Patent Application
20170159637
This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2015 224 132.7, filed on Dec. 3, 2015 in Germany, the disclosure of which is incorporated herein by reference in its entirety.
The disclosure relates to a hydrostatic axial piston machine having a cylinder drum and having a control disk.
In principle, a hydrostatic axial piston machine can be operated as a motor or as a pump. An axial piston machine of swashplate type of construction has an inner cylinder drum with multiple cylinder bores in which pistons are movable in an axial direction. The stroke of the pistons is realized owing to the mounting of the pistons on a swashplate by way of slide shoes. On the side of the cylinder bore ducts, there is seated between the cylinder drum and the housing a control disk, on which a high-pressure side or low-pressure side is realized on a region of a first semicircle. The opposite pressure side (high/low pressure) in relation to the first semicircle is situated on a second semicircle.
Owing to the high operating pressures of up to 500 bar, the control disk is pressed against the cylinder drum. During engine operation, pressure medium is pressed, at the high-pressure side, into the stroke chambers, whereby the cylinder drum is set in a rotational motion by way of axial movement of the pistons and oblique positioning of the swashplate. The torque that is generated is output to the drive-output shaft. By contrast, during pump operation, by way of an input torque at the driveshaft, the pressure medium is drawn from the low-pressure side into the cylinder bores by the pistons. Owing to the rotation of the cylinder drums and a fixed pivot angle, the pressure medium in the stroke chamber is displaced to the high-pressure side as a result of the axial movement of the piston, which results in a pressure increase.
Owing to the friction forces of the outwardly sliding pistons on the high-pressure side, the pressure on the control disk is increased during pump operation in relation to motor operation. To reduce friction forces and wear between the rotating cylinder drum and the static control plate, hydrostatic relief means are known from the prior art.
A hydrostatic axial piston machine having a control disk, having a permanent relief field and having at least one auxiliary relief field for the cylinder drum is described in the document DE 10 2010 006 895 A1. In said document, one or more auxiliary relief fields can be activated along with the permanent relief field, or deactivated, in a manner dependent on the selected operating state (pump/motor). During motor operation, a lesser relief action is sought, such that one or more auxiliary relief fields can be switched into an unpressurized state. This prevents the relief action from becoming too intense during motor operation, while however ensuring an adequate relief action during pump operation.
A disadvantage of said solution is that the relief action can be adapted only to particular operating states. Other disruptions resulting from load alterations, such as arise for example in a load cycle in motor/pump operation with varying rotational speeds, cannot be eliminated by way of the features of the document DE 10 2010 006 895 A1.
The present disclosure is therefore based on the object of realizing a hydrostatic axial piston machine with hydrostatic auxiliary relief means, which solves rotational-speed-dependent problems that arise for example within a load cycle, such that disruption-free and tilt-free rotation of the cylinder drum on the control disk can be ensured.
According to the disclosure, said object is achieved with a hydrostatic axial piston machine which has a control disk against which a rotating cylinder drum can be held in contact, wherein, in addition to a permanent relief field, at least one hydrostatic auxiliary relief field is realized which can be supplied with pressure medium by way of a pressure medium supply. According to the disclosure, the relief pressure of the at least one auxiliary relief field is set in a manner dependent on a rotational speed of the cylinder drum. With an embodiment which is simple in terms of apparatus, the relief pressure of the at least one auxiliary relief field can be activated and deactivated in a simple manner.
The hydraulic permanent relief field has a hydrostatic and a hydrodynamic component. At rotational speeds below 250 rpm, the hydrodynamic component of the permanent relief action breaks down. The remaining, hydrostatic fraction of the permanent relief action is not sufficient to maintain liquid friction. Instead, boundary/mixed friction or even solid-state friction arises, in the case of which, without the rotational-speed-dependent auxiliary relief action according to the disclosure, results in severe wear phenomena on the control disk. An advantage of the solution according to the disclosure is that the omitted hydrodynamic component of the permanent relief action can be replaced by a purely hydrostatic component of the at least one auxiliary relief field.
