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Post by silverfox on Jan 24, 2015 10:31:10 GMT
Chatting to a chap the other day re clearances on the crankpins. Now I am under the impression that the connecting rod crankpin should have as little tolerance as possible, now as the B17 middle cylinder runs off the leading axle, i would also assume that this needs as liitle slop as possible, otherwise, in my childlike mind the axles would be slightly out of sync and therefore the valve events would be a bit skew. He recommended that the clearance of up to 10 thou is permissible on all the pins. Now he is one of the 'old farts' ( See thread on Ally Pally) and i thanked him for the information. So i thought i would put this up for the collective for their input, before i start milling our the rods. I will of course, knock up some dummy rods before putting cutter to metal on the real thing My way of thinking is to have the leading and coupled axle as close a spossible without binding, and have the trailing axle to take up ( wrong word i know) the slack so i get a really good rotation.
So what does the team think?
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Post by Cro on Jan 24, 2015 10:40:36 GMT
I was always taught to get everything as close as possible, if you build it with 'slop' then its only going to get worse quicker. I would almost say it sounds silly to build it to get the leading and coupled axles good and then leave the trailing to be not quite as good, why not get them all great? This is just my opinion as I hate seeing sloppy motion.
Adam
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Post by andyhigham on Jan 24, 2015 11:05:30 GMT
The problem with to close fit on the coupling rods is as the suspension moves, the centre distance between the axles changes. The options are :- 1) Increased clearance on coupling rod bearings 2) Increased clearance between axle boxes and horns 3) A good fit on all bearings and risk binding up or even breaking something over a bump
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Post by Deleted on Jan 24, 2015 11:24:43 GMT
Hi guys------ Siverfox, you've come up against the same problem that dear Roger ( and to some extent, Joan) is battling with ie}--- these drawings for models are not Engineering Drawings as understood in to-day's world..."So just what clearances DO I use?" I hear you say......OK, consider these points}-----1) A 1" pin (for example) will NOT fit into a 1" hole because they are the SAME size...You'll need a few 'thou clearance just for this to happen alone.....2) Oil occupies space so this will need some clearance as well...........These 2 alone will need approx 0.005" I would say.....Incidentally you can see 5 thou. on a ruler.......Find one with 1/100's on it and compare the space between the graduations with the thickness of a graduation...Notice how they're the same ??....By splitting this graduation you now have + or - 0.005"............... Next, consider just how good is your machinery ( when was the last time you checked the tailstock alignment with a "clock" ?? ), how accurate is your tooling and finally---and don't take this personally---- how good are YOU ?......................Shim your axleboxes with 0.005" either side ( to maintain centrality ) and use a new reamer for your bushes................. However, your crank quartering MUST be accurate ( ie all 3 must be at the SAME angle, even though it might not be exactly at 120 degrees. )............ and the Con. rod pitching MUST be identical to the wheel pitching........ Either trammels or a DRO will give you this .............. Finally, I'm 65 and suffer from a very active digestive system... This latter one gives me ( in tech. speak ) a "Vent(Heated)to Atmosphere".......Thus making me an Auld Fart, I guess ?? ........ Hope this has helped a little ??
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Post by Deleted on Jan 24, 2015 11:26:23 GMT
The problem with to close fit on the coupling rods is as the suspension moves, the centre distance between the axles changes. The options are :- 1) Increased clearance on coupling rod bearings 2) Increased clearance between axle boxes and horns 3) A good fit on all bearings and risk binding up or even breaking something over a bump ---------------------- thanks Andy,had forgotten to include that consideration as well.....( Bump, or more likely a drop in rail height at a joint....This is more pronounced with a model as 99% of the time you'll be running on Non-Scale trackwork..Unless the GL5 people invite you along that is...).......
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Post by miketaylor on Jan 24, 2015 11:44:57 GMT
Hold it,
With 6 inches between axle centres a differential of one eighth inch in relative levels will increase the effective length of the coupling rods by 1.3 thou.
One thou clearance in each Coupling rod bearing should provide all the clearance needed to avoid binding due to track irregularity. Allan's 5 thou would be much more than required.
In practice, I would expect to do considerably better than 5 thou but a one thou clearance all round on the pins should be OK.
I would have though a bigger problem would be providing enough play for transverse angular movement of the axles and the need to accommodate this in the Conrod end bearings at crankpin and crosshead. One thou clearance in a quarter inch wide bearing will only provide for about quarter of a degree of twist. This is unlikely to be enough for practical operations. One eighth of an inch differential level across an axle would induce a twist of around 1.4 degrees.
Mike
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Post by Deleted on Jan 24, 2015 12:38:49 GMT
Hello matey, have you been at that Albanian plonk again ??..LoL........... OK, I hear what you say, but if Silverfox opts for the closest fit he can "Engineer" then at least he'll have the option at a later date to ease any tight spots etc....
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Post by Deleted on Jan 24, 2015 12:39:04 GMT
I always think that this necessity for large clearances in coupling rod bushes etc. is greatly exagerated. In practice, the difference between the wheel heights on a half decent track is going to be very small. The track would have to be in extremely poor condition to get even 1/8" difference and probably not fit to run on anyway, certainly not at any speed! The only time you need a large clearance in coupling rod bushes is if the quartering of the wheels is out.
Make everything a nice running fit. A few laps of the track will soon loosen things up if necessary.
