Process and apparatus for coating motor pistons

The coating of the cylindrical periphery of engine pistons with coating compound, in particular low friction materials, is performed by means of screen printing. The pistons can be coated in the vertical position around their vertical central longitudinal axis as they rotate past a screen printing stencil.

FIELD OF THE INVENTION 
The invention relates to a process and an apparatus for coating the 
cylindrical surface of engine pistons or the like with a coating compound, 
in particular low friction materials. 
BACKGROUND OF THE INVENTION 
Coating of piston surfaces with lead, tin, graphite or similar coatings 
having a predetermined thickness used to prevent the seizing of the 
pistons in the event of a failure in oil lubrication in the cylinders of 
an internal combustion engine and also in some cases to reduce engine 
knocking. The coating is applied in varying thicknesses and with varying 
coating patterns to achieve the desired results. 
Hitherto the coating of engine pistons has generally been applied by 
spraying, which is expensive and unsatisfactory because of high losses and 
low efficiency. Furthermore, spraying is detrimental to the environment as 
large quantities of solvents are atomized. 
Since the peripheral surfaces of the pistons are coated using masks when 
spraying, sharp contours of the piston surfaces cannot be reached. In 
addition, the spray mist causes undefinable coating contours under 
spraying masks which are not close together. Furthermore, the thickness of 
the coating, which is determined by adjusting the spray nozzle, normally 
has large tolerances. 
SUMMARY OF THE INVENTION 
An object of the invention is therefore to create a process and an 
apparatus for coating the cylindrical periphery of engine pistons or the 
like with a coating compound without the above-mentioned deficiencies and 
with a relatively simple process and structure. It should also be possible 
to produce the desired coating thickness based on predetermined values at 
any time for different pistons. 
This object is achieved by the invention by applying a coating by means of 
a screen printing process. 
With this screen printing process a uniform coating with minimum tolerances 
can be consistently applied to the cylindrical peripheral surfaces of 
engine pistons or the like, with environmentally damaging emission, such 
as, for example, solvent evaporation, being completely eliminated. With 
this process it is also possible to apply several layers of coating with 
the same or different thicknesses on top of one another in one process 
sequence. If the viscosity of the coating compound remains constant, the 
thickness of the coating on the peripheral surface of the piston can be 
predetermined via the characteristics of the woven cloth of a screen 
printing stencil used in the process. 
With the process specified by the invention one or several coating-free 
areas can also advantageously be provided in a coated surface for 
lubrication bore reliefs and/or as observation ports in the form of slits, 
holes, recesses, strips or such configurations. Such free areas perform a 
double function. Firstly, they serve as measurement holes or observation 
ports for the measurement and consequently the monitoring of the coating 
thickness and secondly they improve the lubrication of the piston in 
normal operation, as they act as grease bearings. In contrast to known 
spraying methods, in the screen printing process specified by the 
invention, such free areas can simply and without great expenditure be 
produced in any size and form with very precise thickness and edge 
contours. This is the case especially for very small coating free areas. 
With the process specified by the invention the pistons are expediently 
coated in an upright position and around their vertical central 
longitudinal axis while they rotate past a printing stencil while abutting 
the stencil, as the screen printing stencil is also moved in the direction 
of rotation of the peripheral surface of the piston. 
Of course, it is also possible to place the pistons in a horizontal 
position during coating as, for example, in experiments and with small 
loads. 
The coating can also be advantageously applied in a wedge shape with the 
coating thickness decreasing at its lateral and/or upper and lower edges. 
As a result of this method, hard edges or steps are avoided and the coated 
surface is smoother. In this way a better adaptation to the rounded shape 
of the piston is achieved with a correspondingly reduction in friction. 
This is true in particular for the lateral edges, but this is also 
advantageous for the upper and lower edge with respect to the direction of 
movement of the piston. The coating compound can be brought to the screen 
printing stencil of the screen printing unit while it is continually 
rotated and mixed together. 
The invention also relates to an apparatus for performing the screen 
printing coating process, which is characterized in that it comprises one 
or more screen printing stations each having a screen printing stencil and 
an applicator blade. The screen printing stencil with which the pistons 
are to be coated can be moved with the piston's cylindrical peripheral 
surface to be coated. The pistons, contained in receptacles, can be 
rotated around their central longitudinal axes, with the printing stencil 
tangentially aligned thereto and without slipping in the direction of 
rotation of the peripheral surface of the piston. In addition, an 
applicator blade with its blade edge extending parallel to the central 
longitudinal axis of the piston to be coated is positioned opposite the 
peripheral surface of the piston abutting the screen printing stencil 
during the printing operation. Alternatively, the pistons can be 
positioned so that they are upright or lying horizontally. 
In one configuration the screen printing stencil and the applicator blade 
extend vertically and the pistons are disposed vertically in the 
receptacles. 
The hardness of the blades can also be adapted to the required thickness of 
the coating to be applied. If there are several screen printing stations 
located one behind the other in the path of the pistons to be coated, the 
blades of these stations can have the same or different hardness depending 
upon the application.