The hydrostatic auxiliary relief field is particularly highly suitable for compensating rotational-speed-dependent disturbance variables. Such disturbance variables may for example be forces and moments. Such disturbance variables may arise in particular in the range of very low or very high rotational speeds. A disturbance variable is for example, as described above, the increased friction as a result of omission of the hydrodynamic component of the relief action at very low rotational speeds close to zero. A further disturbance variable is the reversal of the friction force vectors upon a change from pump operation to motor operation. Here, in the worst case, the cylinder drum may be pulled from the control disk. A third disturbance variable is a tilting moment which arises as a result of very high rotational speeds of the axial piston machine. Here, the centrifugal forces that act on the piston become continuously higher, such that, owing to the pistons being deployed to different extents, the running surface of the cylinder drum becomes increasingly obliquely inclined on the running surface of the control disk. When a lift-off rotational speed is reached, the tilting moment becomes so high that the cylinder drum lifts off from the control disk at one side.
An exemplary embodiment is particularly preferable in which the pressure medium supply has a pressure medium flow limitation means.
By way of the pressure medium flow limitation means, the pressure that is available to the auxiliary relief field is reduced. As a result, a limiting pressure medium flow in combination with the pressing force of the cylinder drum against the control disk ensures a smaller gap dimension between the running surfaces of the cylinder drum and of the control disk. The nozzle may be used in combination with an auxiliary pump as pressure medium supply.
It is preferable for a non-adjustable nozzle to be arranged upstream of the pressure medium inlet of the hydraulic auxiliary relief field, which nozzle limits the pressure medium flow and reduces the relief pressure. Here, a nozzle diameter of 0.4 mm is particularly preferable, though nozzle diameters from 0.1 to 0.8 mm are basically also conceivable.
As an alternative to a non-adjustable nozzle, use is particularly preferably made of an adjustable nozzle for controlling the relief pressure in the auxiliary relief field and thus the auxiliary relief force. By way of the adjustable nozzle, the pressure supplied from the high-pressure side of the control disk can be reduced.
An embodiment is particularly preferable in which the pressure medium is drawn or picked off via a high-pressure side of the axial piston machine.
A particular advantage of this solution is that it is possible to dispense with further hydraulic components such as pumps or accumulators. In the case of the pressure being picked off on the high-pressure side of the axial piston machine, the above-described adjustable nozzle is preferably used. Owing to the coupling of the auxiliary relief field to the high-pressure side of the axial piston machine, an adjustment capability is thus realized for enabling the relief pressure of the auxiliary relief field to be controlled, according to the disclosure, in a manner dependent on the rotational speed. There are basically two adjustment types that may be realized. Firstly, the pressure may be raised, by way of the adjustable nozzle, until complete relief of the auxiliary relief field with simultaneous minimization of the pressure medium flow is realized, or it is possible, by way of the dimensioning of the auxiliary relief field, for the hydraulic resistance thereof, which is defined by the gap dimension between the auxiliary relief field and the cylinder drum, to be lowered. In this way, the pressure medium flow is determined primarily by the resistance characteristics of, for example, the nozzle, which impart a defined pressure medium film height to the auxiliary relief field.
The auxiliary relief field is preferably connected to a hydraulic accumulator and/or to a hydraulic pump, in particular auxiliary pump.
Since, in the case of these embodiments, there is no coupling of the hydrostatic auxiliary relief field to the operating pressure of the axial piston machine according to the disclosure, it is possible for the relief pressure of the auxiliary relief field to be controlled by way of the adjustability of the auxiliary pump. Alternatively to or in parallel with the auxiliary pump, a hydraulic accumulator may also be used. The latter may release the stored hydraulic energy back to the system, in particular may release said hydraulic energy to the auxiliary relief field. In this embodiment, the abovementioned non-adjustable nozzle is preferably used.
