GB2031160A

GB2031160A - Environmental testing apparatus

Info

Publication number: GB2031160A
Application number: GB7839433A
Authority: GB
Original assignee: Derritron Electronics Ltd
Current assignee: Derritron Electronics Ltd
Priority date: 1978-10-05
Filing date: 1978-10-05
Publication date: 1980-04-16
Also published as: GB2031160B

Abstract

An environmental testing apparatus comprises a vibrator 2 having a vibrator table 6 and a test chamber 4. The vibrator table carries a number of support pillars 12 which pass through e.g. a cover plate 14 sealing a hole in the floor of the chamber and which in turn carry an extension vibrator table 16. The chamber is sealed against the atmosphere by e.g. a sealing bellows 20 around each pillar. An inflatable air bag 22 or spring (not shown) between the vibrator table and the bottom of the chamber exerts a biasing force on the vibrating parts equal and opposite to the force exerted on them by virtue of a reduced pressure in the chamber. <IMAGE>

Description

SPECIFICATION Environmental testing apparatus The invention relates to environmental testing apparatus comprising a vibrator, particularly an electromagnetic vibrator, connected to an environmental test chamber and relates also to apparatus for the connection of vibrators to environmental test chambers.

Modern environmental testing, for instance a simulation of the stresses undergone by equipment used in aircraft, may require the simultaneous simulating of vibration or shock together with the conditions experienced at high altitude, such as low pressure and temperature.

Conventional environmental testing apparatus generally utilises an electromagnetic vibrator having a vibrator table to which an extension table is bolted which extends through a hole in the floor of the environmental chamber. The vibrator may have a power requirement of up to 100 kilowatts of even more, and since much of this energy is dissipated as heat within the vibrator some cooling is necessary. This is generally achieved by the circulation of oil or air through the vibrator. The vibrator is isolated from the chamber, normally by a flexible diaphragm attached to the extension table, in order to protect the vibrator from moisture produced during temperature cycling, whilst the space between the vibrator and the chamber is sealed against the atmosphere, for instance by means of a stainless steel bellows or a silicon rubber sealing ring.

It will be appreciated that when there is a vacuum inside the environmental chamber the vibrator table and its extension would tend to have a low pressure acting on its upper surface and a higher presure, normally atmospheric, acting on its lower surface. This pressure differential would tend to "suck" the vibrator table into the chamber, and for this reason a pressure equalising pipe is provided between the chamber and the inside of the vibrator. As a result it has in the past not been possible to use air cooled vibrators in conjunction with environmental testing chambers and it has been necessary to provide an oil mist filter in the equalising pipe to prevent the chamber becoming contaminated with cooling oil.In addition it is necessary to provide a heavy duty pump for the cooling oil to overcome the low pressure prevailing in the vibrator and some thermal control in the cooling oil circuit to compensate for low temperatures in the chamber.

According to one aspect of the present invention environmental testing apparatus comprises a test chamber and a vibrator, the vibrator having a vibrator table carrying a support pillar which passes through a hole in a wall of the chamber and which in turn carries an extension vibrator table, the apparatus including sealing means surrounding but permitting movement of the pillar while sealing the interior of the chamber from the atmosphere and from the interior of the vibrator, and biasing means adapted to exert a force on the vibrating parts opposite to a force that may be executed on them by virtue of a reduced pressure in the chamber.

Preferably the biasing means is substantially friction free and may be, for example, an inflatable air bag.

In a preferred embodiment there are a plurality of support pillars each of which passes through a hole in the wall of the chamber, which may be a cover plate removably secured in a larger hole in the wall of the chamber, and an equal number of sealing means, e.g. generally cylindrical sealing bellows.

According to a further aspect of the present invention apparatus for connecting an environmental test chamber to a vibrator having a vibrator table to provide apparatus for environmental testing includes a cover plate for a hole in the wall of the chamber, at least one support pillar, means for securing it to the vibrator table so as to extend through a hole in the cover plate, an extension table adapted to be carried by the pillar or pillars, sealing means adapted to surround the or each pillar and resilient biasing means adapted to exert a force on the vibrating parts of the vibrator.

