Steam loco wheel wobble

[Paul Townsend]

I have recently returned to loco building after a long hiatus due to boring health issues.

My current project is a Bill Bedford CSB chassis kit under an ancient MPD?Maygib body for a S&DJR "Bulldog" aka 3F ( b$%£^& Bachmann )
The whole original kit was built some 5 years ago and put aside after painting due to lousy chassis performance.
The new BB chassis seems a much better bet and is giving me an easy route into CSB building.
The chassis is largely built with High level gearbox and Mashima 1620, no pickups yet, Ultrascale wheels pinned to homemade ground MS axles (This is new territory for me, triggered by the supplied UScale axles being too short and two other old locos slipping there Loktited quartering recently) and assembled with trusty GW quartering jig.

So it has reached the stage of running in on pure dc on rolling road, very smooth and negligible variation in current draw, so coupling rods behaving well.
Next imminent stage is dcc and pickups, however...

I am miffed to note that the wheels are concentric but two have a pronounced sideways wobble.
This is something one has got used to with AG (press tyres off and reassemble) and Sharman (tweak!) drivers but a new one on me for Ultrascales.

As the chassis has no pickups yet I don't know how much effect this will have on track holding.
Can anyone quote a tolerance that is proven to be workable? (Drivers are 5'3")

I raised a similar request for wobble tolerance on coach and wagon wheels recently in a thread on a Yorkshire or Manchester group thread about their wobble test jig but have had noone give a figure yet.

We know a bit about B-B tolerance but that assumes parallel tyres and mine aren't!

Have I caused this by repeated axling and deaxling, by pinning or what?
How can I cure it, simple tweak a la Sharman doesn't move anything. Remember they are now pinned to axles although wheelsets drop out on request.

[Mike Garwood]

Paul
I don't have a solution for you, I just wanted to sympathise. For years I've suffered the same fate when it came to adding the wheels to the chassis. I use the GW tool as you do, and I've tried various ploys of adding plastic card spacers to the rim of the wheel, filing off the boss at the rear etc, to get the wheel on true. This is after using a rosebud burr lightly on the back of the wheel to get the axle to centre and then put a slight chamfer onto the edge of the axle so that it doesn't dig into the wheel as it's pressed home. I'm afraid that my frustration has spilled over to the extent that I now have someone else wheel my chassis for me...and he uses a toffee hammer and a scalpel!
I'm waiting for some enterprising person to come up with a solution that works, unfortunately it's beyond me!

Thinking of you in your plight..

regards

Mike

[Paul Townsend]

[quote="Mike Garwood" said][/quote]

filing off the boss at the rear etc, to get the wheel on true

I haven't tried or considered that. I would have thought the more wheel boss thickness we have the better for true mounting onto axle.

This is after using a rosebud burr lightly on the back of the wheel to get the axle to centre and then put a slight chamfer onto the edge of the axle so that it doesn't dig into the wheel as it's pressed home.

I haven't done the former but the axle making process involves lathe work so slightly chamfered ends were certainly done as frequently recommended.

I will try and measure the wobble; currently unsure if it is due to the placcy spokes not orthogonal to axle or the tyre moved off true from the spokes which may be due to use of GW wheel puller??

tolerance anyone?

[Tim V]

The back to back on the prototype had an error of plus/minus 1/32".

I am surprised that the Ultrascales wobble, a light chamfer would mean about 0.75mm at 45 degrees to me.

[Philip Hall]

Paul,

I suspect that repeated removal and refitting of the wheels may have had a detrimental effect on the hole in the wheel. Ultrascale are made of a much more rigid plastic, so you're right, heaving them square won't do a lot of good. I would not do anything to the wheel before putting them on, there is no need, but a decent chamfer on the axle as Tim suggests is always a good idea, and I always round the chamfer with a no. 6 file so there is no possibility of the axle taking a shave off the wheel as it goes in. I have just measured an axle from a set of wheels and it's 0.124 in; hopefully your steel is the same.

