Bassett Lowke 3/4″ Scale Traction Engine – See How Freely the Crankshaft Runs
Crankshaft Alignment Challenges
I experienced exactly the same issues with this little 3/4! scale traction engine model that I did with my big 1/2 sized Little Samson Traction Engine.
In both cases no matter how careful I was to machine everything square when it can to assemble the crankshaft and the connecting rod/crosshead together with the cylinder on the boiler it was very difficult to get the crank to turn freely.
And this despite machining the crank bearing holders at one time with the hornplates bolted together.
On the big 6″ scale Little Samson Traction Engine the problem was greatly exacerbated by the gasket under the cylinder saddle sliding all over the place because it was liberally covered with Steam Seal.
On the little 3/3″ Basset Lowke engine a gasket isn’t needed the cylinder saddle just sits directly onto the boiler.
If you’ve seen the page in this website “Cylinder Assembly On Boiler”
you’ll appreciate that I machined the cylinder and saddle with the utmost of care to get everything square.
And yet when I came to spin the crank it hit some very tight spots.
I’ll talk more about that below but first just look at how sweetly the crank spins now in the video below.
What No Soundtrack?
Did you hear anything when the video was playing? Well, that’s not because I cut out the sound. The sound recorder was left on. It’s quiet because there is no noise!
How Did I Make it Run So Sweetly?
I did two things, both of which I did on the big Little Samson Traction Engine.
1) I lapped in all bearings, the crank bearings an connecting rod big end bearings with very fine Timesaver lapping compound.
2) I moved the crankshaft support bearing nearest the crank itself until the crank ran freely. I was surprised by how much I had to move it. Over 1mm (1/32″). I had to drill new holes for the bearing retaining screws.
How Can The Crank Bearings Be So Far Out?
After (for both my engines) I took such great care to machine everything square how could the bearings be so far out? Look at it like this: The machined components, the Crank, Bearings, Connecting Rod. Cylinder and Cylinder Saddle are all precision-machined parts.
So you put that precision assembly onto a boiler and hornplate assembly that’s not machined, not precise and is an agricultural level of fabrication.
It’s like trying to use blacksmith forged plated to hold switch watch movement.
Anyway that’s my excuse.
Anybody who has built a Traction Engine and got the crank to run sweetly first time please share your experiences below. Ditto anyone who struggled as I did.
Hi Les, can’t remember exactly where I bought the Timsaver but it was at the time I was living in the UK so 10 years ago at least.
I bought a ‘Trial Kit’ which has 8 small tims of powder – four grit grades Coarse –> Very fine and for each grit grade a tin for soft metals (brass, bronze) and one for hard metals (steel).
Although Ive used it a great deal and wouldn’t be without it the tins are still nearly full. you only need a tiny bit of it to go from a very tight fit to a free running fit.
Hi Alan
The crank assembly is looking good. Interesting method of demonstrating the rotation with a bit of string.
It always frustrates me as no matter how hard I try to measure, align and check when you finally set up a crankshaft in its bearings they always end up out of alignment.
The engine is looking good and real progress can be seen.
I hope to send a few photographs soon so you can update a few of the projects on the web site.
Could you not machine the bearings to size in situ after fitting them to the frame?
Hi Paul, you mean like lineboring?
I could put the engine on my big mill and bore out the crank bearing bushed, yes. But how would I line up the cetreline of the cylinder in two axes?
Looking good Alan. As usual, your craftsmanship is impeccable.
Above comment by Les who posted it on the home page and I copied it here.
The finish you see on all the parts is as machined. I slight touch-up with 500 grit wet & dry on the steel parts but that’s all.
As to fits I would mention:-
– Obtaining good fits ie running fits of bearing surfaces is never a problem. The problem is getting the machined parts to line up when fixing them to unmachined plates and tubes. (ie hornplates and boiler barrel)
– I use Timesaver lapping compound because it doesn’t embed in the components as would valve grinding paste (which is far too coarse anyway) but instead breaks down into a non-abrasive dust. I wouldn’t be without it.
Hi Alan,
Thank you for moving the post, good to know about Timesaver lapping compound, the valve lapping compound I have is basically brasso, it is super fine not the course stuff we normally use for valves, however I agree the abrasive is hard to remove and normally takes several cleaning passes to remove all traces from the bearing after finishing.
I checked lazada and can not find Timesaver lapping compound, where do you purchase it from?
Wow looking very nice, I like the finish to the parts, I’m guessing lots of time with wd40 and fine wet&dry.
Regarding bearings and crankshaft alignment. I made all my bearings (bronze inserts) tight then used some very fine valve grinding paste I had left over from rebuilding my Bronco engine.
I used a bit of grinding paste and some wd40 then manually rotated the shafts and axles until they turned freely then cleaned everything with white spirit to make sure all the grinding paste was removed.
I was also surprised even though I took my time making sure everything was square when I first put the axles in place they did not line up perfectly.
I drilled and marked out all the plates using cnc and used the edge finder and DRO on the mill for the bearing blocks, I think the errors came from bending the plates, even though I scribed the bend lines on the cnc, bending the plates on my 5 ton press introduced +/- 0.5mm.
I did all the bending calculations so I allowed for the metal creep during bending but still the plates ended up slightly off size.