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Norman Bain
03-12-2017, 05:57 PM
This thread is inspired by Brian Rupnow's recent overhead cam build. I do not wish to offend and/or clutter that fine build with a discussion on a change to the crankshaft design, plus am certainly not clear enough in my own mind as to how to go about it, so have started this thread.

I do thank Brian for getting my juices going on this.

If I can I would like to hold the discussion to pressed assemblies; this being to avoid need to turn a large block of metal into a one piece crank.

For most of us HSMers turning shafts of any length is often problematic. Getting the fit and finish right on the running surfaces of a critical part like a crankshaft is difficult.

In my mind I keep coming back to desirability of using shafts (both the main and the rod big end shaft) to being pressed in "standard" (if there is such a thing) bar. But how to do this given that whenever I have tried a pressing of any consequence (especially when the overall length is 8 inches or so) the result is far from desirable alignment/runout in the 0.001" to 0.002" range.

One part of the puzzle maybe to make the crankshaft webs (as normal) in two pieces. One of the webs to have a temporary boss to "rest" against the other half to maintain alignment while pressing. The boss'd portion would have a bore larger than the shaft being pressed thru in order that it can be safely cut away in the lathe (or even in a mill) later. The boss'd portions would be both at the con-rod bearing area and the main shaft area. A cross drilled hole thru the bosses may provide visual confidence as the cut-away action is being performed.

I guess the boss could be large and be located at the fringe of the web throw.

I have it that the main shaft needs to be pressed in one piece all the way thru to maintain alignment. If the webs do not stay adequately aligned then the sides of them can be skimmed later. Downside of this is the pressing (of the main shaft) is 4 or 5 inches and may that result in scoring of the shafts. If so, can that be reduced.

Does some chamfer of the lead-in edge of the hole and lead-in edge of the shafts assist the pressing process and reduce galling?

Is the press fit (especially if the fit is made less so to avoid galling) able to be made "tight" and maintain alignment thru use of pins? If so, what kind of pin.

Norman

Bob Fisher
03-12-2017, 06:20 PM
I made a crankshaft like you describe for my Galloway hit and miss.i used Locktite and tapered pins. It had Shoulders on the crank portion to obtain the correct spacing. Seems solid enough. Bob.

MattiJ
03-12-2017, 06:57 PM
this is pretty much standard solution in small to medium sized 2-stroke engines, ie chainsaws, mopeds, motocross bikes , waterjets and so on. with small engines you Usually press the arts together and worry about alignment later, ie https://en.wikibooks.org/wiki/PRD_Fireball_Shop_Manual/Bottom_End#Align_Crankshaft

your biggest problem are going to be the required tolerances and surface quality for the small parts.
for a 6mm shaft you want something like 0,006mm oversize shaft or 0.25000 inch hole with 0.25025 in shaft.

Norman Bain
03-12-2017, 07:32 PM
this is pretty much standard solution in small to medium sized 2-stroke engines, ie chainsaws, mopeds, motocross bikes , waterjets and so on. with small engines you Usually press the arts together and worry about alignment later, ie https://en.wikibooks.org/wiki/PRD_Fireball_Shop_Manual/Bottom_End#Align_Crankshaft


I had a read of the web page at the link. It details a reassembly/alignment of production parts for a go-cart. The technique for alignment of the two crank webs is valid only if the main shaft pins are properly aligned to the webs.

Norman

J Tiers
03-12-2017, 07:32 PM
....
I have it that the main shaft needs to be pressed in one piece all the way thru to maintain alignment. If the webs do not stay adequately aligned then the sides of them can be skimmed later. Downside of this is the pressing (of the main shaft) is 4 or 5 inches and may that result in scoring of the shafts. If so, can that be reduced.

Does some chamfer of the lead-in edge of the hole and lead-in edge of the shafts assist the pressing process and reduce galling?

Is the press fit (especially if the fit is made less so to avoid galling) able to be made "tight" and maintain alignment thru use of pins? If so, what kind of pin.

Norman

Make the press fit area LARGER than the rest of the shaft by a half mm or so, then there is obviously no problem.

Or make it press on the other direction. I think the long way is better, though, because you can press the shaft flush with the web, the press is on a shorter portion, and it seems nicer overall.

Might also be a case where heat is appropriate to get better ease of assembly and a better grip. With that small hole, it might need to be pretty hot to expand enough.