Pinning shouldn't make any difference unless perhaps you've not properly cleaned off the burr left by the drill as it comes out of the axle. I only pin the wheels after all the testing, merely as insurance. Neither should the use of the GW press, although Ultrascale don't seem to care for the use of one. I've always used one for my wheels without a problem, but be aware that the crankpin is not fully supported at the face of the wheel because the crankpin bush partly enters the wheel, centring it, but I've never found that a problem. Certainly it shouldn't produce wobble.

All is not lost, however, as you can cut off the axles and ask Ultrascale to remould the centres for you. This is possible in many cases and no doubt they can advise if it can't be done.

Philip

[Will L]

I suspect that repeated removal and refitting of the wheels may have had a detrimental effect on the hole in the wheel.

That would be my guess. One advantage of your CSB fitted chassis is that the wheels drop out, so there should be no need to ever take them off the axles once fitted. As a bonus, if you fit the wheel once only they usually don't need pinning.

If you do have to pin there is also the method I got from Don Rolland's. Start by cutting a 0.5 slot in the end of the axle with a cutting wheel in your mini drill. This should be at an angle of 30 degrees aprox to the axle end, starting on the axle end face and ending a little way down one side. You can then drill the wheel once on the axle, fit a pin, cut off flush with the axle face and fill the rest of the slot. As you don't need to drill hard axles, expenditure on drills in much reduced.

Will

[James Moorhouse]

I raised a similar request for wobble tolerance on coach and wagon wheels recently in a thread on a Yorkshire or Manchester group thread about their wobble test jig but have had noone give a figure yet.

We know a bit about B-B tolerance but that assumes parallel tyres and mine aren't!

I did post this diagram on the WYAG News thread:

wheel-wobble.GIF

This shows the maximum wobble permitted for a wheelset that falls within the P4 back to back (BB) tolerance. Note that I have stated a plus and minus tolerance of 0.02 mm, but perhaps it would have been better expressed as plus 0.04 mm and minus nil, as the effective BB dimension across any part of the wheelset should not be less than the specified minimum P4 BB. However, given that the dimension between checks (BC) is 17.47 mm in P4, there is room for quite a bit of wheel wobble.

Incidentally, the BB tolerance on the prototype is plus 1 mm and minus nil. In 4mm scale this equates to 0.013 mm. Comparatively speaking, the P4 BB tolerance is very generous.

[45609]

The diagram James has posted is useful but it is worth thinking about the practicalities of how you measure the wobble (or run out) of any particular wheel. If using a dial test indicator (DTI) on the diagram above resting the needle close to the outer diameter touching on the inside flange face of the wheel it will record a total indicated range (TIR) of 0.04mm. However the allowable TIR per wheel will be variable depending on the TIR of the companion wheel. For instance if the first wheel has 0mm TIR then the other wheel can have up to 0.08mm TIR and the back to back will still fall within the tolerance range. That assumes that you've gauged to the minimum back to back at the point of maximum narrowing due to wheel wobble. It is the sum of the TIR's of each wheel that should be considered. But not only that, the interaction between the wheel set and the track gauge also needs to be taken into account. In other words the variable back to back + effective flange (BB+EF) of the wobbling wheelset in combination with the track gauge and turnout flangeways. Digest 1.2 does talk about clearances being included in the derivation of back to back dimensions to account for "....the accuracy to which the wheelset BB could be reasonably be expected to be maintained....". A pragmatist would include wheel wobble within that statement. I'm sure some of my early loco efforts in P4 had more than 0.08mm (just over 0.003") total wheel set wobble and they ran well. I'd say that anything more than 0.005" TIR (which is visually very noticable) would be unacceptable.

Cheers....Morgan

[Paul Townsend]

lots of helpful stuff here...ta for many responders...I will return to this next week and report how my wobbles stand in relation to the suggested tolerances.
Meanwhile I have been seduced by photos of Highbridge and TPO's....

