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By B. R. LEFFLER Bridge Engineer, New York Central Railroad, West of Buffalo
In this paper are set forth the merits of a fat disk center for turntables, with grease as a lubricant. Diagrams of energy consumption, taken in December, 1920, and a detail plan of the center are shown. An attempt is made to separate, approximately, the friction losses in the center from the losses in the tractor.
In Volume 17 of the Proceedings, Appendix F, are shown diagrams and tabulations of energy consumption for various centers; the experi. ments for them were made in May, 1915. Item 2, of the tabulation, pertains to the center under discussion in this paper.
The Aat disk center was installed late 1913. It has been in service (and is still) for seven years. It has given excellent results and should be good for many years more About forty engines a day are turned. It will thus be seen that the center has been thoroughly tried. It can now be judged in the light of experience.
Quite complicated centers are in use, mostly of the roller kind, running in oil. The roller center has many parts. It is probable that, in the most carefully fabricated roller center, the load is not equally distributed among the rollers, and it is certain that this condition soon arises under service. Some designs have a large segment of a cylinder for centering and distributing the load; such devise is inadequate. The centering can be done accurately only through a pin.
I found the roller center difficult to maintain and inspect, and that, under present heavy loads and frequent service, it gave trouble.
The stock argument for the roller center is the low energy coi?sumption as compared with the disk center. It is the intention to refute this argument.
Lenticular disk centers, sliding in oil, are also used. The advantages of a lenticular disk are not apparent. Why lenticular?
A center, running in oil, is not self-cleaning' and the devises for retaining the oil and keeping out the dirt, complicate the design. Simplicity of design makes for continuity of service and ease of maintenance.
In the flat disk center, as developed by me, the annual maintenance cost is very nearly nothing. It is not necessary to clean the center, or open it up, as must be done for roller centers to pick out the broken thrust washers of the conical rollers, and to make repairs.
On account of the present heavy engine loads, it is difficult to keep the load, a linear inch of roller, within safe working limits; special steel is used for the high loading. But even such precautions are insufficient and add to the cost.
The drawing for the flat disk center is clear, but it is well to call attention to some of the features. The load is accurately centered through a pin. The grease grooves are large, have rounded corners and are cut on the upper sliding surface. The phosphor-bronze disk is without grooves and is swept by the lubricant at every rotation. The lubricant is forced by compression screw cups, with handles, through the feed pipes and center. A grease gun may be used.
The following instructions are issued to the turntable attendant:
Rules for Lubricating Disk Centers of Turntables 1. Use cup graphite grease, soft consistency. Do not use oil or
other greases. 2. Keep the grease cups, ducts and grooves filled with grease. 3. Screw compression grease cups should be used. One cup should
be installed at one end of the pipe or duct; the opposite end of
the duct should have a screw cap. 4. To lubricate the disc, unscrew the screw cap and then turn the screw lid of the compression cup until grease oozes out of the
Do this once a week or oftener, depending on the number of times a day the table is used. 5. It is necessary to keep the grease in a soft condition. If allowed
to remain too long in the grooves and ducts, it will cake or harden.
Hence, another reason for Rule 4. 6. While the grease is being compressed, the turntable should be
rotated. (No engine on table.) 7. If the grease does not ooze out of the open end, attempt to clean
the ducts and grooves with a wire, removing all caked crease. If it is impossible to clean the duct, etc., with a wire, the table must
then be lifted and grooves cleaned. It is essential that the lubricant be kept plastic. It must not be allowed to harden or cake.
Several Alat disk centers are now in use on the New York Central Lines. In one case, attention was not given to proper lubrication. I yellow grease was used. The grease caked. It was necessary to lift the table so that the center could be cleaned. Graphite grease, of soft consistency, gives excellent results.
For the disk, subject of this paper, graphite grease was used.
Trunnion bascule bridges are now in successful service. The trunnio: lubricant is grease. There is every reason for a disk being as practicable as a trunnion: both slide and can be similarly lubricated.
Coming now to the energy consumption, a properly designed and lubricated sliding surface improves with service; the friction decreases through use. In three years' time, the frictional torque on the motors of one of the Calumet River lift bridges of the New York Central Railroad decreased from 940 ft. Ib. to 600 ft. Ib. A marked decrease in the friction of the flat disk center will also be found.
The input of energy to the tractor (this is what the railroad pays for) is used for three purposes: (1) to overcome the friction in the center; (2) to overcome the friction of the tractor; and (3) to cover the electric losses in the tractor. The last two items represent the total loss in the tractor. A particular value of this total loss, I have called M in Table I. M is defined as the loss in the tractor on the supposition of a frictionless center. It is an ideal value, and it can be calculated from the test data as will be shown. The actual loss in a tractor is always greater than M because the pull, necessary to move the table, increases the losses in the tractor. It is not possible to calculate the exact value of the pull of the tractor.
The use of M, in place of the actual losses in the tractor, penalizes the center, as the calculated losses in the center are somewhat greater than the actual losses.
