Hydraulic drive with rapid stroke and load stroke

A hydraulic drive including a differential cylinder that has a first pressure chamber and a second pressure chamber and a differential piston, a first hydraulic pump that includes a pump intake and a pump outlet, a directional control valve having a first switching position and a second switching position, a high pressure tank, and an additional hydraulic cylinder that includes an additional pressure chamber hydraulically connected with the pump intake and with the high pressure tank and an additional piston limiting the additional pressure chamber, the differential piston being movably coupled with the additional piston, wherein in the first switching position of the directional control valve the first pressure chamber is hydraulically connected with the second pressure chamber, and in the second switching position of the directional control valve the second pressure chamber is not hydraulically connected with the first pressure chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a hydraulic drive having a differential cylinder. The invention moreover relates to a method for operating such a drive.

2. Description of the Related Art

Hydraulic drives are well known from the current state of the art. In practice it is desirable for hydraulic drives, in particular for hydraulic drives for hydraulic presses, to provide a hydraulic drive that on the one hand provides a rapid movement of a drive piston in a so-called rapid stroke or rapid movement with low force, and with which on the other hand a slower action with great force is possible in a so-called load stroke or load movement.

Various drives are known for this purpose from the current state of the art. In one drive with a so-called throttle control, a pump is driven by a motor at constant speed. The control and changeover between rapid stroke and load stroke through control of the volume flow occurs hereby via flow resistances, for example, via valves. A disadvantage of such a drive with throttle control is the low efficiency due to the occurring flow losses.

Drives having a so-called displacement control system are moreover known from the current state of the art. A drive of this type may for example have a variable speed motor that drives two pumps having opposite delivering directions. The two pumps are connected via a hydraulic cylinder in such a way that the pump takes in hydraulic oil from one piston chamber of a hydraulic cylinder and moves hydraulic oil into the other piston chamber. The changeover from rapid stroke to load stroke, or respectively the speed control of the hydraulic drive occurs through changing of the displacement volume of the pump or respectively through a change in the rotational speed of the motor. A disadvantage of such a drive with a displacement control system is that the motor must provide a higher speed for the high speed in the rapid stroke and a high maximum torque for the high force in the load stroke mode. When using a fixed displacement pump, the motor must be designed accordingly because of this high so-called peak performance, and therefore it becomes large, heavy, slow and expensive.

What is needed in the art is a hydraulic drive that can be operated in a rapid stroke and a load stroke mode such that efficiency losses can be avoided and the motor can be produced cost effectively.

SUMMARY OF THE INVENTION

The present invention provides a hydraulic drive that includes a high pressure tank and an additional hydraulic cylinder that has an additional piston limiting the additional pressure chamber, wherein the differential piston is movably coupled with the additional piston. The additional pressure chamber is hydraulically connected with the pump intake and with the high pressure tank. The first pressure chamber is hydraulically connected with the pump outlet, and in the first switching position of the directional control valve the first pressure chamber is hydraulically connected with the second pressure chamber and in the second switching position of the directional control valve the second pressure chamber is not hydraulically connected with the first pressure chamber. In the second switching position the second pressure chamber can be connected with a tank or a low pressure tank. The low pressure tank can have a tank pressure of approximately 2 to 10 bar.

Pump intake and pump outlet are understood to be pump connections of the hydraulic pump. The hydraulic pump may be driven, for example, by a variable speed electric motor, whose direction of rotation is reversible. Consequently, in one direction of rotation of the electric motor hydraulic fluid can be sucked in at the pump intake, whereas hydraulic fluid can be discharged from the hydraulic pump at the pump outlet. When reversing the direction of rotation, hydraulic fluid can be sucked in at the pump outlet, whereby hydraulic fluid can be discharged at the pump intake. For this purpose the hydraulic pump can be designed as a so-called four-quadrant pump. It also conceivable that the hydraulic pump can be designed as a piston or gear pump. Instead of one single hydraulic pump, it is however also conceivable to provide two two-quadrant pumps that deliver in an opposite direction and which are driven by an electric motor whose direction of rotation is also reversible. The two two-quadrant pumps can provide identical delivery volumes. By changing the rotational speed of the electric motor the movement speed of the hydraulic drive can be influenced.

