# How much slack in the connecting rods?



## rexcadral (Jan 20, 2016)

I am building an 0-6-0 at 1/32. The locomotive has suspension. I've noticed while testing that the connecting rods have to be "exactly" the right length in order to not bind while turning the wheels. 

Given that suspension adds ~1/8" +/- the "ideal" height of the drivers in relation to each other, I'm guessing there needs to be some slop between the connecting rods.

Additional information:

Wheels are 50mm apart on center.
Rods are drilled 5/32, and fit on 5/32 pins.
The middle axle serves as the "joint" - it has separate connecting rods between the rearmost and frontmost drivers.

If the front drivers are not in perfect horizontal alignment with the middle drivers, they are no longer exactly 50mm apart, and will bind when the mutual pin distance exceeds the 50mm length of the connecting rod.

What is the "correct" solution to this problem.

Thanks for the expert help in advance!


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## Gary Armitstead (Jan 2, 2008)

This is a photo of my 1-1/2 inch scale ten wheeler. A perfect example of what you are asking. The side rods on my engine are split just as you describe and all drivers are on a leaf spring suspension. The distance between centers of this engine is 10.5 inches or a rigid wheelbase of 21 inches. The split side rod is to accommodate the lifting of the drivers, just as you describe. On my engine, the bearings for the pins in the rods are within .0015" tolerance and the engine doesn't bind. If you do the math showing the arc of the center pin of the rod and the split of the side rod, the amount of change in length between centers is so small that it become irrelevant, even on my 1/8th scale engine . Hope this helps.


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## David Leech (Dec 9, 2008)

Many years ago, at a G1MRA meet in the UK, my brother made the comment, "You can always tell the models made my model engineers - they're the ones that don't run very well!"
He went on to explain that since they follow the plans exactly, and everything is a perfect fit, then everything become just too tight a fit.
Amateurs that built 'sloppy' fitting engines have much more success.
However, having said that, there must be a point at which the 'slop' become excessive and things will wear out quicker.
I have no answer for you in facts and numbers, but a little slop seems to work well!
Cheers,
David


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## rexcadral (Jan 20, 2016)

*@Gary - How much vertical travel in your springs?*

Granted they're not the same scale, but I'm wondering how much vertical play there is on that gorgeous loco of yours? I've planned for a full 1/4" travel, but am thinking that's practically a 4x4 at 1/32 and I really need something much smaller.


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## David Leech (Dec 9, 2008)

I don't know if you are familiar with the G1MRA 'The Project' 0-6-0 loco.
It has no springs and they run very well without.
In fact a friend in Seattle built his some 30 years ago decided to build his WITH springing.
He had nothing but troubles with that and eventually took all the springs out.
I am convinced that springs really don't make a lot of difference in Gauge 1 scale locos.
If they are too soft and bottom out, then they do very little but perhaps allow the wheel to drop into a hole.
If they are too strong, then you might as well have a rigid chassis.
So, what loco are you building?
All the best,
David Leech, Delta, Canada


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## Gary Armitstead (Jan 2, 2008)

rexcadral said:


> Granted they're not the same scale, but I'm wondering how much vertical play there is on that gorgeous loco of yours? I've planned for a full 1/4" travel, but am thinking that's practically a 4x4 at 1/32 and I really need something much smaller.


The engine ONLY (boiler and frame, drivers and lead truck, etc.) weigh about 375 to 400 pounds. The amount of movement in the springs is probably less than 1/8" to 5/32". It's actually quite rigid. I suppose if I forced the center driver upwards, I could get as much as 3/16". I haven't run this ten wheeler yet, but our 2-6-0 of the same design, flexes about this much. 1/4 inch travel on your small loco seems a little excessive.

Like David commented above, I was told to make everything a little sloppy when it came to the valve gear and the side rods. In this case, a couple of thousandths in the crankpin clearance. You definitely hear some "rod clank" on the better running steam engines. Railroad machinists were NOT considered small tolerance mechanics....i.e. they were usually a little sloppy. You try to get as close as possible to cut down on bearing wear.


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## rexcadral (Jan 20, 2016)

*@David - Suspension & Equalization*

Hi David, 

re: which loco - Nothing specific. A freelance "camelback" that would be somewhat prototypical for New England.

I plan on equalizing the axles with simple teeter-totter style fake leaf springs between the axles, so I can pretty much set the axles to "prefer" the middle of their travel in the suspension. The springs are there mostly to dampen it a little.

