BACKGROUND

The invention relates to sheet steel or tinplate pails of the full open head type and lids of the same material, of the multi-lug type. In particular the invention concerns a lid and lid-to-pail interconnection providing improved resistance to leakage when subjected to internal pressure.

The need has been specified in certain countries to require full open head pails with lids used for the transport of dangerous goods contents to be capable of withstanding internally applied hydraulic pressure (in a test simulating abnormal service conditions) , the pail and lid interconnection being regarded as satisfactory for the carriage of dangerous liquiform goods if it is capable of withstanding a pressure of 100 kPa for five minutes, without leaking. Such a test is specified by certain national regulations for the packaging and transport of dangerous goods by road, rail or sea. At such test pressures the pail ends permanently distort to a very considerable degree.

Such tests have so far been found very difficult to pass using pails of the full open head type, unless made of steel so thick by comparison with previously commercially accepted standards as to be now unacceptable.

The present invention has been found particularly applicable to open head type pails of tinplate or mild steel having lug type lids of the same material in which the following particulars obtain:

Pail Capacity 15 to 25 litre Pail Internal diameter

(nominal) 286mm

Pail height Dependent on capacity Pail outside diameter

(approx) 300mm

Thickness of pail sheet From 0.35 to 0.5 mm

Thickness of lid sheet From 0.43 to 0.5 mm

Accordingly it is an object of the present invention to provide a pail lid for a pail-to-lid interconnection when assembled to withstand the above described pressure test, the dimensions of the pail and lid components being generally within the parameters as set out above.

SUMMARY

The invention consists of a lid for use in an open head pail and lid assembly, said lid being of tinplate or sheet mild steel of thickness in the range of 0.43 to 0.5 mm, the lid having a gasketted peripheral groove to be complementarily sealed to a peripheral rim of the pail by the inward and upward deformation of lugs around the peripheral of the lid around the curled rim of the pail; a generally planar central region and a drawn periphery region where the gasketted groove is offset from the plane of the central region; characterised in that the central region of the lid is provided with a plurality of upwardly and outwardly deformed beads each having a cross-sectional profile asymmetrically offset wherein the maximum upward deformation or crest of each bead is radially outwardly offset from a radial centre line of that bead and wherein the amount of offset of the central region of the lid from a plane containing the most distant extremity of the gasketted groove, that is, the depth

of draw, is 5 to 7% of the outside diameter of the lid and the amount of offset of the planar central region of the lid from the plane containing the crest of the beads is approximately 23 to 27% of the depth of draw.

Preferably the number of beads provided is two to four. Preferably the number of beads provided is three. Preferably the beads are circular. Preferably the beads are concentric. Preferably the beads are circumferentially continuous. Preferably the bead cross-section comprises two straight lines joined by a radius wherein the straight lines are of unequal length and the radius is not more than five times the thickness of the tinplate or steel sheet.

Preferably, of the two straight lines comprising the bead cross-section, the radially more outward line intersects the planar portion of the lid at an angle in the range of about 80 to 90 degrees

DESCRIPTION OF THE DRAWINGS

Notwithstanding all the forms that fall within the scope of the invention, one particularly preferred form is now described with reference to the accompanying illustrations in which:

Figure 1 shows a portion of both a cross section and a projection of an elevation of a pail lid according to the invention, before attachment to the top rim of a pail.

Figure 2 shows a portion of an elevation of a pail lid and lid-to-pail interconnection according to the invention.

DETAILED DESCRIPTION

Referring to Figure 1, the circular pail lid (10) comprises twenty lugs (11) around the periphery of the lid, downwardly depending from the bottom edge (12) of a downwardly depending peripheral skirt (13) . A gasket groove (14) is defined by a circumferentially extending radiused groove which in use occupies to a suitable depth by a cellular compressible gasket (not shown) for sealing against a complementary curled rim of a pail (see Figure 2) . The plane (14) defines the axially uppermost portion of the lid. Radially inwardly of the highest point (15) a downwardly depending draw (16) of the lid extends further downwardly than the skirt (12) , before blending via a radius (17) into a central lid portion (18) bounded radially outwardly by radius (17) .

