US 3289556 A
Description (OCR text may contain errors)
Dec. 6, 1966 c. D. RUSSELL 3,289,556
MANHOLE FRAME AND COVER Filed April 14, 1964 INVENTOR C layfon D. Russe!!! Attorney United States Patent 3,289,556 MANHOLE FRAME AND COVER Clayton D. Russell, Piedmont, Calif., assignor to Phoenix Iron Works, a corporation of California Filed Apr. 14, 1964, Ser. No. 359,646 1 Claim. (Cl. 941-34) This invention relates to manholes and covers such as those used in connection with the distribution of utilities, sewage, drainage systems, and the like. They are utilized to provide access to underground vaults and since the latter are often located under roadways over which heavy traffic must travel, the construction of the manhole frame and cover must be such that the cover would be easily removable but at the same time strong enough to withstand highly concentrated loads.
In addition to the foregoing the covers should be able to withstand the expansion and contraction caused by changes in temperature, but at the same time the engagement between the frame and cover should be such that dirt and other foreign matter would not be objectionably lodged there-between. A further problem is encountered in the case of loose fitting covers which vibrate and rattle when trafc passes over them. In addition to the inherent mechanical weakness under such conditions, the noise occasioned is very objectionable in residential areas, particularly at night.
Various types of construction have been resorted to in the past to overcome these problems. These have comprised the utilization of resilient cushions or shims, selflocking slots or lugs between frame and cover, axial serration in the cover, two-part rectangular frame and triangular cover construction employing three point support, etc. None of these have completely solved all the problems enumerated above and in addition have created problems of their own and in almost every case are expensive to manufacture and maintain.
It is therefore an object of my invention to provide a manhole frame and cover in which the cover would be many times stronger than those now in use and still be light enough for easy removal and replacement.
It is another object of my invention to provide a manhole frame and cover which would be reasonably impervious to dirt and water and still not be adversely affected by thermal expansion and contraction.
It is still another object of my invention to provide a manhole frame and cover in which the cover would not rattle and create objectionable noise when subject to surface traffic.
It is finally an object of my invention to provide a manhole frame and cover which would possess all the foregoing advantages and still be economical to manufacture and maintain.
I have discovered that by utilizing a cover of solid construction, having my particular conliguration, and adapting it in the manner and in the dimensional relationship described below, I am able to achieve the desired results. In particular, and especially with regard to the strength of the cover, I have discovered that the use of reinforcing ribs on the under surface, as is now the case with most existing covers, adds nothing to the strength of the cover.
These covers are customarily made of cast iron which is known to have a low tensile strength and because of their shape and method of support it is diicult to design them using customary methods of engineering stress analysis. The use of reinforcing ribs would seem to be a logical and natural expedient, but I have discovered through experimentation that this is erroneous and that a solid cover having a curved inner surface approximating that of a sphere, or other solid of revolution, is preferable, and
in addition to the greater strength it may actually be made lighter in weight, contrary to previous belief.
I have discovered further that when utilizing a cover of my construction I am able to make the cover thinner and by further adapting it to t within the frame in the novel manner which I describe below, its strength is actually augmented and I am able to obtain a dimensional relationship between cover and frame so that I obtain tight or snug lit between these two, which is highly desirable, without encountering expansion and contraction diiculties.
This will become evident to those skilled in the art from the description and figures which follow in which:
FIG. l is a top view of the cover of my invention.
FIG. 2 is a cross-section along lines 2--2.
FIG. 3 is a longitudinal section through the manhole frame.
The top surface 1 of the cover is characterized by a diamond shaped tread for skid resistance purposes which is typical of many conventional manhole covers. The pick holes 2 are conveniently located through the cover to facilitate removal and replacement. I have discovered that if I locate these about 31/2 inches from the outer edge and have their axes inclined at approximately 45, I eliminate the possibility of foreign matter in the form of long rods from falling into the bottom of the vault. The external inclined peripheral surface of the cover is shown at 3, while the lower horizontal machined surface is shown at 5. The bottom surface of the cover 4 is generally spherical in geometry, although it may also be a part of a paraboloid or other solid of revolution. When a spherical surface is used I may make the diameter of the sphere approximately four times the diameter of the cover, which is indicated at small d. The thickness of the cover at the outer periphery is indicated at T-l, while the thickness at the center is indicated at T-Z on the drawings which will be explained below.
