Part built Allchin 1.5 inch

Pulling my head out with the final stages of the engine, knocking sounds seems to be bearings, however even after replacing the knock appears again, there seems to be a tiny wobble on crank that unevenly wears the bearings.

the crank shaft is something I can’t fix so looks like the slight knock is something I’m going to have too live with.

shame because I wanted it to run smooth, I had other parts that required work such as crank alignment and loose cotter pin all fixed but bearing wear pretty quick after making new ones.

i cant keep chasing my tail and going in circles so I think that’s the best I can have.

Maybe someone clever has an idea of a fix, I think the crank has a slight wobble so the bearing wears ever so slightly and I can pull the crank up and down causing a knocking sound.

if I replace the bearing I get the same result after a while of running, any suggestions 🙏

kind regards,

Del

As it’s far away from the heat, could I use PTFE plastic bearings inside the brass’s so not to have metal on metal and possibly remove the knock?

Derek,

PTFE is OK with heat, I use it as valve liners for water gauges and drain cocks etc, swells when it gets hot though and goes brittle over time. I guess the loads on a 1.5" scale are fairly light but even so I think you might be pushing it a bit. Given you haven't steamed it but are test running on air I am surprised the bearings are wearing that quickly, are you using brass or bronze? What is the nature of the wobble on the crank? Is it built up or machined from one piece? Can you not set it up true one end and then lightly skin the other to square it up and use an undersized split bearing where you have skimmed it? Afraid I am not familiar with the design so not easy to give good advice!

Paul.

Hi Paul

the crank has a tiny wobble, and seems to wear brass or bronze bush after an hour or so, the cranks is a one piece crank.

I fit new bush and seems ok but get worse and worse.

very frustrating....

I’ve just read that cast iron makes a far better bearing than bronze or brass, but not sure if it will help me,

.

Not in my comfort zone, Paul, but ... curiosity led me to this photo from Derek’s album:

**LINK**

https://www.model-engineer.co.uk/albums/member_photo.asp?a=49637&p=818310

MichaelG.

Any suggestions on how to fully true up the crank shaft or us this near on impossible not it’s finished? I’m concerned that I make new bearings but because of the runout it egg shaped the bearings yet again.

i hasn’t measured the run out/wobble yet but it’s very small, maybe this is normal and acceptable?

kind regards

Del

Derek,

Looking at the picture Micheal kindly linked too there are two possibilities that occur to me. Either it was cut form flat bar and machined or it was a casting. I am guessing this came ready made when you got the engine and you are not aware of its origin? I am guessing that originally it would have been turned between centres and if it was a casting it would have had a support cast in opposite the big end for turning the outer journals which would then be cut out to set it over to turn the large end. If it is out of true there are a couple of possibilities, if it was a casting the outer ends were finished to size, the ligament cut and at that point stress was released that induced a bend. If cut from bar stock possibly there was too much pressure from the tailstock when the large end was turned that induced a bend.

The crank for my half size (many times larger) was an SG iron casting. Order of play was to put in the two sets of centre drillings in the end blocks, set between centres and rough turn the outer ends and leave for a few weeks to relax. Turn the outer ends almost to size, cut out the ligament and set up between centres with wood blocks between the webs and the end blocks to give support from bending and finish turn the large end journal. Then reset to the original centres with a packer between the webs to support and skim the outer details to finished size. It did change shape very slightly between steps (after leaving to relax and after cutting out the ligament) but following that process meant the final item was straight and true.

How you can or can't deal with this depends really on the material of the crank, if it's cast bending it back isn't likely to be an option, cos it will likely snap! If it's bar stock you have more chance.

First thing is to identify against a clock where the error likely is. If the run out is all in the plane of the large end heating and tweaking if it's cast might correct it. Very first crucial thing to understand is exactly what is wrong with it, then you can make an informed decision on how or if to correct it. If it were mine I think I would set up a couple of thin vee plates at the bearing locations and rotate against a clock and mark the high / low spots at the ends or set between centres and clock the bearing locations to get a picture of what is going on.

Paul.

An important detail is missing from that photo, Derek ... So, I must ask:

Are the two ends of the crankshaft centre-drilled ?

A job like that should have been turned between centres: But it may be that the original builder was over-confident about what they could do in a chuck.

MichaelG.

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Edit: Good to see that Paul got-in with some informed advice whilst I was typing 

Edited By Michael Gilligan on 01/06/2020 12:36:26

Michael,

it may have been turned between centres even if there are no centres! It's usual for cast cranks to have blocks or webs at the ends to take the various centre drillings, once machining is complete they are cut off! I have put the centres back in after as they can be useful for other ops like tailstock support for spline milling but some may not do this.

Paul.

Hi Guys

thanks very much for responses, it has holes drilled in the end so must have been done between centres.

i think I need to set it up as Paul suggests to get an idea how far out it is, I’ve never noticed the run out before which is odd.

is it normal to have any sort off run out or would all cranks be totally spot on?

It’s possible it runs out by 0.1mm but I need to confirm if I can later.

Derek,

Depends who made it and how well! Normally you would expect a crank to be "spot on" certainly within half a thou. However they are tricky things to machine being spindly and long and easy to put a bend into. Machining the large end needs a long tool overhang with a fairly slender tool so it's easy to get chatter, dig ins etc! It's a case of sharp tooling, right speed and patience. Also of course adopting the right sequence if it's cast as it's likely to have internal stresses which can be released as machining progresses, hence the roughing and finishing processes and setting aside during for it to relax and find its line.

Paul.

.

Yes, I realise that Paul ; but it seemed a reasonable first line of questioning.

... Happy to back-out of the discussion now.

MichaelG.

Thanks Paul, all very interesting! I think I will avoid ever trying a crank!

looks like mine could be far more than half a thou out, I will attempt to check tonight.

Michael, I’m very glad if you have any ideas how to remove a few thou run out?

i know in the book it says to make the crank from bar stock, but I’m unsure how mine was made.

Derek,

Maybe check commercial suppliers and see if anyone offers a cast crank? If not you can be confident it is probably bar stock in which case it's a good candidate for straightening. If you have a decent size lathe, stick it between centres, clock it, put the clock opposite the bend, lump of brass in the tool post and give it a tweak with the cross slide. Don't do this in a watch makers lathe though! A good size lathe should be fine.

0.1 (0.004" is too much I would say!

Michael,

Wasnt decrying your question re centres, not everyone appreciates how the throw and the centres are achieved in the first place. Just pointing out that whatever centres are present now may not be those it was machined on. In fact as later additions the may not even be "in the centre" depending on how and why they were put there lol.

Paul.

Bloody smiley thing!

Edited By Paul Kemp on 01/06/2020 13:07:00

Many thanks, I shall give that a try 👍

My first move would be to mount the crankshaft between dead centres, and measure the bearing surfaces as thoroughly as possible [taper, run-out, eccentricity, surface finish], so that you understand the magnitude of the problem, and where it lies.

BUT ... As Paul so rightly emphasised, this pre-supposes that those centres are nominally the centres of the main bearings.

MichaelG.