Bearings – Rotary bearing – Plain bearing
Reexamination Certificate
2000-08-21
2002-05-21
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Plain bearing
C384S300000, C384S908000, C029S898055
Reexamination Certificate
active
06390682
ABSTRACT:
This invention is concerned with plain bearings, in particular bearings which require to operate with (high) loading levels and low viscosity liquid lubricant.
In particularly heavily loaded bearing conditions wherein a low viscosity lubricant is used, as may be experienced for example in vehicle suspension parts such as dampers, and/or where fluid flows are severe, such as gear pumps, there is a need to provide a bearing surfaced with a lining material that exhibits both fatigue strength and resistance to erosion by such liquid.
Traditionally such bearings have been developed with lining materials of one or more layers of metal alloys chosen for their physical characteristics within the operating environment, but there is developing a trend to avoid using bearing metals as bearing linings, because of both cost and environment factors, and to use polymer based materials as bearing linings.
Plain bearings are known in which a backing strip of steel or comparable metal has formed thereon a lining layer comprising a porous metal matrix of sintered bronze and a low friction polymer-based material that incorporates fillers, hereinafter referred to as a filled polymer, infiltrated into the interstices of the sintered matrix forming a relatively thin overlying skin, or polymer layer.
It is known to infiltrate the low friction polymer-based material as a “mush” with an organic lubricant which is pressed into the interstices of the bronze matrix and heated to drive off volatile components and solidify it by a sintering action. It is also known to infiltrate the polymer-based material as a so-called “dry paper”.
Such backed, infiltrated sintered bronze bearings are used both in oil lubricated bearings and also in dry bearings, wherein there is minimal lubricating fluid.
Polytetrafluoroethylene, hereafter referred to as “PTFE”, has good tribological properties and is often used as, or in, a lining material for plain bearings as the main polymer or “base” polymer on which any mixture is based; however, PTFE is soft and weak and has an unacceptably high wear rate when used alone and for practicability requires the addition of so-called wear resistant fillers and/or other materials which reinforce the lining material against rapid wear and low load bearing strength.
Whereas in a dry or marginally lubricated bearing, and to a lesser extent an oil lubricated bearing, wear occurs as rubbing erosion which is directly responsible for removal of the filled PTFE, the rate of removal being determined by the roughness of the surface of the body borne by the bearing and thus the dynamic and break-away friction between the body and bearing, in an oil lubricated bearing, wear results from the lubricating oil bringing about cavitation erosion of the filled PTFE.
However, where fatigue strength and erosion resistance are important, polymer compositions which have been developed for improving either one of these properties have been found to exhibit a worsening of the other property. That is, the polymer compositions, PTFE-based or otherwise, developed for increased strength have been accompanied by reduction in their frictional abilities, i.e., an increased coefficient of friction and lowered erosion resistance, and vice versa.
Patent specification number GB-A-2166142 describes a PTFE-based polymer lined bearing exhibiting enhanced erosion resistant properties by virtue of a filler of an ionic fluoride, particularly a finely divided form such as calcium fluoride.
Patent specification No. GB-A-2279998 (WO95/02772) the contents of which are incorporated by reference, describes a plain bearing of the filled PTFE infiltrated sintered bronze type that is specifically intended as an oil lubricated bearing. The specification summarises the perceived situation in the art regarding reinforcement and filler materials that provide strength and wear resistance, namely stating that the use of “smooth” filamentary materials such as glass and aramid fibres per se that do not adhere to PTFE should be considered as unsuitable, before concentrating on the suitability of fibrillated aramid filaments (hereafter also referred to as aramid “fibres”) and in addition thereto particulate ionic fluoride fillers to achieve a lining material having improved wear against cavitation erosion. It is found in practice that notwithstanding a wide range of variation in the amount of aramid fibres that provide a useful product, there is in addition to the aforementioned conflict of properties, restrictions brought about by employing such aramid fibres at the upper end of the range as the mush has a tendency to become tough and intractable, making it difficult to spread in other than a thick overlay.
It is an object of the present invention to provide a plain bearing that has a lining layer of reinforced, filled PTFE-based polymer infiltrated into a sintered metal matrix and has a higher fatigue strength and liquid erosion resistance better than hitherto, and a method of manufacturing such a bearing.
According to a first aspect of the present invention a plain bearing comprises a metallic backing, a sintered porous metal layer bonded to the backing and a lining layer infiltrated into the pores of the porous metal layer and overlying the sintered metal layer, said lining layer comprising PTFE containing 10-30% by vol. particulate wear-resistant filler, 2-10% by vol. fibrillated aramid fibres and 2-10% by vol. of a melt processable fluoropolymer.
Preferably the melt processable fluoropolymer is a copolymer of the group tetraflouoethylene-hexafluoropropylene (referred to herein as FEP), tetraflouoethylene-perfluoroalkylvinylether (referred to herein as PFA) and monofluoroalkoxy (referred to herein as MFA). More preferably the melt processable fluoropolymer is a copolymer of said FEP.
Preferably, the melt processable fluoropolymer is present in the lining layer to an amount of 5 to 8% by vol., and more preferably to an amount of 7% by vol. Furthermore, it is preferred that the fibrillated aramid fibre is present in the lining layer in an amount at least 50% by vol. of the melt processable fluoropolymer.
The filler may be any particulate filler known in the art as providing wear resistance in polymer bearing materials. Conveniently, it comprises principally or wholly an ionic fluoride, such as CaF2, MgF2, or SrF2. preferably the inorganic filler is present in the lining layer to an amount in the range 15-20% by vol.
The fibrillated aramid fibres preferably have an average length in the range 0.2 to 1.0 mm and are present in the lining layer to an amount of 3 to 5% by vol.
In respect of the fibrillated aramid fibres and general method of manufacture the invention follows the aforementioned Patent specification No GB-A-2279998, namely the average length of the aramid fibres used in the invention may be 0.2 to 1.0 mm, their diameter prior to fibrillation may be 0.012 mm-0.015 mm, and the degree of fibrillation (as measured as Canadian Freeness) may be 200 (Du Pont method TM 0894-84, Reference TAPPI-T-227M-58).
In respect of the manufacture, and according to a second aspect of the present invention, a method of manufacturing a plain bearing comprises forming a laminar metallic backing strip, bonding thereto by sintering a porous metal layer, forming a mush of fibre-reinforced, filled, PTFE-based bearing material in a volatile lubricant, spreading the mush onto the porous metal layer, pressing the mush to infiltrate a part thereof into the porous metal layer and leave a lining layer overlying the porous metal layer, heating the combination to a first elevated temperature to remove volatile liquid components and a second elevated temperature to effect sintering of the PTFE component and cohesion of the bearing polymer, preparing the mush by mixing together:
(1) an aqueous slurry of particulate, wear resistant filler containing 10-30% by volume solid, (2) an aqueous dispersion of fibrillated aramid fibre containing 2-10% by volume solids, (3) an aqueous solution of melt processable fluoropolymer containing 2-10% by solids, (4) the remainder being an a
Johnston Janette
McMeekin Kenneth Macleod
Glacier Garlock Bearings Inc.
Hannon Thomas R.
Harrington John M.
Kilpatrick & Stockton LLP
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