WO2000022239A1

WO2000022239A1 - Lubricated structural bearing arrangement

Info

Publication number: WO2000022239A1
Application number: PCT/GB1999/003245
Authority: WO
Inventors: Nigel Henry New
Original assignee: Federal-Mogul Engineering Limited
Priority date: 1998-10-10
Filing date: 1999-09-30
Publication date: 2000-04-20
Also published as: TW412628B; AU6108599A; GB9822070D0; GB2342408A

Abstract

A structural bearing arrangement (20), for interposing between relatively displaceable parts of a structure (5) such as a bridge, comprises a hard polished part (22) overlying a softer bearing material (27) that is formed on rigid carrier (28). A series of transversely extending channels (29) form grease-filled pockets to limit erosion wear of the bearing material, and the grease is replenished by means of a distribution manifold (46) connected, via end plate (51), to a reservoir (50) for the grease. A small superatmospheric pressure (less than about 3 Bar) is maintained in the grease to effect replenishment of the channels by an accumulator (44) comprising a piston (55) and spring (56). The accumulator is charged by compressing the spring by charging the reservoir with grease from a pressurised source, the accumulator spring thereafter maintaining a positive, if decreasing pressure, channel-replenishment pressure in the reservoir as grease is drawn from the channel by bearing operation. The accumulator may comprise a gas spring or a tandem arrangement of springs to maintain pressure for longer. The reservoir may be recharged with grease periodically.

Description

Lubricated Structural Bearing Arrangement

This invention relates to structural bearing arrangements, that is, bearing arrangements disposed at an interface between components of structures which undergo limited relative motion in the plane of the interfaces due to variations in forces applied to the structure, such as, for example, at the interface between a bridge deck and support pier where reciprocation due to thermal expansion and contraction of the deck materials occurs, and in particular relates to lubrication of such bearing arrangements.

Structural bearings for such an application are known comprising a first part which sits on the effectively stationary pier and comprises a rigid carrier on which is formed a bearing layer of relatively soft bearing material, such as polymer or bearing alloy, and a second part, which is attached to the moving deck and comprises a substantially smooth plate that overlies, and rests on the bearing layer. Such bearing construction requires minimal levels of lubrication which is often provided by a solid or semi-solid lubricant contained in the bearing material, either intimately infiltrated into small pores thereof or contained in larger pockets arrayed across the surface.

Because the load on the bearing is very high, specific loading of 30 MPa (300 Bar) is typical, the smooth plate substantially closes the lubricant pockets to loss or contamination of the lubricant and the relatively small and infrequent movement means that only a very small quantity of lubricant is drawn out by the movement to prevent the relatively soft bearing layer from being eroded by friction.

Indeed, polymeric bearing materials containing PTFE may, notwithstanding the poor strength of the material to erosion and deformation per se if not lubricated by the provision of such lubricant pockets, have a working life of some 20 years before absence of lubricant results in significantly accelerated wear.

There are some circumstances in which a longer operating life without intervention is necessary and/or circumstances in which the frequency of such reciprocation movement is significantly increased, for example, a railway bridge wherein there is a bearing movement each time a train crosses the bridge and it may therefore be necessary to consider means of extending the unattended operating life of the structural bearing arrangement.

Examples of such bearings are shown in GB-A1 -2156914, GB-A-1585998 and DE-B1- 2952614, which bearings also include provision for introducing a lubricant.

It is further more known from DE-C2-3034710 to provide forced lubrication for a structural bearing by having associated therewith a lubricant reservoir and a pump which is driven directly by motion of a bridge deck to force lubricant into the bearing, elements of the pump reciprocating with the structure deck by way of a linkage which must be freely operating throughout the operating lifetime of the bearing arrangement. Such an arrangement is complex and susceptible to deterioration of the linkage.

It is an object to provide a structural bearing arrangement of simple and cost effective design that is capable of providing unattended operation for longer than hitherto.