In Table 1 are given the energy consumption figures for the flat disk and the tractor. The basic figures are taken from Volume 17 of the Proceedings, item 2, and the wattmeter diagrams accompanying this article. The interval of time between the sets of tests is more than five years. The figures are for friction losses in the center, and all losses in the tractor.
Throughout this article, the figures and data of energy consumption are for 360 degrees of rotation of the turntable at a constant velocity. For this condition, the input of energy balances the loss of energy.
The calculation of M will now be shown. The loss in the tractor in excess of M, is in proportion to the pull of the tractor on the table. The pull on the table is in proportion to the load on the center. Hence, the excess losses are proportional to the loads on the center. These statements are strictly true for all friction losses, and are nearly true for the electric losses. Unvarying coefficients of friction, for any particular test, are assumed.
Referring to Table 1, 1920 tests, the following equations are written:
166000p + M=1.53.
1738100p + 4M = 7.76 in which p is the proportional factor. The terms, with the p factor, represent the excess losses. The solution of the equations gives M equal to 1.27 kw.m. The other values of M, shown in columns 1 and 6, were calculated similarly,
When the uncertainty of the engine loads is considered, the nearly equal values of M are remarkable. An average value of 1.24 kw.m. will be used for the remaining discussion.
Referring again to the article in Volume 17 of the Proceedings, I have calculated all the values of M for the tests. For the purpose of making comparisons with the roller center, Table 2, showing values of M and other matter, is presented.
TABLE 2-ENERGY CONSUMPTION FIGURES FOR ROLLER CENTERS
Summarizing, a total load of about 728,000 lb. was turned with a loss in the roller centers of 2.61 kw.m. The loss of 100,000 lb. is 0.35 kw.m. From the Table, I selected the weights of the turntables only, for the loads.
Referring to Table 1, the loss in the flat disk center for a load of 166,000 lb. is 1.53—1.24=0.29 kw.m. For 100,000 lb., the loss is 0.17 kw.m.
The flat disk center is now more efficient than the best roller center, item 1, of Table 2. At the time the roller center was tested, in 1915, it was new and in excellent condition. The disk center was also new, but its friction losses were high. By giving regular and proper attention to the lubrication, the losses in the disk center, for a load of 166,000 lb., are reduced from 1.51 kw.m. to 0.29 kw.m. The tractor losses were, also, greatly reduced. See Table 1.
The coefficient of friction, f, for the disk center will now be calculated. The disk is 24 inches in diameter. The frictional resistances are assumed as acting in a circular path of a diameter 16 inches and a length of 4.2 ft. The equation is (4.2) (166000) f= 12760. f=0.0183. The right side of the equation is the equivalent in foot pounds of 0.29 kw.n. Taking the total weight of 1738100 lb. and the corresponding total losses for the center, 2.80 kw.m., f equals 0.1169. In 1915, the coefficient of friction obtained by a similar calculation was 0.086.
I will again calculate f by a crude method. A Mallet engine was placed on the table, and given an angular velocity of 360 degrees a minute. The power was then cut off. The table, with its load, drifted through 360 degrees and came to a stop in about 120 seconds. The total load on the center was 680,000 lb. The retarding forces are the friction on the disk and the drag of the tractor. The disk friction has an arm of two-thirds of a foot; the tractor drag, 43/2 feet.
The drag of the tractor will now be considered. After the power is cut off, the tractor is retarded by its own friction. The tractor also has kinetic energy, due to rotation of its parts and its translation as a body. The kinetic energy overcomes part of the friction. The remaining friction must be balanced by the pull of the table on the tractor.
It is very difficult to calculate the kinetic energy of the tractor, but its friction losses, for a frictionless center, have already been calculated. The quantity M covers the losses. M is equal to 1.24 kw.m., or 54560 ft. Ib. The tractor moved through a distance of 273 feet, the length of the path around the turntable pit. Hence, the drag on the tractor must be no greater than 200 lb.; it may be considerably less. I will calculate f by, first, neglecting the drag and, second, by considering it.
A PRINCIPLE OF MECHANICS.—The sum of the moments of the retarding forces acting on a rotating body equals the moment of inertia of the body about the axis of rotation times the angular retardation.
The angular retardation, using the second as the unit of time, is six degress divided by 120, or 0.05 degrees, or 0.0009 radians.
The moment of inertia may be taken equivalent to that of a rigid body 75 ft. long, six and a half wide, and weighing 680000 lb. The mass is 21120. The rigid body is an approximate substitute for the combined table and engine. Using these numbers, the moment of inertia is found to be 10000000 closely.
All of the quantities necessary for applying the principle of mechanics are now known. The equation is: 2 - (680000) f = (10000000) (0.0009). f equals 0.019. 3 The left hand side is the sum of the moments of the retarding forces acting on the table and engine. The right hand side contains the equivalent moment of inertia, and the angular retardation. Considering the greatest drag of the tractor, the equation becomes : 2
(680000) g + (200) (10000000) (0.0009). f equals 0.0006. 3 The approximations of this method are such as to penalize the center. Both methods show a low coefficient of friction.
The drifting of the heavy engine, through an angular distance of 360 degrees is an indication of low friction losses in the center and tractor. The drifting test is a rough and ready way of determining the efficiency of a center and tractor.