Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Agitating or moving electrolyte during coating
Reexamination Certificate
2001-02-08
2002-11-26
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Agitating or moving electrolyte during coating
C205S170000
Reexamination Certificate
active
06485627
ABSTRACT:
The present invention relates to a method and apparatus particularly, though not exclusively, for the electro-deposition of metal coatings on plain bearings.
Plain bearings such as are used as crankshaft bearings in internal combustion engines for example, are generally semi-cylindrical in form, a pair of bearings forming a cylinder around each shaft journal usually being used. Each individual semi-cylindrical or half bearing generally comprises two or more layers including: a strong backing material such as steel for example; a lining of a bearing alloy such as an aluminium or copper alloy for example; and, frequently a so-called overlay coating of a softer metal alloy based on alloys of lead, tin, cadmium or zinc for example on top of the bearing alloy lining to provide bearing properties such as conformability and dirt embeddability for example.
Presently, the most commonly used method of providing the overlay coating is by electroplating from solutions containing ions of the required metals in appropriate concentrations in conjunction with either an inert or a consumable anode of the appropriate alloy. In the prior art plating processes, pairs of half bearings are held in stacks in jigs which allow free access of plating solution to the bearings. The jigs then being immersed in open tanks of various chemicals appropriate to the plating process being carried out. A typical prior art plating process may comprise the steps of: initially immersing the jigged bearings in a tank of cleaning fluid; transferring the cleaned bearings in their jig to a water wash tank; immersing the washed bearings in an acid etch tank; transferring the etched bearings to a water wash tank; transferring the washed bearings to a nickel plating tank; transferring the nickel plated bearings to a water wash tank; transferring the washed bearings to a tank for the electro-deposition of an alloy overlay coating; and finally transferring the overlay coated bearings to a water wash tank. Generally the transfer stages between tanks are effected by overhead cranes or conveyors and result in the fluid from one tank draining out of the jig as it is being raised with consequent splashing and risk of contamination and injury to the people operating the process. It must be born in mind that many of the cleaning, etching and plating solutions used comprise extremely toxic and harmful chemicals which, in addition to the danger posed by direct contact with people, may also produce harmful fumes.
A further disadvantage of the prior art process is that much of the time from initially jigging the bearings to removing the plated bearings from the jig is taken up in the transferring from tank to tank and in allowing time for each solution to drain as much as possible to minimise cross contamination between tanks.
A further disadvantage of conventional tank plating systems is that although the plating solutions in the tank are generally stirred, the current density able to be employed without “burning” of the overlay deposit is relatively low leading to relatively long plating times.
GB-A-2 181 744 shows a method of plating an internal surface of a hollow object by providing an internal anode and filling the internal volume of the object with a plating solution. However, the plating solution is only changed if it becomes exhausted and no indication of process conditions are given.
An object of the present invention is to provide a method and apparatus for the electroplating particularly, though not exclusively, of plain bearings, the method and apparatus being less hazardous to people than existing methods and apparatus.
A further object of the present invention is to provide a method and apparatus for the electroplating of plain bearings which is more economic and faster than known methods and apparatus.
Other objects of the present invention will become apparent from the disclosure of the present invention hereinbelow.
According to a first aspect of the present invention, a method for electroplating of a metal coating of at least a part of the surface area of an article includes the steps of: placing the article in a vessel, the vessel being provided with means to allow access of fluid to an interior volume thereof in which volume said article is contained and, means to allow egress of said fluid; providing said vessel with anode and cathode means such as to enable said article to become cathodic with regard to an anode extending into said interior volume; providing means to cause at least two different fluids including at least one plating solution to be introduced into said vessel in sequence, flow through said interior volume and to exit therefrom for a sufficient time to allow a required thickness of said metal coating to be deposited, said at least one plating solution flowing continuously through said vessel during deposition of said metal.
In the case of overlay plating of half bearings for example, the method may typically comprise the steps of clamping the half bearings together in either a generally cylindrical or semi-cylindrical stack for example within the interior volume of the plating vessel; passing a cleaning fluid through the vessel and past at least the bores of the bearings; then pumping water through the vessel to remove the cleaning fluid; next acid to etch the surface on which the overlay is to be deposited; next water to remove residual acid and wash the bearings; next a plating solution to deposit a thin layer of a so-called interlayer such as nickel or copper onto the bearing surface to act as a diffusion barrier between the bearing lining alloy and the overlay; next water to remove the residual interlayer plating solution; next overlay plating solution to deposit the overlay alloy per se; and, finally water to flush away residual overlay plating solution.
It should be emphasised that the above plating sequence is merely exemplary and that more, different or fewer fluid passing steps may be employed depending upon the specific bearing alloy being plated, the overlay alloy composition and the resulting bearing structure which is desired.
In the case of cylindrical or semi-cylindrical articles such as bearings for example, the anode has an elongate form extending into the plating vessel and may be generally coaxial with the bearing bore axis or vessel axis. It has been found that the method of the present invention is inherently a factor of at least about two or three times faster than in the conventional plating process where the jigged bearings are moved from tank to tank due to the higher current densities which may be employed allowing deposition of a given amount of metal in less time. However, it has been found that the speed of the electroplating deposition step per se may be further significantly increased by providing means to enable the anode surface to move such as by rotating, oscillating or reciprocating motion for example.
In one embodiment of the method of the present invention, the anode is provided with a paddle wheel at a position adjacent the fluid inlet means such that the fluid flow itself causes the anode to rotate. The higher the fluid flow rate, the faster the rate of rotation of the anode.
In another embodiment of the method and apparatus according to the present invention, the anode is driven by means of an electric motor thus, allowing independent control of rate of anode surface movement and fluid flow rate.
The use of high plating fluid flow rates such as in the range from about 15 to 40 l/min together with the use of a moving anode surface has enabled higher current densities in the range from about 5 to about 100 A/dm
2
for any given step in the plating process to be employed thus producing rapid plating and an overall increase in the speed of the plating process by a factor of four or five. In the prior art plating processes, current densities were limited to about 7 A/dm
2
otherwise “burning” of the plated coating occurred as evidenced by black sooty deposits forming on the surface. It is envisaged that flow rates of up to about 160 l/min may eventually be emplo
Carey John
Johal Charan Preet Singh
Dana Corporation
Wong Edna
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