It's a first project for me, new to working with metal, new to working with locos. I have a donor Roundhouse engine providing the boiler and cylinders. So far it's a roller coaster, big victories and lots of frustrating little failures. - I should probably start a thread chronicling the build, most of the pics are up on Facebook already.


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## Pete Thornton (Jan 2, 2008)

rexcadral said:


> Granted they're not the same scale, but I'm wondering how much vertical play there is on that gorgeous loco of yours? I've planned for a full 1/4" travel, but am thinking that's practically a 4x4 at 1/32 and I really need something much smaller.


Vertical play of 1/4" on a 1/32nd loco seems way too much. 1/16" seems reasonable! Then a slight slop in the rod bushings will prevent binding.

And if you have equalized the frame properly, you don't need springs at all!


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## RP3 (Jan 5, 2008)

Echoing what Pete just said and expanding upon it, American prototype Aster locomotives for some time now have been equalized but not sprung. The vertical movement only needs to be enough to handle normal track irregularities, so 1/16" or just touch more will probably be all you need -- with no springs on the drivers.

Good luck,

Ross Schlabach


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## BigRedOne (Dec 13, 2012)

Rex, I think there would need to be a minute amount of fore-and-aft play in the axle bearing block fit in the frame. This is sufficient to allow the side rods to not bind as the suspension travels.

My (unfinished) Roundhouse kit has what appears to be no slop in the side rods - they are somewhat tight and either need work with a polishing compound, or perhaps will just run-in with some time.


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## rexcadral (Jan 20, 2016)

BigRedOne said:


> Rex, I think there would need to be a minute amount of fore-and-aft play in the axle bearing block fit in the frame. This is sufficient to allow the side rods to not bind as the suspension travels.
> 
> My (unfinished) Roundhouse kit has what appears to be no slop in the side rods - they are somewhat tight and either need work with a polishing compound, or perhaps will just run-in with some time.


My Sammie (used a bit) is the same way; the tolerances on the rods are very close, but there is wiggle along the shafts. Of course, Sammie is not equalized or sprung.


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## BigRedOne (Dec 13, 2012)

It's a bit of geometry to determine the free play needed.

In an exact horizontal, each side rod is 50 mm long (center to center.) If you have a 3 mm vertical displacement of the wheels relative to each other, the rod would have to be 50.09 mm to be the proper length. That's a difference of 0.0035 inch.

Still, there needs to be some freedom for the rods to rotate on the crank pins, and sufficient clearance for the bearing blocks to move vertically.

Perhaps obviously, the range of motion in the suspension will increase the slop needed in the rods. For this reason it would seem beneficial to limit the travel to that which is necessary for all wheels to remain in contact with the rails.


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## Taperpin (Jan 6, 2008)

Rex,
Normal method is to make the drive axle rod bearings a close running fit drill and then ream, with the rod ends [in your model] front and back rod ends about + 0.05" to 0.06" larger in the bushes.. that will allow the suspension to work and the main drive pin / connecting rod will work quietly without slop..thus the valve gear will remain accurate longer..


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## David Leech (Dec 9, 2008)

+0.050" to 0.060" seems rather excessive to me?
Regards,
David Leech, Delta, Canada


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## Taperpin (Jan 6, 2008)

David ,
Yes that's what happens doing metric /Imperial conversions in a hurry!

I meant to write 0.005 to0.006 " 

Gordon.


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## rexcadral (Jan 20, 2016)

Taperpin said:


> David ,
> Yes that's what happens doing metric /Imperial conversions in a hurry!
> 
> I meant to write 0.005 to0.006 "
> ...


OK, so if I'm drilling 5/32" holes in the rods, how the heck do I get them .005" wider? My tool set (and knowledge) is a bit limited. I obviously can't grab an 11/64" bit (which seems an unusual size) because that's .015" of play.


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## David Leech (Dec 9, 2008)

rexcadral said:


> OK, so if I'm drilling 5/32" holes in the rods, how the heck do I get them .005" wider? My tool set (and knowledge) is a bit limited. I obviously can't grab an 11/64" bit (which seems an unusual size) because that's .015" of play.


