Curved Flywheel Spokes

[sheppard4682]

I have always been curious why some wheels, e.g. flywheels, pulleys, etc., have curved spokes while others do not. Is this just a matter of aesthetics? Or are there design principles or manufacturing practices that are involved? The only reference I have found is a paragraph or two in "U.S. Navy Foundry Manual", Chap. 2 that suggests it is to avoid hot tears where the small cross-section of the spoke intersects with the heavier rim, yet most wheels I have seen over the years feature straight spokes. Any thoughts?

OK, its a Saturday night and things are a bit slow....

 

[kf2qd]

Aesthetics a swell as i suppose it would allow for a bit more expansion/contraction of the casting without creating as many stresses. Some of those flywheels were built back in the day when aesthetics were as important as the function. They made it pretty because they could.

 

[tel]

The early curved spoke flywheels were made that way to prevent the spokes of the casting breaking during the contraction period of the process.

To be correct, a flywheel, or driving pulley with curved spokes should run with the outer end of the spokes pointing forward. A driven pulley should have 'em pointing backward. This puts the spokes under compression.

 

[stevehuckss396]

The early curved spoke flywheels were made that way to prevent the spokes of the casting breaking during the contraction period of the process.

To be correct, a flywheel, or driving pulley with curved spokes should run with the outer end of the spokes pointing forward. A driven pulley should have 'em pointing backward. This puts the spokes under compression.

Never really thought about it but that makes perfect sense.

 

[IanN]

Hi,

It is my belief that the curve on the spokes of some flywheels are purely decorative and serve no engineering function.


Two points raised against this view are the issue of contraction and casting fracture, and the issue of ensuring the cast iron is in compression.

Point 1 - Contraction.

If a casting shrinks by a factor (say 5%) then it will change by this factor in all dimensions - there is no way the spoke will shrink more than the rim. If you are happy with this, skip to point 2, if not then consider the initial diameter when hot to be D then the circumference when hot is:-

Circ = D * pi

If the shrinkage is 5%, the new D will be 0.95*D
So the new Circ to perfectly fit the new diameter will be:

Cold Circ = D * pi * 0.95

If you now think of the rim of the flywheel as a bar of length Circ (where Circ = D * pi), bent into a circle, then on cooling this bar will change to 0.95 of its hot length - i.e. the Circ * 0.95 which is a perfect match for the change in diameter previously calculated.


Point 2 - Ensuring the spoke is in compression.

Ignoring the obvious - the UTS of cast iron is approx 17 tons/sq inch, and I cannot imagine any steam engine producing sufficient acceleration or deceleration to tear the spokes apart - if the spoke is curved then while part of the spoke will be compressed, there must be some part in tension or shear if the spoke joins the hub on a radial line. If you want to ensure the spoke is always in compression then the spoke must be joined to the hub at a point away from the radial centre line (think of a bike spoke in reverse - the wire spoke is only strong in tension and so the mounting point on the hub is off the direct radial).

Finally consider the wheel of a steam locomotive - these never have curved spokes but are designed to transfer huge forces.

Ian.

 

[kf2qd]

Discussing contraction of the casting - As a casting cools trhe various sections of an object like a flywheel may not (and probably won't) cool at the same rate. A curved spoke would actually have some small amount of give in it as it is not a linear connection between the hub and the rim. In the day when many of these items were made there was still a bit of by-the-seat-of-the-pants engineering and design involved in making the patterns. A casting with nice consistent cross sections will have few stresses cased by uneven cooling, but a part like a flywheel may have cooling problems because of the relatively heavy rim, smaller cross section spokes and then a thicker chunk at the hub. I have seen the bent spoke flywheels used on less expensive applications where the engineering was probably done by the pattern maker instead of a lot of engineering going into what would be a relatively low cost/high volume machines.

This was also at a time when a crank for a hand operated machine was cast with a nice "S" curve - didn't help the function, it just looked more elegant.

 

[shred]

Somebody posted a period article on flywheel self-destruction and in that was a section on why flywheels used to have curved spokes. If I can find it, I'll put a link here.

 

[Maryak]

I don't know much anything about the technicalities of metallurgy but I do know that cast iron is brittle and will break long before steel when subjected to rapid acceleration and deceleration.

This was the main reason its use was discontinued by the RN in ships machinery, as the force of an underwater explosion would shatter cast iron, whilst ships with steel castings would continue to function.

If, for whatever reason your "bloody great wheel," (made of cast iron), stops suddenly, it's far less likely to chase you round the factory if it has "S" spokes.

In the north west of the UK, people were quite often chased around by broken belts and failing pulleys on lay shafts in the cotton mills, just ask my mum.

Best Regards
Bob

 

[shred]

Here it is

http://www.homemodelenginemachinist.com/index.php?topic=3472.msg35231#msg35231

 

[tel]

Hi,

It is my belief that the curve on the spokes of some flywheels are purely decorative and serve no engineering function.


Two points raised against this view are the issue of contraction and casting fracture, and the issue of ensuring the cast iron is in compression.

Point 1 - Contraction.

If a casting shrinks by a factor (say 5%) then it will change by this factor in all dimensions - there is no way the spoke will shrink more than the rim. If you are happy with this, skip to point 2, if not then consider the initial diameter when hot to be D then the circumference when hot is:-

Circ = D * pi

If the shrinkage is 5%, the new D will be 0.95*D
So the new Circ to perfectly fit the new diameter will be:

Cold Circ = D * pi * 0.95

If you now think of the rim of the flywheel as a bar of length Circ (where Circ = D * pi), bent into a circle, then on cooling this bar will change to 0.95 of its hot length - i.e. the Circ * 0.95 which is a perfect match for the change in diameter previously calculated.


