Built up crankshafts

[ivanj]

I am building a King (4 cylinders) so there is a two throw crank between the wheels. I have made one but I am not too happy with its rigidity. I built it up using loctite and parallel pins to lock it in place. Is there a better way apart from machining from solid which in this case would be difficult as the cams for the valve gear are mounted during assembly of the shaft. Another problem I am having is getting all of the elements concentric despite taking great care with the machining.

[baggo]

Hi Ivan,

The secret with built up crankshafts is to leave the axle in one piece until the whole assembly is loctited and pinned together. The axle is then sawn out between the crank webs. Doing it this way ensures that everything is perfectly lined up afterwards. I've made two crank axles for three cylinder locos this way and they have both turned out fine.

The alternative to Loctite is brazing but the heat involved can cause distortion and the crank assembly may require straightening afterwards.

The photo below shows one of the axles after assembly but before the centre piece was sawn out.

John

[ivanj]

Thanks for that. Is it best to use pins or to drill and tap at three points. I have already had one attempt at this and I did get alignment but I am not confident that the structure is rigid, especially as there are two inner throws.

[baggo]

Hi Ivan

I actually use taper pins. The holes for these are drilled and reamed after the Loctite has had chance to set properly, say 24 hours. In the two cranks that I have made I have only put pins through the main axle and not through the crankpin as well. For your two throw crank I would suggest pinning the crankpins as well.

I wouldn't worry about lack of strength or rigidity using Loctite - it's very strong stuff if used correctly. Just make sure that all the joint surfaces are clean and free from grease or oil.

Incidently, there's a good description of building a similar crank axle for a 5" Bulldog by Keith Wilson in ME volume 144 issue 3595.

John

[Malcolm]

There is a lot to be said for rough machining everything oversize, brazing up or silver soldering to choice, and then treating the assembly as a forging or casting and finish machining to size. This eliminates the distortion that you get when brazing finished components.

[havoc]

There is a lot to be said for rough machining everything oversize, brazing up or silver soldering to choice, and then treating the assembly as a forging or casting and finish machining to size.

Isn't this just as complicated as turning it from a solid blank? Maybe less swarf, but I don't see other advantages. Or do I miss something?

[Malcolm]

You certainly are missing something! Try turning a decent sized crankshaft from the solid and you will find out. There is heavy stress on the lathe from the interrupted cuts, even more stress on the user, and enormous amounts of swarf. Do as I suggested, and only light finishing cuts are required. The throw pieces have to be made in either case.

[havoc]

Well, I was looking at it from the point of view of setting up the job, supporting the job to avoid stress in it etc.

[davidimurray]

Just out of interest - why pin after loctiting?? Assuming that your using 638, it is enormously strong as long as it doesn't get too hot. In the event that the loctite did fail surely the pins would fail as well !?

To give you an idea how strong 638 is a friend of mine has a Volvo car engine connected to the hydraulic pump of his HIAB crane using a simple shaft loctited onto the pump!!!

Cheers

Dave

[waggy]

For those who doubt Loctite may I suggest a test?

Take two pieces of steel, say 1/2" thick and an inch or so wide by six or so inches long, drill and ream a 1/2" hole at one end of each bar. Take a piece of 1/2" round bar long enough to go through both pieces and leave a small gap between the sandwich. Ensure the pin is a sliding fit in the hole. Using 601 or 638, bond the pin and pieces together making sure there is a gap so no face to face contact occurs. Leave to cure for a day or more, the curing times on the bottles are for ideal conditions of which I have no knowledge.
After curing secure one bar in the vice and try to turn the other on the pin. You'll be very surprised at how much force it takes to break the bond. This test should instill confidence in any of those who doubt.

Waggy.

[baggo]

Hi Dave/Waggy,

Believe me, I've every confidence in Loctite! The only reason I pinned the crankpins is because it was suggested for the 2-1/2" A1 that I am building and being somewhat of a belt and braces person did so.

I've had two cases of being caught out by the Loctite (both 601 and 638) going off sooner than expected when quartering wheels and can vouch for it's strength when trying to remove the offending wheel again. The first time I just managed to get the wheel off the axle by violent twisting but the second time I wasn't so lucky and had to heat the axle to get the wheel off again. This only a few seconds after the Loctite going off. Both cases were, I think, caused by making the axle too close a fit in the wheel as the smaller the gap, the faster the Loctite sets.

Actually, this is the one thing that I do have against Loctite i.e. the speed at which it sets. It doesn't leave you much time to get the quartering etc right even though I do use a jig. Perhaps there is a slower setting version? Perhaps I'm just getting slow in my old age!

John

[Malcolm]

I think that Baggo has highlighted the major problem with Loctite. If you make the fits tight enough to give reasonable accuracy, then the stuff sets far too quickly for comfort. Stick to Loctite's recommendations, and you get poor concentricity. I won't use it any more on crankshafts for that reason.

[Tel]

You need to compromise here - make the journals a closish fit and turn a relief to Loctite specs in the centre of the area

[baggo]

Hi Tel,

thanks for that tip, I'll try doing that in future

John

[chris vine]

A trick which works with Loctite 638 if it is going off too quick is to chill the parts in the fridge or freezer. This slows up the action so that plenty of time is available for adjustments.

Chris.

[chris vine]

PS I was advised by Loctite that the action is much quicker on copper alloys.

C

[baggo]

Chris,

The freezer trick sounds a good idea - I'll give that one a try.

John

[SteveW]

Guys,

It's just occurred to me that a super-cooled part could attract condensation. Will this be a problem with Loctite?

[chris vine]

Hi Steve,

Yes I wondered about the condensation aspect. However I just wiped all the bits with kitchen towel before putting the stuff on and the joints were fine. (so far!!!......)

Chris.

[ivanj]

Thanks to Baggo, waggy and others for all your help on this topic. I have now built the new crankshaft and it is accurate and seems to be rigid. I got the webs cut by water jet and used loctite and pins to secure it all.

Thanks again

Ivan