DETAILED DESCRIPTION OF THE INVENTION 
In the preferred embodiment represented in the drawings, the pistons are 
supplied by a continuously rotating conveyor belt 15 with their central 
longitudinal axis X vertical then are sequentially removed from the 
conveyor belt by a rotating head 16 having tongs 17, which grasp the 
pistons 1 in their grooves with jaws (not shown), and are transferred 
parallel to one another via a epicyclic gear system 9, 12A, 12B, 14A, 14B 
to position controlled receptacles 18, which are disposed on a rotary 
table 13. The pistons 1 are held in these receptacles 18 so that they 
cannot rotate by upwardly projecting pins 6, on to which the pistons 1 are 
placed, and by rotating the rotating table 13 they are brought up one 
after the other by a step device to three screen printing stations A, B 
and C positioned behind one another in the path of the pistons, in which 
stations they receive the predetermined coating Al, B1, Cl on their 
cylindrical surface one after the other. The coatings are applied by means 
of screen printing stencils 2, which by means of applicator blades 10 come 
to tangentially abut the peripheral surface of the pistons to be coated. 
During the printing operation the pistons together with their receptacles 
18 rotate around their central longitudinal axis X (FIGS. 3 and 5), in 
which the printing stencil 2 can be aligned, without slipping, in the 
direction of rotation of the peripheral surface of the pistons 
tangentially thereto. The arrows in FIG. 2 shows the direction of rotation 
and direction of movement of the peripheral surface of the pistons and of 
the screen printing stencil 2 during the printing operation. During this 
operation the applicator blade remains with its blade edge extending 
parallel to the central longitudinal axis X of the piston 1 to be coated 
opposite the peripheral surface of the piston abutting the screen printing 
stencil and at the same time the coating compound is supplied to the upper 
region of the screen printing stencil via a hose line 8 and applied 
through the stencil to the peripheral surface of the pistons by applicator 
blades 10. 
Just one part of the peripheral surface of the piston can be coated, 
depending on the design of the screen printing stencil. In the exemplified 
embodiment shown in FIGS. 3 to 5, the head of the piston 1 is provided 
with a circular coating C3 by the screen printing station C and in the 
region of the periphery of the piston on the sides adjacent to the piston 
pin bore 3 with coatings for surfaces Al, B1, Cl by the coating stations 
A, B, C. 
The coated surfaces shown in the exemplified embodiment are given as 
examples. In many cases fewer coated surfaces are adequate. Thus, for 
example, only two lower coated surfaces Al may be provided, which are 
located opposite one another and extend over a peripheral region of 
approximately 90.degree. . The two coated surfaces Al may be provided as 
buffers, for example, for the reduction of engine knock and with a coating 
thickness of 10 .mu.m. 
Two coated surfaces Cl, which are also located opposite one another and are 
mutually spaced and disposed parallel to coated surfaces Al and also 
possibly the coated surfaces B1 may be provided in particular for 
emergency lubrication in the event of the failure of the engine's 
lubrication system. Coated surfaces Cl (and possibly B1) may also extend 
over a peripheral region of 90.degree. and may have a thickness of 20 
.mu.m, for example. 
As can be seen from FIG. 5, the coated surfaces Al, B1 and Cl form a wedge 
shape 4 at their edges. 
As shown in FIG. 3 and the enlarged representation in FIG. 4, the coated 
surfaces can be provided with small uncoated free areas 5. For this 
purpose rectangular, ellipsoidal and round recesses 5, for example, are 
provided in the coated surfaces Al and Cl. Of course further coated 
surfaces having corresponding coating free areas may be provided, for 
example, in the form of holes, slits, strips, ellipses, ovals, circles or 
similar shapes. 
Graphite powder having a resinous binding agent or a resin is preferably 
used as the coating compound. The following proportions for the mixture 
have proved successful: 
Graphite 30-50% by weight, preferably 40% 
Resin 30-50% by weight, preferably 60%. 
The viscosity of the coating compound should preferably be within a range 
from 6,000-8,500 cp. Very good results are achieved with a viscosity of 
approximately 7,000 cp. 
To regulate or, if necessary, change the viscosity, the use of isopropanol 
or ethylglycol have proved successful. 
Excess coating compound running off the screen printing stencil is 
collected in a storage vat lI located under the stencil and is supplied by 
means of a pump 7 via a hose line 8 while being continually mixed back to 
the screen printing stencil. By mixing and continuous supplying the screen 
printing compound it is possible to prevent the tendency of the coating 
compound to change its viscosity by decomposition. 
After the end of the coating application, the blade 10 is withdrawn from 
the screen printing stencil 2, and thus loses contact with the coated 
peripheral surface of the piston so that the screen printing stencil can 
be withdrawn from contact with the piston into its tangential initial 
position. 
After the coating of the pistons has been effected in the three screen 
printing stations A, B and C, they are returned to the rotary head 16 and 
removed via the tongs 17 from the receptacles 18 and conveyed on the 
conveyor belt 15 to be transported to another location.