Point 2 - Ensuring the spoke is in compression.

Ignoring the obvious - the UTS of cast iron is approx 17 tons/sq inch, and I cannot imagine any steam engine producing sufficient acceleration or deceleration to tear the spokes apart - if the spoke is curved then while part of the spoke will be compressed, there must be some part in tension or shear if the spoke joins the hub on a radial line. If you want to ensure the spoke is always in compression then the spoke must be joined to the hub at a point away from the radial centre line (think of a bike spoke in reverse - the wire spoke is only strong in tension and so the mounting point on the hub is off the direct radial).

Finally consider the wheel of a steam locomotive - these never have curved spokes but are designed to transfer huge forces.

Ian.

Regardless of all that, that IS the reason they were made that way, and the correct orientation for running them. I didn't necessarily claim they were right!

 

[tel]

http://www.communitywebs.org/~AHMRC/documents/CurvedSpokes.pdf

 

[sheppard4682]

As a follow-up to what has been already posted, I decided to look in some old books and reprints I have.

My first observation is, with the exception of model engines, curved spokes were not that common on flywheels of steam engines. On the other hand, it was frequently used on driving wheels, e.g. the old traction engines used in early farming would typically use a large, straight-spoked flywheel on the engine while the driving pulley would have curved spokes. I find this especially interesting as it seems unlikely the reason would be aesthetics. I wasn't able to determine the direction of rotation or how the load was applied, i.e. what side of the pulley had the belt in tension. It seems very likely the thought was to put the spokes in tension, as tel suggested, as the spoke gradually curved away from a radian at the hub to meet the rim at an angle somewhat less than 45 degrees

Second observation is that straight spoke designs appear heavier with greater cross-section area in the spokes than the curved spoke design. It may be that more mass was desirable in the flywheel so conserving mass in the spoke served no purpose if the trade-off was more mass in the rim, especially so if casting difficulties were introduced where thin sections joined heavy sections.

My last observation is that all designs used six spokes per wheel whether curved, straight or solid with equally spaced holes in the disk. Obviously, more spokes would result in smaller spoke cross-section but would be harder to cast. I suspect the pattern-makers were behind the six-spoke design.

 

[sheppard4682]

P.S. Make that "compression"

I also found some curved spoke handwheels that are used on gate vlaves 12 " and larger while straight spoke designs are used on 10" and smaller. These curved spoke wheels used a spoke that projected along the radian from the hub, then curved to the right, back to the left and intersected the rim along another radian. a similar design is found on old Singer sewing machines. Curious....

I would like my models to reflect at least some relevance to historical engines and I hope that explains my unusual curiosity in the design of the flywheels. All designs have purpose: some are more artful than others.

 

[shred]

Going with the historical accounts that say casting issues were the reason for curved spokes, it makes sense that big flywheels and the like which weren't cast in one piece, but built-up wouldn't have curved spokes, while the little wheels that were one-piece do..

 

[Dusty1]

I have seen locomotive, at least 1/8 scale drive wheels with cracked spokes.

These were as I understand from cooling issues. Spokes on one side of the wheels had hair line cracks only one side.

Later wheels were left in the mold longer to cool and came out perfect.

The spokes being thinner cool and shrink first, and something has to give.

These scale locomotives, model Pacific's, are works of are built by a life long
model maker, and machinist.

Later locomotive wheels (full size) were cast in steel.

Kap Pullen

 

[IanN]

Hi Tel,

Thanks for the document proving me wrong about the issue of casting cooling contraction - not the first time I've got things wrong and certainly not the last!

The reason I did not think that this would not be an issue was because I assumed that the people working in the casting business would have the skill and experience to design dies and molds that compensated and made allowance for the faster cooling rate of thinner sections.

Ian.

 

[Kermit]

While reading I found the section of a book which makes reference to the practice as well. From A Manual of the Mechanics of Engineering...
http://books.google.com/books?id=Io...eering"&lr=&as_brr=1&as_pt=ALLTYPES#PPA573,M1

The link will take you to the start of the reference which is around 1 page long. It dances around some sketched figures as well. The other thing mentioned besides the need for contraction while cooling the casting is that ellipse shaped spokes were used instead of flat when the speed of rotation would be very high to cut down on wind resistance...? I don't quite understand that except the writer didn't like pulleys that behaved like fans.. ???


Back to the books,
Kermit

 

[tel]

S'OK Ian, I knew where you were coming from, but you must bear in mind that the curved spoke evolved in the relatively early years of wheel casting. The iron masters didn't quite have it nutted out at that time and wheels were still being fabricated at a late stage.

 

[mikeh935]

Hello guys, I've just been looking over the posts, re: curved spokes and shrinkage, I think our Aussie friend Tel just about nailed it . In theory they all will shrink at the same rate that is provided they are the same relative thickness. In the case of the flywheel the rim is normally thicker than the spokes therefore they would shrink at a faster rate. The Foundery men overcame this problem I think in the last 20 years or so of the 19th century.

 

[David Morrow]

Is it at all possible that it was just a marketing issue ? Something for the salesmen to brag about. If you have to have spokes, they might as well be beautiful and at least look fast and powerful.