Page:TheHorselessAge Vol15 No2.pdf/6



40

diameter of shait with a view to effect- ing a secure anchorage for levers.

It is plain enough that shafting will best serve the purpose in motor car construe- tion if it is of large diameter and hollow. But hollow shafting comes as cold drawn tubing—not exactly to size—and hence an allowance must be made for finishing. It is low in carbon, hence it must be of pro- portions suitable for mild steel.

In one case the writer noted the use of a hollow shaft 50 inches tong, 2 inches in diameter with a 1g inch hole. This shaft transmitted a load of 400 pounds, quiescent load, measured at 1 foot radius, The shaft was made of 20 carbon steel—not hardened. Assuming a safe torsional strain equal to 1,000 pounds per square inch, the ability of this shaft was

Pa 0.1963 2

X 1000 — 496

pounds pull at ; foot radius, in which P is the force or resistance act- ing to twist the shaft; a the radius in feet of the point of application of the force or resistance: Pa the torsional moment; d* the fourth power of the outside diameter im inches; di‘ the fourth power of the di- ameter of the hole in inches; d the out- side diameter of the shaft in inches, and S the safe torsional shearing resistance in pounds per square inch. The value of o is taken at 1 foor radius as @ convenience in making comparison. The shaft in ques- tion was subjected to some bending, and transmitted the power of a 90 horse power motor, including the effect cf a flywheel, the rim of which weighed 98% pounds, at a mean radius of 11 inches. The shaft proved fully capable, and while the tor- sional shear must have been more than 1,000 pounds per square inch, *t is believed, nevertheless, that the walls of the tube could still have been reduced a little.

In a Mercedes 90 to 10g horse power car the solid differential shaft is about 114 inches in diameter. There are conditions under which all the power of the motor moutst_be transmitted by this shaft. The shaft is of Krupp nickel-chrome-carbon steel, hence very capable. Nevertheless, the torsional shear is very considerable, be- cause

Last Pa — 0.1963 X25" ¥. 1000

= 380 pound: torsional moment at 1 foot radins. Assuming the matar to he capable of exerting 100 horse power at 1,000 reva- lutions per minute, the pull—torque—at_ foot radius of the motor is

HE P. % 33,000 __ 09 * 33,000

Ree ae 6 28 * 1000.

22.3 pounds. 380 $22 : : 1.000 X = 1.4780, the torsional shear assuming a pull of $22.3 pounds at 1 foot radius. Consider-

Hence

THE HORSELESS AGE

ing shock and allowing that the magni- tude of the shock equals the quiescent load the torsional shear becomes 2,957.8 pounds per square inch. This same shaft

supports the differential brake, hence it

takes the braking strain. Assuming the load on the rear wheels to be Go per cent. of the total load, and assuming the car to be within the racing limit, i. €4 1,000 kilograms—2,204 pounds—then a pull of

aes ae = 661 2 pounds comes at a radius of 1.5 feet, and allow: ing that the tractive coefficient is as good a8 0.5, we have

661.2 X 05 X L5 = 4959 pounds

equivalent pull at 1 foot radius, thus showing that the effort put upon the dif- ferential shaft’ through the brake is less than the effort on the part of the motor. This result, however, is obtained under the assumption that the braking effort on the two rear wheels is equal. It is also another way for saying that the motor is somewhat larger than the car requires, but it is highly improbable that the torque of the motor in question is as high as the figures would indicate. In other words, 100 horse power, if realized, must be obtained at a higher speed.

Tt will be noted that in the illustrations no account is taken of bending moments. In this connection it may be emphasized that bending moments should be elimi- ions, except- ing the unavoidable moments set up in the crank shaft and that duc to gear pres- sure. In other words, the deformation of the chassis frame should not be traus- mitted to the shafts.

nated in motor car transmi:

The Mercedes car above referred to furnishes an illustration of fight con- struction. If we refer to Kent, page 869, we find that for too horse power, 1.000 revolutions per minute, prime moy- ers a 2% inch diameter solid shaft would be used. We have here, therefore, a fur- ther illustration of the fact that ordinary materials and conventional methods of machinery designing will not produce satisfactory motor cars. It would not be feasible to employ a 234 inch diameter of th differential shaft in a motor car, even if the question of weight was ieft out; but the question of weight cannot be overlooked with impunity, nor can we say that the Mercedes results are bad. Hence we must say that motor car de- signers niust solve their problems inde- pendently of what has been accomplished in other lies of machine construction American designers who accept the di- mensions of parts in foreign cars as well chosen must consider the quality of the material used. else the results will be dif- ferent—possibly disappointing. The best makes of foreign cars are built of special grades of material, which accounts in a large measure for their success; and in this connection it is but fair to say that

Vol. 15, No 2.

the most competent American designers are also paying great attention to the question of the quality of material, Some of the well known foreign cars of even a year ago used pressed steel frames made from mild steel. Many of these frames “wilted” in ordinary service and were found to be quite unserviceable for the work required of them, Side frames, to be light and strong, but above all rigid, must be of steel very much better than that produced by cold pressing low grade mild steel. Soft, pliable stect plates may be easy upon the dies used in the process, but deformability in side frames can scarcely be recommended as a desirable quality.

‘American steel mongers seem to agree that “all grades of steel may be shipped from one pilé or lot,” and seem to over- look the fact thal the purchaser may have views. The writer finds that no matter what kind or grade of metal the order calls for it is rarely ever filled properly; for upon analyzing the metal it is generally found to be “something clse.”

In Germany the steel maxers turn out various brands of metal especially adapted for use in motor cars, thus making it pos- sible for motor car desiguers to definitely limit the weight of cars, yet allowing « definite frctor of safety. True, Germany is not far away. and in cast of necessity American car builders can import their raw material. In the meantime it is believed it is a waste of time to figure upon using a special grade of metal unless the metal is tested before using. In motor car con- struction it is believed constructors should have at their disposal the following metals:

( Gun Tron Steel Aluminwn Copper. Alnminum Nickel, Phosphor Bronze. Bearing Bronze. ‘Malleable fron.

s U j Mild Steel. r i “4 l

Castings.

Medium Steel. Nickel Steel.

Mild Steel.

Medium Steel.

High Steel.

Nickel Steel. Chrome Nickel Steel Tool Steel.

Rollings.....

‘The nickel steel products should be avail- able as follows: Mieket Per Crt.

a 0.75 1.80 15 8,00 8.00 4.50 0.75 18 15 3.0 3.0 4.5 0.7% 18 16 8.0 3.0 An 07 1.5 1.5 3.0 0.40 0.50 80 45 