My apologies to James Moorhouse for not seeing his previous posted diagram....My visits to this forum are nearly always driven by the "View new posts" switch.
For some odd reason the WYAG thread and indeed this one didn't come through. I only found the responses in this thread here too by "manually" revisiting it.
Good job I did as it is full of gold dust!

I am raising this Forum tecchy issue in a separate thread.

[Paul Townsend]

I suspect that repeated removal and refitting of the wheels may have had a detrimental effect on the hole in the wheel.

That would be my guess. One advantage of your CSB fitted chassis is that the wheels drop out, so there should be no need to ever take them off the axles once fitted. As a bonus, if you fit the wheel once only they usually don't need pinning.
Will

All true of course, the drop out of wheel sets is newish to me major advance in chassis techniques. The reasons for pinning were triggered by 2 old chassis recently going out of quarter, as not dropout it was a pig to renovate, then I found the new USc wheels for this chassis had axles that were too short by .8mm which looked awful, only realised after mounting so that was the first pull-off for all 3 axles. ( Writing this makes me think back and wish I had just stuffed some filler into the axle end holes! Still I wouldn't then have received so much invaluable advice! )
I then decided it worthwhile on learning how to pin ( under orders from local guru, Tim V. &-} ) and evenyually had a problem with the gear positioning on driven axle which required another pulloff for one axle only. So all 6 wheels have been mounted twice and 2 have thrice. One wobble is on the driven axle, another wheel wobbles on another axle.

If you do have to pin there is also the method I got from Don Rolland's. Start by cutting a 0.5 slot in the end of the axle with a cutting wheel in your mini drill. This should be at an angle of 30 degrees aprox to the axle end, starting on the axle end face and ending a little way down one side. You can then drill the wheel once on the axle, fit a pin, cut off flush with the axle face and fill the rest of the slot. As you don't need to drill hard axles, expenditure on drills in much reduced.

I will try that method soon, maybe very soon if I end up sawing axles
Two steps forward and one back again but increased adversity justs spurs me onward!
The day I can't learn new tricks I might as well be dead.

[Paul Townsend]

The diagram James has posted is useful but it is worth thinking about the practicalities of how you measure the wobble (or run out) of any particular wheel. If using a dial test indicator (DTI) on the diagram above resting the needle close to the outer diameter touching on the inside flange face of the wheel it will record a total indicated range (TIR) of 0.04mm. However the allowable TIR per wheel will be variable depending on the TIR of the it's companion wheel. For instance if the first wheel has 0mm TIR then the other wheel can have up to 0.08mm TIR and the back to back will still fall within the tolerance range. That assumes that you've gauged to the minimum back to back at the point of maximum narrowing due to wheel wobble. It is the sum of the TIR's of each wheel that should be considered. But not only that the interaction between the wheel set and the track gauge also needs to be taken into account. In other words the variable back to back+ effective flange (BB+EF) of the wobbling wheelset in combination with the track gauge and turnout flangeways. Digest 1.2 does talk about clearances being included in the derivation of back to back dimensions to account for "....the accuracy to which the wheelset BB could be reasonably be expected to be maintained....". A pragmatist would include wheel wobble within that statement. I'm sure some of my early loco efforts in P4 had more than 0.08mm (just over 0.003") total wheel set wobble and they ran well. I'd say that anything more than 0.005" TIR (which is visually very noticable) would be unacceptable.

Cheers....Morgan

Yes that is me...a pragmatist.

Ta for this and James' diagram which makes it clear and simple. I had assumed that would be the case but thought it worth asking in case experts came up with some other theoretical aspect.

Now looking critically at each wheel I reckon that 5 are Ok or just acceptable; it isn't easy to measure well as caliper pressure for an interference fit causes spoke flexure, but one has a wobble of over .25mm.
That wheel will have to go so now the fun of attempting to press out the pin, if that fails the axle and both wheels are dead, fortunately I have replacements to hand.

As these are Ultrascale the new ones will have the same crank throw as the old so all else should be hunkydory

[Mike Garwood]

Been following this with a lot of interest, but surely the best way forward is a method of getting the wheels on without any wobble to start with?