According to the present invention a structural bearing arrangement, for disposition at the interface between components of a structure superimposed under load and capable of limited reciprocation in a plane containing the interface, comprises first and second parts, adapted to be secured one each to said superimposed components, having contacting surfaces arranged to extend in said interface plane, said first part having a bearing surface on a rigid carrier, the bearing surface having formed therein an array of lubricant holding pockets disposed at intervals in the direction of said relative reciprocation of the structure components and extending across the surface with respect to said direction of relative reciprocation, and said second part having a smooth co-operating surface arranged in operation to overlie said bearing surface and provide substantially a closure for said lubricant pockets, and lubricant replenishment means comprising lubricant distribution means comprising at least one distribution manifold connected to each of the pockets, lubricant reservoir means, arranged to store a quantity of the lubricant, and characterised by an accumulator of mechanical energy having at least one displaceable element driven thereby against the lubricant in the reservoir to define, with said bearing arrangement in operational disposition relative to the structure components, a superatmospheric pressure in the lubricant replenishment means. The accumulator may be rechargeable.

It has been found that it is possible to obtain a significant increase in the working life of the bearing arrangement by replenishing the lubricant in the bearing surface pockets at a pressure of less than 3 Bar, permitting simple and compact lubricant replenishment means.

The lubricant reservoir means may be defined in the carrier and the accumulator may be contained in the reservoir as spring means, as a gas spring and/or one or more physical spring elements.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which

Figure 1 is a sectional elevation through a portion of a bridge structure showing superimposed structure components separated at their interface by a structural bearing arrangement of known form,

Figure 2(a) is a sectional elevation, similar to that of Figure 1 but showing a structural bearing arrangement, in accordance with the present invention, in a first embodiment in which lubricant reservoir means is contained within the bearing part carrier, the energy accumulator comprises a diaphragm within the reservoir in the form of a piston that is urged by a spring, and coupling between reservoir and distribution manifold is by way of an end closure plate,

Figure 2(b) is a plan view of the lower bearing part of the arrangement of Figure 2(a),

Figure 3 is a plan view of part of a second embodiment in which coupling means is contained within the carrier,

Figure 4 is a plan view, similar to that of Figure 2(b) of a third embodiment of structural bearing arrangement in which the lubricant reservoir means is external of the carrier,

Figure 5 is a sectional elevation of part of a fourth embodiment of structural bearing arrangement, showing an energy accumulator of tandem spring and diaphragm form, and triggering mechanism therefor, Figure 6 is a sectional elevation, similar to Figure 5 but of a fifth embodiment of bearing arrangement having an alternative triggering mechanism for the tandem energy accumulator,

Figure 7 is a cross-section through an alternative form of reservoir management containing and energy accumulator of spring and piston form,

Figure 8(a) is a sectional elevation through a further form of reservoir arrangement incorporating an energy accumulator in the form of a volume of gas and diaphragm in the form of a flexible membrane, being chargeable from without by way of a non-return valve,

Figure 8(b) is similar to Figure 8(b) but shows charging of the accumulator by way of a permanently coupled high pressure flask,

Figure 9 is a cross-section, of yet another form of reservoir and energy accumulator therein, comprising a centrally disposed volume-defining membrane, and

Figure 10 is a sectional elevation through a sixth embodiment of structural bearing arrangement, similar to Figure 2(a) but where the energy accumulator comprises a spring compressor to charge the accumulator as a result of applying load to the bearing.

Referring to Figure 1 , a bridge structure 5 comprises a fixed pier 6 on which is supported superimposed a deck component 7, the interface 8 between them being formed by a structural bearing arrangement 10 of known form. The bearing arrangement 10 comprises a first part 11 supported on, and secured to, the pier 6, and a second part 12 secured to the deck component 7. The second part of the bearing arrangement has a surface 14 that is smooth and hard, being a polished surface of the part which is a solid plate of stainless steel. The first part has a surface 16 of a layer of bearing material 17 which is soft, relative to the surface 14, and supported on a rigid carrier 18. The bearing material 17 contains an array of depressed pockets 19 which open to surface 16 and contain a viscous lubricant, such as grease.

The components 6 and 7 of the structure reciprocate in the plane of the interface due to inter alia thermal expansion and contraction of the deck 17. The specific pressure/load between the parts of the bearing in use is large, and the lubricant is effectively trapped in the pockets, to be released only slowly by the drag associated with the relative movement between the surfaces.

This represents a typical known structural bearing arrangement.

Referring to Figures 2(a) and 2(b) there is shown a similar structure comprising components 6 and 7 reciprocable in the plane of interface 8 and at the interface a first embodiment of structural bearing arrangement 20 in accordance with the present invention.