Other combinations, such as for example non-adjustable nozzle, high-pressure pick-off, adjustable nozzle, auxiliary pump or hydraulic accumulator are however also conceivable.
In general, in a hydrostatic axial piston machine, internal losses (leakage) of pressure medium or oil, for example an oil spray process in the housing of the axial piston machine, occur. Said discharged oil is preferably used for providing a supply to the hydrostatic auxiliary relief field.
A first auxiliary relief field is preferably arranged in a tilting direction of the cylinder drum on the control disk.
This exemplary embodiment has the advantage that the specific operational disruption of the tilting of the cylinder drum is substantially prevented. As already described above, centrifugal forces are exerted on the drawn-out pistons in particular in the presence of very high rotational speeds of the cylinder drum of the hydrostatic axial piston machine, which centrifugal forces lead to tilting of the cylinder drum on the control disk. Said tilting is characterized by the cylinder drum lifting off at one side, resulting in a punctiform residual pressing force of the cylinder drum against the control disk. At that point of the control disk at which the punctiform residual pressing force (tilting point) is localized, there is arranged an auxiliary relief field for the purposes of compensating the acting residual pressing force. In this way, the axial piston machine can be operated without disruption in a relatively high rotational speed range. The auxiliary relief field preferably extends from the tilting point to both sides in the circumferential direction of the control disk.
The arrangement of the auxiliary relief field preferably enlarges a support circle radius in the tilting direction on the control disk by virtue of the auxiliary relief field being arranged radially at the outside at the edge of the control plate.
This has a particularly advantageous effect in preventing the tilting of the cylinder drum at very high rotational speeds. In addition to the compensation owing to the hydrostatic action of the auxiliary relief fields, the arrangement of the auxiliary relief field in the tilting direction enlarges the support circle radius. There are types of axial piston machine in which the cylinder drum, when it lifts off from the control disk, pivots about the outer edge of a support circle which is characterized by the outer diameter of the permanent relief field. By definition, the cylinder drum lifts off if the punctiform residual pressing force lies outside the support circle radius. Owing to the arrangement of the auxiliary relief field outside the permanent relief field, the support circle radius is enlarged. In this way, the tilt angle of the cylinder drum is reduced, and the lift-off rotational speed is shifted into a higher rotational speed range.
In many cases, the tilting direction is inclined at an angle (α) relative to a dead center axis or central axis of the control disk, wherein the value of the angle may lie in a range from 5-45°. The two dead centers are defined by the positions of the piston in the cylinder at which no axial movement is performed and the direction reversal of the piston takes place. The top and bottom dead centers are situated diametrically with respect to one another. If one transfers the two dead centers onto the control plate and connects these by way of a line, this forms the dead center axis or central axis. The auxiliary relief field is particularly preferably oriented correspondingly to said angle (α).
In a preferred refinement, a further auxiliary relief field is arranged diametrically with respect to the first auxiliary relief field.
A particular advantage of the diametric arrangement of two auxiliary relief fields is an additionally generated stability moment which serves for the compensation of the operation-dependent disturbance variables. In the event of tilting of the cylinder drum and associated oblique positioning, the pressure on the respective auxiliary relief field arranged in the tilting direction increases. By contrast, at the oppositely arranged auxiliary relief field, the pressure decreases. The resulting forces accordingly generate a stabilizing stability moment.
In principle, any relief field always involves a self-regulating effect. If the gap dimension between cylinder drum and control disk running surface becomes smaller in the presence of a constant volume flow, this leads to a pressure increase. The increased pressure results in an increase of the relief force and acts counter to the cylinder drum, which in turn leads to an increase of the gap dimension. With the gap enlargement, however, the pressure falls again, whereby the gap dimension decreases. This yields a type of self-regulating mechanism which is realized by way of the hydrostatic auxiliary relief field according to the disclosure. With the arrangement of two auxiliary relief fields arranged diametrically, these act with opposing self-regulation with respect to one another.
