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, 431⁄2 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. lb. 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

3

(680000) f = (10000000) (0.0009). f equals 0.019.

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

3

(680000) g + (200) (10000000) (0.0009). f equals 0.0006.

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.

The cost of power for turning an engine on a flat disk is very small. Referring to Table 1, the cost for turning any of the engines did not exceed one-eighth of a cent with power at 3 cents a kw.-hr. Really, the power cost is a very small item for the usual center, disk or roller. Hence, a center which is low in first cost and maintenance, is the most desirable. A simple flat disk meets the requirements.

Now, how about turning by hand? If the table is at a point where only a few turns a day are made, a roller center is desirable for hand power. Although, a well-worn and lubricated disk center will answer as well. A new disk center would not receive enough wear to make it smooth. But wherever a tractor is used, I see no economy in a roller center. If there is a failure of power, the resistances are too great for hand operation, if the tractor is allowed to remain attached to the table. The experiments show very clearly the relative great loss in the tractor. It is easily seen why it is difficult to turn, by hand, a table with the tractor attached.

A short time ago, I made an examination of a flat disk center which had been in service, for three years, at a point where about 100 engines a day were turned. The disk was very smooth; there was scarcely any

wear.

I have modified the design of the disk center. The thickness of the disk has been reduced to 21⁄2 inches. Other minor changes were made. The pedestal has been made lower. Recent bids for the modified design are as low as $400.00.

If any trouble arises in a properly designed flat disk center, it will be found due to a lack of a little common sense. This sense should be applied with the lubricant.

Why use a complex devise, like a roller center, when a simple disk is more economical?