GB2086047A - Deadweight pressure gauge tester - Google Patents

Abstract

In a deadweight tester hydraulic pressure is applied by rotating shaft 10 in nut 11 to move the piston 13 forward. To allow the hydraulic system to be rapidly bled the nut 11 is slidable in a tube 12. This permits the piston to be axially reciprocated freely before pressure is applied. <IMAGE>

Description

SPECIFICATION Deadweight tester This invention relates to so-called deadweight testers, which are precision instruments for pressure and vacuum measurement and for the calibration and checking of pressure and vacuum gauges.

The invention is particularly concerned with deadweight, liquid-operated, pressure teSters of the kind employing a piston gauge, in which calibrated weights to provide an equivalent test pressure are loaded on top of a vertically mounted piston of known effective area to balance a force provided by the pressure of the liquid medium.

The basic arrangement of such deadweight pressure testers is well known and, as shown in Figures 1 to 3 of the accompanying drawings, normally comprises a cast base plate 1 having a single vertical piston, or a double vertical piston 2, 3 as shown, one of said pistons 2 being for low pressure work (up to approximately 500 psi) and the other piston 3 being for high pressure work (from 500 psi up to possibly 1 5,000 psi). The base plate 1 accommodates a reservoir 4 (see Figure 2) for an appropriate incompressible liquid such as oil, alcohol, etc. from which the pressurised liquid is fed through the system via a valve 5, a suitable pipework arrangement, and valved openings to two cylinder sleeves 6, 7 for the l.p. and h.p.

pistons respectively and to a testing station 8, at which the pressure device for checking or calibration, e.g. a pressure gauge 9 as shown, can be detachably fitted. If required, two testing stations 8 can be provided to enable the devices to be tested simultaneously, or if a comparison test between two devices is required.

The pressure of the liquid e.g. oil, on the system side of the valve 5, is controlled by a reciprocating movement of a ram screw 10, which is normally threaded along its length, and is supported in a screwed ram nut 11 fixed to the baseplate 1 and pressed into one end of a guide barrel 12, the free end of the ram screw being connected to a "rambler" 12 which is slidingly sealed in a barrel via a seal 14, the sealed space 1 5 of the inner end of the barrel being suitably connected to the valved connected for the cylinder sleeves 6, 7 and the testing station 8. Reciprocating movement of the ram screw 10 is controlled by rotation of a capstan 1 6 in one or other rotational direction, which is rotatably fixed to the extreme outer end of said ram screw.

In use, after priming the reservoir 4 with its incompressible liquid as discussed below, the pressure device to be tested is fitted and sealed to the testing station 8. Selected weights 1 7 are then added to the appropriate piston 2 or 3 to a value corresponding to the actual pressure to be tested (the weights are normally marked with their equivalent pressures to facilitate selection).

The capstan 1 6 is then rotated to screw-in the ram screw from an initial outwardly extending position and the weights 1 7 are spun manually by suitable drive means at approximately 50 r.p.m.

When the spinning weights 1 7 rise to a level mark 1 8 on an indicator rod 1 9 projecting from the base 1 (see Figure 2) the balanced pressure in the system corresponds to that of the test pressure required and the test drive can then be visually checked.

One disadvantage of such a deadweight tester is the time taken, particularly for initial priming of the reservoir 4. Thus, to achieve priming it is usual to open the reservoir, which is normally provided with a transparent cover 20 (see Figure 3), by unscrewing a priming rod 21 from a valve seat 22, which thereby automatically shuts off the valve 5, and to fill the reservoir with the selected liquid.

The reservoir is then closed, the priming rod replaced to open the valve 5 (in the described tester, by screwing down the priming rod 21). In this condition, the testing station 8 is closed by a suitable plug (not shown) and the capstan is then rotated first in one direction and then the other, repeatedly, to reciprocate the ram screw 10 fully in and out between limit stops until all the air has been bled from the liquid, this being indicated by the fact that no air bubbles can be seen in the liquid in the reservoir 4, which is at ambient pressure, through the cover 20. In practice, such a priming procedure is time consuming and normally requires at least six, and perhaps ten or twelve, reciprocating movements of the ram screw 10 to bleed out the air. For initial setting up, the priming procedure can take up to ten minutes to perform.

A principal object of this invention is to provide a deadweight tester with a modified ram/barrel assembly which enables priming to be carried out effectively and in a fraction of the time taken for the above procedure.

According to this invention, there is provided a deadweight tester of the kind including a reservoir for incompressible liquid and a ram/barrel assembly for priming, and thereafter controlling the pressure of the liquid, said assembly comprising a ram screw which is threadedly engaged in a ram nut and rotational drive means for the ram screw, characterised in that said ram nut is slidingly located in the barrel so as to be reciprocable therein by appropriate means, the ram nut, via its threaded engagement with the ram screw, causing axial reciprocating movement of the latter independently of movement caused by said rotation drive means.

In order that the invention will be readily understood and further features made apparent, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a general view of one known form of deadweight pressure tester described above, Figure 2 is an enlarged, fragmentary, sectional view of the typical arrangement described above of the reservoir and l.p. and h.p. piston assemblies, Figure 3 is a view similar to Figure 2 of the typical ram/barrel assembly described above, Figure 4 is a general view of said one embodiment of deadweight pressure tester according to the invention, and Figure 5 is an enlarged, fragmentary, sectional view of the modified ram/barrel assembly of said one embodiment.

Referring to Figure 4, the general arrangement of said embodiment is basically the same as that described hereinbefore with reference to Figure 1 and includes a baseplate 1, low and high pressure pistons 2, 3 respectively, a testing station 8, a capstan 1 6 for the ram/barrel assembly described below, a transparent cover 20 and priming rod 21.