The bearing arrangement comprises first and second parts 21 and 22 (corresponding to 11 and 12), the second part 22 comprising a flat plate of stainless steel secured to deck component 7 and having polished surface 24 facing a surface 26 of bearing material 27 of the part 21 , the bearing material being formed as a layer on a rigid carrier 28 of steel. The bearing layer includes an array of lubricant holding pockets 29 which comprise channels extending across the surface with respect to the direction of relative reciprocation. The direction of relative reciprocation is indicated by arrow 30 and in this embodiment the channels extend orthogonally thereto separated from each other in the reciprocation direction by less than the amplitude of the reciprocation.

The bearing arrangement 20 is specifically distinguished from the prior art by including lubricant replenishment means, indicated generally at 40, which comprises lubricant distribution means 41 , lubricant reservoir means 42, coupling means 43, connecting the reservoir means with the distribution means, and an accumulator of mechanical energy 44.

The lubricant distribution means is, of necessity, formed within the carrier 21 and comprises a distribution manifold 46 extending in the direction of channel separation and at least one supply duct 47 connecting each channel to the distribution manifold.

The lubricant reservoir means 42 comprises a cylindrical chamber 50 extending through the carrier, similar to, and conveniently parallel to, the distribution manifold but of greater cross-section. Both reservoir and distribution manifold open to one end 21' of the carrier and are coupled by the coupling means 43, the latter taking the form of a plate 51 secured to the carrier and having a surface recess 52, connecting the reservoir and manifold, and a non-return connector 53 by which lubricant can be loaded into the reservoir and a super- atmospheric pressure established as discussed hereinafter.

The accumulator 44 is defined within the chamber 50 and has at least one displaceable element driven thereby against the lubricant to define a superatmospheric pressure therein which urges lubricant towards the channels. The displaceable element comprises a diaphragm 55, in the form of a substantially rigid piston, and compressible spring means 56, in the form of a helical compression spring, which bears against the piston and a blind end 57 of the chamber. The part of the chamber between the end 57 and the piston is exposed to atmospheric pressure, by way of vent passage 58, which conveniently forms a passage for an indicator rod 59, the exposed length of which is directly related to amount of lubricant remaining in the reservoir.

In operation, and with the bearing arrangement installed between structure components 6 and 7, but prior to the structure loading effectively closing the channels 29 from above, a source of lubricant, such as a grease gun, is attached to coupling 53 and grease is pumped into the system, filling in part of the reservoir, the distribution manifold and channels. At this time the grease is free to exude from the channels so the pressure is 1 Bar and, within the reservoir, the spring 56 is fully extended and piston displaced to maximum extent. After the channels 29 are closed by structure loading, more grease is pumped into the system, in practice into the reservoir chamber by displacing the piston against spring pressure until a predetermined pressure of 2 to 3 Bar is established, following which the grease gun is removed. The "filling" pressure exerted by the gun therefore charges the accumulator with mechanical energy by compressing the spring, and the substantially closed-off channels prevent the spring from expanding other than as grease is used up.

During the operating life of the bearing arrangement, as grease is drawn from the channels 29 it is replaced by that in the reservoir under superatmospheric pressure. Such replenishment is accompanied by displacement of the piston and indication rod, and continues for as long as the spring is able to exert a superatmospheric on the grease in the reservoir chamber.

It has been found, by accelerated life testing, that a relatively low pressure of up to 1.5 Bar is sufficient to effect replenishment and almost double the unattended operating life of the bearing arrangement and, furthermore, such low pressure requirements place minimal constrains on maintaining the reservoir chamber sealed.

It will be appreciated that the force exerted by the simple spring 56 is a function of its length compression and falls as the piston is displaced. Therefore, the length and spring rate may conveniently be chosen, having regard to chamber dimensions, to give about 1.5 Bar pressure towards the maximum useful extension, and about 3 Bar when fully compressed, that is, when reservoir filled at commencement of operation and the accumulator charged. Such relatively low initial pressure is advantageous in posing few constraints on sealing efficacy, but such pressures and dimensions may, of course, be varied to suit different circumstances.

It will be appreciated that even when reservoir pressure is reduced to 1 Bar, as may occur if and when the spring extends fully, there is still the prospect of many years operation as a conventional pocket-filled lubricated bearing. However, the end of pressurised grease replenishment may be attended to by pumping more grease into the reservoir to recharge the accumulator and commence another period of unattended operation.

