View Full Version : Crankshaft Balancing for Steam Engine

02-12-2004, 06:01 PM

I’m working on small 2 cylinder simple steam engine, and was wondering the best way of balancing the crankshaft?. Are there any good rules of thumb for determining the mass of the counterweight, or good design sources (books or websites) that explain the easiest way to calculate it?

Your help is really appreciated.

02-12-2004, 08:28 PM
First, I've never made a crankshaft, but I'm hoping to soon!

That said, it sounds like you have two parts to your question: 1) what is the design of a ballance two cylinder crank? 2) once I machine the crank to spec how do I ballance it?

On the first, if you build a 180degree crank for a two cylinder it will be ballanced because it is symetrical.

On the second, I built a simple ballancer using two stright knifes. Build some kind of sturdy U-shaped support such that the two ends of the crank can sit on the top of the arms accross the gap. Mount the two knifes, edges up on the top of the arms as the bearing edges for the crank. Now, this is the hard part. The U-mount must be will supported so it doesn't move. The knife edges must be adjusted so that each is *perfectly* level. A nice round rod laid on the edges and accross the gap should not roll one way or the other.

To use: lay your machined crank accross the gap with the crank ends on the edges. Make sure the crank centerline is square to the edges. Then see which way it rolls. The heavy side will be at the bottom. Drill a small hole in the heave side and retest. If the crank was carefully made to be symetrical this should only require a small amount of metal to be removed.

As I said, I've never made a crank, but I've used the above jig and method to ballance wheels, and rotating baskets. Works pretty well.

02-12-2004, 08:36 PM
Most steam engine cranks are at 90* so the engine will be self starting. A 180* crank will lock up and not start.
How big is the engine? With most small model engines, balance is not a problem as not that much weight is involved and speeds are low. With a larger engine, the entire assembly including crank, eccentrics and flywheel(s) can be balanced in a static balancer as described above, drilling the flywheel to effect balance or adding weights similar to those seen on railroad engine wheels.

02-12-2004, 09:29 PM
BTW, I just read a biography of Charles E. Taylor, the Wright Brother’s machinist and mechanic during the period they were experimenting with flight. Charles Taylor built the engine for the 1903 flyer as well as engines for later flyers.

Both the Wrights and Taylor worked on the design and development of the 1903 engine. It was Taylor, however, who did the entire in house fabrication and assembly of the engine. The only requirements that the Wrights gave Taylor were; the engine had to produce at least 9hp, weigh less than 200lbs, and have four cylinders of 4”x4” bore and stroke. The reason for four cylinders was a desire to reduce vibration.

The crank was a 180 degree design. Taylor produced the crank from rectangular steel bar stock. First he laid out the design on the side of the stock. Then he drilled out the perimeter of the throws and then knocked out this excess metal using a hammer and chisel. It appears that the 180 degree design was chosen for simplicity and it’s natural balance. A simple analysis of this choice, however, shows that the advantages of a four-cylinder were mostly negated because two of the cylinders fired at once acting as a big two-cylinder engine. The only advantage may have come if one of the paired cylinders didn’t fire, the other one would produce torque to keep the engine running.

The block was cast of aluminum, the first use of that new and exotic material in a flying machine. Apparently the Wrights may have learned of aluminum from Mr. Hall, the inventor of the Hall process, at nearby Oberlin college. The casting was done at a local foundry.

The gears were made at a local machine shop that probably also made sprockets for the Wright’s cycle business. Taylor made all the other parts in house. The amazing thing about this is what Taylor achieved considering the paucity of tooling. He had available: a 14” swing lathe with riser blocks that allowed for 18” swing operation, a 20” drill press, a bench grinder, assorted hand tools. That’s it! This is what I have in my small lab shop. It was the use of the riser blocks that allowed Taylor to bore the block on the lathe.

Another amazing feature of this story is apparently the Wrights and Taylor never considered that this engine wouldn’t work as planned! Not only did it work but it produced not 9hp but 12hp. And Taylor finished the majority of the work in six weeks!

