Crankpin problems.

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Tim V
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Crankpin problems.

Postby Tim V » Wed Oct 13, 2010 5:53 pm

I've had a lot of bad experiences with commercial crankpins, so for some time I've made my own on the lathe.

First requirement is to understand what is wrong with the commercial ones, and then come up with a solution.

I have no objection to the 14BA screws that form the basis of the pin, unlike Chris Pendlenton who uses 12BA.

A list of problems, in no particular order.

1. The hole through the crankpin bush is sometimes oversize, or off centre leading to non consistent crank throws
2. The crankpins are all the same length - lack of flexibility, or rods fouling wheel centres
3. Crankpins coming loose - caused by poor length of thread

Any ideas for more?
Tim V
(Not all railways in Somerset went to Dorset)

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Russ Elliott
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Re: Crankpin problems.

Postby Russ Elliott » Wed Oct 13, 2010 10:29 pm

I don't understand your point 2, Tim.

HowardGWR

Re: Crankpin problems.

Postby HowardGWR » Fri Oct 15, 2010 6:10 pm

I enter this discussion in trepidation as I am no engineer. I have always thought the use of plastic stuff for wheel centres to be dubious because it can flex, Thus a crank pin, made of a non flexible substance, such as brass or steel will win the unequal struggle when the two are put under strain. I have not heard that Romford technology gives much trouble.

Am I off beam here?

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grovenor-2685
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Re: Crankpin problems.

Postby grovenor-2685 » Fri Oct 15, 2010 6:56 pm

Am I off beam here?


Only in that Romford technology is not available in P4 standards.

I do remember, years ago granted, people making a living turning Romford wheelsets to make them round and wobble free, not noticed such adverts lately, perhaps the quality has improved, but no use if you can't get them in the required standard.
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Keith
Grovenor Sidings

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Tim V
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Re: Crankpin problems.

Postby Tim V » Fri Oct 15, 2010 8:50 pm

Russ Elliott wrote:I don't understand your point 2, Tim.


The length of the crankpin can be customised to suit the width of the coupling rod, by the time a coupling and connecting rod are on the same pin, they exceed the normal longest commercial crankpin. Hence I make my own.
Tim V
(Not all railways in Somerset went to Dorset)

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Russ Elliott
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Re: Crankpin problems.

Postby Russ Elliott » Sat Oct 16, 2010 12:49 pm

Tim V wrote:The length of the crankpin can be customised to suit the width of the coupling rod, by the time a coupling and connecting rod are on the same pin, they exceed the normal longest commercial crankpin. Hence I make my own.

Are you talking about pins or bushes, Tim?

HowardGWR

Re: Crankpin problems.

Postby HowardGWR » Mon Oct 18, 2010 7:38 pm

My point about Romford wheels had nothing to do with their roundness, merely that the idea of metal on metal is less likely to flex than metal on soft plastic. But what do I know? I'll answer that. Very little. :-)

Philip Hall
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Re: Crankpin problems.

Postby Philip Hall » Mon Oct 18, 2010 9:02 pm

Howard

You're quite right, a metal wheel centre is in theory going to be stronger than plastic. But in practice, given the loads that we place on wheels, the right kind of plastic will grip a smooth axle and be plenty rigid enough. Gibson wheels, like Sharmans of old, are a bit more flexible than Ultrascale, but all of them will hold a crankpin quite satisfactorily. Given the right kind of plastic (and this is where I have no idea of the engineering) strength and rigidity are not a problem - witness the esteemed High Level gearboxes, with high quality plastic gears.

Having said all that, Alan Gibson did make brass centred wheels for a few prototypes at one time (and I think Colin Seymour has an idea to bring them back sometime) but this was more to cater for the split axle enthusiasts amongst our brethren than to increase the strength.

Philip

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Tim V
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Re: Crankpin problems.

Postby Tim V » Sat Nov 27, 2010 10:00 pm

I was reminded at the Skills Day in Portsmouth, that this thread needs to be continued, so I shall be reaching for the lathe.
Tim V
(Not all railways in Somerset went to Dorset)

martin goodall
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Re: Crankpin problems.

Postby martin goodall » Sun Nov 28, 2010 4:12 pm

Can you also complete the 45XX thread, Tim? It was very useful and I would like to read about the final completion of the project.

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Tim V
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Re: Crankpin problems.

Postby Tim V » Sun Nov 28, 2010 4:52 pm

Was it unfinished?
Tim V
(Not all railways in Somerset went to Dorset)

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Tim V
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Re: Crankpin problems.

Postby Tim V » Sun Nov 28, 2010 6:12 pm

First requirement with small machine tools is sharp cutting tools. Recently I've found diamond honing stones in the 99p shop! Give a first class finish on HSS steel.
Tim V
(Not all railways in Somerset went to Dorset)

HowardGWR

Re: Crankpin problems.

Postby HowardGWR » Sun Nov 28, 2010 10:37 pm

Isn't it b. marvellous. There we were, three people unknown to each other, sat next to each other on Phil's demo table, and we didn't mention the subject once, at least nobody said anything to me. I've only just caught up with the plastic vs metal thing here.

Still, nice meeting you and I am busily replacing crankpins, bushes and goodness knows what else on my American method pannier. Will keep you informed on some thread or other.

I thought the format of Expo EM South not bad, not bad at all.

HowardGWR

Re: Crankpin problems.

Postby HowardGWR » Sun Nov 28, 2010 10:48 pm

I should hastily add tfhat I was the only unknown of course!

Philip Hall
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Re: Crankpin problems.

Postby Philip Hall » Wed Dec 01, 2010 10:12 pm

Sorry if I didn't recognise any of you at Expo South. David Dornom kept me busy all day guiding him through his M7 conversion...

Philip

martin goodall
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Re: Crankpin problems.

Postby martin goodall » Thu Dec 02, 2010 5:13 pm

Tim V wrote:Was it unfinished?


The last post on the 45XX chassis of which I was aware was on 19/9 showing another Comet chassis (followed by some comments concluding with Geoff Brewin's note of 30/9). Having found this thread extremely interesting and useful, I was looking forward to one or more further episodes leading to illustration of the fully completed chassis, together with any observations on testing and adjustments when tried on the layout. [Some people are never satisfied!]


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