Suggestions?

Mike

[Tim V]

You first

[steamraiser]

One thing I did not like about the GW models press / quartering tool was that all the pressure on the wheel was around the centre, and no support further out.
I have cut a round hole in a square of plasticard that glued on to each of the two faces of the tool. The hole is centred on the axle spigot on each face of the jig.
This provides support support to the wheel further out from the centre, hopefully reducing wobble.

Gordon A
Bristol

[Will L]

One thing I did not like about the GW models press / quartering tool was that all the pressure on the wheel was around the centre, and no support further out.
I have cut a round hole in a square of plasticard that glued on to each of the two faces of the tool. The hole is centred on the axle spigot on each face of the jig.
This provides support support to the wheel further out from the centre, hopefully reducing wobble.

Hum..

I understand where your coming from but

I find they do go on quite square enough (i.e. no visibly perceivable wobble) using the jig as it comes and I suspect that the problem your worrying about it more conceptual than real, specially if you try hard to press them on once and once only.

What I do find is that it is important to keep the sprung axle stubs in the jig adjusted so they hit the stop when flush with the face of the jig. If not you tend to find that the axle does not go an equal distance through both wheels (i.e stands proud of the face of one wheel and is recessed in the other one), which is very aggravating. How you going to stop this happening with your amended jig I do not know.

Will

[Paul Townsend]

One thing I did not like about the GW models press / quartering tool was that all the pressure on the wheel was around the centre, and no support further out.
I have cut a round hole in a square of plasticard that glued on to each of the two faces of the tool. The hole is centred on the axle spigot on each face of the jig.
This provides support support to the wheel further out from the centre, hopefully reducing wobble.

Gordon A
Bristol

I have now tried this and it certainly supports the rim and I have successfully replaced my wobbly wheel but I had another problem.....
I was dreading pin removal but in the act it pushed out easily enough so the old wheel may be salvaged by bushing the centre another day.

The new wheel didn't quarter accurately because the combination of the plasticard spacer and previously shortened CP meant the CP didn't reach into its slot properly. Anyway quartering tweaked by eye and the chassis is now running again smoothly on the rolling road pending repinning the new wheel, dcc chip and pickups. This will now wait for a week as other happenings will intervene.

[Philip Hall]

I think Allan Sibley in an old MRJ article also made up some Plastikard packing to support the rim of a wheel in the GW press, but like Will I've never had any problems with not doing so; after all the wheel is supported where the pushing is going on, at the centre, and the rim should almost just go along for the ride.

However, if you do it's important only to make this packing sufficient to take up the amount that the boss of the wheel protrudes beyond the rim - and this will vary between wheel types. If you make it any thicker you may well have the problem with previously finished crankpins that Paul describes; perhaps the solution would be to fit a temporary crankpin for assembly.

Philip

[Paul Townsend]

perhaps the solution would be to fit a temporary crankpin for assembly.

Philip

Now why didn't I think of that?
In practice most users will have full length CPs when wheel mounting; it is only twerps like me who have to remount again and again...

[Tim V]

Cut them to length after all work done....

[James Moorhouse]

Morgan's post on this thread is excellent. As Morgan says, it is the sum of the total indicated range (of wobble) for each wheel that should be considered. I thought a few diagrams showing wheel wobble variations might be useful.

wheel-wobble-variations.GIF

Wheel wobble variations - in figures 1 and 2 the points of both wheels' maximum wheel wobble correlate (this is the worst possible orientation that must be accounted for)

Figure 1. Both wheels are equally wobbly. If the total range of wheel set wobble were 0.08 mm, then the total wobble for each wheel would be 0.04 mm.
Figure 2. Wheel A is more wobbly than wheel B. Assuming the same range as above, if the total wobble for wheel A were 0.06 mm, then the total wobble for wheel B would be 0.02 mm.
Figure 3. Wheel A has zero wobble, therefore wheel B can have maximum wobble.
Figure 4. If both wheels were equally as wobbly, one wheel could be rotated around the axle until the point of its maximum wobble correlated with the other wheel's point of minimum wobble and chosen back to back (BB) dimension could be achieved across the whole of the wheel set. However, for loco driving wheels that have to be quartered, this is not an option.

