Design of etched chassis

[burgundy]

The last couple of chassis that I have put together, to EM gauge, have both needed to have washers added outside the frames to reduce the amount of sideplay on the wheels. The back to back measurement for EM is 16.5mm, but checking a number of chassis from kits, the front to front measurement across the frames seems to come out at a little over 14mm. I may be doing something wrong, but the 2+mm of rattle space seems to make it much harder to get the connecting rods running smoothly, as it allows the driven axle to move laterally in response to torque from the worm and the other coupled axles to move independently and introduce binding.

I understand that you need some flexibility in a chassis to allow it to negotiate corners, turnouts, etc., but how much is enough? And if 2mm is excessive, how do those with more experience deal with this? Using the P4 spacers, that are usually included in a kit, would widen the frames a bit, but is that storing up trouble in some other way? Or am I missing something obvious?

I should appreciate any suggestions.

Many thanks

Eric

[Il Grifone]

I noticed this building a Comet 57xx chassis. I think it stems from the 00 practice of ½" frame spacing. My solution was the addition of washers between the spacer and the frame to push them out a bit. In EM only about a 1mm of sideplay is necessary (it depends on radius) so about 15/15.5mm over the frames should be about right.

[Adam]

Goodness alone knows. I have certainly used P4 spacers in EM before without any issue for precisely that reason. I suppose that the EMGS have some standards for this? I can only see potential issues with certain long wheelbase types with lots of wheels and judicious sideplay. For 0-4-0s and 0-6-0s without sideplay I can't see what the difference would be between wider frames and more washers. Pick-ups perhaps?

 

Of course, some loco's had wider frames than others: Bulleid Pacifics and Swindon's class 14s were rather narrow.

 

Adam

[bertiedog]

It may be the designer is allowing for full springing with hornblocks, which are thicker than the frames and stick out about a 1/2 mm each side.

 

Various designs exist and making the frames slimmer helps with leaving a bit of play over and above the fitting of the hornblocks, and or any washers needed behind the wheels to ensure the tyre is clear of the frame, as not all wheels are flush backed.

 

They may also have allowed for the possibility of fitting plunger pickups, which need about .5mm per side or more.

 

If the frames is flush, then the flush backed wheel must have a washer added to the axle to ensure clearances exist. Some wheels have a boss on the back which pushes it away from the frame a bit, but most makes these days are flush.

 

Working clearances vary, but are relatively tight on EM and very tight on P4/S4 standards, only a working clearance on lead and trailing drivers, with a bit more on middle, for a six coupled locomotive.

 

So basically most times you will need a selection of brass or other metal washers to trim the clearances down to the amount that you need to suit your layouts curves.

 

If it was made too tight, with wider etched frames, then others would complain about having to trim the etchings to get bigger clearances, it cuts both ways. Adding washers is the best practical answer.

 

This is why it is always best to dry run the assembly and check out what the sizes are going to be before soldering the chassis together, it is easier to adjust the frames at an early stage rather than finding out later on it does not suit.

 

For making thin washers use flat brass sheet, say 10 to 15 thou (ot tinplate etc), , drill 1/8 holes and trim the thin brass with scissors to shape, as they are hidden they do not have to be exactly round, or you can even leave the square, still works fine. file flat and remove burrs before fitting.

 

Hope the notes help,

 

Stephen.

[billbedford]

These frame spaces were originally drawn up in the late 70s early 80s and I don't think anyone as looked at the design since. These spacers were designed to accommodate profile milled frames which have a minimum thickness of 0.8 mm. Add to that a bearing flange which may be 0.5mm and the spacers are too wide. Even with etched frames of 0.45 (18 thou) and bearings with flanges or wheels with bosses on the inside, there isn't the excessive of slop in the axles.

[craigwelsh]

It could be that the designer had no knowledge or interest in EM gauge and relied on 3rd parties or just guessed it too. Perhaps left it safer being narrow..

 

I'd say adding washers is the poorer solution as it looses some of the benefit of a more scale gauge in allowing you to put the frames in the right place. Wider frames gives you more room for springing and a wider selection of gearboxes soon. Not sure what sprung hornblocks Stephen is meaning but most hornblock systems i've seen do not stick out past the frames.

 

The S4 Society has notes in its handbook about calculating sideplay requirements for different wheelbases and i'd be surprised if the EMGS cdrom didn't have some similar maths that people could use as a guide.

 

Some of the BR standards like the 9F also had narrow frames as they followed the design of hornblock pioneered on the Bullied designs that straddled the frames. Frame stretchers on a cl14 were 3'2 wide.

[bertiedog]

The guide way may be on the inside, but usually the bearing block is made thicker to project so that flush backed wheels do not rub the frames, especially as they do not just go up and down, but twist against the frame side as the other end of the axle rises and falls.

So there must be a working clearance that allows for the flanges never touching the frames in extreme positions, and a washer is the simplest way if the bearing block is flush, which you say, most are.

Stephen.

[bertiedog]

The guide way may be on the inside, but usually the bearing block is made thicker to project so that flush backed wheels do not rub the frames, especially as they do not just go up and down, but twist against the frame side as the other end of the axle rises and falls.

So there must be a working clearance that allows for the flanges never touching the frames in extreme positions, and a washer is the simplest way if the bearing block is flush, which you say, most are.

Stephen.

[ArthurK]

My own underframes and designed to the following standards. The values are the measurements over the outside of the finished frames.

 

OO 12.5mm

EM 15mm

P4 16mm

 

These distances are adequate for normal model track radii (about 30" in OO and about 36" in P4). there are problems when the leading bogie has wheels are too large to pass under the front frames. Then we have to resort to angling the front frames inwards to give sufficient clearance. The alternative of using undersize wheels does mar the appearance.

 

P4 modellers sometimes move the frames out to 16.5mm but this does lead to very tight tolerances. It is very close to prototype clearances.

 

Arthur

[burgundy]

Thanks for all the replies to this query. At least now I know that it is not just me doing something wrong, or missing something obvious!

I have checked locos from a number of kit makers and the issue seems to be common to all of the etched chassis; it looks like I need to invest in a stock of washers for future projects. I will also see what happens if I use the P4 spacers next time.

Again, many thanks to all.

Best wishes

Eric