It will be appreciated that there are many ways of effecting the reservoir means and/or accumulator, both within and without the carrier, and some of these will be described. In Figure 2(b) there is shown ghosted a second chamber 50' which also opens to carrier face 21' and communicates with the recess 52 in the cover plate. Clearly the size and distribution of such plural reservoir chambers may depend upon the dimensions and strength of the carrier.

Figure 3 shows in part a second embodiment of bearing arrangement 60 in which carrier 61 differs from that 21 of Figure 2(b) in that the reservoir chamber(s) 50 and distribution manifold 46 are closed by a simple blanking plate 51' or individual plugs (not shown) and the connecting means 43' is effected by a cross-drilling 62 which terminates in non-return coupling 63.

Referring now to Figure 4, this shows in plan view a third embodiment of bearing arrangement 70, in particular carrier 71 which contains only the distribution means 72, the reservoir means 73 being contained totally in a plate 74 secured to the carrier (or optionally remote therefrom). The reservoir means 73 comprises a single chamber 74 which contains two opposing diaphragm pistons and spring arrangements 75, 76, as well as a central outlet duct 77 and non-return coupling 78. A single piston and spring may be employed but in either event it will be seen that operation is exactly as described above.

Referring now to Figure 5, this shows a part of a fourth embodiment of bearing arrangement 80, in particular, a part of reservoir 81 and accumulator 82. This is similar to that of Figure 2(b) except that the accumulator comprises a pair of diaphragm pistons 83, 84 and springs 85, 86 in tandem. Each piston 83 and 84 has associated therewith an indicating rod 87 and 88 respectively, disposed coaxially with respect to each other and being coupled by a detent arrangement 89 which holds them in relative axial disposition. As shown, the detent arrangement holds the spring 85 fully compressed whilst the spring 86 is permitted to extend and displace the piston pair. The end of the vent passage 58 in the carrier is shaped, as is a limb 89' of the detent arrangement, such that at a predetermined displacement of the indicating rods/piston pair, when the pressure exerted by spring 86 has fallen, the wall displaces the limb 89' and releases the detent, thereby permitting spring 85 to act on pistons 83 and 84 and, with spring 86, increase the pressure in the reservoir until that spring 85 is also fully extended.

Figure 6 shows part of a fifth embodiment of bearing arrangement 90, similar to that of Figure 5 except in respect of detent arrangement 99 which is formed integrally with the outer coaxial indicator rod 98 and, upon extension of spring 96 and displacement of piston 94, operated by having a limb 99' lifted by engagement with the carrier wall to release the inner indicator rod 97 and attached piston and spring combination 93, 95.

Referring to Figure 7, this shows an alternative form of reservoir means 100 and accumulator 101 ; it may be contained within a cover plate, as in Figure 4, or within the carrier. The reservoir chamber 102 contains an accumulator in the form of two opposed piston and spring combinations 103, 103' and 104, 104' but one of the pistons, 104, also contains a pressure actuated detent arrangement106. As shown, a detent pin 107 is spring loaded in a radially inward direction by detent spring 108 but may be forced outwardly against detent spring force by pressure from the reservoir 102 acting on detent piston 109.

In operation, when grease is applied to the reservoir by way of coupling 43 at elevated pressure, the pistons 103 and 104 are forced apart as the springs 103' and 104' contract. The detent piston 109 also forces the pin 107 against the reservoir chamber wall. At a predetermined pressure/piston displacement the detent pin locates in a recess 110 which physically restrains the spring 104' from extending. Thus during the life of the bearing arrangement, the accumulator is first charged, replenishment pressure is applied to the lubricant by piston 103 as spring 103' extends. When reservoir pressure falls below a predetermined value, say 1.5 Bar, the detent spring is able to overcome resistance from the detent piston and release the pin, thereby permitting the piston 104 and spring 104' to pressurise the reservoir for a further period of operation. Clearly there are other ways of ensuring that accumulator pressure is ramped up after a particular force applying means, such as a simple spring, is depleted.

It will also be appreciated that a simple spring arrangement is not the only form of compressible spring means. A more complex spring arrangement may be employed permitting prolonged release of energy, such as a wound spring used in a clock, or a spring form other than a physical spring may be used.