If anyone has any doubt that the Wrights were geniuses and Mr. Taylor an expert machinist, let me put them to rest. The more I read about the invention of the airplane, the more astounded I become. Truly this is one of the great achievements in history and deserves all the laurels that are accorded.

02-12-2004, 11:16 PM
I look at it this way,the wieght of the crankpin and throws should equal the wieght of the conrod.May not be perfect,but it is a start.

02-13-2004, 12:48 AM
What do auto engines get balanced at? Something like half the weight of the piston/rings/wristpin plus half the weight of the small end of the rod plus the weight of the big end of the rod equals a bobweight to be attached to the crank throw and then the crank is balanced? There is definitely a formula something like this but I am not sure I remember it accurately. Does this jog anyones memory?

02-13-2004, 01:07 AM
But remeber,IC engnes and steam engines are like apples and oranges,Ic's use only one side of the piston where steam uses both,plus in steam there is no detonation driving the piston back,just steam pressure.

02-13-2004, 02:34 AM
Arcane-you are correct, the bobweight is equal to the rotational mass + 1/2 reciprocating mass unless you're into racing where over-balanced engines are used, say 51-52% recip. mass. But auto cranks must be balanced wth flywheels and harmonic balancers attached also. Most auto engines are externally balanced, meaning there is an internal imbalance offset by the harmonic balancer. In crankshaft balance according to my Stewart-Warner manual, there are 3 types of imbalance: static, kinetic, and dynamic.

STATIC IMBALANCE-The center of gravity of a perfectly balanced rotor is always located on the rotors axis of rotation. Adding weight to a rotor at a distance from the axis causes the center of gravity to shift from the axis resulting in static unbalance.

KINETIC UNBALANCE-Kinetic unbalance exists when a statically unbalanced rotor is rotated. For narrow inflexible rotors static and kinetic unbalance can be treated alike but in longer, more flexible rotors, the bending will increase the unbalanced amount and must be treated as such.

DYNAMIC UNBALANCE-A condition caused by two equal forces acting in opposite directions in two separate planes in the rotor. The rotor can have two weights away from the axis of rotation of equal mass but 180 degrees opposite each other and at opposite ends of the rotor. At rest the rotor is in static balance but when revolved these masses create a centrifugal force acting in the opposite direction of each other forming what is called a couple.

Almost all unbalanced conditions are a combination of kinetic force and dynamic couple and have to be separated and corrected in their respective planes.


Rich Carlstedt
02-13-2004, 12:14 PM
Simple balance: Use 1/2 weight of con rod.
More complicated: use 1/2 weight of con rod and 1/2 weight of throw.

Weston Bye
02-13-2004, 01:59 PM
canonicalman wrote:
...advantages of a four-cylinder were mostly negated because two of the cylinders fired at once acting as a big two-cylinder engine.

A properly designed four cylinder, 180° crank (2 at TDC when 2 at BDC) operating as a four-stroke will provide two power strokes per revolution. If cylinder 1, at TDC, is beginning an intake stroke, 2 and 3, at BDC are beginning compression and exhaust, and 4 is beginning the combustion. A half a revolution later, 2 is beginning combustion and so on. 4-2-3-1. More power impulses per revolution provides the least vibration. Running the engine 2&2 might give more torque but would indeed yield much more vibration.

perhaps Taylor got it wrong.


02-13-2004, 02:08 PM
This is a steam engine right? Every stroke is a power stroke unlike an IC engine, unless this steam engine is not double acting, which I doubt.

02-13-2004, 02:18 PM

You are correct. The 180degree crank does infact give two power strokes per rev for a 4-stroke engine. I didn not find an indication of how the timing of the engine was set up in my not so thourough reading. I merely assumed co-ignition as I didn't realize the arrangement you described. Most likely you are correct, but it would be interesting to find out for sure.

One thing is sure, it was the very first engine that any of the three had built.

Al Messer
02-13-2004, 09:51 PM
WD, Is your steam engine single acting or double acting? If it is single acting, it will run and run well with the crankpins at 180 degrees. If it is a double acting engine, a la Stanley Steamer, JC Hannum is absolutely correct.