Crankshaft...

[Davek0974]

Hi all,

I need to fabricate a single throw crankshaft for a traction engine model. The shaft & journal are 38mm and the webs are 48x19mm ba
r.

There are several methods mentioned including loctite and pins or press-fit and pins, also welding.

I dont really trust the first two and was wondering if i had the stuff to weld it. I ahve my trusty migmate 190pro and a 90A arc welder.

I know this is small kit but does that mean it cant be done?

I was thinking a good v-notch around all the joins and a couple of heavy beads. Surely that would be stronger than loctite?

Its for a steam engine so not stressed heavily, and only quite low rpm's, probably about 3/4 - 1 hp.

I cant use the one-piece casting offered as it is too big for my lathe.

Any input?

Thanks

Dave

 

[shenion]

You could use press fit. Make the hole a bit smaller than the pins. Heat up the outside part, put inside part in freezer and press together. I bet the "big boys" here would know the how to calculate the proper diameters for a press fit.

Hard part is getting the press fit right the first time, once together very hard to get apart.

The timing gear on my mustang distributer was that way. Was a bear to get right, but never came loose.

 

[piman]

Hello dave,

it won't be true if you weld it together, and i your lathe isn't big enough you can't true it after welding.. As Shenion says , press fit will do it (Arial Arrow cranks were built that way, by the way).

Alec

 

[Drains]

Press fit all the way. The heat of welding will throw it out, unless you have a very precise procedure to ensure even heating/cooling. Really not worth the bother or risk, IMO.

Alternatively get the one piece casting and get someone with a bigger lathe to machine it!

 

[snowcat]

No NO NO dont weld it, split journals, interferance fit or loctite, we were having this disscusion on friday night regarding the fly cranks on the romulas 0-4-0, I must find another pub to frequent

 

[Davek0974]

Snowcat, can you enlighten me on the 'split journals' idea.

Also, is Loctite *really* that good, if so then it would be the way to go. I could jig it up on the mill table for assembly, then glue it and leave to set. I believe 'loose' sliding fits are used for Loctite joints, which makes it even easier.

As i said, although 6", its a small engine so not too much power going through it.

Dave

 

[MrFluffy]

You can do both, suzuki gsx cranks are pressed together from the factory with nothing but a interference fit to hold them and with helical drive gear, when you start to make more power this tends to destroy things so the helical gears are replaced with a straight cut drive gear and a weld put on the crank pins on assembly to stop them twisting with the torque from a tuned engine. Factory theyre about 114bhp, tuned depends on what you do, but its not unheard of to see them putting 300+ hp out with some degree of reliability still.

Having said that, your building a traction engine, is it going to be seeing 30+bhp at 12000 rpm? the factory suzuki crank is good for that and a bit more with just the interference fittings and they go on for 80-100k miles without issue. I know potentially you will have a lot of torque, but I do not think your crank will live in conditions as extreme as these do.
I can think of lots of examples of built up cranks (anything with roller bearing mains mostly, as you have to split them to change the bearings) in high performance engines, that just use interference to hold it. Go back a few years more to british bike engines, and almost everything used a built up crank pressed together.

 

[Davek0974]

I never realised press fit were used on cranks.

It will be about 0.5 to 1hp but with quite high torque due to 30kg flywheel on end and boiler feed pump on other. Max revs only in the hundreds.

I would prefer to loctite as i have no heating/cooling/assembly setup for press fits.

Dave

 

[MrFluffy]

You can assemble them cold with a press, or rig something up to that effect.
Factory tool to assemble tiger cub crankshafts was a flat bit of bar that bolted to the crank webs, and a central bolt that pushed the pin in/out of the wheels courtesy of a tapped hole in the bar. Mind you, reconditioning alignment proceedure consisted of making some pencil marks on the webs, assembling it to these, then hitting the flywheel halves with a lead hammer till the assembly ran true.
Freezing the crankpin in the freezer overnight will help lots though...
Id be nervous about the loctite fit. Its good, but is it THAT good? Ive never heard of anyone having a sliding fit on a crankpin that is loctited in place on a production engine and it would be a singificant cost saving to do it that way.

 

[Davek0974]

I'm intrigued by this loctite theory, i might do a mock-up and stress it a bit.

If it works, it will definately be useful.

The item is Loctite 638 high strength retainer.

Dave

 

[piman]

Hello Dave,

Loctite is fine for items with little or no torque, but I think you are wasting your time using in this application. Machine accurately for an interference fit and heat the main parts of the crank in an oil bath to assemble or take it to an engineering shop with a hydraulic press.

Another approach is to machine to a slightly looser tolerance that you can assemble your self then drill and ream both the pin and the webs to take a suitable dowel. Do this axially along the pin so you effectively have a round key fitted.

Alec

 

[snowcat]

Ah think the lack of sleep had me going mad last night split journals are used on the crank pin's so they can be split after there asembled,
As for the crank a tight interferance fit should be fine ,Iv proberbly got a maching spec for the fit somewere, and a drop of threadlock, there's loads of diffrent types my favourate is shaft lock good for upto 350 C. the problem with welding and iv found this from loco frames is the HD caused by the welding.

BTW 6" scale i take it? a decent sized engine which one is it?

 

[Davek0974]

This is very interesting stuff coming in..

Would that be taper or parallel dowells?

I was thinking of using loctite backed up with two taper pins going across the shaft at each joint either axially or radially. Maybe one but two sounded better. In a 38mm shaft, they could be quite beefy pins?

I can see the method of the dowells, fitting the flanks to the shafts will be easy as it will be obstruction free to drill but drilling the flank/shaft assemblies to the journal pin will be blocked by the shafts sticking out.

There is a final working solution coming, i can feel it

 

[Davek0974]

Ah think the lack of sleep had me going mad last night split journals are used on the crank pin's so they can be split after there asembled,
As for the crank a tight interferance fit should be fine ,Iv proberbly got a maching spec for the fit somewere, and a drop of threadlock, there's loads of diffrent types my favourate is shaft lock good for upto 350 C. the problem with welding and iv found this from loco frames is the HD caused by the welding.

BTW 6" scale i take it? a decent sized engine which one is it?

'HD' whats that?

Its the Savage 'Little Samson' steam tractor.

 

[snowcat]

'HD' whats that?

Its the Savage 'Little Samson' steam tractor.

Heat destortion..

 

[Davek0974]

Ah, thanks snowcat.

As a side point, what spec steel would be good for a low power crankshaft?
It runs in brass/bronze bearings with manual lube.
EN16 or higher?


Thanks for advice so far.

Dave

 

[snowcat]

I would have thought EN8 would have been fine, the phos/bronze should be far softer than the bright..

 

[Davek0974]

Sounds good, nice and easy to work with, i like that.

So its a cold assembly with loctite and dowell pins made from EN8 then.

Thanks all.

Dave

 

[piman]

Hello Dave,

do aim for a light press fit at the least or the dowels will fret rather quickly (As they are round in section, unlike a key which can accommodate some slight movement without deteriorating. A key would be better but machining keyways takes more machinery to achieve.). And I still believe that Loctitie is of no use in this application.

Alec

 

[Davek0974]

Hi all, before posting here, i put the Loctite question to Loctite Technical and have just got the reply...

********************************************
Loctite 638 should be ok for this application
Max gap fill is 10 thou Min about 2 thou
Make sure all parts clean and dry
If steel to steel then expect 30 minutes fixture & 6 hours full cure at room temp

Regards,

Henkel Loctite Adhesives Ltd
Technologies House
Wood Lane End
Hemel Hempstead
Herts HP2 4RQ
********************************************

So it does sound possible. I might do some testing.

Dave