Rand Society of
Model Engineers
RSME Newsletter
April 2012
From the editor 2
Farewell to Brian and Ria 2
From the Secretary’s Desk 3
Notes on Shrink fits 4
A new type of boiler 6
New Member 8
1st Sunday of each month – club day
3rd Sunday each month – family day
On both days members and their friends are welcome to visit the club and participate in running locomotives or just enjoying a picnic . The public are also welcome.
From the Editor
First of all – a lot of thanks to all members who sent contributions. Without your help, I cannot put this newsletter together. Many thanks also to Collin who is the unofficial language editor. He tries to prevent me from reinventing the English language.
Let's face it, English is a crazy language. There is no egg in eggplant, nor ham in hamburger; neither apple nor pine in pineapple. English muffins weren't invented in England or French fries in France . Sweetmeats are candies while sweetbreads, which aren't sweet, are meat. We take English for granted. But if we explore its paradoxes, we find that quicksand can work slowly, boxing rings are square and a guinea pig is neither from Guinea nor is it a pig.
And why is it that writers write, but fingers don't fing, grocers don't groce and hammers don't ham? If the plural of tooth is teeth, why isn't the plural of booth, beeth? One goose, 2 geese. So one moose, 2 meese? One index, 2 indices? Doesn't it seem crazy that you can make amends but not one amend? If you have a bunch of odds and ends and get rid of all but one of them, what do you call it?
If teachers taught, why didn't preachers praught? If a vegetarian eats vegetables, what does a humanitarian eat? Sometimes I think all the English speakers should be committed to an asylum for the verbally insane. In what language do people recite at a play and play at a recital? Ship by truck and send cargo by ship? Have noses that run and feet that smell?
How can a slim chance and a fat chance be the same, while a wise man and a wise guy are opposites? You have to marvel at the unique lunacy of a language in which your house can burn up as it burns down; you fill in a form by filling it out; an alarm goes off by going on.
English was invented by people, not computers, and it reflects the creativity of the human race, which, of course, is not a race at all. That is why, when the stars are out, they are visible, but when the lights are out, they are invisible.
I am getting all steamed up about this, which is a bad thing. If I get my loco’s boiler to steam up, on the other hand, that is a good thing. You see why I am getting confused.
PS. - Why doesn't 'Buick' rhyme with 'quick'?
(From an e-mail sent to me by Prof. Pieter van Jaarsveld)
FAREWELL TO BRIAN AND RIA
On Sunday 15th I said farewell to Brian and Ria Armstrong who, by the time you read this, will have relocated to the United Kingdom.
We will certainly miss both of them as they were very active members who participated enthusiastically in the society’s activities. Brian in matters engineering and Ria in matters culinary – assisting the ladies in the kitchen who look after the refreshment and victualling needs of the members and visitors on track and family days as well as on special occasions hosted by the society such as the Regional and National Steam Meets.
Brian’s association with RSME goes back to after the 2nd World War in the late 1940s when, as a youngster of about 11 years of age, he was involved with his father, Johnny Armstrong, in the construction of the society’s layout at the old Rand Easter Show grounds at Milner Park.
The Armstrong family subsequently joined Johannesburg Live Steamers and Brian renewed his association with RSME in 1986 when the society was obliged to relocate to Len Rutter Park in Roodepoort when the Milner Park grounds were taken over by Wits University and helped in the re-establishment of the infrastructure in the park.
He qualified as a Fitter and Turner and with the encouragement of his father, applied these skills to his model building that was prolific and of a very high standard of workmanship. He recalls building 9 models in his time.
He served on the society’s Executive Committee and was generous in sharing his wisdom and experience in model building with new members and could always be relied upon to throw his weight into projects that improved the facilities at the club premises.
On behalf of the membership, I wish Brian and Ria a happy and prosperous life in the UK and that Brian finds a model engineering club whose membership will embrace his skills and dedication to the hobby.
Colin Retief
FROM THE SECRETARY’S DESK
We still await the renewal of our lease with Propcom, the Johannesburg City Council’s property management company and have been reluctant to invest in further development of the facilities until this comes to pass.
However, we cannot wait any longer and are driven by circumstances to undertake certain urgent work that requires labour resources in addition to the inevitable financial input.
To enable the committee to plan accordingly, we are appealing to members who are prepared to assist in giving their time either for work parties on a Saturday or for those who are retired, during the week, please to get in touch with the secretary stating what you are able to do. The secretary’s telephone number is 083 307 1475 or Email;
Financial contributions will also be welcome in lieu of your labour.
The following projects, in progress or planned, need to be addressed and we are looking for members who can volunteer their time and skills to assist.
· Removal of rotting, overgrown and fallen trees in the stream area
· Completion of the library – book cases installed but books require to be catalogued and filed. – in progress
· Conversion of old meeting room and kitchen to a workshop and a secure storage room for valuable tools, equipment and materials – in progress
· Conversion of old store room to additional accommodation for Daniel – toilet, washbasin and shower to be installed and solar water heater installed to reduce electricity consumption that has become a significant operating cost.
· Repair of exhibits and equipment in the museum that have been vandalized by theft of copper cable and metal fittings.
· Isolation of museum from casual entry via main door or rear service entrance to prevent theft and vandalizing of exhibits as well as ensure safety of members and visitors using toilets.
· Repainting of the club premises inside and outside. It looks very run down.
· Refurbishment of bogies on passenger wagons. Wheels and axles have been ordered.
· Completion of construction of 2x Lawley 7.25 inch club locomotives urgently required for steam haulage of passengers – awaiting completion of workshop.
NOTES ON SHRINK FITS
Shrink fits are useful in Models, for Wheels on Axles, Steel Tyres on Cast Iron Wheels, Crank Webs on the Shafts of Crank Axles, and so on. But be careful, too much shrink can be disaster, as can too little.
