Lapping between webs

[DiogenesIIParticipant @diogenesii]

I need to size and finish a big end journal between the webs of a one piece single throw crank for an IC engine.

The pin is 10mm x 10mm, the throw also.

Plan A is to make a spilt lap of the 'wash tongs' type

– What is a good width to make the lap to help avoid either barrelling or waisting of the pin?

– opinions in the literature seem to vary from 'accurately-finished full width' to 'narrow, work it up and down to remove high spots'.

I can see potential pitfalls in both of these methods, I'm sure that there is some happy middle ground.

As I already have some hours invested in the part, I'd rather not experiment myself if anyone has achieved consistent and trustworthy results from a particular design/dimension – there's little point re-inventing the wheel..

I'd love to hear from you.

[DiogenesIIParticipant @diogenesii]

Finishing the pin of a one piece crank

[Howard LewisParticipant @howardlewis46836]

In this instance, the object is probably to produce a good surface finish, rather than to finish to a particular size?

If the bearing were pressure fed lubricated whitemetal (Such as Aluminium Tin, Aluminium/Silicon ), subjected to high rubbing speeds and loads, the clearances would probably be 0.002 to 0.004" (0.050 to 0.1 mm) with a surface finish of not more 6 micro inch Centre Line Average

The parts of a model are dimensionally small, so clearances of the order of 0..001 – 0.002" (0.025 – 0,05 mm ) would probably be more likely to be suitable.

In the case of a model, the lubricant is unlikely to be pressure fed, but to be applied manually to a steel / yellow metal bearing, not subject to high speeds and loads, but the pin will still need to be polished to a fine finish to avoid wear.

Barelling or taper, of the order of a few microns is unlikely to prove much of a problem.

Maintaining a small smooth radius in the fillets smoothly blended between pin and web will be beneficial avoiding stress raisers, although, again, fatigue failure should not be a worry in a model

The Great Western Railway were quite precise in their manufacture of locomotive parts, but when, many years later, King George Vth was rebuilt, it soon ran a hot box, because the clearances had been made too small!

Howard

[DiogenesIIParticipant @diogenesii]

Thanks, Howard

The main object is to produce a good bearing surface, sizing being a secondary consideration. I'm currently +0.1mm over target 10mm, I need to increase corner radius / examine the score marks / apply a slip to my 'stuck-out' parting blade before comitting to another pass – interesting watching the lead corner 'pull-in' on either traverse!

I digress – yes, I need make it 'round', straight, and polished – I have a nice piece of HE15 with which to make a conrod, I have a measure of cconfidence that this will run acceptably on a steel journal provided everything is as it should be. (..on a model, of course..)

[JasonBModerator @jasonb]

I tend to finish crank pins with fine emery backed by a stick or flat metal bar which is a couple of mm narrower than the gap between the webs. Others tend to use a pair of flat sticks or oil stones and pinch the crankpin between those.

[DiogenesIIParticipant @diogenesii]

Thanks, Jason, that sounds good, will give it a go.

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