The Steamboating Forum

Another approach that is used are pinch bolt designs.... the bolt goes in between the crankpin and the shafts, so this works better for longer stroke engines. The shafts are keyed, and the crankpin is not. With a proper rigid engine frame, this sort of crank works well - and since no heat is involved, rolling contact bearings can be used.

This does require extra length between the bearings, though.

- Bart

Hmm. I never thought of doing that on a crank.

I built some pitman arms for use on unlimited hydroplanes some years ago. This was basically a 12" inch long tiller attached to a two inch rudder post. Bouncing across the water at upwards of 180 mph one can only imagine the reversing shock loads involved.

To my interest, the design did not call for any sort of spline or keyway or any sort of set screw. The arm was machined out of 2" aluminum plate with a 2" reamed hole for the rudder post and slit so that a grade 8 nut and bolt clamped the arm to the post. According to the boat builder, these never shifted. In the past, they had at least one fracture at a keyway so they never did that again!

The trick, with a crankshaft, if I'm visualizing this correctly, would be to machine the slot between the main shaft hole and the crank pin hole. Or is each web made in two pieces? That doesn't seem as sound.

Mike

Lopez Mike wrote: The trick, with a crankshaft, if I'm visualizing this correctly, would be to machine the slot between the main shaft hole and the crank pin hole. Or is each web made in two pieces? That doesn't seem as sound.

Mike

Yep - just cut it (a bandsaw or hack saw would do in a pinch ).

I'm trying to find a link to a design I saw like this... somewhere on
the web.

- Bart

Sounds pretty barbarous to me. I'll have to think about what can go wrong.

Jack? Have you fainted yet? (grin)

Mike

The American walking beam engine in my sidewheeler has a built up crank, pressed together, and fitted with “poor boy” keys. The interference fits are 0.001 on a 1.25 inch shaft diameter, and the maximum design torque is 270 LBf-Ft, a healthy number. The crank webs are ordinary low carbon steel, and the shafts are common, unhardened drill rod. This crank is not overstressed, and the fits are not overloaded.

The level of machine integrity described by steamboatjack is first class, but for many applications it may be much more than is necessary. To have machined my crank from solid, especially the high strength stuff steamboatjack states as a minimum strength (600 N/mm2 = 87,000 PSI) would have cost a small fortune compared to what I was able to produce in my small home shop.

Many two stroke racing motorcycle engines use pressed together cranks, so they can use needle bearings on the big end of the connecting rod. These cranks hold up well, under stress conditions that generally far exceed those of our steam hobby.

Design of machinery should always be a compromise, trading simplicity and ease of manufacture where prudent to allow reasonable cost. A forged one piece crank may be necessary for high performance engines with peak cylinder pressures of 1000 PSI or more, but is beyond the reach of the one-off home shop builder, unless of course a ready made IC engine crankshaft is adapted to the engine being built.

Folks,

This discussion is healthy for all concerned, I believe that every one should strive to make the best job they can when building an engine, perhaps this is why mine seem to take years? I believe the cost of a piece of EN8 (U.S. 1040 approx) for a solid crank shaft in this size (without weights) would not be too much relative to the value of a completed engine. I would do 90% of the machining on a milling machine with a dividing head & tail stock and only use the lathe for completing the mains less grinding allowance (I always have all the journals ground). I am NOT against fully built cranks or the use of Loctite (for other jobs). I just know some of the published drawings/designs show crankshafts which are poorly detailed and doomed to failure without guidance.
Regarding Mike's comment regarding dead blow hammers, I am a bit lost here? A shrink fit crank is not trued this way, if it could move at all it would be a failure, Careful assembly should leave it close enough for grinding, its obvious that on any assembly the main shaft is left as one piece until assembled and the crank spaces cut out afterwards. The Brazed crank was a pinned assembly and machined and ground afterwards (crank-pins ground only) The Pinched crank idea was used on Hasbrouck engines I believe.
Regards Jack

steamboatjack wrote:Folks,

This discussion is healthy for all concerned, I believe that every one should strive to make the best job they can when building an engine, perhaps this is why mine seem to take years? I believe the cost of a piece of EN8 (U.S. 1040 approx) for a solid crank shaft in this size (without weights) would not be too much relative to the value of a completed engine. I would do 90% of the machining on a milling machine with a dividing head & tail stock and only use the lathe for completing the mains less grinding allowance (I always have all the journals ground). I am NOT against fully built cranks or the use of Loctite (for other jobs). I just know some of the published drawings/designs show crankshafts which are poorly detailed and doomed to failure without guidance.
Regarding Mike's comment regarding dead blow hammers, I am a bit lost here? A shrink fit crank is not trued this way, if it could move at all it would be a failure, Careful assembly should leave it close enough for grinding, its obvious that on any assembly the main shaft is left as one piece until assembled and the crank spaces cut out afterwards. The Brazed crank was a pinned assembly and machined and ground afterwards (crank-pins ground only) The Pinched crank idea was used on Hasbrouck engines I believe.
Regards Jack

Concerning the lead hammer truing, I believe Mike was referring to the way you true Harley Davidson cranks. The Harley crank has two flywheels and three shafts. The shafts are tapered (to fit the tapered holes in the flywheels) with threads on the ends for a nut. The nuts are snugged up then either on a truing stand, or V blocks or on a thick piece of glass using feeler gauges the flywheels are manipulated with a lead hammer into true. Nuts are tightened again and the trueness rechecked then more taping with the hammer to keep true and the nuts are tightened to torque.

"Concerning the lead hammer truing, I believe Mike was referring to the way you true Harley Davidson cranks. The Harley crank has two flywheels and three shafts. The shafts are tapered (to fit the tapered holes in the flywheels) with threads on the ends for a nut."

While the Harley Davidson cranks are as described above, several Japanese 2-stroke motorcycle engines use a simple pressed-in crankpin on a three piece crankshaft assembly, with no tapers, no nuts, no keys and no pinch bolts. These cranks are pressed together, then aligned with a lead hammer which smacks the crank web. Alignment is checked by mounting the crankshaft between centers, and placing a dial indicator on the main shaft, near the crank web.

The photo shows the American Walking Beam Engine crankshaft being checked the same way. Fortunately I did not need to smack anything with the lead hammer, but if alignment was not right I would have.

That is exactly how I was shown with the Villiers (s/cyl 2 stroke) so many years ago.

It was 'clocked' at at least four points, two on each main shaft.

Note that the high torque area in a crankshaft is the shaft - web connection; the crankpin will have no net torque so long as the crank is properly supported by the main bearings. In the pinch bolt design I mentioned earlier in this thread, the main shafts are keyed into the webs, but the crankpin ends are not.

- Bart