Reduction of failure incidence of metallic diaphragms for compressors

In a compressor which includes an upper and lower head portions and an interposed diaphragm, provisions are made for increasing the service life of the diaphragm by reducing the incidence failure of the diaphragm due to fretting products and airborne contaminants, which collect at the outer edge of the diaphragm which results in stress concentrations at the outer edge of the diaphragm, and specifically by providing a plurality of concentric grooves in the upper head of the compressor to remove the fretting products and gas borne contaminants from build up at the outer edge of the diaphragm.

BACKGROUND OF THE INYENTION 
1. Field of the Invention 
This invention relates to metallic diaphragms for compressors, and more 
particularly to provisions for increasing diaphragm life by reducing the 
failure incidence arising from the results of fretting and stress 
concentrations on the surface of the diaphragms. 
2. Description of the Prior Art 
The use of metallic diaphragms in non-lube diaphragm compressors offers 
many advantages over the use of elastomeric or plastic diaphragms in that 
higher pressures can be attained, higher temperatures can be accommodated, 
and less deformation is experienced. However, metallic diaphragms are more 
prone to failure from stress concentrations, which may occur due to dirt, 
chips or accretion of foreign material such as dust or extremely fine 
particles which become trapped between the diaphragm and the head of the 
compressor. The outer edge of the diaphragm cavity is the region where the 
combined stresses of bending moment and diaphragm stress occur, and where 
most diaphragms fail. Rarely do failures occur towards the center of the 
diaphragm, even though the movement is largest at the center. 
Another phenomenon which appears at the outer edge of the cavity is that of 
fretting, which is caused by the sliding action of the dry metallic 
diaphragm against the dry metallic head. Chemical analysis has shown this 
to be oxide of iron in the case where the materials of construction for 
the head and/or diaphragm contain iron (such as any of the austenitic 
stainless steels, i.e. 304, 301, 316, etc.). This oxide slowly builds up 
to a point where the shape of the cavity is altered, increasing the 
bending moment which ultimately results in diaphragm failure at the outer 
edge. 
It has also been observed that diaphragm failure occurs in the fretted area 
due to small particles of metal being "picked" out of the diaphragm. This 
"picking" action is probably the result of "cold welding" of the diaphragm 
to the head. 
There has been no way yet developed to dramatically reduce or eliminate 
this fretting action, many solutions have been proposed such as hard 
facing the mating parts, plating, nitriding, and surface treatments of the 
heads and diaphragms, to no avail. The problem, of course, is accentuated 
by the presence of dust in the gas, which even with most careful gas 
filtration, still builds up over long periods of time. 
Frankenberg in U.S. Pat. No. 1,712,657 discloses the use of radial grooves 
which provide a path for gas to exit from the head, and also discloses 
using one or more wide concentric stress risers in communication with the 
radial grooves to also distribute the gas pressure, but does not disclose 
grooves adjacent the outer edge of the chamber. The use of wide concentric 
stress risers in a high pressure compressor would result in considerable 
loss of compressor efficiency due to the dead space provided by the wide 
concentric stress risers. 
Bowen in U.S. Pat. No. 3,668,978 discloses a typical compressor head with 
diaphragm for high pressure pumps, but contains no provisions for dealing 
with the problems of fretting and stress concentration. 
My invention provides a storage place for the products of the fretting 
action and normal particles carried by the gas, to be accommodated, 
thereby reducing failure due to stress concentrations, and greatly 
extending diaphragm life. 
SUMMARY OF THE INVENTION 
This invention relates to provisions for reducing the incidence of failure 
of metallic diaphragms due to fretting action and stress concentrations 
and more particularly to the providing of concentric grooves on the upper 
head of the compressor to provide a storage place for the products of the 
fretting action and particles, thereby reducing stress concentration and 
preventing failure of the diaphragm. 
It is the principal object of the invention to make provisions in the 
compressor head so as to reduce failure of the diaphragm due to fretting 
and stress concentrations. 
It is a further object of the invention to make provisions aforesaid which 
is applicable to compressors having flat or curved interior head 
configurations. 
It is a further object of the invention to make provisions aforesaid which 
are applicable to both high and low pressure compressors. 
Other objects and advantageous features of the invention will be apparent 
from the description and claims.