In general, a distinction can be made between three relief types (κ) which can be presented as per the formula:
K
pump=κmotor+κauxiliary relief.
The permanent relief corresponds to the motor relief κmotor. If the axial piston machine is operated as a pump, higher forces are generated on the control disk, whereby the motor power alone is not sufficient, and disturbance-free operation is not ensured. Accordingly, with the aid of the auxiliary relief field or auxiliary relief fields, additional relief κauxiliary relief in relation to the motor relief is realized. This altogether increased relief is referred to as pump relief κpump.
Here, values in the range of for example κmotor=90-100% and κauxiliary relief=1-10% are preferably sought.
What is particularly preferable is a motor relief of approximately κmotor=96% and an auxiliary relief of approximately κauxiliary relief=5%, which leads to a pump relief of κpump=101%.
Particularly preferred exemplary embodiments of the hydrostatic axial piston machine according to the disclosure are illustrated in the drawings. The disclosure will now be discussed in more detail on the basis of the figures of said drawings, in which:
The auxiliary relief field 1 is arranged on a control disk 2 at an angle of approximately 90° relative to the top dead center TDC of the piston position. In the exemplary embodiment as per
The relief pressure that prevails in the auxiliary relief field 1 is defined by pressure division by way of the nozzles 3 and 6. The latter nozzle 6 particularly preferably has a diameter of 4 mm. If, for example, the throughflow cross section of the nozzle 3 is equal to the throughflow cross section of the nozzle 6, the pressure in the auxiliary relief field 1 is equal to half of the pump pressure. If, by contrast, the cylinder drum lifts off slightly from the control disk, the throughflow cross section of the nozzle 3 becomes larger, and the pressure on the auxiliary relief field decreases. If the cylinder drum approaches the control disk in relation to a position with half pump pressure in the auxiliary relief field, the throughflow cross section of the nozzle 3 becomes smaller, and the pressure on the auxiliary relief field increases. This results in self-regulation of the pressure in the auxiliary relief field and thus self-regulation of the action of the auxiliary relief field. If the relief of the cylinder drum changes in the surface region surrounding the high-pressure kidney-shaped control port and the low-pressure kidney-shaped control port owing to a change in the rotational speed or the working pressure, the relief by way of the auxiliary relief field changes oppositely thereto.
A further secondary line with a 2/2-way valve 16 branches off from the main line. In an open position of the 2/2-way valve 16, the pressure medium can flow off out of the main line via the secondary line into the tank 14. The pressure in the main line and thus also in the auxiliary relief field is then the tank pressure. The auxiliary relief field is inactive. In the closed state of the 2/2-way valve 16, the pressure level and the main line is maintained.
The pressure prevailing at the auxiliary relief field 1 can be measured by way of a manometer 10. The manometers 8 and 10 are provided primarily for testing purposes.
The pressure level at the auxiliary relief field 1 can be measured by way of the manometer 10.
The control disk 2 from
A control disk 2 with two diametrically arranged auxiliary relief fields 1 and 40 is shown in
1 Auxiliary relief field
2 Control disk
3 Nozzle
4 Auxiliary pump
6 Nozzle
8 Manometer p1
10 Manometer p2
12 Pressure-limiting valve
14 Tank
15 Nozzle
16 2/2-way valve
17 Branching point
18 Nozzle
20 Cylinder drum
22 Cylinder
24 Piston
26 Swashplate
28 Slide shoe bearing arrangement
30 Bearing point
31 Tilting point
32 High-pressure kidney-shaped control port
34 Low-pressure kidney-shaped control port
38 Permanent relief field
40 Second auxiliary relief field
42 Small support circle radius
44 Large support circle radius
46 Pressure profile
48 Rotational speed profile
α Angle
TDC Top dead center
BDC Bottom dead center
Mges Tilting moment
y Dead center axis or central axis
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 224 132.7 | Dec 2015 | DE | national |