The movable coupling of the differential piston and the additional piston can be mechanical. The differential piston and the additional piston can share a common piston rod. The piston rods of the differential cylinder and the additional hydraulic cylinder may also be, for example, axially aligned with one another and welded together. It is however also conceivable that the piston rods of the differential cylinder and the additional hydraulic cylinder are arranged parallel to one another and are movably coupled with one another by a yoke or a pressing tool that is arranged on the piston rods.

The hydraulic drive according to the present invention can also include a pre-load drive due to the connection of the additional pressure chamber with the high pressure tank. Moreover, due to the interconnection via the directional control valve, a changeover can be achieved from a so-called rapid stroke, having a high speed and comparatively low force to a so-called load stroke having a lower speed and comparatively high force. If, in the first switching position the first pressure chamber is hydraulically connected with the second pressure chamber and if, during operation of the hydraulic drive, hydraulic fluid is moved (pumped) at the pump outlet into the first pressure chamber and hydraulic fluid is moved (sucked) out of the additional pressure chamber at the pump intake, the movably coupled differential pistons and additional pistons can move together in rapid stroke, since the hydraulic fluid that is necessary for filling the first pressure chamber can be provided partially by the hydraulic fluid that is displaced out of the second pressure chamber due to the hydraulic connection of the first and the second pressure chamber. Thus, only the additional hydraulic fluid that cannot already flow out of the second pressure chamber into the first pressure chamber has to be provided by the hydraulic pump at the pump outlet.

If, in the second switching position the first pressure chamber is not hydraulically connected with the second pressure chamber and if, during operation of the hydraulic drive hydraulic fluid is moved (pumped) at the pump outlet into the first pressure chamber and hydraulic fluid is moved (sucked) out of the additional pressure chamber at the pump intake, the movably coupled differential pistons and additional pistons can move together in a so-called load stroke, since the hydraulic fluid that is necessary for filling the first pressure chamber must be provided entirely by the hydraulic pump. At an unchanged pressure provided by the hydraulic pump, the hydraulic pump can now act upon a larger effective surface of the first pressure chamber. The differential piston and the additional piston can operate at greater force and lower speed compared to the rapid stroke.

Since the additional pressure chamber is hydraulically connected with a high pressure tank, cutting shock damping can be provided during the operation of the hydraulic drive by the pre-loading of the additional pressure chamber, in an opposite direction of the effective direction of the drive or respectively in an opposite direction of the pump direction in the load stroke mode. The force resulting from the pressure in the additional pressure chamber can thus, for example, prevent that—in the event of a punch tearing through a metal sheet work piece—the punch makes uncontrolled movements. Cutting shock damping can be adjusted by adjusting the pressure in the high pressure chamber.

A further development of the hydraulic drive provides that the first pressure chamber includes a first hydraulic effective surface, the second pressure chamber includes a second hydraulic effective surface, and the additional pressure chamber includes an additional hydraulic effective surface, wherein a differential surface is consistent with the first and the second effective surfaces of the additional effective surface. The first effective surface is desirably larger hereby than the second effective surface. If the directional control valve is positioned in the first switching position and the pump moves hydraulic fluid into the first pressure chamber (pumps) and moves hydraulic fluid (sucks) out of the additional pressure chamber, the performance of a synchronized speed cylinder can be replicated in the rapid stroke, since then the effective differential surface of the first and the second effective surface that results due to the hydraulic connection of the first and second pressure chamber is consistent with the additional effective surface. Based on the use of a four-quadrant pump or based on two interconnected two-quadrant pumps having identical delivery volumes, and which deliver in opposite directions, a hydraulic drive can be provided wherein the delivery volumes of the hydraulic pump in the rapid stroke mode are adapted to the hydraulic effective surfaces of the pressure chambers. It can thus be achieved that no or almost no hydraulic fluid has to be subsequently sucked from a tank or pressure tank.

If, during the load stroke the delivery volume of the hydraulic pump or respectively the delivery volumes of the hydraulic pumps are no longer adapted to the effective hydraulic surfaces of the differential cylinder and the additional hydraulic cylinder, in other words if more hydraulic fluid is delivered (pumped) into the first pressure chamber than can be moved (sucked) from the additional pressure chamber, a torque assistance of the electric motor driving the hydraulic pump can be achieved via the hydraulic connection with the high pressure tank. The pressure in the additional pressure chamber acts upon the pump intake of the pump, so that the pump can be partially driven as a hydraulic motor, and the electric motor that drives the pump can be sized accordingly smaller.