Referring to Figure 8(a), this shows in part a further form of reservoir means 120 and an accumulator 121. The accumulator is provided by a volume of compressed gas 122 acting on a diaphragm membrane 123 in the form of a flexible or analogous diaphragm form, such as an aforementioned piston, that is, a gas spring.

Broken lines 123' or 123" illustrate respectively possible disposition as the accumulator is fully charged (that is maximum compression of gas) and depleted. Charging may be effected by having a closed volume and pressure applied by way of the lubricant and membrane to compress it by reducing that volume. As shown, a non-return valve 124 permits the volume to be charged from an external gas source (not shown).

It will be appreciated that instead of providing a valve 124 for charging the spring means with gas, a flask of compressed gas may be permanently in communication with the volume as illustrated in Figure 8(b), at 124'.

Furthermore, a gas spring means may include a physical spring element, such as shown ghosted at 125, to supplement the force exerted by gas pressure on the membrane.

Such a gas spring may take other forms, for example the reservoir 125 and accumulator 127 as shown in Figure 9 which is analogous to the form shown in Figure 4, except that the gas pressure of the accumulator exerts a force towards the ends of the reservoir. The gas volume may be charged by way of valve 124 or a flask 124' of compressed gas.

Referring to Figure 10, this shows in sectional elevation part of a sixth embodiment of structural bearing arrangement 130 which comprises a second part 22 secured to the structure part 7 and first part 21' secured to structure part 6. The first part 21' is substantially identical to the part 21 of Figure 2(a) but differs in the form of the chargeable accumulator, indicated generally at 131. A diaphragm 132, in the form of a piston, separates the lubricant 133 from the remainder of the reservoir 133 which in turn is vented to the atmosphere by passage 134. A spring 135 extends between the diaphragm piston and a support 136, conveniently in the form of a piston like member. A toothed actuating arm 137 extends from the support through the vent passage 134 and meshes with a gear wheel 138. The gear wheel also meshes with a drive arm 139 extending through the bearing layer and proud of the surface 26 towards the bearing part 22 such that as the bearing is put under load, that is, part 22 brought towards part 21', the drive arm 139 is displaced, which displacement is transferred by gear 138 into displacement of arm 137 and compression of spring 135, thereby charging the accumulator. Operation is thereafter as described above.

As indicated above, the reservoir means, and/or energy accumulator may be provided internally or externally of the carrier, the common feature of all embodiments of the invention being that a simple stored energy accumulator is employed to develop preferably low superatmospheric replenishment pressure.

The lubricant may of course be other than grease, possibly a synthetic oil which is able to remain in use for long periods without deterioration.

It will be appreciated that as the channels in the bearing surface are effectively closed by the bearing plate before superatmospheric pressure is applied, the distribution manifold may be formed as an open channel in the bearing surface.

Also, although it is convenient to form the lubricant holding pockets as channels, which are an elongate component in the direction orthogonal, they may be individually non-extensive provided there is overlap in a said orthogonal direction or separation in the reciprocating direction less than the actual amplitude of the body parts to ensure that the whole bearing surface is lubricated.

It will be appreicated that the features described in the preceding paragraphs may be effected by way of a meandering or serpentine channel in the bearing surface.

It will also be appreciated that because the bearing arrangement is capable of being self- contained in operation, the carrier part including the lubricant replenishment means may equally be mounted on either (or both) parts of a structure. That is, the pockets and lubricant replenishment means may be defined within bearing surface formed by the stainless steel plate, rather than the softer bearing material, or the parts as described hereinbefore may be inverted in their relative disposition.

Claims

1. A structural bearing arrangement (20; 60; 70; 80; 90; 130) for disposition at the interface between components (6; 7) of a structure superimposed under load and capable of limited reciprocation in a plane containing the interface, the structural bearing arrangement comprising first and second parts (21; 22), adapted to be secured one each to said superimposed components, having contacting surfaces arranged to extend in said interface plane, said first part having a bearing surface (26) on a rigid carrier (28; 61; 71), the bearing surface having formed therein an array of lubricant holding pockets (29) disposed at intervals in the direction of said relative reciprocation of the structure components and extending across the surface with respect to said direction of relative reciprocation, and said second part (22) having a smooth co-operating surface (24) arranged in operation to overlie said bearing surface and provide substantially a closure for said lubricant pockets, and lubricant replenishment means (40) comprising lubricant distribution means (41;

