The Stationary Engine List is an internet mailing list which specialises in world-wide
computer "conversationsĒ about engines. It was decided sometime last year that the GEM readership would enjoy
some of the discussions which take place, so I pick one of the many topics covered over the past month to pass
on. This month starts with a query from New Zealand:
~ I've got a question for all you welding experts out there. I've had a 3hp National for some time now, complete
but with three broken spokes so itís sat at the back of the shed. Just yesterday I was given a flywheel with one
cracked spoke. Iíve tried for a long time to find a good one but they all seem to be connected to complete engines.
Whatís the best way to weld the spoke?
When I collected all the responses together, I had around 6,000 words to deal with, so I have disregarded all the
tales of havoc wreaked by broken flywheels on cars and tractors (this is, after all, an ENGINE magazine!) and tried
to stick to the subject.
~ Clean up the flywheel real good. Vee out the crack real good heat the flywheel on a forge or in an oven and get
it good and hot (around 600 degrees will work) braze it, put it back in the oven and cool it down slowly. Grind
and file the braze job and put it to work.
I have had a great deal of success so far with this method. The other option is to use the best one as a pattern
and get an new one cast. The reason for the preheat and cool down is sometimes if you localize the heat and just
braze it you can make matters worse. The preheat dries out the iron and helps to prevent too much stress building
up in the braze area.
~ If there is only a crack in one spoke, I would do nothing. Trying to weld it could stress other areas and actually
you could end up with more than one crack. At 150 to 175 RPM one cracked spoke shouldn't present a problem. Now,
if you are planning on running this thing like a Maytag (5000 RPM !! Maybe not quite this fast but when the smokeís
rolling they seem so) at a high RPM (400-450) I would possibly have second thoughts. For "play" purposes,
you should be fine. (This from a guy who has
personally watched two single cylinders "explode"!!)
~ Seriously, I'd look that flywheel over REALLY REALLY GOOD (one crack could mean two, depending on what caused
the crack you can see!) and if it is indeed only one cracked spoke, I'd do nothing. Run it slow and fun.
My JD E 1.5 has had a cracked spoke (just one) since 1979 and has made a show nearly each year, often running a
pump or something. Do look it over, however, as a crack can exist that you won't see easily.
~ Just a word of caution to everyone. You never know what these old engines did or how they were treated by their
owners 80-90 years ago. Give them careful inspection, especially in the cylinder/base/flywheel area, to assure
that the 100 year old iron can handle the stress.
~ The subject of welding flywheels gives me a headache. I'm taking an aspirin and going to bed!
~ The wheel in question has six curved spokes with a rather heavy rim the crack
is near the rim. The engine should be happy below 150 rpm being 88 years old, I think she deserves a rest .
~ I have had flywheel spokes repaired using a stitching method. The method uses no heat. A slot is milled perpendicular
to the crack then a specially shaped wedge is inserted. Then overlapping holes are drilled along the line where
the wedge was inserted. These holes are tapped and plugs screwed in. The plugs are then flushed off. In the end
after a coat of paint you will not be able to tell there was a crack.
This process is widely used for heavy press and shear repairs.
~ Metal spraying torches are for build-up work only. They should never be used for this type of repair. They have
been used for decades for motor shaft build-up etc.
~ To weld something like a cracked cast iron spoke you need to heavily grind the broken ends to about 45 degrees
to allow the weld rod to get down into the break and build up a good thick weld. It's not done like soldering.
~ It does help to preheat the part but it's not as critical as the old ways.
~ Metal spraying is not the best for welding broken flywheels. It is great for building up shafts or bores or filling
in holes. It bonds pretty well to the part but doesn't have the same tensile or tear strength. It can be ground
to size and if done right there are no pin holes or separation. It's hard to even pick the edges.
~ Arenít you supposed to NOT grind cast iron with a normal disc?
I was told the carbon is deposited from the disc which results in a crack point after welding. Vee out using rotary
burrs instead, or use gouging rods instead - lotsa neat big sparks..
~ A machinist friend of mine repaired a flywheel with two cracked spokes by drilling down from the outside rim,
through the center of the spoke, past the crack. He then counter bored the hole down to the crack. The top of the
hole was also enlarged.
What he wound up with was a hole down through the spoke with three steps. The bottom diameter was the correct size
for a 3/8 tap, the mid-section was for clearance for a 3/8 bolt, the top opening formed the shoulder for a special
bolt that he made on a lathe.
Can you see where this is going? After all this machine work, he coated the long threaded bolt with some kind of
epoxy, stuck it down through the spoke, cranked it in tight and, presto, the crack disappeared. The top of the
bolt was cut off flush with the rim and the outside face was then machined smooth to get rid of the rust pits.
When the repair was done the only evidence of it is two round spots on the rim, where you can see the difference
in color between the steel bolt and the cast iron.
When finished, the machinistís only comment was, if he thought about it a little more, he would have made the top
counter bore deeper and installed a cast iron plug above the pin, so that nothing would have shown on the flywheel
This repair was done after considering all kinds of ideas, from metal stitching to welding. It was a lot of work
to align the wheel correctly for the drilling process. But the end result was better then expected and I would
guess that the repaired spokes are now stronger then the other four.
~ The only fix I'd consider is no heat, no heat stress!
Ottawa log saws are bad about getting over on their side and busting flywheels. I've seen two different saws with
repaired flywheels and a steel ring shrunk around the outside.
As is common with the Mailing List, the direct responses to the questions caused a great many discussions to go
off at tangents; the safety aspects of running an engine with such a repair being a primary one. This in turn led
to complicated mathematical equations (many impossible to reproduce with a normal 102-key computer keyboard!) to
calculate the speed at which a flywheel runs, and the force with which it could burst apart. One calculation compared
the rotational inertia of one 66 lb., 2 ft. diameter flywheel at 500 rpm to the muzzle energy of a 6.5x55mm Swedish
~ From what I can gather, all that pounding on ancient cast iron could cause fractures and the resulting blow ups
discussed on the list. The presence of a broken flywheel is forbidden at my club here because of obvious liability
concerns. I just wouldn't do it - to me it is like bringing a bomb to class.
~ If you heat a flywheel to repair it, out of fairness to the public and the people that run the shows you should
keep the engine at home and run it at your own discretion
~ I agree with you, 100%. Perhaps the person making the repair will be aware, but what about the next generations
of unsuspecting collectors? I feel that a welded spoke is a time bomb due to stresses that will invariably be imposed
~ Imagine the forces on a shaft that has been built up by metal spray, or a cylinder bore - it's not too bad and
they are supported and contained. The forces are in shear. But consider the centrifugal force on the spokes of
a flywheel where the force is a direct tensile pull on the two parts of the spoke. Metal spraying is a bit doubtful
in that application.
In the light of these comments, one would certainly think twice about a flywheel repair. So, with that thought
in mind, Iíd like to wish you all a very happy, and SAFE show season.