Piston rods.

The diameter of a piston rod needs to be considered in conjunction with the method of fastening the piston.

A "rule of thumb" is piston diameter x 0.16 to x 0.2.

The basic formula is : A = P
F
Where:
A = cross sectional area of the piston rod in square inches
F = stress in piston rod (a usual figure = 3,000)
P = maximum pressure on piston.

If the piston is fastened to the rod by a cone (as above) and lock-nut, or is screwed (and lock-nut) to the shaft, this formula is generally satisfactory.

If the piston is on a section of the shaft that is reduced in diameter, it must be remembered that this smaller section will need to be of the size calculated by the above formula.
It must also be remembered that the whole pressure of the piston is then applied to the small face that is the difference between the two diameters (on the "down" stroke), and in the reverse direction, the whole force is acting on the thread of the lock nut.

It is usually necessary to use a fine thread to leave the largest core diameter.

The securing nut should always have a lock nut, or be secured from undoing by mechanical means, such as a split pin through a castle nut.

Connecting rods.
For slow speed engines, the same formula as for piston rods can be used (see previous page).
For larger engines, a rough guide is to use the above formula + 50%.

The following formula is sometimes used:

36,000 x A
P =
L²
1 +
560 d²

Where:
A = smallest cross sectional area of rod
d = mean diameter of rod
P = total load on rod in lbs.
L = connecting rod centres in inches

A factor of safety of between 5 and 10 is desirable. (i.e., use 5 P to 10 P).

Connecting rod length.

The length of the connecting rod should normally be
between 2.5 and 5 times the stroke. This can be reduced to as little as 1.75 x stroke in high speed engines, but this increases stress.

Bolts.
Connecting rod bolts should be of the same total cross sectional area as the con rod if they are of the same
material. If they are of high tensile material the strength should be calculated to be at least that of the con rod.

CRANKS & CRANKSHAFT.
Crankshaft size.
D = C x ³v HP/N
Where:
C = 5.1 for single crank, 4.7 for 2 crank, 4.5 for 3 crank
engines
D = Diameter of shaft
HP = Maximum possible HP
N = RPM

Crank webs.
Thickness = 0.625 x shaft diameter
Breadth = 1.33 x shaft diam. And T = v 0.12d² / h
Where: d = crankshaft diam.; h = thickness of web:
T = thickness of metal around eye-holes.
All dimensions in inches.

GUIDE BARS.
In full-size locomotives, it is common practice to make the guide bars slightly closer at the piston end to allow for expansion due to heat, thus making the bars parallel at normal working temperatures.

The clearance between the guide bars and the guide blocks on  locomotives was often of the order of 3 to 5 thou.

GUIDE BLOCK PRESSURE.
The pressure on the guide blocks should be less than 80 lbs. per square inch, or 60 lbs. per square inch for locomotives or traction engines, or where subject to dust.