in the first cylinder hole <NUM>, and the seal holder <NUM> of the first piston <NUM> abuts against the second damper 96b. That is, the first piston <NUM> is controlled to be in the "upper end position", and the workpiece W is released at the predetermined position (see <FIG>).

In the above description, it is assumed that the first piston <NUM> is controlled from the "upper end position" to the "adjustment position" and only the uppermost workpiece W is attracted among the plurality of workpieces stacked on the floor surface. However, it is also possible to control the first piston <NUM> to the "lower end position" to attract a plurality of workpieces, and then control the first piston <NUM> to the "adjustment position" to attract only one workpiece W among them. For example, this is useful in a case where a plurality of workpieces are simultaneously transported to another place, and thereafter only one workpiece W is retained.

In this case, the force with which the second piston <NUM> pushes up the piston rod <NUM> via the stopper <NUM> by the air supplied to the third air chamber <NUM> may be set so as to exceed a combined force of the force with which the piston rod <NUM> is biased downward by the air supplied to the second air chamber <NUM> and the magnetic force of attraction between the first piston <NUM> and the workpiece W.

According to the magnetic chuck <NUM> of the present embodiment, the first piston <NUM> can be controlled to be in three positions: the "upper end position" where no magnetic force acts on the workpiece W, the "lower end position" where the workpiece W is retained with the maximum magnetic force, and the "adjustment position" where the workpiece W is attracted with a predetermined magnetic force smaller than the maximum magnetic force. Therefore, only one workpiece W among the plurality of stacked workpieces can be attracted, and the workpiece W can be retained with the maximum magnetic force and can be safely transported.

Further, by changing the position of the stopper <NUM> relative to the piston rod <NUM>, the magnetic force acting on the workpiece W at the "adjustment position" can be set to optimum magnitude. Since the pressure of the air supplied to the third port <NUM> is the same as the pressure of the air supplied to the second port <NUM>, the air supply system can be simplified.

Next, a magnetic chuck <NUM> according to a second embodiment of the present invention will be described with reference to <FIG>.

The magnetic chuck <NUM> according to the second embodiment is different from the magnetic chuck <NUM> in the relationship between the inner diameter of the first cylinder hole <NUM> and the inner diameter of the second cylinder hole <NUM>, and the relationship between the pressure of the air supplied to the second port <NUM> and the pressure of the air supplied to the third port <NUM>. Specifically, the inner diameter of the second cylinder hole <NUM> is the same as the inner diameter of the first cylinder hole <NUM>, and the pressure of the air supplied to the third port <NUM> is higher than the pressure of the air supplied to the second port <NUM>.

The "upper end position" is achieved by supplying air to the first air chamber <NUM> and exhausting air from the second air chamber <NUM> and the third air chamber <NUM>. The "lower end position" is achieved by supplying air to the second air chamber <NUM> and exhausting air from the first air chamber <NUM> and the third air chamber <NUM>. The "adjustment position" is achieved by supplying air to the second air chamber <NUM> and the third air chamber <NUM> and exhausting air from the first air chamber <NUM> (see <FIG>).

When the first piston <NUM> is controlled to be in the "adjustment position", air is supplied to the second air chamber <NUM>, whereby the first piston <NUM> and the piston rod <NUM> are biased downward. When air is supplied to the third air chamber <NUM>, the second piston <NUM> is biased upward. The stopper <NUM> integral with the downwardly biased piston rod <NUM> abuts against the upwardly biased second piston <NUM>.

The piston area S2 of the second piston <NUM> receiving the pressure by the air in the third air chamber <NUM> is equal to the piston area S1 of the first piston <NUM> receiving the pressure by the air in the second air chamber <NUM>. However, the air pressure in the third air chamber <NUM> is higher than the air pressure in the second air chamber <NUM>. The force with which the upwardly biased second piston <NUM> pushes up the piston rod <NUM> via the stopper <NUM> exceeds the force with which the piston rod <NUM> is biased downward. Therefore, the second piston <NUM> is displaced to the uppermost position in the second cylinder hole <NUM>, and is held at a position where the fourth damper 96d abuts against the top cover <NUM>. Thereafter, when the magnetic chuck <NUM> approaches and comes into contact with the plurality of workpieces stacked on the floor surface, only the uppermost workpiece W is attracted.