Well - 5/32 is 0.1562"
+0.005" = 0.1612"
#20 drill is 0.1610"
or
#21 drill is 0.1590"
Not sure that I would trust a drilled hole for a bushing as it will still be a little rough so you really need something to ream it out after drilling.
Cheers,
David Leech, Delta, Canada


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## Gary Armitstead (Jan 2, 2008)

Rex,

You never use a drill to open the diameter of a hole. You use a reamer. In your case, buy an "adjustable" reamer. You can set that for EXACTLY what you need. To do this job "properly", you would fasten your rod in place on a surface so that it cannot move. Use a center-drill to "spot the correct location. Then use a number drill with the size you really need for clearance on your rod pins (as David suggested). The reason you don't want to open a drilled hole with another drill is that you won't have material to keep your drill centered in the previously drilled hole. The drill WILL wander and the hole WILL NOT BE ROUND. I only mention this because you sound like you might be new to some machining operations. I'm an old retired tool and die maker/die sinker....spent about forty-five years in the trades.


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## rexcadral (Jan 20, 2016)

Gary Armitstead said:


> Rex,
> 
> You never use a drill to open the diameter of a hole. You use a reamer. In your case, buy an "adjustable" reamer. You can set that for EXACTLY what you need. To do this job "properly", you would fasten your rod in place on a surface so that it cannot move. Use a center-drill to "spot the correct location. Then use a number drill with the size you really need for clearance on your rod pins (as David suggested). The reason you don't want to open a drilled hole with another drill is that you won't have material to keep your drill centered in the previously drilled hole. The drill WILL wander and the hole WILL NOT BE ROUND. I only mention this because you sound like you might be new to some machining operations. I'm an old retired tool and die maker/die sinker....spent about forty-five years in the trades.


You are correct, I haven't worked with metal at all before.

I do have a drill press, and it has a vice. I have since learned about scribes, pricks, and center punches as well as centering drill bits. (mostly the hard way). I haven't seen any adjustable hand reamers that are under 1/4", at least not in the local big-box stores. I've noticed the centering/countersinking bits tend to leave a much nicer interior finish, and I used those on my axle bearings, which are very close tolerance, but also spin quite freely, so I have used them to "ream" out holes before.

Apparently I need to see what the Internets can give me.


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## Gary Armitstead (Jan 2, 2008)

rexcadral said:


> You are correct, I haven't worked with metal at all before.
> 
> I do have a drill press, and it has a vice. I have since learned about scribes, pricks, and center punches as well as centering drill bits. (mostly the hard way). I haven't seen any adjustable hand reamers that are under 1/4", at least not in the local big-box stores. I've noticed the centering/countersinking bits tend to leave a much nicer interior finish, and I used those on my axle bearings, which are very close tolerance, but also spin quite freely, so I have used them to "ream" out holes before.
> 
> Apparently I need to see what the Internets can give me.


I just checked McMaster-Carr and the smallest adjustable reamer they have in stock is 1/4" AND they are pricey at $85 apiece. Just stick with center-drilling and numbered or lettered drills for the odd sizes. Close enough for the scale you are working in. 

A tip: don't "push" that drill....let IT do the cutting. Also "peck drill"...go down a little and back off and then drill a little more and back off. And don't let the drill "dwell" in the hole. You will dull the drill quickly doing that. Also check on using drills with short shanks and very little length on the flutes. Keep everything as short as possible and in the chuck as far as it will go. You want to cut down on any run-out toi get those holes as accurate as is possible with your drill press.


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## rexcadral (Jan 20, 2016)

Gary Armitstead said:


> I just checked McMaster-Carr and the smallest adjustable reamer they have in stock is 1/4" AND they are pricey at $85 apiece. Just stick with center-drilling and numbered or lettered drills for the odd sizes. Close enough for the scale you are working in.
> 
> A tip: don't "push" that drill....let IT do the cutting. Also "peck drill"...go down a little and back off and then drill a little more and back off. And don't let the drill "dwell" in the hole. You will dull the drill quickly doing that. Also check on using drills with short shanks and very little length on the flutes. Keep everything as short as possible and in the chuck as far as it will go. You want to cut down on any run-out toi get those holes as accurate as is possible with your drill press.


Thank you very much for the advice!


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## Totalwrecker (Feb 26, 2009)

Cutting oil can help a lot in keeping the hole true and as an aid to lift the chips out.
If you can't find cutting oil use a little beeswax, it will stick to the drill rather than being thrown off.
Cutting oil is High sulfur oil. Wear gloves and wash after.
John


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## Taperpin (Jan 6, 2008)

Rex, 
A standard 4mm drill is 0.397"diameter , 5/32" is 0.394" probably the easiest way to get the clearance you need,, if it is ok then you could go back and buy a 4mm hand reamer and make new bushes for the final fit..using a 5/32"drill and finishing with the 4mm reamer.