72) comprising at least one distribution manifold (46) connected to each of the pockets, lubricant reservoir means (42; 73; 81; 100; 120; 125; 133), arranged to store a quantity of the lubricant, and characterised by an accumulator of mechanical energy (44; 82; 101; 121; 127) having at least one displaceable element (55, 56; 75, 76; 83, 84; 93, 94; 103, 104;

123; 132, 136) driven thereby against the lubricant in the reservoir to define, with said bearing arrangement in operational disposition relative to the structure components, a superatmospheric pressure in the lubricant replenishment means.

2. A structural bearing arrangement as claimed in claim 1 characterised in that the accumulator is chargeable by way of force exerted on a displaceable element thereof by way of pressure in the lubricant.

3. A structural bearing arrangement as claimed in claim 1 or claim 2 characterised in that the reservoir means (44) is defined in the carrier.

4. A structural bearing as claimed in any one of claims 1 to 3 characterised in that the accumulator comprises compressible spring means (56; 85, 86; 95, 96; 122; 127) within the reservoir and the displaceable element comprises a diaphragm separating the spring means from the lubricant.

5. A structural bearing arrangement as claimed in claim 4 characterised in that the spring means comprises a volume of compressed gas (122; 124').

6. A structural bearing arrangement as claimed in claim 4 characterised in that the spring means comprises at least one compression spring (56; 85; 95; 104') disposed between a diaphragm (55; 84; 94) and an abutment (57) which, in operation of the bearing arrangement, is relatively stationary.

7. A structural bearing arrangement as claimed in claim 6 characterised in that the spring means comprises a tandem diaphragm and compression spring arrangement, responsive to a predetermined displacement of a first diaphragm (84; 94) by a first spring (86; 96) to effect displacement of a second diaphragm (83; 93) by a second spring (85; 95).

8. A structural bearing arrangement as claimed in any one of claims 4 to 7 characterised in that the compressible spring means includes a visual indicator (59) of lubricant pressure.

9. A structural bearing arrangement as claimed in claim 8 when dependent upon any one of claims 6 or 7 characterised in that the visual indicator (59) comprises a rod attached to a diaphragm and extending therefrom out of the reservoir, to indicate, by the exposed length of the rod, the position of the diaphragm within the reservoir and the spring induced pressure thereon.

10. A structural bearing arrangement as claimed in any one of the preceding claims characterised in that the accumulator is arranged to define a pressure of less than 3 Bar in the lubricant throughout the operating life of the bearing arrangement.

11. A structural bearing arrangement as claimed in any one of the preceding claims characterised in that the lubricating pockets (29) comprise one or more elongate channels extending across the bearing surface with respect to said direction of relative reciprocation.

12. A structural bearing arrangement as claimed in claim 11 characterised in that the channels in the surface of the second part are isolated from each other and separated in said direction of relative reciprocation by less than the amplitude of said reciprocation.

13. A structural bearing arrangement as claimed in any one of the preceding claims characterised in that the distribution manifold means (46) extends through the carrier body (28).

14. A structural bearing arrangement as claimed in any one of the preceding claims characterised in that the distribution manifold is arranged to extend in a direction parallel to the direction of said relative reciprocation.

15. A structural bearing arrangement as claimed in claim 14 characterised in that the reservoir means extends through the carrier substantially parallel to the distribution manifold means.

16. A structural bearing arrangement as claimed in claim 15 characterised in that the reservoir means and distribution manifold means are connected to each other by coupling means defined externally of the carrier.

17. A structural bearing arrangement as claimed in claim 16 characterised in that the reservoir means and distribution manifold means open to an end face of the carrier and the coupling means is defined in a closure plate (51) therefor secured to the end face of the carrier.

18. A structural bearing arrangement as claimed in claim 16 or claim 17 characterised in that the coupling means includes a non-return valve (53) arranged to permit the introduction of lubricant into the reservoir means and, if applicable, charging of the accumulator.

19. A structural bearing arrangement as claimed in any one of the preceding claims characterised in that the bearing surface of the first part comprises a polymer composition infiltrated into, and overlying, a porous sintered metallic material formed adhering to the carrier and the second part comprises a plate of polished stainless steel.