View Full Version : It looks good, but is it the best answer?

David Powell
09-17-2010, 10:14 AM
I am completing a 2" scale British Style model traction engine which I began many years ago, sold to another fellow and have now got back. Originally I built the boiler with the hornplates as extensions of the firebox sides, it was welded professionally. The other fellow CUT THEM OFF !. I was left with replacing them somehow. I could have simply restayed the boiler with longer threaded stays and fitted the hornplates on them,as most models are made. which would have given me 6 10/32 bolts in a 3" square pattern on each side. I decided to bolt my new hornplates on to the firebox sides with 4 40 allen heads. I now have the new plates fitted with 24 4 40s on each side in a 6" square pattern. The 10 32stay heads will be hidden under a dummy side plate. My question is simple. Is what I have done as strong as the other solution assuming the rest of the arrangements are the same.?( The firebox sides are machined flat and square to each other within about 5 thous, the plates will be separated from the firebox sides by a sheet of thin gasketing to reduce heat transfer). I use my models a lot and they lead a hard life. Regards David Powell.

Paul Alciatore
09-17-2010, 11:53 AM
Your post raises more questions in my mind. I have done some RR modeling, but in smaller scales like HO. You say it is a "2 inch" scale. Is that 2" to the foot (1/6th scale) or a 2" track gauge (about 1/28 scale using US standard gauge - likely different in England)?

From what I can understand you are asking if 24 4-40 bolts are as strong as 6 10-32s. I have not worked with any live steam models and am not completely familiar with the structural requirements or the stresses that act apon these hornplates. Are they just supporting the boiler against gravity? Or are there other stresses? To prevent buckling from the steam pressure? And what are the nature of these stresses? Sheer? Tension? Combination?

Oh, and you say, "The 10 32stay heads will be hidden under a dummy side plate." If you are not using any 10-32s, what are you hiding? Or was that the idea if you had used them, which you didn't?

Also, what grade of bolts? Are we talking grade 3? Or higher, perhaps SHCS? Gut feeling: 24 4-40 SHCSs would be stronger than 6 grade 3 10-32s. But calculations would be needed. And by "square pattern" did you mean actually square or perhaps rectangular? I envision a rectangular part, not actually square. This could matter as the central two screws in a 2 x 3 rectangle would share less of the load than the four corner ones. While a real square pattern would keep all the stresses on the screws much closer to equal.

A picture or drawing would help.

David Powell
09-17-2010, 12:54 PM
I havent yet learnt how to do more than make word postings. Very briefly, the Model is approximately a sixth of full size, it will weigh about 120 lbs, the cylinder is 1 1/4"" Bore by 2" stroke and will be geared down to the rear 1 ft Dia rear wheels about 20 to 1 , In full size engines the horn plates, on which the crankshaft, intermediate and rear axle bearings are mounted are made as an integral part of the boiler, being extensions of the firebox sides. Most models have separate hornplates bolted to the firebox sides. This is where my question arises, what will be stronger, six 10/32 bolts in a 3" approx square pattern or 24 4 40 s in a 6" square pattern ? I went with the 4/40s so I didnt have to disturb the already fitted 10/32 staybolts there is no question regarding the boiler strength for the job,( 1/4" minimum plate thickness etc)The bolts take the weight on the rear wheels together with all the driving forces, which when pulling 3-400 lbs on rough grassland and the front wheels get stuck in a rut, can be considerable. I have seen models which have had problems with bolts loosening or shearing and I do not want any problems. Hope this helps thanks David Powell.

09-17-2010, 03:01 PM
Hi David, so is it just a matter of calculating all the minor diameters? the 10/32's total .107 sq inches and 4 40's total .122 sq inches so 4 40's should be stronger; assuming in each case they all take the load evenly...more force is required to sheer .122 than .107...but that seems too simple?

I'm with Paul, without being closely familair with the way these are built its hard to understand without a pic or a sketch; have you a camera? There's a tutorial thread near the top of the thread listings or i could i help

09-17-2010, 07:36 PM
Why not attach the pics to an email to a member here who can post them to say Photo Bucket for you and either email the links to you to use in a post of your own or have that member post the links to the pics on his own post for you?

Liger Zero
09-17-2010, 08:52 PM
I sent the OP a PM with detailed instructions on how to set up Photobucket and post an image.