The Crankshaft

 

An Article submitted to "The Funnel", The journal of the SBA of Great Britain

 January 2002

 Some thoughts as to construction methods, some interesting calculations, a method of fabrication introducing a way to reduce cumulative error, a failure, and a design for a successful conclusion.

 There have been very few articles in "The Funnel" or "Steamboating" or any of my steam-related literature on how to make your own crankshaft. Perhaps that should have warned me before I even started.

The finished shaft prior to grinding, all 32" of it

 The following account relates my experience to date.

 October 2001 found me seven months into the machining of a 2¾ + 4⅜ + 6½ x 4" triple expansion marine steam engine. The castings had been purchased from the Elliot Bay Steam Launch Co. in Portland, Oregon.

 The castings for the engine are supplied on a "not ready for production" basis which essentially means that if you machine to the drawings it will not all go together. (See Builders Notes)

 I had taken a trip to the U.S. in July 2000 to speak with an engine builder in Colorado who is about 70% finished and to another in Vancouver. The latter finished the engine a few years ago and it runs magnificently in a 23 foot Elliot Bay hull. Convinced that it could be done I bought the castings and they arrived in October 2000.

 The workmanship of both of these men is exceptional and both have been a great help to my amateur, self - taught efforts so far. Another of these engines has just been completed in Maine and five are currently under construction in the UK.

 By October I had completed the cylinder block, bedplate, main bearings, pistons, slidebars, crossheads, connecting rods etc. etc. Progress to that date is shown in an adjacent photo.

Progress at October 2001

 I had, for some time, been losing a bit of sleep thinking about the crankshaft.

 A casting for the 32" long crankshaft is not currently available and the options and conclusions appeared to be: 

  1. Make a pattern and cast it: The traditional method for smaller shafts but although the pattern would not be a big problem, finding someone to actually cast one in the correct material and the large lathe (11" swing over the saddle) required to machine the crankpins ruled this method out for me.

  2. Turn it from the solid: Not too bad for small shafts but machining all that metal from 11" diameter stock and the large lathe required ruled this out

  3. Cut a blank out of  flat stock, twist it to provide the throws and then machine: A novel method being used in Australia for a couple of compound engines but not so far attempted for three throws.

  4. Fabricate and weld/ braze: The method used by the builder of the five UK engines, distortion likely to be a problem, annealing and grinding necessary.

  5. Fabricate by machining webs, pins and journals then "locking" together by splitting the webs and securing by squeezing with socket head screws: The method used for the two US engines, thought to be suitable for up to 20HP. No grinding thought to be required as the shaft is assembled and locked in position in the bearings. I had some doubts as to torsional strength.

  6. Fabricate and secure with Loctite™: Certainly a low machinist-stress option, strength assured say the technical representatives but what happens if you run a hot bearing and the adhesive fails above 150 degrees C?

  7. Fabricate by shrink-fitting: The traditional method for large shafts (ship sized), a bit stressful, thought I, with the chance of a journal locking up half way through a very hot web and wrecking the whole job. The method used in the Canadian engine.

 

I eventually chose the shrink fit after speaking with the Canadian engine builder but decided to eliminate the keying of the journals and webs that he used as I had doubts about my ability to machine them accurately. The impression that I had was that the keyways were primarily there for the purpose of location rather than torsional strength in a shrink fit joint. I would be interested to hear the opinions of others on this matter.

 The diameter of the final shaft was to be 1.500" so I used 1.625" to allow for final machining to eliminate any eccentricity, and grinding. The dimension of the webs was 2.500" wide, 0.875" thick. With the journals and pins in the webs there was 0.750" of material beyond the main journals and 0.450" of material beyond the crankpins. Obviously, there was 0.500" of material on either side of the pins and journals. Drawing 1 shows the essential dimensions.

 These dimensions were taken from the drawings supplied with the engine, which were designed for method five listed above. My assumption that these dimensions were suitable for an alternative method of construction led to a problem explained later.

 My first shrink-fit experiment went surprisingly well, I wondered what all the fuss was about. The web was bored 1.4935", the pin 1.495". Heated with an LPG torch until the hole measured 1.502" the pin fell into the web and after cooling I was unable to hold the shaft firmly enough to see how strong it was.

 My second experiment was to make up the components for a single throw shaft to see if my assembly technique would work: crankpin into hot web, web and pin slid into hot web on a flat surface to produce a "web assembly" and the main journals dropped into the remaining holes. I was a bit careless with measuring the hole in the hot web for the first journal and experienced my first "lock up". The journal went in at an angle, locked halfway and that was that. My confidence was shattered!

 By cutting off the journal close to the web I was able to drill a 1.250" hole axially through the offending journal and was able to knock the remainder out with ease leaving the web intact. Although a bit upset about the failure, I had found the way that I could get out of any future trouble. Previously I had nightmares about dropping the last journal into place on the almost complete shaft, having a lock up, and having to start all over again.

