A Novel Steam Motor Development

By Ray Rolt

CONCEPT

For many years, I have felt that a modern application of the ’Scotch Crank’ using a roller bearing in place of the ’die-block’ could be used to make a compact steam motor. In the past its use has been restricted to driving pump rams and small donkey engines, for driving winches and similar applications. One example of rotative drive is on the “Merryweather” steam fire pumps, where it is mounted between the pump and steam cylinders to drive a crankshaft on which is mounted a fly wheel to give a smoother operation of the pump.

Its lack of popularity is due to the high friction losses and attendant wear compared to the use of a connecting rod. If these disadvantages could be greatly reduced, the elimination of the rotative forces of a connecting rod and attendant need for balancing would be beneficial. By using a single acting, horizontally opposed power unit, the pistons become ‘trunk guides’ and the main moving parts are reduced to two, the pistons and crankshaft. The only balancing would be for the crankpin and roller bearing, which can be accommodated in the disc crank.

In larger applications, the roller bearing would be housed in a circular housing to absorb and spread the impact against the slide faces, but in a model would be in direct contact and the bearing could be a simple ball race. A minimum clearance will be necessary between the slide faces to allow the roller bearing to move up and down without friction.

The ‘Power Unit’ will be in the form of two horizontally opposed units with the crankshaft in between and the cranks at 90 degrees to make it self starting.

Why the name ‘Rolbro’? Though my late brother Norman and I were different in many ways, one thing that we had in common was the need to find out how things worked and a desire to find solutions to technical problems. Many years ago, the Mills brothers traded in the model railway world under the name ‘Milbro’, hence the name ‘Rolbro’as a tribute to my brother.

DESIGN CRITERIA

A) Suitable for both construction in a small workshop and commercially, using laser cut steel and CNC machining, for small volume low cost production to meet a potential market in the model engineering world for a power unit for use in locomotives and road vehicles.

B) A compact crankshaft for fitting between the frames of a 5” gauge locomotive.

C) Using ‘Hackworth’ valve gear for simplicity.

D) Optimum temperature gradient in cylinders using direct steam admission at cylinder head, via simple slide valve, and ‘Uniflow’ exhaust.

DETAIL DESIGN

Two mild steel parallel plates with bearings for the crankshaft and tabs at both ends to engage with transverse end plates supporting the cylinders with MIG welded fillet joints to form a rigid frame for mounting the motor components to.

The end plates to be bored out to receive cast iron cylinder liners, forming the cylinders, with flanged end to engage with bored out recesses to suit in end plates. The liners to be secured by an overlay end plate bearing against the flanges, with a suitable steam seal, which forms the cylinder ends, retained by bolts tapped into main end plate.

The ‘cylinder head’ end plate to be drilled and tapped to receive overlay slide valve faces and flanged valve chest(s) and ports cut through to form inlets to cylinders. Drain cocks to be provided in bottom of valve chest(s) to allow the cylinder ends to be warmed through and condensation in the cylinders to be cleared through the slide valves.

The pistons to be turned from aluminium round stock, for lightness to reduce inertial loading on the ‘Scotch Crank’, slotted to receive rod drive to crank slide faces and drilled for captive ‘gudgeon pin’link. Pistons to be turned down with flange at head to receive ‘Admiralty’ brass sleeves, as ‘sacrificial’ wearing face, with fixing designed to allow them to be rotated to even out wear. The sleeves to have oil grooves and an oil reservoir, at the pin end, in the form of a deep wide groove to take a standard ‘lamp wick’fed via an oil cup on the top of the cylinder. The sleeves to be machined short in length to form a groove, for a ‘Viton’ “0” ring steam seal, inside the end flange on the piston. This allows the “0” ring to be replaced by removing the sleeve.

A direct fabricated rod link is used between the two pistons, incorporating packing shims for adjusting the piston head clearances. The ‘gudgeon pins’allow for any misalignment of the two cylinders. The slides for the ‘Scotch Crank’are mounted below this rod, with suitable bracing, to allow the crankshaft to be located inside any frames. This also allows for a longer drive link to the expansion link of the “Hackworth”valve gear for better valve events. The ‘die-block’ will also be a roller bearing, with shim adjustment to give clearance to the slide faces.

The crankshaft to have overhung disc cranks which will also act as eccentrics for the valve gear, based on the LBSC “Mona” design for a 3 ½” gauge locomotive, running in roller bearings with a central drive sprocket/pinion.