Mcgyver
03-12-2017, 07:38 PM
Matt's right, the fits tolerances will be the challenge.....along with alignment. If a small engine with a simple single throw, you could do a locating fit, with main shaft through both webs then saw it out after. I've always made them from solid myself.

Are hoping to avoid machining after, or is this just a way to avoid all the tedious light cut 'roughing'

when engines get bigger, press fit starts to make more sense - tolerances are more doable and from the solid becomes unrealistic. If you haven't seen it yet, Peter Cowie gives a great description of building the elliot bay triple expansion engine (now THAT's a man sized project!). I couldn't get his web site to come up, but recall he had an issue - for the press fit to be tight enough, it would break or stretch the web.....I have no idea what you are making, but it is a reminder that at some point the web needs some engineering to take the interference fit

MattiJ
03-13-2017, 05:02 AM
I had a read of the web page at the link. It details a reassembly/alignment of production parts for a go-cart. The technique for alignment of the two crank webs is valid only if the main shaft pins are properly aligned to the webs.

Norman

Thats another problem besides required tolerances. Getting the crank pin and crank shaft holes exactly parallel could be tricky. I wouldn't make crank webs as a rectangular sections, I would rather make round flywheel type halves like in many real engines. If you have good, solid milling machine I would imagine its doable. I would probably drill and ream the flywheel halves as one piece and part them after drilling and reaming. That way you need to worry only about getting 2 holes parallel with each other, hole spacing is and perpendicularity to flywheel face surfaces is not critical. Crankshaft stubs to flywheel halves can be fitted with goddammit-tight heat shrink fit because its one-time deal without need to adjust anything. Crank pin needs to be tight but still not totally solid so that you can align the halves without breaking or twisting the crank pin.

http://www.68shovel.com/wp-content/uploads/2012/12/IMAG0155.jpg

brian Rupnow
03-13-2017, 10:27 AM
Norman--I have had good luck with this method on two piece crankshafts where the end with the flywheel, a single web, and the con rod journal form one side of the crankshaft, and the other end of the crankshaft which drives the ignition points and timing gear along with it's web are a separate pressed together sub assembly. An extension on the con-rod journal fits "loosely" into a hole in the driven side web to keep both sides of the crankshaft turning together. I find that for crankshafts 3/8" in diameter, that an interference of .0015 to .002" is as much interference as is required. any more than that, and you are going to bend something. I have an 'undersize' reamer of 0.3735" diameter which I use to make the holes in the webs. I use drill rod for the journals, as it comes in at about .0005" oversize, whereas plain cold rolled shafting comes in at .0005" undersize and doesn't give a hard enough press. Part of the secret to pressing in shafts is to mark the end to be pressed into three sections each 3/8" long. Turn the first section to 0.373" diameter. Turn the next section to .374" diameter. Leave the third section at a full .3755" diameter. Use a strip of 240 grit garnet paper to blend the steps between the different diameters somewhat so there is less of a shoulder to catch on the web. I press my stuff together in a shop vice with a hollow socket to back up the web, so that the first two undersize sections of crank can pass completely thru the web, which aligns everything properly before the last and final full size section enters the web. I coat the shaft with green 638 Loctite first. The force required to press this is far greater than can be accomplished with a 1 ton arbor press---that's why I do it in my vice. After 24 hours to "set-up" the end of the crankshaft which sticks thru the web and protrudes on the other side is trimmed off in the bandsaw. On full length crankshafts which are not designated "two piece" units, this method is rather hit or miss. It is not nearly so easy to maintain alignment with no runout when you make a full crankshaft pressed together.---Brian

old mart
03-13-2017, 05:12 PM
British made Villiers engines used light press fit hollow crank pins and hardened plugs pressed in to fully tighten up the assembly. The assy was trued up before fitting the expander plugs. If the big end was refurbished, the plugs were drilled out, leaving a thin shell in the hole. The assembly could be dismantled easily. Carbide tipped drills were required.

Norman Bain
03-14-2017, 05:07 PM
British made Villiers engines used light press fit hollow crank pins and hardened plugs pressed in to fully tighten up the assembly. The assy was trued up before fitting the expander plugs. If the big end was refurbished, the plugs were drilled out, leaving a thin shell in the hole. The assembly could be dismantled easily. Carbide tipped drills were required.

Nice approach ... like it.

The "thin" portions would need only to be at the locations of the webs. The remainder could remain solid.

I visualise that the alignment of the web would be retained as the pressing in of the pins would be directly into the shafts.

Norman