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The running gear of a locomotive is composed of the following important parts: Wheels, axles, rods, pistons, and the frames which form a connection between these parts.

Wheels. The driving wheels have a cast-iron or steel center protected by a steel tire. Until about 1896, cast iron was universally employed for wheel centers and is yet used for the smaller engines. For engines having large cylinders, where a saving of weight is important, cast steel is now used makes possible a considerably lighter construction. Such a wheel is illustrated in Fig. 72. The universal method of fastening on the tire is to bore it out a trifle smaller than the diameter to which the center is turned, then expand it by heating and after slipping it over the center allow it to contract by cooling. The shrinkage commonly used is 1/80 of an inch for each foot diameter of wheel center for all centers of cast iron or cast steel less than 66 inches in diameter. For centers more than 66 inches in diameter, 1/60 of an inch for each foot diameter is allowed for shrinkage. This gives the following shrinkages:


Shrinkage Allowance

Diameter of Center Shrinkage Bored Diameter of Tire
56 .058 55.94
58 .060 57.94
60 .063 59.93

The American Master Mechanics' Association recommends the following conterning wheel centers:

In order to properly support the rim and to resist the tire shrinking, the spokes should be placed from 12 to 13 inches apart from center to center, measured on the outer circumference of the wheel center. The number of spokes should equal the diameter of center expressed in inches divided by 4. If the remainder is ^ or over, one additional spoke should be used. The exact spacing of the spokes according to this rule would be

3.1416 X 4 = 12.56 inches

Wheel centers arranged in this manner would have the following number of spokes:


Spoke Data - General

Diameter of Centers Number of Spokes Diameter of Centers Number of Spokes
38 10 72 18
44 11 74 19
50 13 76 19
56 14 78 19
62 16 80 20
66 17

Among pattern makers and foundry men, there is an impression that an uneven number of spokes should be used so as to avoid getting two spokes directly opposite each other in a straight line. The following table has been made up on this basis:


Spoke Data - Foundry Rule

Diameter of Center Number of Spokes Pitch
44 11 12.5
48 11 13.6
50 13 12.6
54 13 13.0
56 13 13.5
60 15 12.6
62 15 13.0
66 15 13.8
68 17 12.5
70 17 12.9
72 17 13.3
74 17 13.6
76 19 12.6
78 19 12.9

The spokes at the crank hub should be located so that the hub will lie between two of the spokes and thus avoid a short spoke directly in line with the crank pin hub.

Cast steel driving wheel centers should be preferably cast with the rims and uncut shrunk slots omitted whenever steel foundries will guarantee satisfactory castings. For wheel centers 60 inches in diameter and when the total weight of the engine will permit, the rims should preferably be cast solid without cores so as to obtain the maximum section and have full bearing surface for the tires.

It is difficult to get sufficient counterbalance in centers smaller than 60 inches in diameter so that it will be found very desirable to core out the rims to obtain the maximum lightness on the side next to the crank pin and in some cases on the counterbalance side in order to fill in with lead where necessary.

The American Master Mechanics' Association recommends a rim section as shown in Fig. 73 for wheel centers without retaining rings. The tire is secured from having the center forced through it by a lip on the outside 3/8 inch in width and about 1/8 inch in height, the tire being left rough at this point. The height of the lip, therefore, depends upon the amount of finishing left on the interior of the tire. Accurate measurements of tires after they have been in service for some time, especially when less than 2-1/2 inches in thickness, show that a rolling out or stretching of the tire occurs, and for reasonably heavy centers, these figures will account more for loose tires than any permanent set in the driving wheel center.

Counterbalance. A study of the construction of the driving wheel brings up the question of counterbalance since it is made a part of the wheel center. The counterbalance, Fig. 72, is the weight or mass of metal placed in the driving wheel opposite the crank to balance the revolving and reciprocating weights.

The revolving weights to be balanced are the crank pin complete, the back end of the main rod or connecting rod, and each end of each side rod complete. The sum of the weights so found which are attached to each crank pin is the revolving weight for that pin.

The reciprocating weights to be balanced consist of the weight of the piston complete with packing rings, piston rod, crosshead complete, and the front end of the main rod complete. The weight of the rod should be obtained by weighing in a horizontal position after having been placed on centers.

The revolving weights can be counterbalanced by weights attached to the wheel to which they belong, while the reciprocating weights can only be balanced in one direction by adding weights to the driving wheels as all weights added after the revolving parts are balanced overbalance the wheel vertically exactly to the same extent that they tend to balance the reciprocating parts horizontally. This overbalance exerts a sudden pressure or hammer blow upon the rail directly proportional to its weight and to the square of its velocity. At high speeds, this pressure, which is added to the weight of the driver on the rail, may become great enough to injure the track and bridges.

The best form of counterbalance is that of a crescent shape which has its center of gravity the farthest distance possible from the center of the axle. The counterbalance should be placed opposite the crank pin as close to the rods as proper clearance will allow. The clearance should be not less than 3/4 inch. No deficiency of weight in any wheel should be transferred to another. All counter balance blocks should be cast solid. When it is impossible to obtain a correct balance for solid blocks, they may be cored out and filled with lead, which will increase their weight. In all such cases the cavities must be as smooth as possible. Holes should be drilled through the inside face of the wheel to facilitate the removal of the core sand.

In counterbalancing a locomotive, the following fundamental principles should be kept in mind:

  1. The weight of the reciprocating parts, which is left unbalanced, should be as great as possible, consistent with a good riding and smooth working engine.
  2. The unbalanced weight of the reciprocating parts of all engines for similar service should be. proportional to the total weight of the engine in working order.
  3. The total pressure of the wheel upon the rail at maximum speed when the counterbalance is down, should not exceed an amount dependent upon the construction of bridges, weight of rail, etc.
  4. When the counterbalance is on the upper part of the wheel, the centrifugal force should never be sufficient to lift the wheel from the rail.

The following rules have been generally accepted for the counterbalancing of locomotive drive wheels:

  1. Divide the total weight of the engine by 400, subtract the quotient from the weight of the reciprocating parts on one side including the front end of the main rod.
  2. Distribute the remainder equally among all driving wheels on one side, adding to it the sum of the weights of the revolving parts for each wheel on that side. The sum for each wheel if placed at a distance from the driving wheel center, equal to the length of the crank, or at a proportionately less weight if at a greater distance, will be the counterbalance weight required.

The method of adjusting the counterbalance in the shop is as follows: After the wheels have been mounted on the axle and the crank pins put in place, the wheels are placed upon trestles as illustrated in Fig. 74. These trestles are provided with perfectly level straight edges upon which the journals rest. A weight pan is suspended from the crank pin as shown. In this pan is placed weight enough to just balance the wheels in such a position that a horizontal line will pass through the center of the axle and crank pin and counterbalance on one wheel, and a vertical line will pass through the axle and crank pin centers of the other side, the crank being above. The amount of weight thus applied, including the pan and the wire by which it is suspended, gives the equivalent counterbalance at crank radius available for balancing the parts. This weight found must not exceed that found to be necessary by the formula. Should the counterbalance be left with extra thickness, the extra weight can be turned off with little trouble after the trial described has been completed. This process should be repeated for the opposite side.

The weight of the reciprocating parts should be kept as low as possible, consistent with good design. Locomotives with rods disconnected and removed should not be handled in trains running at high rates of speed because of the danger arising from damage to the track and bridges, due to the hammer blow.

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This page last updated 9/6/99