RESEARCH AND DEVELOPMENT

Having decided on the detail design, it now remains to construct the ‘motor’ and test it, to see if it actually works as anticipated! If there are problems with the offset crank drive, the cylinder units can still be used in a reconfigured unit in the form of a normal horizontally opposed layout.

Draft drawings have been prepared and now need to be finalised to enable the ‘motor’ to be built. Most of the materials have been sourced and when the drawings have been completed, I will be looking for someone to do the precision machining and help with the fabrication work.

As I worked out the details, it occurred to me that the concept was capable of developing in other ways. With the design of the cylinder/ piston assembly as a complete unit, it is capable of production in quantity on CNC machines as stock items in several sizes and sold as such.

Thanks to modern CAD, a variety of configurations can be designed for the main fabrication unit, to suit individual requirements, and used for the production of laser cut components. These can then be fabricated and machined to receive the cylinder units bought ‘off the shelf’, and the other moving components. The configurations can be as multiple ‘in line’, ‘V’ or ‘radial’units, and thanks to the slotted piston with gudgeon pin, conventional ‘con-rod’ variants as well. Similarly, alternatives such as ‘balanced’ slide valves instead of ‘uniflow’exhaust and other variations of valve operation can be used.

Full size applications are possible as ‘servo’ motors or small winch motors, where compressed air is available, or as ‘hydraulic’ motors. Here a metered oil feed would be used to the cylinder oilers and self lubricating materials used to the wearing faces of the valves.

My primary aim is to produce a ‘motor’ for fitting into a 5” gauge steam locomotive, with the future possibility of a multi cylinder 7 1/4” gauge locomotive, such as the ‘Paget’ 2-6-2 built by the Midland Railway, or a doubled up in size ‘motor’ for a large scale ‘under type’ steam lorry!

POSTSCRIPT

In the Autumn 2007 “Oily Rag” ( Issue 91 ), my article was published about a ‘fast track’ 5” gauge Live Steam Bulleid Pacific, using an existing kit for a battery electric version adapted to suit. At the time of publication, I was awaiting a reply about a possible modification of their chassis to accommodate the firebox.

They had had various enquiries about the kit and were intending to produce another batch at the end of the summer. When I phoned them about my query, I was dismayed to learn that the other partner was not willing to proceed with this new batch and the project was being sold on to another party. Though they said that they would pass on a list of names of those who had shown an interest, I have heard nothing since.

By this time, I had a desire to proceed with the ‘steam motor’anyway, building a ‘freelance’ design of 2-8-0 with a simple form of chassis, my Plan ‘B’! Hence the delay in progress.

Meantime, my design for the ‘steam motor’ has been formulated into the “Rolbro” Motor’ the subject of this article. As the main component of this is the cylinder/piston assembly, I found my argument for alternative uses under ’Research and Development’ so compelling, I am now looking at the possibility of using the connecting rod option mounted externally and driving directly onto the driving wheels! With the absence of the dummy outside cylinders and connecting rods of the original kit version, I feel that using an internal ’steam motor’ would be visually less attractive to the public. The “Rolbro” Motor could still be built, if the cylinder/piston assembly proves successful, for use in another project.

To accommodate the four single acting cylinders, I propose to base the locomotive on the Pennsylvania “ Duplex” layout in a 2-4-4-0 arrangement, with the driving wheel cranks of each group at 180 degrees and a single set of valve gear driving both valves, the equivalent of a single double acting cylinder. All the wheels of both groups will be coupled by chain and sprocket, with the crank pins of each group at 90 degrees to the other for self starting. This gives the equivalent of a conventional two cylinder, eight coupled locomotive. As the wheels are directly driven, instead of using a reduction ratio drive from the ’steam motor’, it will be necessary to increase the piston stroke to obtain a good Tractive Effort. For calculation purposes, 60% of boiler pressure is used giving a W.P. of 125 psi. for the boiler for the 75 psi used. With 1.25” bore and wheel diameter of 5”, a stroke of 2.25” is required to give a Tractive Effort of 52 lb. As a T.E. of 70lb is estimated to be able to start 1 ton on level track, this makes the potential for serious hauling, say 10 adults! In reality the W.P. will be reduced to 100 psi, or less, with the effective pressure in the cylinders dependent on the steaming of the boiler. The actual adhesive weight will be the governing factor! Unless we use traction tyres!