UWE Bristol Engineering showcase 2015

Thomas Halford
BEng (Hons) Mechanical Engineering
Stress Analysis of Locomotive Coupling Rods and
Their Suitability for Fabricated Manufacture
Locomotive Coupling Rods
Introduction
The project looks at the feasibility of manufacturing steam
locomotive coupling rods by fabrication as opposed to the
traditional as forged method. Locomotive Coupling rods connect all
the wheel sets on a locomotive together as seen in the picture
above.
The main motivation for this project is the difficulty in finding spare
parts to replace lost or damaged locomotive parts. Several
locomotives are missing their original coupling rods and this will
prevent them being eventually restored. However, the forging
method originally used is not financially viable for small numbers of
components. During the second world war the main locomotive built
by the Germans for the war effort had coupling rods fabricated from
three pieces. Research showed the failure rate of the these
fabricated components was no higher than the forged equivalent.
This sparked the idea that new coupling rods could be fabricated
from three pieces welded together.
The benefits of this method are outlined below:
•No expensive forging dies required
•The three separate parts can be made on smaller more common
workshop machines.
•The fabrication method could be used to repair damaged parts for a
fraction of the price of a new part.
This project will look specifically at the feasibility of manufacturing a
coupling rod for a Hunslet 50550 class locomotive (pictured above).
The rod has been modelled in Catia Version 5 Cad software, this can
be seen below. The project looks at manufacturing the part from
three pieces, with the welded joints where indicated below.
Project Objectives
In order to ascertain whether manufacturing coupling rods by fabrication was
feasible the following work was undertaken:
•Fatigue analysis of original “as forged” rod. The results of this will provide the
bench mark to compare a fabricated rod to.
•Strength case for “as forged rod”- This section looks at the severe loading
which occurs in the coupling rod when a slipping locomotive regains adhesion
suddenly.
•Fatigue analysis of the fabricated Rod- This section carries out fatigue analysis
on the fabricated rod.
•Strength case for the fabricated rod- This section takes the same strength
case applied to the forged rod and applies it to the fabricated rod.
•Production Feasibility-This section looks at the production implications of
manufacturing the coupling rod by fabrication from three pieces as well as
some approximate costing.
Stress Analysis
Loading
In order to carry out any stress analysis the loads the coupling rods experience
had to be calculated. The loads experienced were a product of the cylinder
steam pressure and speed of the locomotive attained. The forces in the rod
are continually alternating between tension and compression . The stresses
seen are a combination of bending stresses caused by the rod’s inertia and
direct stresses caused by torque Transmission. However the stresses change
depending upon the speed, cylinder pressure and direction of the locomotive.
Original Forged Rod-Stress Analysis
In order to analyse the fabricated rod it was first necessary to ascertain the
stresses in the original forged component. Due to the variation in loads seen
depending upon speed, cylinder pressure and direction of travel. A fatigue
spectrum had to be devised based around 12 fatigue cases. From this the max
and min stresses were determined at a number of sections in the regions near
where the welded joints would be ideally placed. The graph showing load
against rotation produced a sinusoidal form that was offset due to the positive
bend moment from the rods own weight. It was therefore necessary to use
the Modified Goodman failure criteria to find the equivalent fatigue stress
otherwise the results would have been pessimistic. A stress life (S-N) curve
had to be constructed for the EN3 material which was deemed to be the
closest material to the original material used for the forgings. This can seen on
the graph highlighted in blue. Miner’s Law for cumulative fatigue damage was
then used to assess the part for fatigue damage. The loads for the strength
case for when the locomotive slips and then regains adhesion were then
determined. This was done by resolving the torques acting on the wheel sets
of the locomotive during this sudden recovery of torque. This produced a
torque reacted by one coupling rod, the compressive force experienced by the
rod could then be worked out. The critical buckling force was subsequently
worked out in both planes of the rod with the parabolic buckling formula. The
critical buckling force in y-y direction being significantly less than the x-x. From
this factors of safety could be worked out for the respective planes of the rod.
Project Supervisor-Rui Cardoso
Fabricated Rod-Stress Analysis
Due to the presence of the welds a stress concentration factor needed to
be applied to the S-N curve. This factored down curve can be seen below
in red. This S-N curve was then used to determine the new damage for
the fabricated rod.
The strength case was then calculated for the fabricated rod by using the
critical buckling load previously determined divided by the stress with
stress concentration factor applied. A stress concentration of three was
used as before in the fatigue analysis.
Production Feasibility
The feasibility of manufacturing a coupling rod from three pieces was
discussed in this section. It was concluded that a jig would be required to
restrain and position the three piece rod during the welding process. It
would also be necessary to stress relieve the fabricated rod post welding
by heat treatment, this would be necessary to remove the residual stress
induced by the welding. During this treatment the rod should remain in its
jig , this would avoid excessive twisting or bending due to the part
relaxing. It was also recommended that the profiles for the three piece
rod were water jet cut to avoid producing a heat affected layer that
would then need to be removed by machining due stress implications.
Project Conclusions
• Fatigue analysis shows that a fabricated rod would be capable of
withstanding the same life in service as the original forged rod
without failure.
• The strength case demonstrated that a fabricated rod still had
healthy margins of safety and was therefore acceptable from a
static strength perspective.
• The manufacturing feasibility study showed that a fabricated rod
could be manufactured with the aid of a jig however, a one piece
profile could be manufactured for the same amount of money.
However it would be more cost effective to repair an existing rod
through the fabrication method.