GB2359345A

GB2359345A - Lubricated structural bearing arrangement

Info

Publication number: GB2359345A
Application number: GB0003674A
Authority: GB
Inventors: Nigel Henry New
Original assignee: Federal Mogul Engineering Ltd
Current assignee: Federal Mogul Engineering Ltd
Priority date: 2000-02-18
Filing date: 2000-02-18
Publication date: 2001-08-22
Also published as: GB0003674D0

Abstract

A sliding bearing arrangement for interposing between components of a structure superimposed under load and capable of limited reciprocation in a plane containing said components' interface, said sliding bearing arrangement comprising first and second parts adapted to be secured one each to said superimposed structure components and having contacting surfaces arranged to extend in said interface plane. Said first part has a bearing surface on a rigid carrier, said bearing surface being of sintered porous ultra-high molecular weight polyethylene or high molecular weight polyethylene having formed therein an array of fluid lubricant holding pockets disposed at intervals in the direction of, and extending across the surface with respect to, said relative reciprocation of said structure components. Said second part has a substantially smooth co-operating surface arranged in operation to overlie said bearing surface.

Description

<U>Lubricated Sliding Bearing Arrangement</U> This invention relates to sliding bearing arrangements and in particular to structural bearing arrangements employed in civil engineering.

The invention is particularly, but not exclusively, concerned with supporting structures associated with railroads, roads and the like which are mounted relatively stationary with respect to the surrounding terrain but which undergo small motions with respect thereto due to diurnal thermal expansion and contraction and also the periodic passage of heavy vehicles. It is well known for such bearing arrangements to be formed at the interface between structural components, comprising a first part which sits on the effectively stationary component 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 component and comprises a substantially smooth plate that overlies, and rests on the bearing layer. Such bearing construction requires a minimal level of lubrication which is often provided by a lubricant contained in pockets arrayed across the surface of the bearing material.

Significant load on the bearing causes the smooth plate to close the lubricant pockets to loss or contamination of the lubricant and relatively small and infrequent movement associated with the bearing 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.

It will be appreciated that in such civil engineering structures there is a considerable specific loading carried by the bearing material, and the materials and structure of the laminated component must be chosen to accommodate this.

It is known from GB-A1-2156914 to employ such a bearing arrangement in which the bearing material is a polymer having pockets to contain a fluid lubricant in the form of grease which forms a film between the components.

It has been found that where the only motion between the structural parts occurs relatively infrequently, and is of significant extent, for example diurnal thermal effects, any static friction is readily overcome and relative movement occurs between the components by way of the lubricant film. Furthermore the extent of the relative motion in relation to the pocket dimensions and spacing ensures that lubricant is drawn from the pockets and spread across the surface to minimise erosion of the polymer. Such relative movement is referred to herein as diurnal motion.

It is found that where the structural components also undergo relatively small amplitude motion, due to frequent passage of vehicles hereinafter referred to as transient motion, there is less opportunity for the lubricant to be properly distributed, with the result of greater static friction which may manifest itself as non-uniform motion of the components, and leading to both inefficient relief of stresses set up in the component by movement and also more erosive wear of the bearing surface.

This situation has been found to occur even with renowned inherently low-friction polymers such as PTFE, notwithstanding that its strength is not well adapted to high specific loading found in civil engineering structures without itself requiring to be contained.

In accordance with the present invention there is provided a sliding 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, the sliding bearing arrangement comprising 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 fluid 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 substantially smooth co-operating surface arranged in operation to overlie said bearing surface and provide substantially a closure for said lubricant pockets, and characterised in that the first part comprises disposed on the carrier a body of sintered porous ultra high molecular weight polyethylene or high molecular weight polyethylene.

The term "sintered porous" as used in this specification means a cohesive body of the material derived by fusing together particles thereof under conditions of elevated pressure and temperature such that the cohesive body retains a porosity for said fluid lubricant. In this specification the term "contacting surfaces" is used to mean surfaces which are disposed adjacent to each other and load transmitting, notwithstanding the presence therebetween of a film of lubricant.

Preferably the lubricating pockets comprise one or more elongate channels extending across the bearing surface with respect to said direction of relative reciprocation.

The channels may be isolated from each other and separated in said direction of relative reciprocation by such a distance that the interface between contact surfaces is sufficiently lubricated to prevent erosion during diurnal and transient motions. Advantageously the separation is less than the amplitude of such transient motion, but if such motion is of small amplitude this may be impracticable. It is to be expected that a channel separation greater than the transient motion amplitude will permit adequate lubrication when in combination with the larger amplitude diurnal motion and porous nature of the bearing material that permits the lubricant to spread other than being drawn from the channels by the contacting surfaces.

In some circumstances the channel may be separated by a distance which exceeds the amplitude of diurnal motion if the porosity of the bearing material in combination therewith effects sufficient spreading of the lubricant. Most conveniently, the bearing arrangement is arranged in operation to contain is said lubricant pockets a fluid lubricant that is of high viscosity, that is, in the form of a grease, and preferably a silicon grease.

It has been found surprisingly that notwithstanding the particularly low coefficient of friction offered by the ultra high molecular weight polyethylene (hereafter referred to as UHMWPE) and by higher molecular weight polyethylene (hereafter referred to as HMWPE) per se, which suggests their unsuitability for such a bearing, a body of UHMWPE or HMWPE in combination with such lubrication as provided by pockets containing a semi-fluid lubricant grease has a remarkably low coefficient of friction and a low wear rate.