Further features and details of the invention will be apparent from the following description of one specific embodiment of an environmental testing apparatus which is given by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic part sectional elevation of an environmental testing apparatus comprising a vibrator in conjunction with an environmental test chamber; Figure 2 is a plan view of the vibrator coupling plates to which the support columns are connected; and Figure 3 is a sectional elevation on the line 3-3 in Fig. 2 showing the sealing bellows, chamber floor and the extension vibrator table.

As may be seen in Fig. 1 the environmental testing apparatus comprises an electromagnetic vibrator 2 and an environmental chamber 4. The vibrator has a conventional circular vibrator table 6 which has a number of holes formed in it in a conventional pattern so that a workpiece, or the like, to be vibrated may be secured to it. Connected to the vibrator table 6 by a number of bolts are a lower and an upper coupling plate 8 and 10, formed of aluminium alloy or magnesium alloy. Secured to the upper surface of the upper coupling plate 10 are six support pillars 1 2 also formed of aluminium alloy or magnesium alloy, ar ranged in a circular pattern, as best seen in Fig. 2.

As seen in Fig. 3 each of the support pillars passes through a respective hole in a cover plate 14 which removably seals a large hole in the underside of the environmental chamber 4. Secured to the top of the support pillars 1 2 is an extension vibrator table 1 6 in which a pattern of holes is formed similar to those in the vibrator table 6.

Around each support pillar 1 2 is a cylindrical sealing bellows 1 8 of silicon rubber, one end of which is connected and sealed to the underside of the extension table 1 6 and the other end of which is connected and sealed to the cover plate 1 4. If there is a reduced pressure within the chamber the full pressure differential appears across the sealing bellows which tends to force them outwards away from their respective support pillars. For this reason each sealing bellows is radially restrained by two restraining rings 20. The interior of the environmental chamber is therefore completely sealed against the atmosphere.

If a vacuum is present inside the chamber 4 this is not applied to the interior of the vibrator by virtue of the provision of the sealing bellows 1 8. However an upward force is applied to the extension table by virtue of the vacuum, equal to the difference between the pressures inside and outside the chamber multiplied by the total plan area of the support pillars 1 2 and the associated sealing bellows 1 8. Although this force is smaller than that which would have been applied if the pressure difference acted on the whole area of the extension table, it is nevertheless significant and would degrade the performance of the vibrator.

For this reason biasing means comprising a friction free thruster 22 is positioned in the centre of the circle defined by the support pillars between the upper coupling plate and the underside of the chamber. The position of the chamber is fixed and the friction free thruster is adjusted to provide a downward force on the coupling plate substantially equal to the upward force exerted by virtue of the presence of the vacuum in the chamber. The vibrator can therefore function isolated from the vacuum in the chamber with its performance substantially unaffected by it. In this preferred embodiment the friction free thruster comprises an inflatable air bag inflated to an appropriate pressure. The air bag 22 has a valve 24 by means of which it may be inflated.The valve 24 communicates with a chamber 26 within the lower coupling plate 1 8 which in turn communicates with a nozzle 28 to which a pressurized air line may be attached.

Thus, in use, the pressure in the air bag will be inversely pioportional to the pressure in the chamber. A gauge (not shown) is attached to the extension table to indicate its alignment relative to the chamber so that the pressure in the air bag can be optimised.

Thus in the apparatus in accordance with the invention the chamber is totally sealed against the atmosphere and the vibrator, and the performance of the vibrator is not degraded by the presence of a vacuum within the chamber. The fact that the interior of the vibrator is not connected to the interior of the chamber means that the vibrator may be either oil or air cooled. In addition, of course, the oil mist filter is not necessary and the circulating pump for the cooling oil does not need to be of heavy duty type.

An advantageous modified construction includes an in-line spool valve connected on one side to a regulated air line and on the other to the air bag. The valve is controlled by the pressure inside the chamber so that when the chamber is evacuated the air bag is inflated and when the vacuum is removed the air bag deflates.

The apparatus may be still further modified to include a load support inflatable air bag positioned to exert an upward force on the vibrating parts, e.g. between the vibratory table and the vibrator body. If it is desired that the extension table should carry a load greater than its normal rated load this may be achieved, when a vacuum is present in the chamber, by inflating the friction free thruster air bag to a greater pressure than that required solely by virtue of the pressure differential. However, when the pressure within the chamber is substantially atmospheric the excess load may be born by inflating the second air bag to an appropriate pressure. The load support air bag is therefore inflated and deflated in anti-phase with the friction free thruster air bag.

