GB2032178A

GB2032178A - Fluid Level Indicating Devices

Info

Publication number: GB2032178A
Application number: GB7840908A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1978-10-17
Filing date: 1978-10-17
Publication date: 1980-04-30
Also published as: FR2439390A1; ES485125A1; IT1129047B; IT7950569A0; DE2942016A1

Abstract

A fluid level indicating device incorporated in the filler cap 7 of a reservoir of a vehicle braking system comprises a hollow member 12 arranged to be internally pressurised by immersing the member into the fluid and to a pressure proportional to the head h between the fluid levels A, B inside and outside the member 12. A diaphragm 15 is exposed to the interior of member 12 and responds to pressure changes therein to operate an electric switch (21, 22) for producing a warning signal. One or more elements 13 may be provided, but as shown the member 12 includes two elements 13 arranged to be inserted into respective chambers of a tandem reservoir separated by an internal baffle so that the device is sensitive to a fall of the liquid level in either chamber. Instead of a switch the diaphragm 15 may be coupled to mechanically actuated warning means (not described). <IMAGE>

Description

SPECIFICATION Fluid Level Indicating Devices This invention is concerned with fluid level indicating devices and in particular to such devices suitable for use in the reservoir of a hydraulic system, for example a vehicle braking system.

A known form of fluid level indicator which is in common use in reservoirs of vehicle braking systems consists of a float located in the reservoir and arranged to close the contacts of an electrical switch to actuate a warning indicator, such as a lamp, in response to the fluid level in the reservoir falling. In the case of a tandem reservoir having two chambers separated by a baffle it is necessary either to provide two indicator devices, one for each chamber, with obvious cost disadvantages, or to reduce the height of the baffle to be below the level of the fluid at which the warning indicator is actuated. The latter arrangement has the disadvantage that fluid can easily spill over the low baffle, for example during severe deceleration of the vehicle, which in turn can lead to failure of an otherwise sound braking circuit through loss of fluid to a failed braking circuit.

According to the present invention there is provided a fluid level indicating device for indicating the level of fluid in a reservoir, comprising a hollow probe member open at a lower end thereof and closed at an upper portion by a diaphragm, and means responsive to movement of the diaphragm due to pressure changes in the hollow probe member for producing a signal dependent upon the fluid level.

When the probe member is inserted into the previously charged reservoir the air within the probe member becomes trapped and is compressed as the probe member is lowered into the fluid. The build up of pressure moves the diaphragm to such a position that a "reservoir full" signal is produced. If the fluid level falls the air in the probe member expands and its pressure is reduced so that the diaphragm moves to a position in which a signal indicative of the low fluid level is produced.

In a preferred embodiment of the invention said means comprises an electrical switch having contacts which are normally held open but which are closed in the event of the fluid level falling below a predetermined level.

One form of device intended for use with a tandem reservoir having two chambers separated by a baffle has a probe member with two tubular elements each open at their lower ends and communicating with each other at their upper ends. The probe elements are arranged to be located one on each side of the baffle when the device is inserted into the reservoir so that the device is sensitive to the fluid levels in each of the two chambers and will be actuated to produce a warning signal if the fluid level in either chamber falls below the critical value. With such a level indicating device the reservoir baffle may be of sufficient height to prevent transfer of fluid across the baffle from one chamber to the other.

Other preferred features and advantages of the invention will become apparent from the detailed description given below, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through a tandem hydraulic reservoir fitted with a device according to the invention; Figure 2 is a vertical cross-section through the level indicating device, shown on an enlarged scale; Figure 3 is a section through another tandem reservoir fitted with an alternative form of level indicating in accordance with the invention.

Referrring to Figure 1, there is shown a tandem fluid reservoir consisting of a housing body 1 fitted with a cover 2 and divided internally into two chambers by a vertical baffle 3 which terminates below the cover 2. In use the two chambers are connected to respective circuits of a vehicle hydraulic braking system through outlet connections 4 and 5, respectively. The cover 2 includes a filler opening which is closed by a filler cap 6 incorporating a fluid level indicating device embodying the invention. As may be clearly seen in Figure 2, the cap 6 includes an annular threaded part 7 adapted to screw onto the reservoir cover 2 and a central, main part 8 with a peripheral flange 9 which bears on the rim of the cover opening through a seal 10.Fitted to the main cap part 8 is an upper, cup-shaped portion 11 of a hollow probe 12 which includes a pair of downwardly projecting tubular elements 1 3 having enlarged lower end portions 14 and being open at the lower ends. The elements 1 3 are separated by a gap which is slightly wider than the thickness of the internal baffle 3 of the reservoir so that the probe can be lowered into the tandem reservoir with one element 13 located in each chamber. A diaphragm 1 5 having a peripheral bead 1 6 is held in the bottom of the probe cup portion 11 by a ring 17 having a downwardly projecting annular rib arranged to trap the bead 1 6 of the diaphragm.

A fiat plate or disc 1 8 rests on the upper surface of the diaphragm 1 5 and is pressed downwardly by a light coil spring 1 9 which acts between the disc 18 and a shoulder on the ring 1 7. The disc 18 serves to support the diaphragm 1 5 and also carries a bridging contact member 20 slidably located on an axial stem 21 fixed to the disc 18 for movement between an uppper, normal position and a lower position in which the contact member 20 connects two stationary contacts 22 mounted on the cap part 8 and having exposed terminals 23. It will be appreciated that the contacts 22 and contact member 20 constitute an electric switch.