The upper horizontal surface of the frame is machined as is the lower horizontal surface 7 to form a ledge as shown. The internal incline surface is shown at 8. The inside diameter of the frame at the top of the ledge is indicated as D-l, while the outside diameter is indicated as D-2 and the thickness of the shell at this point is indicated as t. The inside diameter of the manhole opening, or what is known as the nominal diameter of the manhole, is shown as D. The significance of this lettering will also be explained below.
When the cover is in position the cover machined surface 9 will be llush with machined frame surface 6, while machined surfaces 3 and 5 of the cover will rest snugly against machined surfaces 8 and 7 of the frame.
In order to obtain maximum strength, minimum weight of cover, tightness, and freedom from expansion and contraction problems, I have discovered that certain relations must exist between these lettered dimensions. These have een arrived at through prolonged experimentation and verified by testing as set out in the charts below. First, considering the expansion requirements for a given nominal diameter D, there will be a corresponding diameter D-l and D-2 which -deiine the thickness t of the shell at the top of the ledge. For a given temperature rise, t will become larger, thereby tending to decrease D-l, but the circumference of the frame will tend to get larger also thereby tending to increase D-l. On the other hand, the diameter of the cover d will tend to get larger since the circumference of the cover will also increase. I have discovered that if I maintain the value of t equal to approximately two percent of the value of D, these expansions will tend to balance each other within a wide range of manhole sizes thereby maintaining a snug tit between the cover and frame under all normal conditions. This is important not only from the standpoint of tightness but also from the standpoint of strength, since the snug t tends to stilfen the cover.
I have discovered further that if I make T-l equal to five percent of D and T-2 approximately equal to seven percent of D, I obtain a condition of maximum strength of cover utilizing a minimum amount of material. I have found also that the optimum angle of inclination of the inclined surfaces 3 and 8 to the vertical is elevenand one-half degrees.
Since these dimensions may vary about one-sixteenth of an inch from the ideal given above, the dimensions of a typical, standard manhole frame and cover for a twentyfour inch opening would be as follows:
D equals twenty-four inches D-l equals twenty-ve and three-eighths inches D-2 equals twenty-six and one-quarter inches tequals seven-sixteenths of an inch d equals twenty-five and ve-sixteenths inches T-l equals one and one-eighth inches T -2 equals one `and three-quarters inches Silicon 2.29
Manganese .56 Graphitic carbon 2.86 Combined carbon .63
Phosphorus .336 Sulphur .114 Total carbon 3.49
(The above corresponds to ASTM Spec. A 48-60 T, Class 30) A summary of the test results in a typical test series comprising over forty test runs is given below:
Weight of Breaking Sample No. Type of Construction Cover, Load,
Pounds Pounds 200 43, 000 200 44, 515 210 48, 000 170 117, 500 173 120,000 plus 177 120,000 plus The plus in the above results indicates that this was the capacity of the machine and the cover did not fail under maximum loads produced by the machine.
It is evident from the foregoing that the improvement realized is not just an improvement in degree but is of the order of magnitude of three so far as strength is concerned, while the total amount of material used with my construction was considerably less in each case.
It will be evident to those skilled in the art that minor changes may be made in the disclosure above without departing from the method of my invention and I do not limit myself to the foregoing disclosure, except as I do so in the claim which follows.
A manhole frame and cover comprising:
a cast iron frame of generally cylindrical configuration;
bearing surfaces machined on the upper circumferen tial edge of said frame to form an `annular seating ledge therein,
said ledge comprising upper and lower horizontal surfaces and an internal inclined surface joining said horizontal surfaces and forming a predetermined angle of inclination with the vertical;
a cast iron cover of generally cylindrical configuration;
bearing surfaces machined 0n the outer periphery of said cover,
said bearing surfaces comprising a lower horizontal surface for engagement with said lower horizontal surface of said frame and an external inclined surface having a predetermined angle of inclination to the vertical for engagement with said internal inclined surface of said frame; pick holes through said cover to facilitate removal of said cover,
said pick holes being located relatively close to the periphery of said cover; said pick holes being further characterized by having their axes form an angle of approximately forty-five degrees with the surface of said cover.
References Cited by the Examiner UNITED STATES PATENTS 1,926,595 9/1933 Mulvihill 94-35 FOREIGN PATENTS 59,727 2/1954 France. 120,752 6/ 1927 Switzerland.
OTHER REFERENCES Neenah Construction Castings, Catalog R 4th Ed., copyright 1963, page 34 relied on.
CHARLES E. OCONNELL, Primary Examiner.
JACOB L. NACKENOFF, Examiner.
N. C. BYERS, Assistant Examiner.