Patent Application
20140041733
This application claims priority from German patent application serial no. 10 2012 214 388.2 filed Aug. 13, 2012.
The invention concerns a venting device for a hydraulic or electro-hydraulic control unit.
As is known, air entrapped in a hydraulic system generally has an adverse effect on the control and regulating behavior of the hydraulic system. This air can on the one hand be so-termed primary air, which enters the hydraulic system during the assembly of its components and which, in the ready-assembled unit, undesirably remains trapped at some point or another. On the other hand, air can also make its way into the hydraulic system at a later time, during the operation of the ready-assembled unit.
From DE 19717043 C2 it is known that water present in the hydraulic fluid of a hydraulic unit and gases dissolved in the hydraulic fluid of a hydraulic unit can be separated off by pervaporation methods. In this case the gas or water evaporates at an oil-permeable membrane fitted at a suitable point in the hydraulic system.
DE 19933620A1 describes an oil filter in which is incorporated a semi-permeable membrane for separating off air fractions present in the oil. A semi-permeable membrane of this type is also suitable for venting other dead spaces in which air can or does collect.
DE 4210979 C2 describes a water-tight housing for an electric control unit, which is vented by a semi-permeable wall integrated in a plug. For this a spark-plug is used, whose electric contacts are cast and soldered to a plate inside the housing, whereby the plug together with the housing form a unit which is sealed relative to the outside.
A further possibility known from practice for venting valve-housing components of a hydraulic system immersed in oil is to use venting diaphragms—for example fitted with a sieve-screen insert—at suitable points in the control unit, for example in an outlet duct of a hydraulic valve. Although such a device reliably vents the hydraulic space concerned, this takes place at the cost of a continual oil leak which has to be allowed for in the design of the oil supply to the control unit and generally leads to some undesired loss of performance in the system. Besides, in some operating situations it can happen that air from the outside is drawn back into the hydraulic control unit by way of the venting diaphragm.
The purpose of the present invention is to provide a venting device for a hydraulic or electro-hydraulic control unit by means of which a space in the control unit that contains oil can be reliably vented to the outside without significant oil leakage and without air from the outside being able to make its way back into the space in the control unit to be vented.
According to the invention, this objective is achieved with a venting device having the advantageous design features of the invention.
According to these a venting device of a hydraulic or electro-hydraulic control unit is proposed, which is configured in the form of a diaphragm inset into a housing bore of a housing of the control unit so that air collected in the control unit can escape to the space outside the housing by way of the diaphragm bore. According to the invention, it is provided that in the diaphragm is inset a membrane that, when under pressure, is permeable to air and to oil and which closes off the diaphragm bore relative to the outside so that the housing bore and diaphragm form an inside space which can be vented to the outside by way of the membrane, the space being permanently bounded during operation, on its side opposite the diaphragm, by an oil column passed by the control unit into the housing bore and maintained by the control unit continually under a defined pressure, the pressure being such that the oil present in the housing bore does not pass through the membrane, or does so only in very small amounts, whereas an oil column remains permanently in the housing bore so that at least to a large extent only the air fraction present in the oil is vented to the outside while no air can be drawn back from the outside into the control unit.
In a further development of the invention it is proposed to provide additional design means in the hydraulic or electro-hydraulic control unit, in the area of the oil feed to the housing bore, which prevent the oil column from draining out of the housing bore when the control unit is not operating. This can ensure that at no time can air from outside be drawn into the hydraulic system of the control unit via the venting device.
Below, the invention is explained in more detail with reference to the figures, which show:
The invention is based on the recognition that PTFE (polytetrafluoroethylene) membranes known from the prior art conform to a mathematical venting curve.
Now, if in accordance with the invention such a membrane is used to close off a space or a housing bore of the housing of a hydraulic or electro-hydraulic control unit, which space can be filled with oil from the control unit, then up to the discrete pressure threshold typical for the membrane used neither oil nor air can emerge out of the space or housing bore and escape to the outside. Air dissolved in the oil will collect in front of the membrane and will only pass through the membrane if the pressure applied by the hydraulic control unit to the oil column present in the space or housing bore to be vented exceeds the threshold. This ensures the desired venting function.