An additional arrangement of the hydraulic drive provides that the changeover from the rapid stroke to the load stroke can occur when a pressure in the piston chamber is exceeded. A hydraulic control line that hydraulically connects the directional control valve with the first pressure chamber is can be provided for this purpose. In the event of the pressure limit being exceeded, the directional control valve can be moved—against the force of a spring—from the first switching position into the second switching position. When falling below a reset pressure that is below the pressure limit, the directional control valve can then again be moved through spring actuation into the first switching position. If now, for example, a stamping or pressing tool that is driven by the hydraulic drive impinges upon a work piece during the rapid stroke, the pressure increases in the first pressure chamber. If the pressure in the first pressure chamber exceeds the pressure limit, the directional control valve is moved into the second switching position.

A variable speed electric motor may drive the first hydraulic pump. Also, the direction of rotation of the electric motor may be reversible. The rate of movement of the hydraulic drive can be influenced with such an electric motor via the rotational speed. By reversing the direction of rotation of the electric motor a reversal of the direction of delivery of the hydraulic pump or respectively the directions of delivery of the hydraulic pumps can moreover be achieved so that a return stroke of the hydraulic drive can be realized.

The high pressure tank can have a tank pressure of approximately 50 to 150 bar. The low pressure tank then can have a tank pressure that is less than the tank pressure of the high pressure tank. It is for example conceivable that the low pressure tank has a tank pressure of approximately 5 bar.

A hydraulic auxiliary circuit can be provided that includes a feed pump and an auxiliary circuit directional control valve. In a first switching position of the auxiliary circuit directional control valve, the feed pump is hydraulically connected with the high pressure tank.

An additional electric motor may hereby provided which drives the feed pump. In the first switching position of the auxiliary circuit directional control valve, the pressure tank can be filled by the feed pump. The auxiliary circuit directional control valve can include three switching positions, whereby in a second switching position, hydraulic fluid can only drain off into the auxiliary circuit via check valves and pressure relief valves.

In a third switching position of the auxiliary circuit directional control valve, the hydraulic fluid can be directed into the auxiliary circuit for cooling and/or filtering. In the third switching position the system can furthermore be rendered unpressurized.

The hydraulic auxiliary circuit may include a cooler and a filter. In the third switching position of the auxiliary circuit directional control valve, the hydraulic fluid can be directed through the cooler and filter of the auxiliary hydraulic circuit and can consequently be cooled/and or filtered.

A pressure sensor can be provided that is hydraulically connected with the first pressure chamber. The pressure from the first pressure chamber can be used for a stamping force diagnosis. It is moreover conceivable not to design the directional control valve to necessarily be hydraulically actuated whereby the directional control valve is controlled via a hydraulic control line that is connected with the first pressure chamber. Rather, an electric control of the directional control valve can be provided whereby exceeding of the pressure limit can be detected with the pressure sensor that is connected with the first pressure chamber, in order to switch the directional control valve.

A pressure sensor can be provided that is hydraulically connected with the additional pressure chamber. With such a pressure sensor, the pressure in the additional hydraulic cylinder can be monitored, whereby for example monitoring of cutting shock damping that is enabled by the additional pressure chamber can be realized.

A position sensor for measuring the piston position of the two pistons is advantageously provided. If pressure sensors and one position sensor are provided, the hydraulic drive can be pressure-and-position controlled.

An additional embodiment of the hydraulic drive provides that a pressure relief valve can be provided to protect against excess pressure in the first hydraulic pump. A pressure relief valve can be provided to protect against excess pressure in the feed pump.

In a method to operate a hydraulic drive, an additional pressure chamber is supplied in a rapid stroke with hydraulic fluid from the high pressure tank, whereby the hydraulic pump moves hydraulic fluid out of the additional pressure chamber and whereby in the rapid stroke, the first and the second pressure chamber are hydraulically connected, whereby the hydraulic pump moves hydraulic fluid into the first pressure chamber. Due to the hydraulic connection between the first and second pressure chamber, the hydraulic pump only needs to move the differential volume into the first pressure chamber, whereby hydraulic fluid from the second pressure chamber can overflow into the first pressure chamber.

