Binding motion on a new build

[figworthy]

I'm part way through building a High Level Barclay 0-6-0 (my first attempt at a full loco kit), and I've got to the point of testing out the motion gear.

 

The chassis is rigid, and was built using a Poppy jig, with the motion rods placed on the ends of the dummy axles.  The wheels were quartered using a jig.

 

I've got the motor and gearbox installed, and it is driving the rear axle.  As the final drive gear was tight on the axle, I haven't added the grub screw yet (but will do on the final assembly).  The plan was to run it without rods, then with the rear rods, both sets of rods, and finally with the cylinder rods, moving on a stage when it runs smoothly.

 

With no rods connected, that works fine.  I've run it for an hour or so at various speeds and in both directions, and it is nice and smooth.  Today I put the rear rods on (making it a 2-4-0) and it isn't good.  On every revolution it appears to be binding at one spot (same point in both directions).  The bind is sufficient for the final drive gear to slip on the axle.

 

Suggestions are welcomed on where to start looking.

 

Thanks

 

Adrian

[Brassey]

Could be many things but most likely the quartering is out.  Which wheels do you have?  might be worth opening up the rods a bit with a tapered broach.

 

Suggest you slacken the drive gear too so you can rotate the wheels and use the grub screw when sorted.

[Fen End Pit]

Who's wheels are you using and which quartering jig? If the answer is Alan Gibson do you get different behavior with the crankpin bosses loose and tighted? can you detect that one pin is binding when it sticks? Try wiggling each end of each rod along the boss and see which if any is stuck. Just how tight is everything? Obviously it needs to be tight but not so tight that things bind up.

 

Your approach of making it roll as a 2-4-0 first is a good one.

 

Good luck and don't get disheartened..

 

David

[Michael Edge]

It sounds as though the wheels are out of quarter, check the rods in this sequence:

Put one rod on one side (as a 2-4-0 in your case) and run the wheels round together with your hand, there should be on tight spots at quarter to and quarter past. If there are there is something wrong with the rod length or crankthrow.

Repeat with the other side rod (I should have said to mark the rods LH and RH first).

If there are no tight spots try (again by hand) with both rods on, using the driven axle .

Tight spots will apear at quarter to and quarter past at one side (top and bottom center on the other side), adjust one wheel on the non driving axle until they disappear.

Repeat the process with the other rods but this time only adjust the wheel you have just added - don't disturb the others.

 

It's a simple procedure but needs to be done in an exact sequence checking each time, if everything checks out your loco will run first time under power (all mine do).

[Brassey]

What I failed to remember recently was that the GW wheel press will not accept full size new Alan Gibson crankpins (or other makes!); these have to be cut down first to fit in.  I pushed on some outside cranks without realising and I think the wheel press bent the crankpins and/or pushed the quartering out to compensate for the long crankpins.

 

When the axle hole is tight it is hard to push the wheel fully onto the jig and its easy to forget that it might be the overlong crankpin that's stopping the wheel being pushed all the way home.

[figworthy]

Thanks for the various replies, they give me something to work with.

 

The wheels are Gibson's (the only ones available in that size), and the quartering jig is the GW models one.  Noting the comment about long crank pins, the pins had already been shortened to length.

 

I think before going any further, I'll remove the motor so that I can turn the motion over by hand, that should make it a bit easier to detect the tight spot(s).

 

Adrian

Edited August 12, 2021 by figworthy

[Izzy]

It’s unusual to have problems with a 4-coupled loco, even if the quartering is a bit out they mostly still run okay. If you have used Gibson wheels just check they are all the same type/crank throw. There are several different 3’8” available with the crank throw varying between  2.6mm and 4mm and although unlikely you may have got mixed sets somehow.   I’d also check your using the matched rod halves if the wheelbase isn’t symmetrical ( I don’t know what it should be). I offer these thoughts on a ‘been there done that’ basis!

 

[doilum]

Always eliminate the most unlikely. You have picked up the correct coupling rods? Some locos have just a small difference between the front and rear sections. In 4mm this might be almost imperceptible.

 

[figworthy]

10 hours ago, doilum said:

Always eliminate the most unlikely. You have picked up the correct coupling rods? Some locos have just a small difference between the front and rear sections. In 4mm this might be almost imperceptible.

 

 

Thanks for the tip.

 

In this case there is a noticeable difference in the wheel base between the front and back pair of axles, so I'm pretty sure that I've got the right pair of rods.

 

Lack of progress report, I didn't have time to investigate further today, hopefully over the weekend.