Such a control system, by means of which the air bag (or air bags) is automatically inflated and deflated, is particularly advantageous for test programmes requiring rapid evacuation and restoration of the chamber pressure in conjunction with vibratory testing.

In the preferred embodiment the chamber is provided with a drain valve (not shown) which may be used to drain liquid condensed in the chamber during temperature cycling.

It will be appreciated that a great many modifications may be made to the construction described above. For instance the bellows could be replaced by any convenient sealing means, any convenient number of supporting pillars may be used, and the friction free thruster may be, for example, a coil spring.

The invention also relates to apparatus for connecting a vibrator to an environmental test chamber. Such apparatus may comprise all the components described above and illustrated with the exception of the vibrator itself and its table and the environmental test chamber.

Claims

1. Environmental testing apparatus comprising a test chamber and a vibrator, the vibrator having a vibrator table carrying a support pillar which passes through a hole in a wall of the chamber and which in turn carries an extension vibrator table, the apparatus including sealing means surrounding but permitting movement of the pillar while sealing the interior of the chamber from the atmosphere and from the interior of the vibrator, and biasing means adapted to exert a force on the vibrating parts opposite to a force that may be exerted on them by virtue of a reduced pressure in the chamber.

2. Apparatus as claimed in Claim 1 in which the biasing means is adjustable so that the force it produces may be substantially equal and opposite to the force that may be exerted by virtue of the reduced pressure.

3. Apparatus as claimed in Claim 1 or Claim 2 in which the biasing means is substantially friction free.

4. Apparatus as claimed in Claim 3 in which the biasing means is an inflatable air bag.

5. Apparatus as claimed in any one of the preceding claims including a plurality of support pillars each of which passes through a respective hole in the wall of the chamber and is surrounded by respective sealing means sealing the interior of the chamber from the atmosphere.

6. Apparatus as claimed in Claims 4 and 5 in which the support pillars are arranged in a substantially circular configuration and the inflatable air bag is positioned substantially in the centre of the circle.

7. Apparatus as claimed in any one of the preceding claims in which the or each support pillar passes through a respective hole in a cover plate removably secured in a larger hole in the wall of the chamber.

8. Apparatus as claimed in any one of the preceding claims in which the or each sealing means comprises a generally cylindrical sealing bellows extending around its associated support pillar.

9. Apparatus as claimed in Claims 7 and 8 in which one end of the or each sealing bellows is connected to the inner surface of the cover plate and the other end is connected to the extension vibrator table.

10. Apparatus as claimed in Claim 8 or Claim 9 in which the or each sealing bellows is radially restrained by one or more restraining rings.

11. Apparatus as claimed in any one of Claims 4 to 10 in which an indicator is attached to the extension vibrator table, the position of which relative to the chamber may be used to determine the extent to which the air bag should be pressurised.

1 2. Apparatus as claimed in any one of Claims 4 to 11 including a control system which automatically inflates the air bag when a reduced pressure is present in the chamber and allows it to deflate when the reduced pressure is removed.

1 3. Apparatus as claimed in any one of the preceding Claims in which a second inflatable air bag, inflatable independent of the first air bag, is provided to exert an upward force on the vibrating parts to enable the extension vibratory table to carry a load greater than its normal working load.

14. Apparatus for connecting an environmental test chamber to a vibrator having a vibrator table to provide apparatus for environmental testing as claimed in any one of the preceding claims including a cover plate for a hole in the wall of the chamber, at least one support pillar, means for securing it to the vibrator table so as to extend through a hole in the cover plate, an extension table adapted to be carried by the pillar or pillars, sealing means adapted to surround the or each pillar and resilient biasing means adapted to exert a force on the vibrating parts of the vibrator.

1 5. Apparatus as claimed in Claim 1 3 in which the biasing means is an inflatable air bag.

1 6. Apparatus as claimed in Claim 1 3 or 1 4 including a plurality of support pillars and equal number of sealing means.

1 7. Apparatus as claimed in any one of Claims 1 3 to 1 5 in which the sealing means comprise generally cylindrical sealing bellows.

1 8. Environmental testing apparatus substantially as described herein with reference to the accompanying drawings.

1 9. Apparatus for connecting an environmental test chamber to a vibrator to provide apparatus for environmental testing substantially as described herein with reference to the accompanying drawings.