Further, by changing the position of the stopper <NUM> relative to the piston rod <NUM>, the magnetic force acting on the workpiece W at the "adjustment position" can be set to optimum magnitude. Further, since the inner diameter of the second cylinder hole <NUM> is the same as the inner diameter of the first cylinder hole <NUM>, the wall thickness of the second cylinder tube <NUM> can be made the same as the wall thickness of the first cylinder tube <NUM>, and reduction in size is easy.

Next, a magnetic chuck <NUM> according to a third embodiment of the present invention will be described with reference to <FIG>. In the magnetic chuck <NUM> according to the third embodiment, constituent elements that are the same as or equivalent to those of the above-described magnetic chuck <NUM> are denoted by the same reference numerals, and detailed description thereof is omitted.

As shown in <FIG>, a coil spring <NUM> that biases the first piston <NUM> downward is disposed in a second air chamber <NUM>. A cylindrical spring guide <NUM> is disposed inside the coil spring <NUM>. A lower surface of the spring guide <NUM> abuts against the upper surface of the seal holder <NUM> of the first piston <NUM>. The spring guide <NUM> is made of a paramagnetic material such as an aluminum alloy.

A lower end of the spring guide <NUM> includes a spring receiving portion 118a that protrudes radially outward. A lower end of an intermediate cover <NUM> includes a spring receiving portion 120a formed of a stepped portion. An upper end of the coil spring <NUM> is supported by the spring receiving portion 120a of the intermediate cover <NUM>, and the lower end of the coil spring <NUM> is supported by the spring receiving portion 118a of the spring guide <NUM>.

The piston rod <NUM> extends upward from the intermediate cover <NUM> through the center of the spring guide <NUM> and the center of the intermediate cover <NUM>. A damper <NUM> is attached to an upper surface of the spring guide <NUM>. When the first piston <NUM> ascends, the damper <NUM> comes into contact with the intermediate cover <NUM> to alleviate shocks. The intermediate cover <NUM> does not include a rod packing or a damper.

The inner diameter of the second cylinder hole <NUM> is the same as the inner diameter of the first cylinder hole <NUM>. A second port <NUM> is configured as a port that is open to the atmosphere. That is, the second air chamber <NUM> is always open to the atmosphere via the second port <NUM>. The "upper end position" is achieved by supplying air to the first air chamber <NUM> and exhausting air from the third air chamber <NUM> (see <FIG>). The "lower end position" is achieved by exhausting air from the first air chamber <NUM> and the third air chamber <NUM> (see <FIG>). The "adjustment position" is achieved by supplying air to the third air chamber <NUM> and exhausting air from the first air chamber <NUM> (see <FIG>).

When the first piston <NUM> is controlled to be in the "adjustment position", the first piston <NUM> and the piston rod <NUM> are biased downward by the biasing force of the coil spring <NUM>. When air is supplied to the third air chamber <NUM>, the second piston <NUM> is biased upward. The stopper <NUM> integral with the downwardly biased piston rod <NUM> abuts against the upwardly biased second piston <NUM>.

The force with which the upwardly biased second piston <NUM> pushes up the piston rod <NUM> via the stopper <NUM> exceeds the force with which the piston rod <NUM> is biased downward. Therefore, the second piston <NUM> is displaced to the uppermost position in the second cylinder hole <NUM>, and is held at a position where the fourth damper 96d abuts against the top cover <NUM>. Thereafter, when the magnetic chuck <NUM> approaches and comes into contact with the plurality of workpieces stacked on the floor surface, only the uppermost workpiece W is attracted.

Further, by changing the position of the stopper <NUM> relative to the piston rod <NUM>, the magnetic force acting on the workpiece W at the "adjustment position" can be set to optimum magnitude. Further, since there is no need to supply air to the second air chamber <NUM>, the air supply system can be further simplified.

