6" Burrell Gold Medal Tractor

The route I  taken to arrive at a 6" Burrell Gold Medal Tractor(GMT) is not straight forward and has many twists and turns.  I did not start out looking to build either a Burrell or at 6" scale.  I have spent the last few years building large scale Traction Engines using pre-machined kits supplied by ModelWorks and Steam Traction World.  Prior to that I had scratch built several stationary engines and pieces of workshop equipment.  The kits were intended to be a brief interlude so that I could learn about boilers and steam generation.  My final rite of passage on the subject of boilers was the building a boiler for a  5" Gauge Railmotor locomotive.  It therefor should not come a a surprise that I am scratch building again.

Driving my 5" Butch

I am using the term "Road Locomotive" with care and have just got out my copy of Geoffrey' Gilbert's excellent book "Traction Engine Design and Construction 1900-1930" to avoid any slip ups.  I find it strange that agricultural traction engines are the most frequently modeled sub type but often presented as if they were road locomotives.  To some extent I have been guilty of this with "Little Beastie" as I have painted and us it as if it were a road locomotive.  For me, the engineering is paramount so I decided to make a thoroughbred road engine.

4" scale McLaren 10nhp Road Locomotive Brutus

It soon became apparent that I was not ready to go solo and design and make my own casting and I started looking at the established designs.  When I saw the 4" McLaren presented as a road locomotive (as opposed to a showman's)  I swang in that direction.  It met all my objectives except for weight.  At 15cwt it was too heavy to be safely towed with my existing trailer and car.  I pursued the 4" McLaren for a while but then another change occurred.  I saw the 3" McLaren in steam and was impressed with its performance and easy of steaming.  At 60 inches in length and 7cwt the 3" McLaren is a big 3" engine and steams more like a 4".  Guess what I switched to the 3" McLaren.

I am not too sure exactly when and why I switched to the 6" Burrell Gold Medal Tractor.  Perhaps I was not convinced the 3" McLaren was big enough though with the prospect of now building a 6" engine I may regret dropping the McLaren.

Dave Bennion's 6" Gold Medal Tractor

The Burrell Gold Medal Tractor was a small engine as it was designed to meet the requirements of a light locomotive which, at the time, were limited to under 5 tons.  If we assume the weigh of a model goes according to a cube law the weight of the 6"GMT would be 12 1/2 cwt but I know it will be heaver.  After a little research I concluded that the weight and size of the 6"GMT is similar to the 4" McLaren.  I had researched a tentative solution to transport the 4" McLaren and I realised I could use a short  wheel base (SWB) van to transport the 6"GMT.  Buying a SWB van with a 1.4 tonne payload proved difficult as few manfactures put the big axles on their SWB van.  LDV offer a suitable van but in the second hand market it realy comes up as most first time buyer want volume as well as space on go for the LWB variant.

What impressed me with the McLaren was the design of the block.  Most of the steam passages were cast in and this is one reason miniature McLarens are such good performers.  Edward has taken this one stage further with his GMT and has fully cored the cylinder much as per fullsize.  I am hoping this will give excellent thermodynamic performance as there is far less metal to heat up.

I am finding hard to come up with a believable weight for the 6" GMT.  I find the casting much heaver than the equivalent 4" Burrell parts.  I can lift one of my 4" wheels but con only just lift the individual items that go to make up the 6" GMT wheel (spokes,rims & hub).  I am told that Dave Bennion's 6" GMT came in at 16 cwt but being skeptical won't be sure until I see my engine on the scales.

Along the way I have produced this table to help me decide if the completed engine will be the "right size".  Distances are easy to scale whereas weights don't quite follow natures cube law as intended.  This is because it is often necessary to make plates thicker than dictated by mathematical scaling.  In addition to Dave's weight I have estimated weights by scaling up the MJ 2" by 27 (3^3) and the fullsize down buy 1/8.  This gives a weight range of 1400 to 2160 lbs.

 Having  started machining the 6" GMT the biggest concern I have it that I may have a change of heart before I have finished the project!