In use, the filler cap 6 is attached to the reservoir, the probe 12 being inserted so that the tubular elements are located one on each side of the baffle 3. Because the interior of the probe is closed by the diaphragm 15, as the probe 12 is inserted the air contained in it is compressed and the level B of the hydraulic fluid within the probe is much lower than the level A of hydraulic fluid surrounding the probe in the reservoir. The air pressure within the probe 12 is equal to the head h, i.e. the difference in levels A and B, times the specific gravity of the hydraulic fluid, and acts on the diaphragm to lift the disc 1 5 against the force of the light spring 1 9 and raise the contact member 20 to its upper position clear of the contacts 22.Thus, the switch is held open and a warning indicator, such as a lamp, connected in the electric circuit with the switch is not illuminated.

If the fluid level A in either of the reservoir chambers falls, for example due to a failure in the braking circuit associated with that chamber, the fluid levels B in the tubular elements 13 change and the head h is reduced. As the fluid head h decreases the pneumatic pressure in the probe falls in direct proportion with it, and as the pressure acting on the diaphragm 1 5 is reduced the disc 1 8 and contact member 20 are lowered under their own weight and the force of spring 1 9 until eventually at a predetermined head h', the contact member 20 engages the contacts 22 and closes the switch to illuminate the warning lamp.

In the case where the fluid level falls in only one of the chambers, and the fluid head h is reduced to h', the fluid level in the probe chamber 14 in the sound reservoir chamber, rises to compensate for the pressure decrease due to the change in head. The probe sensor in the sound chamber, therefore mimics the probe in the failed chamber, but with only a minimal loss in available fluid volume.

It is to be noted that the level indicating device is equally sensitive to the fluid levels in both chambers of the tandem reservoir while the baffle separating the chambers can be of sufficient height to prevent fluid escaping from one chamber into the other. Furthermore, the device has a built in safety feature insofar as if the diaphragm should fail the switch will close to provide a warning signal which will be found to be due to a malfunction of the indicating device when a check is made.

Although the switch is normally open in the device as described it could, of course, be arranged to be normally closed if required.

Referring now to Figure 3, there is illustrated a tandem reservoir of a vehicle hydraulic braking system, incorporating a fluid level indicating device of similar principle but of different construction to that of Figures 1 and 2. The reservoir body 31 is divided into two chambers 32, 33 by a baffle 34. The chambers have respective outlets 35, 36 and the body 31 has a permanent cover 37 which incorporates the fluid level indicating device. The cover has a main wall 38 with which the two tubular elements 39 of the hollow probe of the indicating device are integral.

As can be clearly seen in the drawing, the probe elements 39 are arranged to be located on opposite sides of the baffle 34 when the cover is fitted to the reservoir body. The cover wall 38 has an upstanding rib 40 surrounded at a small, constant distance by a concentric rim 41. A diaphragm 42 is positioned on the wall 38 and has a peripheral bead 43 which is trapped between the rib 40 and rim 41 by a cap 44 snapfitted onto the rim 41. A flat disc 45 rests on the upper surface of the diaphragm 42 for supporting the latter and is pressed downwardly by a light spring washer 46 acting between the disc and the cap 44. The cap 44 has a central hole through which an actuating element 47 of an on/off switch 48 mounted on the cap extends to bear against the plate 45.

The operation of the level indicating device is essentially the same as for the first embodiment.

When the fluid levels in the two reservoir chambers are normal the air pressure in the probe, due to the iiquid head, is great enough to raise the diaphragm 42 and the disc 45 against the force of spring 46 and hold the switch 48 open. If the fluid level in either chamber drops so will the head and the air pressure in the probe so that the diaphragm 42 and disc 46 are pressed down by the spring 46 and the switch 48 will be closed to produce a warning signal.

The described level indicating devices have the advantages that they are responsive to the fluid level in both reservoir chambers; the reservoir baffle can be of sufficient height to preclude transfer of fluid from one chamber to the other; the device is actuated to produce a warning signal in the event of failure of the diaphragm; and the electrical switch is protected against contamination being isolated from the fluid in the reservoir by the diaphragm. Although described in connection with tandem reservoirs it will be appreciated that, provided with the appropriate number of probe elements, the device of the invention could be used to sense the fluid levels in one or a larger number of chambers. Furthermore, whereas the level indicating devices have been described as including electrical switches they could be used to operate mechanically actuated means for producing a warning signal, in which case the diaphragm could be coupled to said means.

Claims

1. A fluid level indicating device for indicating the level of fluid in a reservoir, comprising a hollow member having an open lower end and an upper end which is closed, the hollow member being arranged to be internally pressurised by immersing the member into a body of liquid within the reservoir and to a pressure proportional to the head of fluid between the fluid levels inside and outside the hollow member, a movable wall exposed to the interior of the hollow and responsive to pressure changes therein, and means sensitive to the position of the movable wall for generating a signal depending upon the fluid level in the reservoir.

2. A fluid level indicating device according to claim 1, wherein the movable wall comprises a diaphragm mounted on and closing an upper part of the hollow member.

3. A fluid level indicating device according to claim 1 or 2, wherein the hollow member comprises a plurality of depending hollow elements for insertion into respective chambers of the reservoir, the elements being open at their lower ends and communicating with each other at their upper ends.

4. A fluid level indicating device according to claim 1, 2 or 3, wherein the signal generating means comprises an electric switch.

5. A fluid level indicating device according to claim 4, wherein a component biased against the side of the movable wall remote from the interior of the hollow member is arranged to operate the switch in response to displacement of the movable wall.

6. A fluid level indicating device according to any preceding claim, wherein the device is incorporated in a filler cap of the reservoir.

7. A fluid level indicating device substantially as herein described with reference to the accompanying drawings.

8. A reservoir for an hydraulic fluid braking system comprising a fluid level indicating device according to any one of the preceding claims.