It is believed that this is in part due to the porous nature of the sintered body, the interstices of which become loaded with the lubricant drawn from the pockets over a period of time. The structure and load bearing capacity of the UHMWPE, or HMWPE to a similar but slightly lesser extent, means that the aforementioned interstices are preserved in operation and, significantly is able to be formed as a bearing component without any guarding structure to prevent extrusion under heavy load, resulting in great economic advantage. By way of example exemplary tests comparing UHMWPE and HMWPE respectively with PTFE are described. Examples 1 and 2 compare UHMWPE with PTFE and Examples 3 and 4 compare HMWPE with PTFE.

EXAMPLE 1.

A block of each polymer material was formed having a rectangular surface measuring 60 x 45 mm, in which surface was formed two grooves extending parallel to the shorter edges, each approximately 10mm from respective shorter edges and 35 mm x 4 mm in dimensions. The surface was disposed in a test rig to be fixed therein whilst a flat counterface material was pressed against and reciprocated with respect to the polymer surface.

The counterface material was a stainless steel with a martensitic structure according to material number 1.4057 (X20 Cr Ni 17 2) and hardness 260 HB. The surface of the steel was finished to roughness values of Ra= 0.4 0.05 and arranged to bear on the sample with a specific bearing load of 30 N/mmZ. The grooves were filled with silicon grease.

The test parameter comprised a reciprocation movement (in a direction parallel to the major edge dimension, that is, orthogonal to the lubricant containing groves) of 25 mm in a time of 1.5 sec with a dwell of 6 sec between each movement, giving a distance per cycle of 50 mm at a sliding speed of 0.017 mlsec, or 12 m/hr or 288 m/day.

The UHMWPE exhibited an initial coefficient of friction of approximately 0.2 which after about 8 m sliding distance fell slightly and remained at about 0.15 for a total sliding distance of about 1000 m.

The PTFE exhibited an immeasurably small coefficient of friction for about the first 10 m sliding distance but then increased to 0.15 after about 36 m sliding distance.

After each test the slabs were tested for change in thickness due to wear of the material. The UHMWPE was found to have worn by slightly less than 22pm, representing a wear rate of 25.74 pm/fkm whereas the PTFE was found to have worn by 120 Nm, representing 137.93 pm/fkm. Thus after a relatively short amount of reciprocation, the lubricated UHMWPE exhibited a coefficient of friction similar to PTFE and after prolonged reciprocation exhibited significantly less wear.

EXAMPLE 2.

The materials, test equipment and conditions were the same as for EXAMPLE 1 with the exception of the specific bearing load being increased to 50 NImm2 and 8 sec dwell time between reciprocations.

In the higher load test the UHMWPE exhibited a thickness change of 137 Nmlfkm whereas the PTFE exhibited a thickness change of 286 Nmlfkm..

In addition to the above measurements, the weight change of the samples were measured. Whereas the PTFE sample decreased in weight by some 860 mglfkm the UHMWPE was found to increase by 500 mglfkm, indicating that the lubricant grease, drawn from the grooves by the reciprocation tended to be forced into the interstices of the sintered material and protect the surface generally from wear.

EXAMPLE 3 Other than substituting HMWPE for UHMWPE, the materials, test equipment and conditions were the same as for EXAMPLE 1 with the specific bearing load being 30 Nlmm' and 6 sec dwell time between reciprocations.

The HMWPE exhibited an initial coefficient of friction of approximately 0.1 which increased gradually with sliding distance to reach approximately 0.2 after a total sliding distance of about 1000 m.

After each test the slabs were tested for change in thickness due to wear of the material. The HMWPE was found to have worn by slightly less than 17j.cm, representing a wear rate of 19.4pmlfkm compared with the PTFE representing a wear rate of 137.93Azmlfkm.

EXAMPLE 4 Other than substituting HMWPE for UHMWPE, the materials, test equipment and conditions were the same as for EXAMPLE 2 with the specific bearing load being 50 N/mm' and 8 sec dwell time between reciprocations.

The HMWPE exhibited a thickness change of 159 Nmlfkm compared to the PTFE thickness change of 286 pmlfkm..

In addition to the above measurements, the weight change of the samples were measured. Whereas the PTFE sample decreased in weight by some 860 mg/fkm the HMWPE was found to increase by 450 mg/fkm, indicating that the lubricant grease, drawn from the grooves by the reciprocation tended to be forced into the interstices of the sintered material and protect the surface generally from wear, although to a slightly lesser extent than that for the UHMWPE sample.

Claims

CLAIMS 1. A sliding 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, the sliding bearing arrangement comprising 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 fluid 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 substantially smooth co-operating surface arranged in operation to overlie said bearing surface and provide substantially a closure for said lubricant pockets, and characterised in that the first part comprises disposed on the carrier a body of sintered porous ultra high molecular weight polyethylene or high molecular weight polyethylene.
2. A sliding bearing arrangement as claimed in claim 1 characterised in that the body is formed of ultra high molecular weight polyethylene.
3. A sliding bearing arrangement as claimed in claim 1 or claim 2 characterised in that the lubricating pockets comprise one or more elongate channels extending across the bearing surface with respect to said direction of relative reciprocation.
4. A sliding bearing arrangement as claimed in claim 3 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.
5. A sliding bearing arrangement as claimed in any one of the preceding claims characterised in that the polyethylene is bonded to a porous sintered metallic material formed adhering to the carrier, and the second part comprises a plate of polished stainless steel.