In the diagrams above it has been assumed that the cause of wheel wobble is the fit of the axle in the bore, however it could, for instance, be the fit of the tyre on the wheel centre or the wheel centre itself that is the cause of wobble.

Thinking again about tolerances we came up with 0.08 mm as the total range of wheel wobble permissible for the wheel set. This happens to be the difference between minimum P4 BB and maximum P4 BB. So, if you use a back to back gauge of the P4 minimum setting (17.67 mm) you have 0.08 mm of wheel wobble to play with until the maximum P4 BB (17.75 mm) dimension is reached across part of the wheel set. What if you use a back to back gauge with a larger BB setting? Again exploring graphically what Morgan said in his post, I'll consider maximum values for wheel wobble for given BB settings.

wheel-wobble-std-a.PNG

The above drawing has been produced by modifying a diagram from Digest 1.2, P4 Track and Wheel Standards, the same nomenclature is used.
A crossing is being traversed at the point of a wheel set's maximum wobble. The rear of wheel B is in contact with the check rail; The flange root radius of wheel A is in contact with the radius of the railhead. If the wheel set were any wobblier, wheel A could ride up the side of the rail head and lose contact at the tread, or in an extreme case of wheel wobble, wheel A could take the wrong route at the Vee - check rails exist to prevent this, but their effectiveness is dependent on the interrelationship between track and wheel standards. The diagram represents the maximum amount of wheel wobble permitted whereby wheel A cannot ride up the side of the Vee.

So, if CGmin is 18.15 mm, EFmax 0.40 mm and BBmin 17.67 mm the maximum wobbly wheel tolerance for the wheel set would be 0.08 mm. If BB is 17.70 mm, then the maximum wobbly wheel tolerance would be 0.05 mm. If the BB is 17.75 mm, then the tolerance for wheel wobble is zero.

As the BB dimension increases, the tolerance for wheel wobbliness decreases. Of course the values given for CG and EF are minimum and maximums respectively, so the tolerances are based on worst case scenarios.

It sounds worrying that there should be a zero tolerance for wheel wobble if you're using the Society recommended 17.75 mm BB setting. So, another diagram I'm afraid:

wheel-wobble-nose-b.PNG

A wheel set is traversing a crossing. The rear of wheel B is in contact with the check rail. Because the BB + wobbliness across the wheel set is greater than CGmin - EFmax, the flange of wheel A is riding up the side of the Vee. As the actual back to back dimension of a wheel set (a wobbly wheel set has a range of BB dimensions) approaches the value of CG the risk of wheel A taking the wrong route at the Vee increases.
So, if BB were 17.75 mm and the wheel wobbliness range 0.08 mm, the flange could ride up the side of the Vee fractionally, but there would be little risk of a wrong route being taken. If a wheel wobbliness range of 0.125 (5 thou) existed, then the risk of wheel A taking the wrong route at the Vee would still be small since the flange could only ride up the side of the Vee rather than on top of it. If a wobbliness in excess of 0.125 mm existed then the risk of the wrong route being taken would be very much increased.

Going back to the practicalities of measuring wheel wobble. In my WYAG post I questioned whether it's actually necessary to measure wheel wobble and suggested that an undesirable amount of wheel wobble could be detected by the human eye. I believe that if a wheel set were to have a range of wobble from zero to 0.125 mm (5 thou) this would be obvious, and therefore any wheel set that is obviously wobbly, is probably not fit for use.
If using a BB setting of 17.75 mm, I think a total range of wheel wobble up to 0.08 mm would be a suitable tolerance; If using a BB setting of 17.70 mm, I’d say a total range up to 0.125 mm would be okay.