Valve Gear For the Stuart Beam Engine

Eccentric and Strap - Part 1 Strap

 

My Drawing of The Eccentric Strap - To Scale

The First part I decided to make for the Stuart Beam was the eccentric sheave, strap and connecting rod.  The strap was the first to be made as the eccentric sheave is made to fit.  In the original kit the strap is made from a casting, but the original was lost and Stuart Models had been sold to new owners at the time, so I decided to fabricate one from solid. Firstly I drew up the strap accurately to scale from the Stuart drawings.  I then used this as a rough cutting template by sticking it to the brass blank with the simple use of PVA white glue.  This holds very well but is eventually easy to remove.

Unfortunately I had no 1/4" thick brass stock but I have some 1/8" flat, so I silver soldered two pieces together using high silver low temperature solder.to make up the thickness.  I did this by flattening a length of round silver solder and made it into a ring around the same diameter as the strap.  The parts were fluxed, the silver solder positioned and the 'sandwich' was made.  In order to prevent the parts moving too much when soldering I arranged that the 'sandwich' was restrained on two sides by firebrick.  The parts were heated and successfully soldered.  I had tried to soft solder the parts but that was not as successful as the use of Silver Solder.  Soft soldering, if properly carried out, is much stronger than most folk realise.  It does no just act as a 'glue' sitting between two surfaces.  A perfect soft soldered joint will have the thinnest possible layer of actual solder as it should actually alloy with the parent metals it is joining at the boundaries of the joint.  It is strong in tension and shear (obviously in compression as well).  However it is weak in 'peel'. therefore those situations should be avoided.

Back to Making

The first stage was to cut the blank across where the joint is to be made. The edges of the two halves were milled accurately.  They were then drilled tapping size.  The hole in the cap were opened out to 7BA clearance while the hole in the body were tapped 7BA. The two parts were bolted together ready to bore out to size.  You should be able to see what I mean in the picture below.

 The blank was then mounted in the 4 Jaw independent chuck and the part centred where the eccentric sheave is to be located.  I drilled out as much as I could, step drilling up to my largest drill. 











The boring bar was then used to bore the strap to size.  It has a groove to locate the sheave so a HSS tool was ground up for the boring bar and the groove carefully bored. 

One point to note is that I ensured that the blank was located in the chuck accurately by using a steel block held against the front of the jaws with the tailstock, and the blank, just held gently in the jaws of the chuck was forced against the block before tightening the jaws of the chuck hence locating it accurately.

After boring the strap was finished by milling and filing to shape using normal practices.  The initial profile was rough milled and then the part was filed to profile with a succession of finer files and abrasives.  It is shown here with it's eccentric before the eccentric and boss were drilled and turned.

It is now just in need of a bit of final bling to get to a high finish.  however I do not want it to be too perfect as I prefer the model to reflect an engine in 'working condition', I'm not to keen on those Highly polished ones I see on the exhibition circuit and in magazines from time to time.