There comes a point when you feel that simply looking at the wheels going round is not enough.... The first simple step is normally a conrod between the flycranks of the axles, then maybe a push rod from what could be a steam cylinder. This is normally as far as most people go -because the next step seems impossible without vast investment in tools and computer CAD modelling software...

SO how do I do valve gear -with just a simple pillar drill, a few sections of K&S brass strip, and some 3mm nuts and bolts????

Well there are SEVERAL DOZEN types of valve gear but they all fall into one of three types...

The Link Motion type -of which the Stephenson is the most famous.

The Radial Gear type -of which the Walschaerts is the most famous.

The Poppet Valve Gear type -of which the Caprotti is the most famous.

The types of valve gear system that I am about to describe are both of the Radial Gear type. This is because all the functions can be calculated very easily (basic trig!) and the system can be made by using a ruler and a centre punch.

The radial gear derives its motion from the driving wheel itself thus all calculations have to based on the fact that the radius of rotation of the flycrank provides the base dimension for all calculations (hence RADIAL). All the following plans and crib sheets are based on a theoretical 0-4-0 with the push rod from the piston to the rear axle OR the centre axle of an 0-6-0.

To use these crib sheets take the radius of of rotation of your flycrank to be ONE and then multiply it by the numbers shown in the drawings to find the correct dimensions for you. You may stretch the dimensions in any ONE direction -but the ratios of the lengths must remain the same to keep the parallelograms in the same kinematic place to each other along this axis.

NOTE: none of these drawing contain plans for revesing gear or the potential for varying lead and lap. If you want to work it out for yourself then please do so.

The Hackworth Valve Gear

This is a very early type -but variations of it existed right up until the demise of steam in the BR days. The main problem with it is the fact that it is incapable of running at high speeds due to vibrational problems -but since the same year as it was developed the Dean of the Royal College of Surgeons proclaimed that: "Travel above 20mph would be impossible due to the fact that the air would be sucked out of the passengers lungs" -I don't think that this is a scale model problem...

If you look at the dimensions closely you will see a 3-4-5 right angle triangle with a 4-4 45 degree right angle triangle with a common 4 side length, there is also a 3 x 1 parallelogram. In the Hackworth gear everything moves forward and backward together. This imposes large structural stresses at high speed added to this the fact that the long arms can go through vibrational modes as well...

It is very simple and has the added advantage that this can be simply "grafted" on to an existing model very easily.

Joys Patent Valve Gear

This is a derivative of Hackworths -but the basic problem with it is that it cannot be "grafted" to an existing model. The decision to use this has to be made at the start -not half way through.... Joy made a very succesful business deal with the LNWR to have his valve gear used by their locos -as each loco proclaimed with its little plate. The word JOYS PATENT had to be in block capitals -it was part of the deal(!) The Joy system has problems with stresses as does the Hackworth -but derivatives remained in use -even on US railways (notably "The Southern"). Due to the fact that it takes it motion from the push rod -this weakened it and could cause cracking. The weight of the loco could also alter the lead timing on the cylinders so JOY locos were given stiff springs (and a reputation for being a very bumpy ride!)

If you study the drawing you will see a parallelogram of 3 x 2 and two right angle triangles of 3 x 1. In the Joy system the valve rod moves in opposition to the push rod from the cylinder which is the more "modern" practice.

The Baker Valve Gear

This is included as a piece of research and demonstration, (I have no idea whether this will actually work).... The Baker valve gear is very popular amoungst Model Engineering builders because it requires no complex machining. It was never used on any BR locomotive (possible because of clearence problems). It is unique that all the movement is the result of direct pivoting and to renew the gear all it takes is to replace the pins and bushes with new ones. It was quite popular in NZ and they reported that it was more accurate than Walshaerts. It was designed in 1903 by Abner D Baker and was originally designed for traction engines. The original drawing that I took my calculations from was "TICH" by LBSC. This is my technical drawing of LBSC's more flowery English!