Next, a magnetic chuck <NUM> according to a fourth embodiment of the present invention will be described with reference to <FIG>. In the magnetic chuck <NUM> according to the fourth embodiment, constituent elements that are the same as or equivalent to those of the above-described magnetic chuck <NUM> are denoted by the same reference numerals, and detailed description thereof is omitted.

As shown in <FIG> and <FIG>, a first cylinder <NUM> includes a first cylinder tube <NUM>, a first piston <NUM>, a bottom cover <NUM> and a first intermediate cover <NUM>. A second cylinder <NUM> includes a second cylinder tube <NUM>, a second piston <NUM>, a top cover <NUM> and a second intermediate cover <NUM>. The first intermediate cover <NUM> is fixed to an upper portion of the first cylinder tube <NUM> via a latching ring <NUM>. The second intermediate cover <NUM> is fixed to a lower portion of the second cylinder tube <NUM>, and is in contact with the first intermediate cover <NUM>.

A tubular second cylinder tube <NUM> has a circular second cylinder hole <NUM>. An inner diameter of the second cylinder hole <NUM> is the same as an inner diameter of a first cylinder hole <NUM>. A second air chamber <NUM> is a space surrounded by the first piston <NUM>, the first cylinder tube <NUM>, and the first intermediate cover <NUM>. A third air chamber <NUM> is a space surrounded by the second piston <NUM>, the second cylinder tube <NUM>, and the second intermediate cover <NUM>. The pressure of the air supplied to the third port <NUM> is the same as the pressure of the air supplied to the second port <NUM>.

The top cover <NUM> includes an annular plate portion 136a and a cylindrical main body portion 136b that extends upward from an inner periphery of the plate portion 136a. The top cover <NUM> is fixed to an upper portion of the second cylinder tube <NUM> on an outer periphery of the plate portion 136a. The main body portion 136b of the top cover <NUM> protrudes upward from the second cylinder tube <NUM>.

A stopper <NUM> includes a cylindrical threaded portion 138a with an internal thread and a cylindrical main body portion 138b that expands in diameter from the threaded portion 138a and extends downward. The stopper <NUM> is inserted inside the main body portion 136b of the top cover <NUM> and is movable up and down relative to the top cover <NUM>. The stopper <NUM> is capable of abutting against the second piston <NUM> at a lower end of the main body portion 138b. At least a part of the main body portion 138b of the stopper <NUM> is covered by the main body portion 136b of the top cover <NUM>. A lower end of the main body portion 138b of the stopper <NUM> is not exposed outside.

The piston rod <NUM> is inserted through the first intermediate cover <NUM>, the second intermediate cover <NUM>, and the second piston <NUM>, and extends upward from the second piston <NUM>. The piston rod <NUM> is in sliding contact with a rod packing <NUM> mounted on the first intermediate cover <NUM> and a rod packing <NUM> mounted on the second intermediate cover <NUM>. The second piston <NUM> is movable up and down relative to the piston rod <NUM>. A fourth damper 96d is attached to an upper part of the second piston <NUM>. When the second piston <NUM> ascends, the fourth damper 96d comes into contact with the plate portion 136a of the top cover <NUM> to alleviate shocks.

The threaded portion 138a of the stopper <NUM> and a lock nut <NUM> are screw-engaged with the externally threaded portion 44a of the piston rod <NUM>. After the position of the stopper <NUM> is adjusted to a predetermined position relative to the piston rod <NUM>, the stopper <NUM> is fixed to the piston rod <NUM>. A scale 138c is indicated on a top outer surface of the main body portion 138b of the stopper <NUM>. In a state that the first piston <NUM> and the piston rod <NUM> are each displaced to an uppermost portion, the position of the stopper <NUM> can be realized by reading the scale 138c that is pointed to by the upper end of the top cover <NUM>.

