The Crank Axle

The Crank Axle


21c11 / 35011 General Steam Navigation was withdrawn from service in 1966, despite having only recently received an overhaul. Subsequently its crank axle was transferred to a classmate 35026, being replaced with a plain axle to allow the locomotive to be towed to the scrap yard. This has left us with the significant task of replacing the crank axle, a not insignificant challenge.


On a steam locomotive the reciprocating (back and forth) motion from the piston has to be transferred to rotary motion to drive the wheels. The change in motion from outside cylinders is via the connecting rods and crank pins off set from the wheel centres.

With internal motion, the drive of the inner cylinder(s) is transmitted to the driving wheel via a crank axle. This involves a pair of radial arms or webs from the stub axle, with a pin between the webs. This pin gives the axle its rigidity and transforms the reciprocating motion of the connecting rod to the rotary / radial motion of the wheels.


The Bulleid Crank Axle


The axle is cylindrical, with the chain driver sprocket attached close to the crank webs. The webs are substantial piece of metal, extending 1’10” from the centre of the axle, 1’8” wide and 5 5/16” deep.

For reasons unknown, the original design of crank axle was for an “unbalanced” design (as seen right); the webs themselves do not have a counterweight attached to them, so this mass was balanced through weights on the inside of the wheels.

The design was applied to the Merchant Navy and Light Pacific designs.


The Crewkerne incident


On 24th April 1953, whilst approaching Crewkerne station at 70-80mph, the crank axle on Merchant Navy 35020 Bibby Line fractured, causing damage to Crewkerne station, the evidence of which can still be seen to this day, yet the train did not derail and no-one was injured.

The incident resulted in the withdrawal of all Merchant Navy class locomotives from service whilst the cause was ascertained. Examination of other class members showed that the fracture, caused by metal fatigue, was a common fault.

The incident resulted in a redesign and replacement of the crank axle, starting in 1954 with 6 of the class (including 35011 General Steam Navigation) having the new crank axle design fitted before rebuilding, the remainder of the Merchant Navies being fitted with the design upon rebuilding (as were the rebuilt Light Pacifics).

A balanced design was used; the webs were extended to the opposing side of the axle to form a counterweight, and the diameter of the axle was increased, both measures being taken to reduce the stress in the axle.


Mission Impossible?


Having had most of her running gear missing and critically no crank axle, has put 21c11 / 35011 General Steam Navigation in a rather unique position compared to her sisters as the cost of restoring her to either rebuilt or original condition are fairly similar. Many years ago, this would have been deemed a challenge too far, but today such challenges have been overcome as seen by other restoration and new build projects.


Aided by significant insight from Andrew Marshall (35006 Locomotive Restoration Society), we have clarity on the design of axle we should be aiming to have fabricated, this is the redesigned, balanced crank axle (as seen right) that was eventually fitted to all the Merchant Navy locomotives following the Crewkerne incident. As 35011 General Steam Navigation had the balanced crank axle fitted prior to rebuilding, we can demonstrate it is applicable to our locomotive. We have had insight and support on the axle material from the wider heritage industry, including our friends at the A1/P2 trust,  the Clan Project  and the B17 Trust,  all of whom have had similar struggles of design to overcome.


We are also working with the University of Birmingham with a project looking at the mechanics of the motion of our locomotive, focussing on the crank axle and chain-driven valve gear and more details of this collaboration can be read at our dedicated project page here.


We are currently missing some of the drawings for the revised design, and we are also having to ascertain the steel to use, but our aim is to have the new crank axle ordered at least in 2021, if not manufactured.


To find out about how you can support us with this unique project click here.