Consequently it can be achieved that the hydraulic pump has to move (pump) comparatively little hydraulic fluid into the first pressure chamber in the rapid stroke, whereby the hydraulic fluid that is moved (sucked) from the additional pressure chamber in regard to its volume is approximately similar to that moved (pumped) into the first pressure chamber.

A further development of the method provides that the additional pressure chamber can be supplied in a load stroke with pressure from the high pressure tank, whereby the hydraulic pump moves hydraulic fluid out of the additional pressure chamber and whereby the first and second pressure chambers are not hydraulically connected in the load stroke, whereby the hydraulic pump moves hydraulic fluid into the first pressure chamber. Since the first and the second pressure chambers are not hydraulically connected in the rapid stroke, the hydraulic pump can now provide the entire volume flow for filling the first pressure chamber, whereby hydraulic fluid can flow from the second pressure chamber into a tank or low pressure tank, unpressurized or almost unpressurized. Consequently, it can be achieved that the hydraulic pump can move (pump) comparatively the same volume of hydraulic fluid into the first pressure chamber at an unchanged pressure provided by the hydraulic pump, in comparison to the rapid stroke. Since the volume of hydraulic fluid moved out (sucked) of the additional pressure chamber is smaller than the hydraulic fluid moved (pumped) into the first pressure chamber, a torque assistance to the hydraulic pump at the pump intake can be realized due to the supply of the additional pressure chamber with pressure from the high pressure chamber. It can thus be achieved that the electric motor driving the hydraulic pump can be sized comparatively small.

Switching from rapid stroke to load stroke can occur when exceeding a pressure limit in the first pressure chamber. If a pressing tool that is mounted on the piston or respectively the piston rod of the hydraulic drive impinges on a work piece during a rapid stroke, the pressure in the first pressure chamber increases. If the pressure limit is exceeded, a changeover from rapid stroke to load stroke can be accomplished, for example, by switching a directional control valve having two switching positions.

After completion of the load stroke, the additional pressure chamber can be supplied with hydraulic fluid from the high pressure tank. After completion of the load stroke, the pressure in the first pressure chamber drops again. Then, a reset of a directional control valve can, for example, occur from its second into its first switching position. The direction of rotation of an electric motor driving the pumps can be reversed, so that hydraulic fluid is moved (pumped) at the pump intake out of the pump and is moved (sucked) at the pump outlet into the pump. Due to the supply of the additional pressure chamber with hydraulic fluid from the high pressure chamber, a braking effect can be provided by the hydraulic pump or respectively the electric motor, so that a non-controlled return stroke of the pistons of the differential cylinders and the additional hydraulic cylinder can be prevented.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one embodiment of the invention and such exemplification is not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIG. 1, there is shown a hydraulic drive10according to the current invention. Drive10includes a differential cylinder12that includes a first pressure chamber14, a second pressure chamber16and a differential piston18that separates the first pressure chamber14from second pressure chamber16. First pressure chamber14has a first hydraulic effective surface20, whereby second pressure chamber16has a second hydraulic effective surface22. First hydraulic effective surface20is larger than second hydraulic surface22. Piston18is connected with piston rod24on which, for example, a pressing tool that is not illustrated in the drawings can be mounted.

Drive10moreover includes an additional hydraulic cylinder26, having an additional pressure chamber28and an additional piston30which limits additional pressure chamber28. Additional pressure chamber28comprises a hydraulic additional effective surface32. Hydraulic additional effective surface32can be as large as a differential surface of the first hydraulic effective surface20and the second hydraulic effective surface22, so that the following may apply:
A32=A20−A22.

Additional piston30can be mechanically movably coupled with differential piston18by a common piston rod34.

Drive10further includes an adjustable speed electric motor36whose direction of rotation can be reversible. Electric motor36drives a hydraulic pump38which includes a pump intake40and a pump outlet42. Hydraulic pump38can be designed as a four-quadrant pump. It can however also be replaced by two two-quadrant pumps that deliver in an opposite direction and provide identical delivery volumes.