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## David Leech (Dec 9, 2008)

Taperpin said:


> Rex,
> A standard 4mm drill is 0.397"diameter , 5/32" is 0.394" probably the easiest way to get the clearance you need,, if it is ok then you could go back and buy a 4mm hand reamer and make new bushes for the final fit..using a 5/32"drill and finishing with the 4mm reamer.


Here in Canada, so it might be different for you, 5/32" is 0.1562" and 4mm is 0.1575"
Regards,
David Leech, Delta, Canada


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## Pete Thornton (Jan 2, 2008)

Rex,
I'm not a metal worker any more than you are, and I rely on my big vertical drill press for lots of things.

I concur with John about the cutting oil, but any oil, water (even spit) will keep the drill cool and allow it to cut without dulling - and the chips stay close which reduces the floor cleanup.

While it would be nice to find a 4mm reamer, I have been known to slightly open up a bearing in a rod by wrapping some fine (600 grit) wet-or-dry emery sandpaper around a toothpick and lightly sanding inside the bearing. Given the geometry, it doesn't need opening all around - you just need to make it slightly oval horizontally to accommodate the axle's vertical movement. 

If you want to make sure the hole is only opened in one plane, put the sanding device in the drill and move it up and down inside the drilled hole. Personally, I put it in a vice and sand it by hand.


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## Taperpin (Jan 6, 2008)

David , sorry , I have been working for 2weeks converting old works drawings from Imperial to metric, nearly cross eyed.. it is of course 0.399 mm and 0.394mm ..

Gordon.


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## David Leech (Dec 9, 2008)

Gordon,
No need to apologise.
We all knew what you meant.
If only there was one system in use!
So, are the works drawings interesting?
All the best,
David Leech, Delta, Canada


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## rexcadral (Jan 20, 2016)

Pete Thornton said:


> Rex,
> I'm not a metal worker any more than you are, and I rely on my big vertical drill press for lots of things.
> 
> I concur with John about the cutting oil, but any oil, water (even spit) will keep the drill cool and allow it to cut without dulling - and the chips stay close which reduces the floor cleanup.
> ...


Hi Pete, thanks for the tip. I've been using spray-on lithium grease to lubricate both my drill and my scroll saw when cutting metal. - It seems to need it much more on the nickel-silver hardware than on the brass stock, but it works like a champ. When I've needed to smooth out the inner sides of holes, I've been drawing 000 steel wool through, twirling as I go. The results are mirror-finish grade and have definitely "deburred" the main wheel bearings quite nicely. - I like the drill press method for ovular holes, or perhaps a new use for my Dremel.

Drilling oil is on the list for the weekend's shopping excursion.


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## rexcadral (Jan 20, 2016)

*The new plan*

OK, so having listened to 3 pages worth of advice, the new plan for the loco is not to suspend it.

In order to keep connecting rod geometry dead simple, I'm going to fix the main driver (rearmost axle in my 0-6-0 and mount the front two axles on a rocking bogie (no side-to-side motion)

This means the only "sloppy" connection is between axle #2 and axle #3 (front-to back of loco) since the geometry between other wheels and the cylinder are fixed.

The rocking bogie won't have much play. ~ 1/16" of an inch deviation for either axle, but should equalize nicely, giving the loco a 3-point stance, which should optimize traction.

Based on last night's drawings, the middle axle is just a little in front of the loco's centerline front-to-back, so weight distribution should be ideal, with the bogie taking all the front weight on a pivot located between the front 2 axles.


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## du-bousquetaire (Feb 14, 2011)

John Van Riemsdijk used to favour pivoting the two outer drivers around the center driver instead of making a real suspension, he showed this to me when he showed me his engines and he insisted on that point. They ran fine. Although a mechanical genius, he wasn't a great machinist and his locos were often quite crude yet they did perform. This more or less confirms what Ross just said about Asters current design practice.


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## Pete Thornton (Jan 2, 2008)

> the new plan for the loco is not to suspend it.


I did recall another neat "suspension" trick used on brass chassis locos. I did it on a recent railbus.

Basically, you set up the axles in bearings/hornblocks that can move, but the top of the movement is the normal level position. So on level track all axles are solid and at the top of their range of movement. Then you add a light spring - piano wire for example - that bears on the top of each of the bearing/hornblock and is enough to push it downwards if there is any deviation in the level track. This has the effect of always making the wheel try to follow the undulations in the track, and thus to not let the flange climb the rail and derail to loco.

Check out section 8.3 on this treatise on suspension: 
http://www.clag.org.uk/41-0rev.html#section7.2


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