Patent Application
20080233296
The present invention relates to parts having a wear resistant coating applied to them and more particularly relates to a method of applying a wear resistant coating to a sheet metal part, such as a forage harvester blower liner.
Many parts are subject to severe service conditions which result in the part becoming unduly worn if steps are not taken to increase their wear life. An example of such a part is the blower liner of a forage harvester, for example. Forage harvester blower liners are:
The exposure to these three conditions call for a liner that has high hardness and one that is friction wear, corrosion and impact resistant. Also, the liner is required to have a smooth working surface to allow low friction flow of forage so that the forage harvester operates in an efficient manner. Finding a liner material that meets all of these criteria is very difficult. The next best solution is to find a coating for the liner surface which meets these criteria.
Examples of coating applied to blower liners using hardfacing methodology are well known. Even though hardfacing has been successful in increasing wear life, the materials are costly, the processes for applying hardfacing are labor intensive and too slow for production applications, and the resulting liner surface is rough and undulating, which is not ideal for efficient forage flow. Also, the known processes of applying hardfacing make it difficult to control the uniformity of the coating thickness.
Thermal sprayed coatings have been found to fail prematurely in service due to spalling and/or have been found to be too expensive to produce due to high material and/or processing costs. Further, it has been found that thermal sprayed coatings could not be applied thick enough (>2.0 mm) to extend the surface wear life since an increase in thickness also increased cohesive and adhesive stresses causing failure. Sprayed coatings can be applied to gradually build up the thickness, but this requires the substrate to be re-heated with each coating application to gradually build up the green coating thickness, with such re-heating and spraying steps adding to the cost and time of producing the coated part.
An example of a coating which has the attributes desired for use in a blower liner application is a slurry coating of highly alloyed powder like that disclosed in U.S. Pat. No. 5,879,743. Specifically, a slurry made in accordance with this patent is prepared by thoroughly mixing a powdered, hardfacing alloy with a PVA binder solution to give the desired alloy-to-binder solution weight ratio. The slurry compositions described in the patent are designed by an eight-digit code. For example, for a “0550/07550” slurry, the first four digits indicate a 5.5 to 1 weight ratio of powdered alloy to PVA solution, and the last four digits indicate a 7.5% (by weight) aqueous solution of PVA as a binder. In this designation, the decimal point is assumed to occur in the middle of each four digit group. Likewise, “1075/1025” means a ratio of alloy to PVA of 10.75 to 1, and the aqueous solution of PVA is 10.25% PVA, by weight, in water.
Thus, the problem to be solved is that of providing an economical process for applying a protective coating to a sheet metal substrate, such as a forage harvester blower liner, which has the characteristics of good friction wear, corrosion and impact resistance while producing a very smooth surface.
According to the present invention, there is provided an improved process for applying a protective coating, having the aforementioned desired characteristics, to a sheet metal substrate.
An object of the invention is to apply a relatively thick (>2 mm) slurry coating to a sheet metal forage blower housing liner such that the coating is applied in a single step after first preparing the liner for receiving the coating and to thereafter cure the coating.
Referring now to
The forage harvester 10 includes a main frame 12 supported on front and rear pairs of wheels 14 and 16, of which only one of each pair is shown. Located at an upper forward location on the frame 12 is an operator's cab 18, which contains all of the controls (not shown) for the harvester. Mounted to the forward end of the frame 12 is a row crop harvesting head 20 which is provided for severing crop, such as corn, or the like, from the ground and directing it to be chopped into forage by knives 21 (FIG. 2—only one shown for the sake of simplicity) of a transverse cylinder cutterhead 22. Chopped forage is delivered rearwardly by the cutterhead 22 to an impeller or blower assembly 24, which, in turn, delivers the chopped forage upwardly into a discharge chute or spout 26 which directs the forage to a container (not shown) of a transport vehicle.
Referring now also to
It is noted that chopped crop exits the cutterhead 22 along a path extending tangentially from the bottom of the cutterhead 22 and tangentially into a lower location of the blower housing 28. Upon being driven counterclockwise, the blower impeller 36 engages crop entering the housing 28 and carries it through an angle of approximately 90° where it expels the crop into the outlet conduit 34.
The blower housing 28 includes a circumferential wall section 42, which extends between opposite side walls of the housing 28, and from the inlet to the outlet of the housing 28. The location of the wall section 42 of the housing 28 results in it being subject to high impact loading due to any hard particles such as sand, rocks and/or tramp metal that may be entrained in the stream of crop ejected from the cutterhead 22. The wall section 42 includes a curved steel plate 44 having a central surface area to which is bonded a hardfacing material 46. An uncoated border 48 surrounds this central area and overlaps a bottom side of the transfer housing 30 and a rear side of the outlet conduit 34. A bottom section of the border 48 is fixed to a hinge 50, while a top section of the border is secured to the outlet conduit by appropriate fasteners (not shown). Thus, by removing these fasteners, the housing wall section 42 may be pivoted to an open position permitting access to the blower impeller 36 for service, or for the purpose of cleaning out clogged material, for example.
Referring now to
Once the slurry is placed in the cavity 54, a striking device, having a straight edge which spans the width of the plate 44, is used to strike off the slurry so as to remove excess slurry and so as to produce a uniform slurry thickness corresponding to the height of the magnetic tape 52. The striking operation should be done carefully not to produce any cracks in the green coating. Specifically, during the striking step, striker oscillations should be kept small and the progression of the striker should be slow, otherwise cracks may result. In order to avoid cracking in the coating after fusion, the striking process should be completed a soon as possible after the slurry is poured on the plate and while the slurry is till wet, i.e., before onset of the curing process of the binder.
A robot or other automated device could be used to perform the striking operation. The straight edge could be constructed as part of a component which is spring mounted so that the straight edge may be oriented parallel to one end of the metal plate 56 and brought into contact with the magnetic tape 52. The robot would be operated to move the component embodying the straight edge about a radius of curvature which approximates the curvature of the plate 56, with the spring mounting of the component maintaining the straight edge in tight engagement with the magnetic tape running along the sides of the plate 56. In addition, the robot could be programmed to undergo a slight oscillation as it progresses along the plate 56.
Once the slurry is in place, the slurry coated plate 44, together with the magnetic tape 52 is placed in a curing oven at a temperature of about 200° F. for 10-15 minutes.
The plate 44 is then removed from the oven and the magnetic tape is removed from the plate 44 leaving a partially cured coating 58. The plate 44, together with the coating 56 are then placed in the oven and heated at 200° F. for at least one hour.
To fuse the coating 58 to the plate 44, the plate 44 and coating 58 are placed in an oven in a hydrogen atmosphere and heated to a temperature of 2000-2025° F. The coating 58 then becomes the layer of hardfacing 46, as shown in
Referring now to
Instead of the fixture assembly 60, other fixture assemblies could be used such as one which employees releasable clamps for fixing the border member 66 to the plate 44, for example.
Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.