 

Adrian

[Simon Moore]

How have you opened the holes out on the rods? Are they all the same size & with enough slop. Sometimes I get a chassis lock up a bit & a quick turn of a reamer usually cures the issue. If the wheels are quartered correctly on a gw press & the wheels are sqaure on the axle then check the rod holes & see if a crankpin is bent because that will tighten up at a certain point. 

 

Good luck I am sure it will be a dead easy fix. Building locos is a learning curve & eventually you know what causes certain issues when they arise.

[Portchullin Tatty]

I have developed the habit of carefully (aka very carefully) measuring the centre to centre of both the coupling rods and the chassis wheelbase.

 

I do this with some vernier callipers.  If you insert the prongs of these into the coupling rods to measure the minimum distance across the holes and then repeat for the maximum distance.  If you sum these and divide by 2, you have the centre to centre distance.  Repeat this for other coupling rod and the wheelbase.  Be very structured about it, do the rear first and get this right, then repeat for the front.

 

It takes a bit of getting used to, the main things being that you can apply more or less pressure to the callipers such that you get different dimensions.  However, I reckon I can measure down to circa 0.02mm.  Even if I can't, I can mismeasure consistently and it does not really matter if I measure the right distance, only that it is consistent.

 

With regard to quartering, I sight through the spokes.  Make sure that you are looking square onto the axle and then see if the one behind lines up.  Again, whether it does or not is irrelevant, what you need is consistency.

 

And one final suggestion.........sign yourself up for the forthcoming Missenden Autumn Weekend (booking for which will open soon I hear) https://www.missendenrailwaymodellers.org.uk/index.php/autumn-weekend/

 

[Michael Edge]

The Poppy jig should show whether the rod centres and axle centres are the same, no need to measure them - and that's very difficult to do accurately.

[michl080]

It is embarassing to admit, but it took me two weeks to detect the reason for a binding at one of my kits. One of the 6 driving wheels had a slightly different diameter, a different nmber of spokes and a different throw. I have no idea how this can be undetected for such a long time

 

Worth checking?

 

Michael

[Michael Hodgson]

I don't see that a different number of spokes would cause it to bind, and even a slightly different diameter should not cause the motion to bind even though the loco might run badly and be prone to derailing.  The killer in there is having a different throw,

[Miss Prism]

I assume there are bushes between the crankpins and the rods?

[figworthy]

23 hours ago, Simon Moore said:

How have you opened the holes out on the rods? Are they all the same size & with enough slop. Sometimes I get a chassis lock up a bit & a quick turn of a reamer usually cures the issue. If the wheels are quartered correctly on a gw press & the wheels are sqaure on the axle then check the rod holes & see if a crankpin is bent because that will tighten up at a certain point. 

 

Good luck I am sure it will be a dead easy fix. Building locos is a learning curve & eventually you know what causes certain issues when they arise.

 

The rods have been opened up with a broach, that might be an issue as it is right at the at the thick end of my second biggest broach, but the biggest is far to big for that job (that's for opening up for wheel bearings).  The crankpin bushes do fit into the holes, but they are a snug fit (unlike the bushes on the pins).

I went into this build under no illusion that the kit would "fall" together and work straight away, so I was expecting it to be a learning experience.

 

2 hours ago, Miss Prism said:

I assume there are bushes between the crankpins and the rods?

 

Yes there are.

 

22 hours ago, Portchullin Tatty said:

I have developed the habit of carefully (aka very carefully) measuring the centre to centre of both the coupling rods and the chassis wheelbase.

 

I do this with some vernier callipers.  If you insert the prongs of these into the coupling rods to measure the minimum distance across the holes and then repeat for the maximum distance.  If you sum these and divide by 2, you have the centre to centre distance.  Repeat this for other coupling rod and the wheelbase.  Be very structured about it, do the rear first and get this right, then repeat for the front.

 

It takes a bit of getting used to, the main things being that you can apply more or less pressure to the callipers such that you get different dimensions.  However, I reckon I can measure down to circa 0.02mm.  Even if I can't, I can mismeasure consistently and it does not really matter if I measure the right distance, only that it is consistent.

 

With regard to quartering, I sight through the spokes.  Make sure that you are looking square onto the axle and then see if the one behind lines up.  Again, whether it does or not is irrelevant, what you need is consistency.

 

And one final suggestion.........sign yourself up for the forthcoming Missenden Autumn Weekend (booking for which will open soon I hear) https://www.missendenrailwaymodellers.org.uk/index.php/autumn-weekend/

 

 

Thanks for the suggestion.  As for Missenden, I'd love to be able to go this autumn, but I'm already double booked that weekend.