 The first hint of possible trouble occurred when I put the trial crankshaft into the dividing head on the mill, clamped a substantial bar to the outermost web and leant on it. The crankpin joint failed with very little effort. This is the joint with 0.450" of web outside it. I was not able to hold the main journal tightly enough to break its grip on the web. Puzzled, I attributed the failure to a poor finish in the holes in the web leaving less surface area to grip.

 I made a further trial "web pair" and clamped one of the webs to the six foot leg of my workshop crane. I clamped an eight foot length of 1.5" square tube to the other web and gradually leaned on the end. The joint failed just before my 150lbs of weight was fully supported on the tube…. I estimated 1000 foot-pounds of turning effort on the joint.

The test rig, less the eight foot lever

 When I inserted two 0.250" silver steel pins through the joints I was able to stand on the end of the tube before the joint failed… I estimated 1200 foot-pounds (150lbs at eight feet) Note that this trial was carried out on the main journal end of a web ie. with 0.750" of material beyond the journal. Also on this trial piece there was only one hole in each web

 It was only then that I calculated the maximum torque likely to be produced by the engine.

 Each of the three engines is likely to impart a turning moment of a maximum of around 180 foot-pounds (eg 180psi on the 2.750 diameter HP piston gives a "push" of 1062lbs at a distance of two inches. This equates to 177 foot-lbs when the crankpin is 90 degrees past top dead centre. (2/12 x 1062))

 With a three cylinder engine, the maximum turning effort appears to occur when one of the cranks is at 120 degrees past TDC and another is 120 degrees before TDC. The other crank is on TDC and cannot generate any turning force. Because the crank is no longer horizontal it's distance from centre is reduced to about 1.750" and therefore the torque at that point is approximately 150 foot-pounds for each of those two cylinders (1.750/12 x 1062).

 300 foot-pounds was just a number to me until I realized that the 5 litre 300HP V8 in my wife's four-wheel drive produces similar numbers.

 I knew that steam engines produce a lot of torque but it still seems a bit hard to believe that my engine designed for 10 IHP @ 300RPM is likely to produce that much twist!

 That figure of 300 foot-pounds is for maximum engine torque. To allow for shock loading such as hitting a rock with the propeller or getting a slug of water in the engine I understand that double that amount is considered a reasonable figure to work with. So with a joint thought to be safe to 1200foot-lbs, a two to one safety factor seemed a bit tight but probably safe.

 Back to the shaft:

 The three pairs of webs were machined then surface ground to final size, the holes for the pins and journals were bored to 1.492" using a substantial boring bar and a tungsten tipped tool to get an excellent finish. The three crankpins were machined to length then the ends were turned to 1.494" and the journal surfaces finished to 1.560" to allow plenty of meat for grinding out any distortion or misalignment.

 The five main journals were machined to length with one end of each turned to 1.494".

 Using the LPG torch and a crayon that changes colour when a certain temperature was reached. I heated up each of the webs to 600 degrees F. At that temperature the bores measured 1.501".

Assembling the "web pairs"

 I assembled the three web pairs as previously described. Into the LP pair I inserted both main journals, into the IP and HP web pairs I inserted only the forward main journals.

Ready to insert a journal into a web pair

 When I placed the aftermost LP journal/web pair/journal assembly in the lathe the runout of the forward journal was about 0.015". I centred this journal and turned it to 1.520" and reduced the exposed end to the shrink fit size.

 This procedure "corrected" the assembly misalignment so that any error did not accumulate down the length of the shaft.

 With this assembly in the dividing head on the mill table and the crank positioned horizontally with a dial gauge I wound it round 120 degrees (The LP trails the IP). With the IP web/ forward journal assembly set up on parallels at centre height I heated up the aft IP web and slid it along the parallels and onto the forward LP journal.

Assembling the crankshaft in correct angular alignment on the mill table

 This IP forward journal of the assembly was "corrected" in the lathe as previously described but using a steady on the IP aft journal.

"Correcting" the crankshaft after assembling the IP web pair

 The same procedure was used to locate the HP web pair/journal assembly, the crankhaft now being complete the forward main journal was "corrected" and I had a three throw crankshaft.

 Seven hours on the crankshaft grinder gave me the finished shaft.  This time would have been dramatically reduced had I not been so pessimistic about distortion and left the crankpins at 1.520" rather than 1.560".

 It looked beautiful and I proceeded to machine the counterweights, turning them from 6 1/2" stock, cutting them in half and milling out the recess. I fitted them all to the webs as a gentle tap fit.

The finished shaft

 It was when I was counterboring the attachment holes in the counterweights that I noticed that disaster had struck... The shaft was not running true, to the tune of about 0.040" The crankpins were not absolutely tight in the webs. My tapping on of the counterweights had upset the alignment.

 To say I felt sick would be an understatement!

 After tapping the shaft back to truth (well, within 0.0015") and pinning the joints with the silver steel pins I could still get movement in the crankpins which lets the shaft run out 0.005"...clearly unacceptable

 So, what went wrong?

 When I went back to my Machinery's Handbook on shrink fits, where to look for some of the answer became clear:

 "The thickness or amount of metal around the hole is the most important factor".