Drive10includes a high pressure tank44and a low pressure tank46. High pressure tank44may have a tank pressure of approximately 100 bar, whereas low pressure tank44may have a tank pressure of approximately 5 bar. Furthermore, drive10includes a directional control valve48having two switching positions. Directional valve48features a hydraulic control line50that is hydraulically connected with first pressure chamber14and can be moved from a first switching position illustrated inFIG. 1against the force of a spring52into a second switching position.

A directional control valve48or position sensor54can be arranged on additional piston30, or respectively in the region of additional piston30with which a current piston position of additional piston30or respectively of differential piston18can be detected. Furthermore, a pressure sensor56can be provided that is hydraulically connected with additional pressure chamber28for measuring the pressure in additional pressure chamber28. Furthermore, a pressure sensor58can be provided for the purpose of measuring the pressure in first pressure chamber14and which is connected hydraulically with first pressure chamber14.

A check valve60can be arranged between first pressure chamber14and second pressure chamber16for switching changeover compensation of directional control valve48. An additional check valve62can furthermore be arranged for switching changeover compensation between second pressure chamber16and low pressure tank46. Furthermore, a pressure relief valve64can be provided for overpressure protection of hydraulic pump38.

Hydraulic drive10can include an auxiliary hydraulic circuit66that includes a feed pump68which in turn is driven by an electric motor70. Auxiliary circuit66can moreover include a cooler72, a filter74, a check valve76, and a tank78. Auxiliary circuit66furthermore can have an auxiliary circuit directional control valve80that has three switching positions. In a first switching position, to the left inFIG. 1, feeding pump68is hydraulically connected with high pressure tank44. In a second switching position, in the center inFIG. 1, hydraulic fluid can only drain off via a pressure relief valve82that is intended for pressure relief of feed pump68, pressure relief valve62, as well as via a pre-tensioned valve84into auxiliary circuit66, in other words through cooler72and filter74. In a third switching position of auxiliary circuit direction control valve80, to the right inFIG. 1, hydraulic fluid can be directed for cooling and/or filtering into auxiliary circuit66.

Hydraulic circuit10lastly may also include a check valve86, which is provided to prevent cavitation during operation, in other words, it is provided for the case that the delivery volume of pump38is not adapted to the volumes of respective pressure chambers14,16,28.

InFIG. 1, pump intake40, as well as high pressure tank44are hydraulically connected with additional pressure chamber28. First pressure chamber14is furthermore hydraulically connected with pump outlet42. In one switching position of directional control valve48illustrated on the right inFIG. 1, directional control valve48is shifted through the spring force of spring52. In the first switching position, first pressure chamber14is hydraulically connected with second pressure chamber48. If a pressure limit that is adjustable via the spring force of spring52is exceed, directional control valve48can be switched into a second switching position that is illustrated right inFIG. 1, whereby first pressure chamber14is not hydraulically connected with second pressure chamber16in the second switching position. Rather, second pressure chamber16is then hydraulically connected with low pressure chamber46.

Hydraulic drive10can operate as follows: during operation of hydraulic drive10, high pressure tank44can be charged in the first switching position of auxiliary circuit directional control valve80by feed pump68to a tank pressure of approximately 100 bar. Due to the hydraulic connection of high pressure tank44with additional pressure chamber28, the same acts as an hydraulic spring, in other words the hydraulic drive10is pre-tensioned upward, in other words in an opposite direction of an arrow88, as shown inFIG. 1.

In one rotational direction of electric motor36, hydraulic pump38moves (pumps) hydraulic fluid at pump outlet42into first pressure chamber14, whereas hydraulic pump38moves (pumps) hydraulic fluid out of additional pressure chamber28at pump intake40. If directional control valve48is in its first switching position first pressure chamber14is connected with second pressure chamber16. If differential piston18and additional piston30that are movably coupled via common piston rod34are moved downward in the direction of arrow88, hydraulic fluid can flow from second pressure chamber16via directional control valve48into first pressure chamber14, due to the hydraulic connection of first and second pressure chambers14,16.

Only the hydraulic fluid for filling first pressure chamber14which cannot already overflow from second pressure chamber16must consequently be provided by the hydraulic pump38at pump outlet42. Since hydraulic effective surface32is as large as the differential surface of first hydraulic effective surface20and second hydraulic effective surface22, a substitute circuit diagram as illustrated inFIG. 2can be realized. Hydraulic drive10acts like a synchronized speed cylinder82that can include additional pressure chamber28and a resulting pressure chamber84.