However, in this embodiment, the baseplate 1 comprises a solid block, which incorporates blind bores in which valved connections, such as those for the reservoir 4, piston sleeves 6, 7 (see Figure 2) and the testing station 8, are located, the pipework connections of the system being provided by small diameter ducts drilled into and through the block and opening into said blind bores to suitably connect said valve connections together. Any open ends of the drilled-out ducts which require to be sealed are counterbored, threaded and plugged e.g. as shown at 22 in Figure 4. It will be appreciated that such an arrangement improves the integrity of the system since no pipework connection pieces, where any leaks would normally occur, are needed.

Referring to Figure 5, the modified ram/barrel assembly basically comprises the same components as described with reference to Figure 3. Thus, the assembly comprises a ram screw 10 which is threaded at 1 Oa along part of its length, a ram nut 11, a guide barrel 12 and a rambler 13 slidingly sealed in the barrel via a seal 14.

However, in its modified form, the barrel 12 is formed by two screwed-together tubes 23, 24 of different diameters, the tube 23 being of the same of similar diameter to the barrel 12 of Figure 3, and the tube 24 being of substantially larger diameter. Instead of the ram nut 11 being fixed to the baseplate 1 at the end of the guide barrel 12 as described in Figure 3, it is slidingly accommodated within the tube 24, which latter is provided over part of the length with an axial guide slot 25. A priming peg 26 which is connected to the ram nut 11, projects radially through the slot 25. The outer end of the tube 24 is closed by a screwed-on end-cap 27, to the central part of which, in turn, is screwed a tubular extension piece 28. The outer end of the extension piece 28 carries a bush 29 which slidingly engages an extending outer end part 30 of the ram screw 1 0.The extreme outer end of the ram screw provides an appropriately splined boss 31 for the capstan 16 (not shown).

It will be appreciated from the foregoing that, after priming, the tester is used as described hereinbefore with reference to Figures 1 to 3, reciprocating movement of the ram screw being effected by rotation of the capstan 1 6 from an outwardly extended position in which the outer end of the ram nut 11 is in contact with the end cap 27 and a collar 32 on the ram screw is in contact with the front end of said nut 11.

However, to initiate a priming procedure, since the ram nut 11 is capable of travelling, instead of being fixed as described with reference to Figure 3, it can be reciprocated directly and positively in the axial direction merely by reciprocating the priming peg within the limits set by the ends of the guide slot 25. Due to its threaded engagement with the ram screw 10, the latter is caused to effect an axial reciprocating movement which is independent of any movement of the ram screw caused by rotation of the capstan 1 6. When the priming peg 26 is not in use, the ram nut 11 and, with it, the screw 10 are urged towards their outwardly extended positions by the pressure within the system. However, to facilitate this return, a compression spring 33 may be provided as shown. Also provision may be made to lock the ram nut 11 in its outwardly extended position e.g.

by providing a lock-nut on the priming peg 26 which screws down onto the outer surface of the tube 24.

Because such a priming arrangement can affect a more direct and quicker sliding reciprocation of the ram screw 10, and thereby the rambler 13, the time taken to bleed out air from the liquid can be very much reduced, with consequent advantages in increased production; it has been found that priming can be effected with just two or three reciprocations in a comparatively short time e.g.

less than one minute.

According to a subsidiary feature of the invention, referring particularly to Figure 5, it can be seen that the modified ram/barrel assembly is inclined at a small angle a to the horizontal. This results in the capstan 1 6 being inclined to the vertical plane, which facilitates handling of the capstan, and also enables a slightly larger diameter capstan to be used than would otherwise be the case with a horizontal arrangement.

Claims (7)

1. A deadweight tester of the kind having a pressure system including a reservoir for incompressible liquid and a ram/barrel assembly for priming and thereafter controlling the pressure of the liquid, said assembly comprising a ram screw which is threadedly engaged in a ram nut and rotational drive means for the ram screw, characterised in that said ram nut is slidingly located in the barrel so as to be reciprocable therein by appropriate means, the ram nut, via its threaded engagement with the ram screw, causing axial reciprocating movement of the latter independently of movement caused by said rotation drive means.

2. A deadweight tester according to Claim 1, wherein the wall of said barrel has an axially extending guide slot, and said ram nut is provided with a priming peg which projects through said guide slot, the arrangement being such that the priming peg is reciprocated within the limits set by the ends of the guide slot to produce reciprocable movement of the ram nut.

3. A deadweight tester according to Claim 1, or Claim 2, wherein said barrel comprises two axially connected tubes of different diameters, the smaller diameter tube being connected into the pressure system of the tester and the free end of the larger diameter tube providing a sealed bearing for the outer end of the ram screw, the arrangement being such that the ram nut is slidably mounted in the larger diameter tube and the inner end of the ram screw projects into the small diameter tube.

4. A deadweight tester according to Claim 3, wherein compression means are provided between the step between the two tubes and the ram nut for urging the ram nut and screw to normal outwardly extended positions.

5. A deadweight tester according to any one of Claims 1 to 4, wherein the ram/barrel assembly is inclined at a small angle to the horizontal.

6. A deadweight tester according to any one of Claims 1 to 5, wherein the pressure system is incorporated in a baseplate, the baseplate being in the form of a solid block of material having blind bores formed therein in which valved connections for the pressure system are located, pipework connections for the system being provided by small diameter ducts extending through the block and opening into said blind bores as appropriate to correct said valved connections, any open ends of said ducts in the surface of the ducts being sealed by plugs.

7. A deadweight tester constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.