The point to bear in mind is, that when the Hub of the Wheel is shrunk on, there is a circumferential Tensile Stress in it, i.e: - a stress around the Hub. The resistance of Steel to Tensile Stress is good – not so with Cast Iron, it is weak! There are many stories of: - ‘I shrank the Wheel onto the Axle, and then as it cooled down, it went plink, and it was cracked!’
So how much do we allow?
The traditional ‘Rule of Thumb’ was to allow 1 thou per inch of Shaft Diameter, (0.001/1”)
There is nothing wrong with it for most jobs! It is certainly OK for Models.
Having been involved with Turbine Rotors with shrunk on Discs, which may be able to come loose at an overspeed of 15% of running speed, due to centrifugal forces, I am cautious.
The better source of Formulae for us is in Machinery’s Handbook, Pg 1532.
For a Cast Iron Wheel on a Steel Axle: -
A = T(2+C)
30000000
Where A = Allowance, in./in diameter.
T = Tangential Stress at the Interface of Hub and Wheel.
C = A Factor which varies with the ratio of the Outer and Inner Diameters of the Hub.
For Cast Iron it is suggested that T be 10 000.
For a Steel Wheel on a Steel Axle: -
A = T(1+C)
30000000
For Steel it is suggested that T be 25000.
For a typical Cast Iron Wheel in which the Ratio of the Hub Diameters is 2, Table 1 on Pg. 1533 gives C as 0.348, giving an allowance of .0008”
If it were a Steel Wheel, the allowance would be 0.0011”
So for a typical Axle Diameter of say ¾” at 0.001”/1”, the allowance would be 0.00075”.
Now we need to know how hot the Wheel must be to give this fit?
The Formula is: -
T = A x D
E
Where: - T – Temperature deg. C
A = Allowance in./in.
D = Diameter in.
E = Expansion Coefficient in./in./degree Centigrade.
For Steel: - E = 0.0000114.
For Cast Iron: - E = 0.0000118
For Brass: - E = 0.0000180
For Bronze: - E = 0.0000180
For Aluminium: - E = 0.0000223
So for our ¾” Shaft with a Cast Iron Wheel, the Temperature increase needed is 66 deg. C. In practice, one would make it somewhat hotter to allow for fitting, if we chose to expand it by 0.003” the Temperature needed is 263 deg. C.
You will have seen from the calculation Method, that the required Shrink for Model sized components is very small, for many parts less than a ‘thou’, and the temperature necessary to expand the Hub (Wheel or Crank) is moderate.
The calculation method fits with the old ‘Rule of Thumb’ of one thou per inch of Shaft Diameter. (in Metric 0.001 mm per mm).
The critical point is that the Hub, once shrunk on, has a Tensile (stretching) Stress going around it, and it is not difficult to cause a failure. An interference of 0.004” to 0.005” is enough to cause failure in a Cast Iron Wheel on a ¾ in. Shaft. So it is particularly important to get the numbers correct for a Cast Iron Wheel. This is because the Tensile Strength of Cast Iron is low – from 10 Tons/sq.in. for a Grey Iron to 30 Tons/sq.in. for an Alloy Cast Iron. To add insult to injury, all too often we don’t know what Grade of Cast Iron we are dealing with.
Even with a Mild Steel Wheel or Crank be careful, it is unlikely that a failure will occur as the material is ductile (elastic). However if the Stress is too high there is a good chance that the Hub can become loose in time, due to it yielding a little more under the reversing working forces.
Opinion: - clearly to hold such a small size difference is not easy – no wonder Loctite is popular. BUT – any Adhesive requires scrupulous cleanliness AND it needs a clearance typically two to five ‘thou’ (0.002” to 0.005”), which is 0.05 to 0.125 mm. Adhesives are analogous to Soldering.
There are four methods for checking if the Wheel is hot enough to shrink on: -
1. No Tech - Guess and hope for the best, or just make it very hot!
2. Low Tech - Wipe with a wet cloth – if it sizzles off it will be about right!
3. Practical Tech 1 – Make a Plug Gauge, turn a short length of bar to bore size plus 0.002 to 0.003 (0.05 to 0.08 mm), when it can enter the wheel’s bore, the wheel is hot enough.
4. Practical Tech 2 – Make a Pin Gauge, a short length of thick wire with pointed ends, made to a length of bore size plus 0.002 to 0/003 (0.05 to 0.08 mm), when it can enter the bore squarely, the wheel is hot enough. The Pin Gauge is a better option than the Plug Gauge as if the wheel cools down quickly, and grips the Gauge, the wire Pin Gauge can be bent to get it out, the Plug gauge may need machining to get it out.
Peter Glanville.
A NEW TYPE OF BOILER FOR LOCOMOTIVES
The boiler used on John Heald’s Darjeeling and Himalayan Railway B-class locomotive is, in 7.25inch gauge, large, to say the least. Which begs the question: what material is best for such boilers? Copper is not economic, stainless steel has safety problems and the usual material, mild steel, has its drawbacks.
These days you look to Australia for boiler design innovation and approval.
It all started when Les Irwin from the Penwood Railway near Sydney, Australia contacted several experts for advice on an alternate to mild steel without the attendant problems of ordinary stainless steels. This group included Dr. Ian Ward, metallurgist, Kurt Staub, welding engineer and Robert Brunton, BSc in Metallurgy and University of New South Wales lecturer.
All recommended use of Duplex SAF 2205 steel alloy developed in Sweden by Sandvik Engineering Group as an alloy with a low expansion rate that would resist cyclic stress cracking at the welds, resist corrosion, and be suitable for pressure vessels.