 

Adrian

[Miss Prism]

3 minutes ago, figworthy said:

The crankpin bushes do fit into the holes, but they are a snug fit (unlike the bushes on the pins).

 

Therein is a world of potential error. One potential error you have probably eliminated is the snug fit of the bushes into the rod holes. That leaves two more potential sources of error:

 

- eccentricity of the bore in the bush;

- slop of the bush on the crankpin (which you state to be not good).

 

 

These errors usually only manifest themselves when the crankpin nuts are nipped up against the bushes.

 

 

 

[figworthy]

25 minutes ago, Miss Prism said:

 

Therein is a world of potential error. One potential error you have probably eliminated is the snug fit of the bushes into the rod holes. That leaves two more potential sources of error:

 

- eccentricity of the bore in the bush;

- slop of the bush on the crankpin (which you state to be not good).

 

 

These errors usually only manifest themselves when the crankpin nuts are nipped up against the bushes.

 

 

 

 

Thanks.

 

Without a close look (which won't happen tonight), I can't say how loose the bushes are on the pins.  If nipping the nuts is likely to be a problem, would having them almost nipped up, enough to stop the bushes sliding up and down the pins, solve the problem ?  I can see that nipping the nuts would stop (or restrict) the bushes from rotating on the pins, which they'd need to do if they are tight on the rods.

 

Adrian

[Miss Prism]

Rods are intended to rotate on the bushes. The bushes are not intended to rotate on the pins.

 

[Izzy]

1 hour ago, figworthy said:

 

The crankpin bushes do fit into the holes, but they are a snug fit (unlike the bushes on the pins).

 

Adrian

If I read this correctly, this may be the basic issue. The rods need to be, at the very least, a free running fit on the bushes, and if the latter are a loose fit on the crankpins, as is so often the case, not locked into place until they are central on the pins. This is the problem this crankpin design can cause as mentioned by Miss Prism. I have only ever used a rat tail file to open out rods, (twisted backwards so they cut only lightly). 

[Brassey]

I bought some Alan Gibson crankpins recently where the bushes were loose on the screws. With the old stock previously the bushes were always tight. I have not used the new crankpins and was advised to send them back. Luckily I still had some old stock. 

Edited August 15, 2021 by Brassey

[doilum]

Eliminate the unlikely. The crankpins are straight and true?

A magnifying glass can prove useful.

[Michael Edge]

Much of the above demonstrates why I never use crankpin systems with loose bushes - crankpins should be plain rods, if bushes are fitted they should be fixed in the rods. This isn't my idea, go and have a look at a full size loco.....

Tightening the nuts on loose bushes inevitably moves them all slightly eccentric and all at random, if you do have to use them back off the nuts a bit and secure with loctite.

[figworthy]

I had a session at the workbench this morning, and the progress such as it was is as follows.

 

I've removed the motor so I can wind the mechanism over by hand.

 

I didn't measure the internal diameter of the bushes or the diameter of the pins.  Whilst the bushes are loose on the pins, the amount of play is not huge, certainly smaller than the diagram posted above by @Miss Prism (which I accept may have been for illustrative purposes).

 

I removed the right hand rod and had a bit of a play with it.  I can now spin the rod around the bushes, so they are no longer tight.  I also measured the distance between the holes (as suggested by @Portchullin Tatty).

 

I then replaced the rod, and repeated the process on the other side.  The right hand rod's mean measurement is 21.96mm, and the left hand rod's is 22.00mm.  I forget to measure the distance on the pins.

 

Having got it all reassembled, I started to turn the wheels over by hand, and for most of the time it appeared to be smooth, but I would get the occasional bind, this was with the left hand rod at "quarter to".

 

I then had a minor disaster.  I heard a ping, and one of the crank nuts had come off (as did the bush).  I managed to find the bush, but not the nut.  I've no spares, but I do have some on order.  I hadn't loctited the nuts, partly because I was expecting to be taking things apart on a regular basis at the moment, and partly because I'm trying to avoid getting things stuck together.  However, it occurs to me, that as I'd tightened the nut, something must have slackened it, which I'm guessing was the bush, in which case, it can't have been rotating on the rod as much as expected.

 

Regarding the straightness of the pins, so far as I can see, they are straight.

 

Thanks again for the suggestions.

[Barclay]

I think there's a good chance that the crankpin system is what is causing your problems. I must admit I never use it and always run the rod straight onto the crankpin - the steel thread has never yet worn out the nickel rods, even on high mileage engines!  By the way have you considered writing up this build over on the Standard Gauge Industrial section, where there will be much interest I am certain?