Scale Traction Engine Speeds

[Anonymous]

I’m trying to get a feel for the maximum rate at which the rear wheels of my 4″ scale DCC Burrell traction engine will be rotating.

 

I assume that the crankshaft will be running at a maximum of about 160rpm? The lowest ratio to the second shaft is 2.739, and the final drive ratio is 3.6. This gives a total reduction of 9.86, and hence a rear wheel rotation rate of a little over 16 rpm. Does this seem reasonable, or have I made a false assumption somewhere?

 

Regards,

 

Andrew

[Anonymous]

[JasonBModerator @jasonb]

Model engines rev faster, you want to be looking at double that and steel feel comfortable. Alloy pistons will also make things run smoother at these revs.

 

Jason

[John OlsenParticipant @johnolsen79199]

Wouldn’t you work backwards from the wheel size and the desired operating speed? I would assume that a top speed of somewhere between 5 and 10 mph would be reasonable for a machine like this. 10 mph is about 15 feet per second, if the wheels are 2 feet diameter then they are near enough 6 feet circumference so at 10mph they would need to do 2.5 revs per second, that would be 150 rpm. Given your overall gear ratio that would require the engine to do 1479 rpm. That might sound high but is not too unreasonable. It will of course be less if your wheels are bigger than two feet, I’m just guessing that a full size wheel might be 7 or 8 feet. My compound launch engine, 3 inch and five inch bores by three inch stroke can easily do 600 rpm with no signs of distress. That was measured by digital tachometer, so is pretty accurate.

 

regards

John

[JasonBModerator @jasonb]

The problem is you can’t easily alter the rations that much if you want to keep everything within scale, there is only a certain size gear combinations that will fit. Did have a look at model rpms and it seems that you would multiply teh full siz espeed buy your scale so full size would be gorverned to 175prm times 3 (12/4) gives you 525rpm you can work your speed out from this.

 

I think something more like 6mph in top gear is the average for a 4″ on rubbers

 

Andrew have a look over on Traction Talk, this subject has been covered a numkber of times, several there have the same engine.

 

J

Edited By JasonB on 11/01/2011 13:44:34

[Anonymous]

Ah, my 267th post! Which will mean nothing to anybody but me, as 267 is the competition number on one of my gliders!

 

John: Actually no, I’m interested in what sort of rpm the rear wheel might do, not the speed of the engine in mph per se. Although of course the two are connected, and you’re spot on, the rear wheels are 2 feet in diameter.

 

Jason: Thanks for the information, and for reminding me about the TE forum. I am a member there, but don’t post much. I don’t know enough to make intelligent posts! The consensus seems to be about 500rpm for the engine and about 50rpm for the rear wheels, give or take.

 

The reason I ask is because I’ve boo-boo’d on machining the final gear castings, which are 14.8″ in diameter. The periphery and bore are concentric to within a thou or two, but the side of the gear has a slight wobble, about 20thou. This won’t affect the mesh of the gears, but I wondered if it will be noticable as the wheels rotate, and should I try and do something about it.

 

Also, in true parliamentary fashion I feel entitled to ask a supplementary question. I am intending to use cast iron pistons, and was going to make cast iron rings, but John Rex recommends bronze rings. What do you think?

 

Regards,

 

Andrew

Edited By Andrew Johnston on 11/01/2011 20:51:07

[JasonBModerator @jasonb]

The wobble won’t show that much, your likely to have as much or more in the wheels anyway.

 

Most tend to run iron rings with either iron or alloy pistons. The Clupet rings also come highly recommended, not heard of any other engines running bronze rings, again a search on TT will give you plenty of reading and don’t be affraid to ask anything there or post some pics of your progress.

 

J

[Anonymous]

Hi Jason,

 

Thanks for the info, when I get time I’ll spend a while searching through the TE forum. I had thought about Clupet rings, but assumed they’d be all but impossible to get in small quantities. I am building two identical engines, but quantities required are still small! Any ideas as to where to purchase them, other than ‘phoning the manufacturer?

 

Regards,

 

Andrew

[Diane CarneyModerator @dianecarney30678]

Andrew

As far as I know you can purchase exactly what you need from Clupet. He is very good – we purchased a set of rings for the Sentinel (admitedly some time ago now). Don’t delay too long though. I have heard that he is looking to retire from the business. He’s at Factory B, Solway Ind Est., Maryport CA15 8NF.

Regards

Diane

[JasonBModerator @jasonb]

As Diane says give him a call and he will advise you of grove size etc, very helpful and will sell just one ring if thats all you want.

 

Tel 01900 818 361

 

J

[Anonymous]

Hi Diane/Jason,

 

Great, thanks for the information. In due course I’ll give Clupet a call. If he’s going to retire I might order some spare rings!

 

I have read that Clupet rings can be prone to breaking if they cross the steam ports. I’m building SCC (yes, I know I said DCC in the original post, that was a boo-boo ’cause it was late) Burrells to the LSM drawings, but using castings from John Rex. His cylinder/liners are significantly different to the drawings, since the cylinder castings have no steam ports cast in, but use thick liners with the steam passages milled on the outside of same. I’m having a bit of difficulty getting details of the passages out of John, so I’ll have to check clearances when I do get details.

 

I’m also confused about whether to use cast iron or aluminium alloy pistons. Obviously the alloy pistons will be lighter, but the recommended clearances seem pretty large. A rattling good fit actually. Decisions, decisions.

 

Regards,

 

Andrew

[JasonBModerator @jasonb]

If your cross head/guides and gland hole are accurate then the piston should not touch the sides of the cylinder, so piston slap is not an issue, think you need about 0.003″ per inch piston dia for clearance.

 

Have you seen Conrad’s site covering the build of his Burrell, there are some photos of the passages being machined into the liner, he’s also on TT as “Rustysteel”

 

Jason

[Anonymous]

Hi Jason,

 

Thanks, and yes I am aware of Conrad’s site; some very interesting techniques therein. From a business perspective it’s not really my problem to sort out the drawings. The supplier has chosen to depart from the LSM drawings; I don’t have a problem with that. I assume that it leads to a cheaper cylinder casting, as it requires fewer cores. However, having gone that route he has an obligation to tell me how to machine the supplied castings and liners. The next time I call in, on my way back from up north to retrieve my glider, I’m planning on picking up the remaining castings, the boilers, and I’m going to sit down with the LSM drawings and mark them up so I understand exactly what to do. I’m not going to mess up an expensive casting because I didn’t have the proper information.

 

Given the recommended clearances for aluminium alloy pistons and cast iron liners it’ll be interesting to see how I get on running my model hit ‘n’ miss engine, as that has an alloy piston and cast iron liner. The clearance as I’ve machined it is much less than 3 thou per inch. Presumably it will seize in fairly short order?

 

Regards,

 

Andrew

Edited By Andrew Johnston on 16/01/2011 20:27:08

[JasonBModerator @jasonb]

They don’t get that hot, I can run mine for quite a while without water, probably cooler than steam engine, can’t remember what I allowed but it was more than the zero clearance of 1.125 bore and 1.125 piston spec’d on the drawing

 

Jason

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