Live steam model on 7¼" gauge of the Württembergische T3 no 924

Welcome to this blog. It will inform you about the progress of designing and building miniature live steam, coal fired locomotives for passenger hauling. Currently I'm working on a 7¼" gauge, scale 1:8, German T3 steam locomotive.

In 2006 I started this new project. This is a small 0-6-0 branch line locomotive of the German KWStE (Königlich Württembergischen Staatseisenbahnen) origin with outside cylinders and Walschaerts valve gear. The loco is about 1.10 metre long and will weigh approx. 100 kg.

On the left you'll find the index where you can browse through the different articles and on the right you'll find all the extra's. On the top tabs you'll find a brief description of my other locos.

Enjoy this site. Erik-Jan Stroetinga. The Netherlands. Europe.

Tuesday, 5 May 2015

Ramsbottom safety valve

The safety valves are perhaps the most important fittings on the locomotive boiler. Two are provided on the boiler. On the original locomotive a Ramsbottom safety valve was placed at first in the cab, but due to the noise this made while blowing off,  in later life of the loco they were placed in front of the cab. This position on the boiler was between the coal bunkers, but when these were extended the valves moved even further forward between the sand box and steam dome. On my locomotive the valve will be positioned in front of the cab.              

The valves are a simple spring loaded  with a 5 mm bore and 6.35 mm ( ¼" ) stainless steel ball. With the nut on top of the spring, the adjusting of the blow off pressure is easy. They are both set to the same pressure. If one is set to blow off at a higher pressure than the other, it may never blow and may become stuck as a result. The ball, valve stem and spring are all of stainless steel. The force on the spring is only about 1.2 kg  (12N) for the working pressure of 6 Bar.  

The valve was designed in Solidworks with the help of some photo's I made in the museum in Heilbronn, were the real loco was overhauled. At the time the valve was on a wooden pallet, so by comparing the size of the planks with the valve I could determine the general dimensions.

The body was milled and turned from a solid bronze bar of 50 mm.

After machining, the radius was filed by hand.

The finished radius and the four holes drilled for fixing it on the boiler. The groove will hold a seal of Teflon tape to make a steam tight connection with the boiler.

The two holes were drilled and taped M10x1 for taking up the valve seating.

The two valves are connected to the boiler by 5 mm holes drilled at an exact angle. Solidworks helps here to determine this angel, without drilling to far out of position or to deep.

A test fitting on the boiler; the position the valve had after moving it from the cab.

A set-up on the rotary  table, for milling the valve body seating to its round shape.

Once this set-up was made, it was a quick job to mill the body. For locating the correct position, the M10 tap was insterted once more in the taped hole, and the tap was than inserted a tool holder in the main spindle of the milling machine. While the rotary table was set in its x=0 en y=0 position (on fore hand determined with a center finder), the vice could move over the table and was locked down on the table when the position was reached.

The rest of shape was made by hand; a file and some elbow grease  finished the part.
A piece of Duck-tape was used to protect the finished sides in the event of a slip of the file. To hold the body in the vice, a rod with screw threat was made. 

The valve base completed

The valve seatings were turned from a bronze rod and screwed in the valve base, sealed with industrial sealant.

Tightening was done by gripping the seat in the lathe with a collet. Without any damage to the surface a secure connection could be made.

The 6.35 mm  (¼") stainless steal ball on the seating.

After curing the sealant, a set-up was made in the milling machine for position, drilling and tapping (M2) the holes for taking up the valve house.

A view of the valve getting its shape. In 1855, John Ramsbottom, later locomotive superintendent of the LNWR, described this new form of safety valve intended to improve reliability.  Although this was a British invention. it was extensively used on German locomotives. 

The slight conical shape was turned, while the valve house was fixed on a mandrel.

Once the valve reached this state of production, it stood on my book shelve for a few months;
 the driving season started, so I was out to steam my other locomotives on several steam meetings in the country and abroad.

The CNC milling machine came to use for making the linkage of the valve. All was milled in stainless steel (304). A new dip-switch setting to the drives of the stepper motors of my Wabeco makes the machine much faster in rapid movements (g-code G0). The motors get now more power.

The cross bar was slotted in this some what strange set-up. This way I could hold the small piece (only 32 mm long) to open out  the hole into small slot.

This lever was characteristically extended rearwards, often reaching into the cab on early locomotives. Rather than discouraging the use of the spring lever by the fireman, Ramsbottom's valve encouraged this. Rocking the lever freed up the valves alternately and checked that neither was sticking in its seat. Even if the fireman held the lever down and increased the force on the rear valve, there was a corresponding reduction of force on the forward valve.
With this T3 loco the lever was cut off after moving the valve outside the cab. So operation by the fire man was not possible any more.

As can be seen in this picture, the valve setting screws are sitting inside the valve house. The area above the valve is as important  as the area of the seating itself. There are 12 holes of 1.7 mm; so the cross sectional outlet area is bigger than the 5 mm bore of the valve and provide an ample escape for the steam.

The valve stems inserted. The nut on top lets you lift the ball of the seating by lifting the lever to test the popper working of the valve while the locomotive is in service.

The top of the valve is made of copper pipe which is flared out on the lathe. A simple piece of round bar and some cutting lubricant are sufficient to preform this operation.  For this metal spinning a modest speed of 500 rpm. is used during this process and the tool is changed in position several times to get the final shape.

The outside springs are fixed by small links. The 12 mm spring is gripped by a small tie bar. 

The almost completed valve. Only the stainless steel M2 bolts and nuts should be inserted to fixe the valve houses on the body.

The valve is set to blow off at 6 bar.

By lifting the bar it's possible for a driver,  to check that the valves are not sticking. On the model you still can reach the bar.