MINNIE TRACTION – Steam link between Cylinder Block and Boiler

[Malcolm FarrantParticipant @malcolmfarrant93389]

[Malcolm FarrantParticipant @malcolmfarrant93389]

I have searched Masons book, but have been unable to find details of where, and what size any hole that may have to be drilled in the top of the Boiler should be.

All Help VERY much apreciated. Malcolm Farrant (Roskrow)

[Brian GParticipant @briang]

Hi Malcolm, I got curious and started looking at the book. On the last page of chapter 7 (page 148 in my 1977 edition) "The gasket can be used to mark out the area on top of the boiler where the cluster of small steam holes occur. These of course must be drilled before finally bolting down the cylinder".

As far as I can see there is no mention of how many holes make a cluster, or how big a small hole is.

Brian G

[roy entwistleParticipant @royentwistle24699]

Malcolm My 1971 copy is the same as Brian G's just mentions a cluster. I've just had a quick look at the drawings,doesn't appear to be anything on there either

Roy

[pgk pgkParticipant @pgkpgk17461]

Of no help at all but i guess a small hole is smaller than a big hole and bigger than a tiny hole…

[Brian GParticipant @briang]

I just took a look at "Metric Minnie" on modelengineeringwebsite.com and although the holes are not on the metric drawings either there is a photo showing the holes made by the builder.

Brian G

[Dave HalfordParticipant @davehalford22513]

They are meant to help reduce priming so what ever drill will fit inside the gasket hole. Jason will no doubt have the correct answer.

[Clive Brown 1Participant @clivebrown1]

If the total cross-sectional area of the holes is comfortably larger than the exhaust blast orifice area, I'd think that was OK.

Re the boiler in the photo; ideally I'd put the holes on the boiler vertical centre-line if possible to maximise the height above the water level.

[JasonBModerator @jasonb]

Jason can't remember what he did on his as it was about 20yrs ago, possibly 5No 1/8" holes

They will need to be off ctr as the void in the bottom of the casting is off ctr

[Brian AbbottParticipant @brianabbott67793]

Hello Malcolm

I just machined a slot to match the pocket in the cylinder, possibly not the best solution to reduce priming but I not going to worry..

[Richard S2Participant @richards2]

I went for 6 off 1/8" diameter holes. 1 in each rounded corner of the chamber and 2 (drilled after pic taken) equispaced in the centre of it.

[SillyOldDufferModerator @sillyoldduffer]

Just a suggestion, but this is one of those questions about achieving a balance. It's about how best to allow wanted steam to flow whilst blocking priming aka foaming, ie unwanted water which reduces performance and can damage the engine.

Priming is caused by sudden demand for steam, dirty water (especially oil contaminated), and too much water in the boiler. Overfilling is one cause, but there's also a temporary effect caused by splashing. Bouncing a model scale engine across rough ground or sudden braking is a likely cause of priming.

Brian's large single slot solution delivers maximum steam flow. This is best for engine performance but provides no protection against water passing into the cylinder. But anti-priming protection may not be necessary if the engine is carefully filled and driven considerately on smooth surfaces.

Richard's six 1/8" holes provide much better protection against priming (if that's a problem), but protection comes at the cost of restricted steam flow. His relatively small aperture might throttle steam and constrain the engine's speed and pulling power. Again this might not matter, provided the engine performs well enough on the ground.

Guessing from the photo, Brian's slot is about 1/2" by 1", providing 0.5 square inches of steam access. In comparison Richard's six 1/8" diameter holes provide 0.07 square inches of access, roughly 15% of Brian's, a lot less. Consequently, Richard's engine is more likely to be throttled than Brian's.

Or maybe not! The passages inside the cylinder and valve also limit the amount of steam that can get to the piston, and these passages are likely to be small – less than 1/4" in diameter? There's not much point making a huge steam hole in the boiler when the piston can't use it. Therefore I suggest the total area of the anti-priming holes should be made about twice the area of the cylinder passage, whatever that is. The boiler port being bigger in area than the cylinder passages will avoid throttling while releasing space under the gasket for more smallish anti-priming holes. It should be possible to prioritise steam flow, whilst providing useful anti-priming. (Ideally there would be a shield inside the boiler as well, but perhaps that's more trouble than it's worth. Do model traction engines suffer badly from priming? Perhaps not. I don't know.)

The optimum diameter and spacing of each anti-priming hole is another issue. I guess small diameter holes will be better at keeping out water (good) at cost of increased friction (bad). Thousands of miniature holes would reduce steam flow far more than a single hole of the same equivalent area. I expect the optimum is covered in the engineering literature if anyone can find it! Although once found the theory may not cover small engines.

In practice, because small engines can never be operated as optimally as big ones, the exact diameter and layout of the holes may not matter much provided they allow enough steam out to satisfy piston demand. 1/8" diameter holes feels big to me, while 1/16" feels too small. How about 2.5mm! Your guess is as good as mine…

Dave

[Former MemberParticipant @formermember19781]

[This posting has been removed]

[Malcolm FarrantParticipant @malcolmfarrant93389]

Whow, You have all been so generous in your helpful messages. I think I am going for Dave's Suggestion. i.e 2.5mm holes, These I will drill as near to the centre line as is possibe.

As and when all is working I will post a message. (Fingers crossed !!)

Again thank you ALL for your advise and help

Very best wishes

Malcolm

[Dave HalfordParticipant @davehalford22513]

By way of an example the Haining 2" scale 16hp plougher has 4off 5/16" holes on 5/8" centers. The boiler is a tad larger at 25 1/2" x 5 3/4"

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