A coil spring <NUM> biasing the second piston <NUM> upward is disposed in the third air chamber <NUM>. An upper end of the coil spring <NUM> is supported by a spring receiving portion 76c of the second piston <NUM>, and a lower end of the coil spring <NUM> is supported by a spring receiving portion 140a of the second intermediate cover <NUM>. A third damper <NUM> is attached to an upper surface of the second intermediate cover <NUM>. When the second piston <NUM> descends, the third damper <NUM> comes into contact with the second piston <NUM> to alleviate shocks.

The "upper end position" is achieved by supplying air to the first air chamber <NUM> and exhausting air from the second air chamber <NUM> and the third air chamber <NUM> (see <FIG>). The "lower end position" is achieved by supplying air to the second air chamber <NUM> and exhausting air from the first air chamber <NUM> and the third air chamber <NUM> (see <FIG>). The "adjustment position" is achieved by supplying air to the second air chamber <NUM> and the third air chamber <NUM> and exhausting air from the first air chamber <NUM> (see <FIG>).

When the first piston <NUM> is controlled to be in the "adjustment position", air is supplied to the second air chamber <NUM>, whereby the first piston <NUM> and the piston rod <NUM> are biased downward. When the first piston <NUM> is controlled to be in the "adjustment position", air is supplied to the third air chamber <NUM>, and the second piston <NUM> receives the biasing force of the coil spring <NUM>, whereby the second piston <NUM> is biased upward. The stopper <NUM> that is integral with the piston rod <NUM> biased downward abuts against the second piston <NUM> biased upward.

The piston area of the second piston <NUM> receiving the pressure by the air in the third air chamber <NUM> is equal to the piston area of the first piston <NUM> receiving the pressure by the air in the second air chamber <NUM>. Further, the air pressure in the third air chamber <NUM> is the same as the air pressure in the second air chamber <NUM>. That is, the force with which the second piston <NUM> is biased upward by the air in the third air chamber <NUM> is the same as the power with which the piston rod <NUM> is biased downward by the air in the second air chamber <NUM>. The force of the second piston <NUM> biased upward that pushes up the piston rod <NUM> via the stopper <NUM> exceeds the force that biases the piston rod <NUM> downward, by an amount of the biasing force of the coil spring <NUM>. Therefore, the second piston <NUM> is held at a position where the second piston <NUM> is displaced to the uppermost position in the second cylinder hole <NUM>.

In this case, the force with which the first piston <NUM> and the piston rod <NUM> are biased downward by the air in the second air chamber <NUM> is set such that the force exceeds the biasing force of the coil spring <NUM> received by the second piston <NUM> when the first piston <NUM> is controlled to be in the "lower end position". The air pressure supplied to the second air chamber <NUM> is set such that the "lower end position" is realized.

According to the magnetic chuck <NUM> of the present embodiment, the first piston <NUM> can be controlled to be in the three positions: the "upper end position" where no magnetic force acts on the workpiece W, the "lower end position" where the workpiece W is retained with the maximum magnetic force, and the "adjustment position" where the workpiece W is attracted with a predetermined magnetic force that is smaller than the maximum magnetic force. Therefore, only one workpiece W among the plurality of stacked workpieces can be attracted, and the workpiece W can be retained with the maximum magnetic force and can be safely transported.

Claim 1:
A magnetic chuck (<NUM>, <NUM>, <NUM>, <NUM>) that attracts and retains a workpiece by a magnetic force of a permanent magnet (<NUM>),
characterized by
a first cylinder (<NUM>) and a second cylinder (<NUM>) that are arranged in series with each other, wherein a first piston (<NUM>) of the first cylinder includes the permanent magnet, a piston rod (<NUM>) connected to the first piston is inserted into a second piston (<NUM>) of the second cylinder, a stopper (<NUM>, <NUM>) configured to abut against the second piston is fixed to the piston rod, and a position of the stopper relative to the piston rod is configured to be changed, wherein
the first cylinder includes a first air chamber (<NUM>) disposed on one side of the first piston and a second air chamber (<NUM>) disposed on another side of the first piston,
the second cylinder includes a third air chamber (<NUM>) disposed on one side of the second piston, and a third port (<NUM>) through which air is supplied to and exhausted from the third air chamber.