A hydraulic effective surface86of resulting pressure chamber84is consistent with the differential surface of first hydraulic effective surface20and second hydraulic effective surface22. Piston89, illustrated inFIG. 2, can be moved in a rapid stroke in the direction of arrow90.

It is important thatFIG. 2represents a substitute circuit diagram of drive10according toFIG. 1, whereby the direction of movement of the drive inFIG. 2is not consistent with the direction of movement of the drive inFIG. 1. If, as explained above, hydraulic fluid can flow from second pressure chamber16into first pressure chamber14via directional control flow valve48, movably coupled pistons18,30can be moved downward in the direction of arrow88in a so-called rapid stroke at comparatively low force and high speed. The movement speed is adjustable via the rotational speed of electric motor36.

If a stamping or pressing tool which is not illustrated inFIG. 1and that is arranged on piston rod24impinges upon an obstacle, for example, a metal sheet work piece, the pressure increases in first pressure chamber14. If the pressure in first pressure chamber14increases to above the pressure limit or switching pressure of directional control valve48which is preset by spring52and which is fed back via control line50to directional control valve48, then directional control valve48is moved from the first switching position into the second switching position—against the spring force of spring52. Second pressure chamber16is now hydraulically connected with low pressure tank46.

The behavior of hydraulic drive10in the second switching position of directional control valve48can also be illustrated by a substitute circuit diagram that is illustrated inFIG. 3. Since the hydraulic fluid can no longer flow from second pressure chamber16into first pressure chamber14, the entire hydraulic fluid that is necessary for filling first pressure chamber14must be provided by hydraulic pump38. At an unchanged pump pressure of hydraulic pump38, hydraulic pump38now no longer delivers onto the differential surface of first hydraulic effective surface20and of second hydraulic effective surface22, but rather only onto first hydraulic effective surface20. Hydraulic drive10thus acts in the second switching position of directional control valve48like a differential cylinder92, as shown inFIG. 3.

It is important that also inFIG. 3a substitute circuit diagram of drive10is illustrated, whereby the operating direction of drive10inFIG. 3is not consistent with the operating direction of the drive inFIG. 1. If hydraulic pump38described above moves (pumps) hydraulic fluid at pump outlet42into first pressure chamber14and moves (sucks) hydraulic fluid at pump intake40out of additional pressure chamber28, movably coupled pistons18,30can be moved in a so-called load stroke downward in the direction of arrow88at comparatively great force and low speed.

Since now the delivery volume of hydraulic pump38at pump intake40and at pump outlet42is no longer adapted to the surface ratio of hydraulic effective surfaces20,32and—as shown inFIG. 3—hydraulic drive10acts like a differential cylinder92, additional hydraulic fluid for filling additional pressure chamber28must be provided by high pressure tank44. Since high pressure tank44has a tank pressure of approximately 100 bar, a pressure acts upon pump intake40in rapid stroke, whereby hydraulic pump38can thus be partially operated as a motor. Due to the pressure at pump intake40, torque assistance can be achieved, whereby in turn electric motor36can be sized smaller.

Since additional pressure chamber28is hydraulically connected with a high pressure tank44, cutting shock damping can be provided during the operation of hydraulic drive10by the pre-tensioning of additional pressure chamber28, in an opposite direction of the effective direction of drive10in the load stroke mode. The force resulting from the pressure in additional pressure chamber28can thus, for example, prevent that—in the event of a punch tearing through a metal sheet work piece—the punch makes an uncontrolled movement. Cutting shock damping can be adjusted by adjusting the pressure in high pressure chamber44.

After completion of the load stroke, the pressure in first pressure chamber14can drop below a reset pressure which is below the pressure limit of directional control valve48. Directional control valve48can again be returned into the first switching position through the spring force of spring52.

If the rotational direction of electric motor26and thereby also the delivery direction of hydraulic pump38is reversed, and hydraulic pump38therefore moves (pumps) hydraulic fluid at the pump intake out of hydraulic pump38, and at pump outlet42moves (sucks) hydraulic fluid into hydraulic pump38, movably coupled pistons18,30can again be moved upward in a return stroke, in the opposite direction to that indicated by arrow88.