Owing to the inherent oil-repellent nature of the smoothed PTFE, its small pore size and its smooth surface, the fabric of the PTFE membrane is able to hold back oil and to a large extent allow only air to escape, provided that the membrane is not acted upon by too high an oil pressure and that the smooth side of the membrane is facing toward the oil. Thus, by using such a smoothed PTFE fabric the oil can be held back to a large extent or even entirely without preventing the escape of troublesome air out of the space or housing bore to be vented. In a particularly advantageous manner, the oil column remaining in the space or housing bore to be vented reliably prevents air from the outside from being drawn into the control unit. If the pressure falls again to a value lower than the discrete pressure threshold, then only the height of the oil column changes, without air being drawn into the pressure system of the hydraulic control unit.
Moreover, the fabric of the usual PTFE membranes is resistant to pressure and temperature, so the membrane has a certain mechanical robustness, allowing it for example to be used in a hydraulic or electro-hydraulic control unit.
Suitable materials for the membrane of the venting device according to the invention are mentioned for example in paragraph [0033] of DE 19717043 C2. It is advantageous to use a smoothed PTFE (polytetrafluoroethylene) fabric with a pore size category such that, for example, its air permeability is around 7 l/m2/sec, for example one with a pore size of 12 μm.
At least when the hydraulic or electro-hydraulic control unit is in operation, there is always in the housing bore 5 an oil column 8 which is kept at a pressure p applied by the hydraulic or electro-hydraulic control unit. This oil column 8 delimits the inside space 7 on its side opposite the diaphragm.
In order for any leakage (undesired in principle) to get through to the outside space 6, the oil held in the housing bore 5 would have to pass through the membrane 3 covering the diaphragm bore 2. Thus, during operation the oil pressure p acting on the oil column 8 is preferably adjusted so that oil in the housing bore 5 does not pass through the membrane 3, or only does so in small amounts, although a section of the housing bore 5 under the membrane remains permanently filled with oil—i.e. there is always an oil column in the housing bore 5—so that no air can be drawn back into the control unit from the outside 6.
It is advantageous for the oil pressure p acting on the oil column 8 to be adjusted by the hydraulic or electro-hydraulic control unit independently of any other working pressures of the control unit, to ensure that the venting device functions reliably under any operating conditions of the control unit. The pressure p can be predefined by the hydraulic or electro-hydraulic control unit as a constant value so that during operation an at least approximately constant oil pressure actually acts upon the oil column 8. Alternatively, however, the pressure p acting on the oil column 8 can also be predefined as a variable value.
The pressure level p to be set by the hydraulic or electro-hydraulic control unit, which acts upon the oil column 8 under the inside space 7 to be vented, is a function of the pore size category of the membrane 3 and a function of the inside diameter d of the diaphragm bore 2. For example, if the membrane 3 is air- and oil-tight up to a pressure of 0.5 bar, then the pressure level p acting on the oil column can be set for example at a constant value of 0.6 bar, which in practice can lead to a minimal oil leakage rate through the diaphragm 1, for example of about six milliliters per hour. For a technical application, as a rule a slight leakage of up to two milliliters per minute is acceptable.
Furthermore, by means of additional design measures (known per se) in the hydraulic or electro-hydraulic control unit in the area of the oil feed to the housing bore 5, it can be ensured that an oil column 8 remains in the housing bore 5 even when the control unit is not operating so that at no time can air be drawn back in from the outside space 6 through the venting device into the hydraulic system of the control unit. As an example of such a design measure, a one-way valve fitted in the inlet line of the housing bore can be mentioned.
Of course, a person with experience of the field will design the outer contour of the diaphragm so as reliably to prevent fitting the diaphragm the wrong way round in the bore or recess that corresponds to its external shape, thus ensuring that the smooth side of a smoothed PTFE membrane always faces toward the oil column.
An example of the use of this venting device is the fitting of the diaphragm with the membrane embedded in it into a valve bore of a hydraulic or electro-hydraulic transmission control unit, which is thereby sealed off from the inside space of the transmission housing in a largely oil-tight but air-permeable manner. The added cost for effecting the necessary venting of the valve bore is decidedly small. In principle, however, the venting device according to the invention is suitable for all hydraulic or electro-hydraulic control units, particularly when very little fitting space is available for the venting device needed. Thus, the venting device according to the invention can be made with an inside diameter of the diaphragm of, for example, 0.25 to 3.00 mm, without problems.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2012 214 388.2 | Aug 2012 | DE | national |