John
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Post by Roger on Jan 24, 2015 14:54:48 GMT
I always think that this necessity for large clearances in coupling rod bushes etc. is greatly exagerated. In practice, the difference between the wheel heights on a half decent track is going to be very small. The track would have to be in extremely poor condition to get even 1/8" difference and probably not fit to run on anyway, certainly not at any speed! The only time you need a large clearance in coupling rod bushes is if the quartering of the wheels is out. Make everything a nice running fit. A few laps of the track will soon loosen things up if necessary.John This is what I've been advised to do. My chassis has as little running clearance as possible, yet I pushed it along a track without any issues. It seems to me that it makes no sense to look at one wheel lifted a large arbitrary distance and then trying to accommodate that when the locomotive is never likely to encounter huge errors in geometry like that. The principle of connecting wheels with rods like this is fundamentally flawed but it works.
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uuu
Elder Statesman
your message here...
Posts: 2,800
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Post by uuu on Jan 24, 2015 16:40:04 GMT
We've rebushed some very sloppy rods at the Pump House. All the harder when they're eccentric. A nice running fit puts it about right. Who needs defined tolerances, when we can't measure holes that precisely. That wasn't meant as a challenge-perhaps I should say when I can't measure holes...
Wilf
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jma1009
Elder Statesman
Posts: 5,896
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Post by jma1009 on Jan 24, 2015 22:16:45 GMT
ron's query specifically concerns loco with divided drive on a 3-cylinder loco.
as the valve gear is driven off the middle driving axle in effect for all 3 cylinders i agree the bearings on the front driving crankaxle and conn rod to same from the middle cylinder, and crankpin bearings for the coupling rods on the front driving wheels ought to made to very close tolerances, the same as you would aim for on the middle driving axle conn rod bearings and crankpin bearings for the coupling rods.
the power from the middle cylinder working on the front driving crank axle can only be transferred to the other driving wheels via those front driving wheel coupling rod bearings. however of equal if not more importance is to get the valve setting correct - which incidentally requires quite fine tolerances on the above bearings. you cant get one without the other!
cheers, julian
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Post by fostergp6nhp on Jan 25, 2015 10:59:22 GMT
0.010" clearance on a models crank pins? I don't think full size were made that loose. I have had my 4" scale traction engine knocking like a good one and that was on less clearance than that in the big end.
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pault
Elder Statesman
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Post by pault on Jan 25, 2015 13:32:48 GMT
personally I go with .002 to .003 clearance
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Post by ianholder on Jan 25, 2015 17:01:23 GMT
Been reading and rereading 4930Hagleys post where he says "shim your axlebox with 5 thou either side to maintain centrality". There's something wrong if you have 10thou clearance in an axlebox to start with, I would expect an engine with that amount of clearance to be well and truly knackered. regards Ian
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jma1009
Elder Statesman
Posts: 5,896
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Post by jma1009 on Jan 25, 2015 17:57:32 GMT
hi ian, there is absolutely nothing wrong in fitting shims between horncheeks and axleboxes. in fact in fullsize this was standard practice in some of the railway works. a shimmed axle box is far better than one that bangs back and forwards like a battering ram against the hornblocks. ive rebuilt quite a few locos built by other people and have often had to resort to shimming axleboxes with the horns. cheers, julian
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Post by ianholder on Jan 25, 2015 18:27:32 GMT
I know that in full size removable faces were fitted to axleboxes that could be replaced to take up wear when the locos were overhauled. But to my mind a replaceable bearing surface is not the same as a shim. I am not surprised that you have shimmed axleboxes when rebuilding locos that have either worn out or not been fitted properly in the first place. Far better to machine the axlebox to a good fit in the first place, regards Ian
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Post by Deleted on Jan 25, 2015 19:28:26 GMT
Been reading and rereading 4930Hagleys post where he says "shim your axlebox with 5 thou either side to maintain centrality". There's something wrong if you have 10thou clearance in an axlebox to start with, I would expect an engine with that amount of clearance to be well and truly knackered. regards Ian -------------- Hello Ian, sorry..my fault for not proof-reading before posting... That's meant to be a TOTAL of 0.005"... so it now aligns in with my earlier comment in that post about lubrication needing it's own space..... With regards to being "Knackered", if in a piece of machinery the original specs. called for Zero clearance as "new" and 0.005" as a max. "in service" size then your point holds............However, if that machinery new specs. start at 0.005" and a max. "in service" spec. is shown as 0.020" then would you authorize scrapping at 0.010" ??........I was a volunteer then self-employed contractor at the SVR Workhops for approx. 20 years and did indeed manufacture and fit Phos. Bronze liners as mentioned to several locomotives... LMS Black Five 5000 in particular comes to mind as I believe it was a Whitworth built engine and had these Bronze liners as standard...........Some locos have tapered shims which are adjusted in much the same way as a Gib strip on your late or miller tables........
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steam4ian
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One good turn deserves another
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Post by steam4ian on Jan 25, 2015 23:09:11 GMT
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Post by Roger on Jan 26, 2015 5:32:39 GMT
How very interesting, I had no idea that this sort of technology was used.
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Post by joanlluch on Jan 26, 2015 7:49:30 GMT
And that was implemented as well on my favorite Catalan made 4-8-4 "Confederation" locomotive.
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