It should, of course, be understood that the description and drawings 
herein are illustrative merely and that various modifications and changes 
can be made in the structure disclosed without departing from the spirit 
of the invention. 
Like numerals refer to like parts throughout the several views. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now more particularly to the drawings and FIGS. 1 to 4 thereof, a 
typical compressor head is shown in FIG. 4 which includes an upper portion 
10 with a curved interior surface 11, a lower portion 12, with a curved 
interior surface 14 and which has an interposed diaphragm 15. The 
diaphragm 15 is illustrated as having a plurality of laminations 15a, 15b 
and 15c. 
The surface 14 provides an oil cavity for the successive application and 
withdrawal of pressure on the underside of the diaphragm 15, with a rim 16 
provided for engagement with the upper face of the diaphragm 15 and a rim 
17 provided for engagement with the bottom face of the diaphragm 15. 
The interior surface 11 provides a gas cavity, with a plurality of small 
radial grooves 18 therein which extend to communicate with a discharge 
hole 19. A suction hole 20 is provided in one of the radial grooves 18. 
The diaphragm 15 is preferably a metallic diaphragm which permits of 
accommodating higher pressures and higher temperatures. The metallic 
diaphragm 15 is prone to failure from stress concentrations, which may 
occur due to dirt, chips or particles or accretion of foreign material 
such as dust or extremely fine particles which tend to collect at the 
outer edge of the diaphragm cavities. This is the region where the 
combined stresses of bending moment and diaphragm stress occurs and where 
most diaphragms fail. 
One of the producers of this foreign material is fretting, which occurs at 
the outer edge of the cavities and is caused by the sliding action of the 
dry metallic diaphragm 15 against the dry metallic head portion 10. 
Chemical analysis has shown this foreign material is an oxide of iron, 
where the materials of constructions for the head portion 10 and the 
diaphragm 15 contain iron, such as austenitic stainless steels. The 
foreign material slowly builds up to a point where the shape of the cavity 
or head portion 10 is altered, which increases bending moment, which 
ultimately results in failure of the diaphragm 15 near its outer edge. 
Another producer of foreign material in the fretted area is due to small 
particles of metal being picked out of the diaphragm 15. This picking 
action is probably the result of cold welding of the diaphragm 15 to the 
head portion 10, and in addition the foreign particles collect near the 
cavity edge and increase the stress on the diaphragm shortening its life 
and causing premature failure. 
While many solutions have been proposed to reduce the fretting action, such 
as hard facing the mating parts, plating, nitriding, surface treatments of 
the head portion 10 and the diaphragm 15, none offers an acceptable 
solution. The problem, of course, is accentuated by the presence of 
contaminants such as dust in the gas which even with the most careful gas 
filtration builds up over long periods of time. 
A solution for the problem is to provide a storage place for the products 
of this fretting action, i.e. the iron oxides and dust and other foreign 
materials to go where they will cause no further trouble. A series of 
small circumferential grooves 25, shown as four in number, is provided in 
portion 10 as shown in FIGS. 1, 2, 3 and 4, for the collection of the iron 
oxides and the dust. 
The grooves 25 may be V-shaped in the preferred embodiment or of a modified 
V-shape, with an included angle in the range from ten (10) degrees to one 
hundred (100) degrees, of a width of from one thousandth (0.001) to 
fifteen thousandths (0.015) of an inch, of a depth of one thousandth 
(0.001) to thirty-five thousandths (0.035) of an inch, and with their 
apices spaced apart in the range from five thousandths (0.005) to thirty 
thousandths (0.030) of an inch, for high pressure machines in the range 
from five thousand (5,000) to forty-five thousand (45,000) pounds per 
square inch. The invention is equally applicable to low pressure 
compressors in the pressure range from one hundred (100) to one thousand 
(1,000) pounds per square inch (not shown). In the low pressure embodiment 
the grooves can also be V-shaped with an included angle in the range from 
ten (10) degrees to one hundred (100) degrees, of a width of from ten 
thousandths (0.010) to thirty thousandths (0.030) of an inch, of a depth 
of from one thousandth (0.001) to thirty-five thousandths (0.035) of an 
inch, with the apices spaced apart in the range from fifteen thousandths 
(0.015) to fifty thousandths (0.050) of an inch. 
Referring now more particularly to FIG. 5, another embodiment of compressor 
is illustrated with a head 100, which has an upper portion 101, with flat 
interior surface 102, a lower portion 103, with a curved interior surface 
104, and interposed diaphragms 105, 105A, and 105B. A rim 106 is provided 
to engage the upper face of diaphragm 105, and a rim 107 to engage the 
bottom face of diaphragm 105B. 
A plurality of small circumferential grooves 108 are provided, similar to 
grooves 25 and which provide a storage place for the products of the 
fretting action and other particles to go without affecting the operation 
of the compressor, and which results in lengthening the life of the 
diaphragm 105. 
It will thus be seen that structure has been provided with which the 
objects of the invention are achieved.