 When I measured the width of the webs (they were surface ground to 2.500") the end with the crankpins all measured 1.501 - 1.5015"

 I originally thought that there was not enough metal outside the crankpins (about 0.450") to stop the web stretching around the journals. Thus, my shrink fit allowance of 0.0015 - 0.002" was reduced to a maximum of 0.001" and a minimum of 0.0005", clearly not enough to lock the pin. The other end, the main journal end that I tested to destruction obviously had enough (about 0.750")

 Some calculations with the assistance of an engineering design consultant in Sydney, an old family friend, showed that my joints were placing a load of somewhere well in excess of 50,000lbs on the steel. Mild steel's yield strength is about 20,000lbs.

 So it appears that the major contributing factor to the problem was my choice of mild steel for the webs. When I had spoken to the Canadian engine builder about his technique and he told me that he had used "4140", an alloy steel, I had assumed that this was for the shafting, not the webs.

 My understanding now is that the 0.002" interference has to go somewhere (the journal is essentially incompressable) and the "bulge in the webs must happen. However I had taken the steel beyond its elastic limit , its ability to return to its original shape and was left with a hole not small enough to grip the journal securely.

 The Solution?

 Start again!

 Although I considered welding the pins to the webs the only proper way to fix the problem was to start again.

 I have decided to make three changes to my original design to strengthen the joint:

 1.      Make the webs wider (3") and longer over the crankpins (I can just get 0.750"). This change in size will increase the amount of metal around the crankpin bore by about 60%.

2.      I will machine the webs from 4140, an alloy steel with a yield strength of around 60,000psi

3.      I will reduce my shrink fit allowance to 0.0015 - 0.0018". This significantly reduces the stress on the steel and should still give sufficient grip to the journal.

 The area of the web between the crankpin and the journal is still the area of greatest stress and the new design does not improve this situation.

The design of the first crank web and the modified MkII version

 There is nothing like a test to confirm a theory so I will assemble a single throw shaft to the new design and stand on the end of my eight foot lever and see what happens. I hope that the joint will support a few of my friends as well!

 Lessons I hope to have learned!

 1.      I will not assume that a design for one method of manufacture will suit another.

2.      I will check the materials specification.

3.      I will do all of my calculations first.

4.      When I run into a problem, I will think again, right from scratch.

5.      When I test a design I will make sure that what I am testing is exactly the same as the final product.

6.      I will not despair over six week's lost work in a two-year project!

 

The Successful Result?

 As I write this I am just finishing the machining of the "trial" shaft to the new design.

 As the deadline for the next issue rapidly approaches you will have to wait until then to hear how strong the new design is.

 

Crankshaft Update 25th April 2002

 In early February I constructed a single throw crankshaft based on the new design using “Holdax”, the 4140 alloy equivalent recommended to me. Using 0.0025” interference I could bounce up and down on the end of the eight foot long test bar with no hint of movement in the joint. That’s 1200 foot pounds of turning force. Looking around for additional weight to find the joint’s final strength I suspended my dividing head on the end of the bar as well. It weighs an estimated 75lbs. When I stood on the bar the joint failed just as my full weight was about to be supported. I estimate that at least 200lbs was supported before failure occurred. That’s 1600 foot pounds or a safety factor of five and a bit for the joint.

 I should mention that I had been experimenting with different interferences and that 0.0015” failed at less than 1200 foot pounds. The final test using 0.0025” would probably have exceeded the 1600 foot pounds achieved if I had not experienced a “lock up” during insertion of the pin into the web so that it was only inserted about 3/4 of the way in.

 With a successful design I was confident to proceed with the construction of the Mk II crankshaft. Fabrication proceeded smoothly with a bit more runout at each stage of assembly than I had experienced with the first shaft. The worst offender was the forward HP journal that was running out 0.050” before “correction”. This was due to a mysterious gremlin that permitted the HP web pair to shrink with a twist. “Correction” in the lathe as previously described sorted this out and the shaft was ground successfully to a finished size of 1.4995”.

 The two outermost main bearings were bored in a fixture in the lathe and the three inner mains were line bored in the engine. This was accomplished on a fixture on the front of the mill table using a flat belt drive from the mill spindle that had been turned horizontally. Thrust was taken from either end of the boring bar with two pieces of 1/4” silver steel propped against a wall and a vice. Feed was applied using the table power long feed.

 At this date the big ends have been machined and fitted and I have reciprocating motion in the engine!

 Despite being a little on the tight side an electric motor coupled to the end of the crankshaft with a large reduction turns the engine at about 200 rpm very smoothly.

 Seeing the engine turn over for the first time was more than adequate compensation for the design and testing hiccoughs taken to get there.

 My earlier comment that the engine would not go together if machined to the drawings could be taken as a criticism. On the contrary, the design and problem solving necessary to get this far have been one of the most rewarding aspects of the entire process. Not always appreciated at the time but the result is very satisfying.

The MkI shaft and the MkII webs
The MkII components
The new shaft in the engine
The Counterweights after cutting
The MkII   crankshaft and counterweights
"Trueing" the counterweight diameter

 

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