Central lid portion (18) is generally planar except that three beads (19) , (20) , (21) are formed so as to project upwardly from the flat central portion (18) .

Each bead is preferably identical in cross-section and, described with reference to bead (19) , comprises in cross-section an approximately vertical straight line (22) , a second straight line (24) , both lines being joined by a radius (23) which is preferably in the range three to five times the metal thickness (the metal thickness being 0.43 to 0.5 mm preferably given that the outside diameter of the lid is approximately 300mm) .

Line (24) is inclined at an angle (25) to the horizontal. A preferable value of angle (25) has been found to be in the range 20 to 30 degrees

Preferably the best results are achieved with three beads as illustrated although it is believed that two may be found sufficient and any more than four would consume unnecessarily large amounts of the sheet metal

lid, requiring a larger blank diameter to ensure adequacy of important dimensions such as the length of the outer skirt (12) and lugs (11) .

The radius (23) defines a crest of bead (19) and it is preferred if the height of the crest measured from plane (18) and the depth of draw from plane (14) to plane (18) are in the ratio of 4.5 to 18.5 or about 1 to 4 or 25%. It is preferred if the depth of draw is at least 5% of the outside diameter of the lid and is optimally in the range 5 to 7%.

With reference to Figure 2, the pail body (32) with its curl (33) is closed by a lug-cover lid having a central portion (18) having a deep-drawn wall (16) rolled over to a rim (36) in which is positioned a gasket (37) . The rim extends downwardly in lugs (11) which are adapted to be clenched under the curl of the pail to secure the lid on the pail as will now be described. Before clenching the lugs (11) extend downwardly from the rim (36) such that the lid may be easily placed in firmly fitting relationship with the pail. Once in position a clenching machine is engaged with the lugs such that each jaw of the tool head forces the lugs inwardly and upwardly with a force finally applied in the direction of arrow (39) so that the lugs are deformed inwardly against the wall of the pail (32) and tucked upwardly so as to form a re-entrant angle (40) between the curl (33) and the peripheral wall (32) of the pail. During the clenching operation the gasket (37) which was previously positioned within the rim (36) on top of the curl is compressed and tends to be forced outwardly and downwardly towards position (41) . This positioning of the gasket material has been found to be particularly beneficial in maintaining the integrity of the seal between the lid and the pail during drop and other performance testing.

It has also been found that the clenching of the lugs inwardly and upwardly to form the re-entrant angle (40) is beneficial in retaining the cover in place on the pail during hydraulic pressure testing. It is a feature of the invention that the curl formed by rotation of the rim of the pail through more than 450° enables a much higher clenching force to be applied to the lug without collapsing the curl on the pail, and therefore allows the re-entrant clenching of the lug up and between the curl and the adjacent cylindrical wall of the pail. The clenching force applied by the lid further forms the curl such that an exceptionally effective combination of pail lid with pail-to-lid interconnection results.

It will be noted after crimping of the lug-cover lid in place, that the final tightening and forming of the curl to the configuration shown in Figure 2 may be achieved during crimping of the lug-cover lid in place on the pail. In this case a clenching machine performs the final stage in the formation of a tightly rolled curl of the configuration shown in Figure 2.

It is convenient and preferable for reasons of tool-making, ease of manufacture and quality control of mass-produced lids if the beads are circular and concentric. For a lid of outside diameter approximately 300 mm suitable diameters for the crests of beads (19), (20) and (21) are approximately 270, 240 and 216 mm respectively and in any event it is preferred if the beads lie within the radially outermost 60% of the diameter of the central portion of the lid.

As an example of the use of the invention it has been found that a lid of outside diameter 300 mm when fitted to an open head pail, using the method of attachment shown in Figure 2 and where the lid is made of tinplate 0.5 mm thick and the pail is made of tinplate 0.38 mm thick in both the body and bottom, then the pail is capable of withstanding an internal pressure of 100 kPa for at least five minutes.