Curved flywheel spokes

[Geoff TheasbyParticipant @geofftheasby]

Hi,

 

Brian Marshall, in “Hot Frivolity” (Current M.E. 4410) says that flywheel spokes were often curved to ensure the spokes were in compression when the flywheel drove a load.

I worked in a company with a foundry in the 1960s and was told that spokes were curved to handle the stresses when the rim and hub of a wheel cooled slower than the spokes after casting.

 

Not being an engineer, I accepted this. Any comments?

 

Regards

Geoff

[Geoff TheasbyParticipant @geofftheasby]

[David Southwell ARPSParticipant @davidsouthwellarps]

Its a long time ago – but from memory the important thing about flywheel spoke design is to ensure when the flywheel is turning the radial deflection of the spoke equals the radial deflection of the rim. If that design criteria is not met then the danger of bursting arises. Another result of getting that balance right is that the spokes are not tending to impart a bending force to the rim.

 

Maybe the curved spoke could have something to do with meeting the criteria. Can’t remember any more than that.

 

David

[Anonymous]

It’s the latter. When hot the metal is weak, so if there is differential cooling, the spokes usually cool first, the spokes may crack at the hub as the hub shrinks. The technical term is ‘hot tears’. It can be avoided by correct mould design.

 

Regards,

 

Andrew

[David Southwell ARPSParticipant @davidsouthwellarps]

Posted by Andrew Johnston on 12/08/2011 14:25:39:

It’s the latter. When hot the metal is weak, so if there is differential cooling, the spokes usually cool first, the spokes may crack at the hub as the hub shrinks. The technical term is ‘hot tears’. It can be avoided by correct mould design.

 

Regards,

 

Andrew

DAVID: I am sure you must be right – I was trying to imagine why curving the spokes would materially reduce any tendency to cause radial deflection but because I couldn’t see any reason it didn’t mean there might not be!! <chuckles>.

[Billy MillsParticipant @billymills]

When iron was first cast the composition of the metal and it’s tempriture was very variable. Many straight spokes cracked on cooling or in service but curved spokes absorbed the stresses by bending slightly. So early handwheels and flywheels have curved spokes. As the art of casting progressed less curved spokes can be found, eventually becoming straight.

 

Billy.

[John Stevenson 1Participant @johnstevenson1]

Geoff.

 

I collect old engineering books and have a decent selection. However depending on which book you read, even given the same period you get different answers so I feel that this is one of those questions who’s definitive answer, if there was ever one ? has been lost in time.

 

I can see both sides to the question and all the answers I have read make sense.

 

As to shrinking on casting I feel that there were far more clever foundries about in Victorian times that later when they were supposed to have sussed this problem out.

 

Personally I feel that curved spokes were selected as they appealed to the Victorian sense of design, They always had to embellish their designs as a matter of personal pride in the job, but we will never know for certain.

 

John S.

[David Southwell ARPSParticipant @davidsouthwellarps]

The Victorians’ designs certainly worked but the logic behind their choices was not often very explicit. Turning (somewhat off-topic) to more modern designs nasa did quite of lot of research into the use of composites for flywheel design. I think I have a paper from them ( it was written in ’76) somewhere if anyone is interested.

David

Edited By David Southwell ARPS on 12/08/2011 22:16:25

Edited By David Southwell ARPS on 12/08/2011 22:16:53

Edited By David Southwell ARPS on 12/08/2011 22:17:34

[Sub MandrelParticipant @submandrel]

My 18th edition of Machinery’s handbook has an entertaining section of flywheel speeds. “The hub and main spokes of the flywheel remained in place, but parts of the rim were found two hundred feet away”. Various designs described, but none with curved spokes. My understanding is the casting stress theory.

 

I had a cast flywheel for Lady Stephanie that came with 3/4″ of a dish, due to unbalanced shrinkage. It’s nailed to my workshop door to remind me to check castings at date of purchase, not two years later…

 

Neil

[David Southwell ARPSParticipant @davidsouthwellarps]

Posted by Stub Mandrel on 15/08/2011 19:56:59:

My 18th edition of Machinery’s handbook has an entertaining section of flywheel speeds. “The hub and main spokes of the flywheel remained in place, but parts of the rim were found two hundred feet away”.

Neil

 

DAVID: Interesting my much earlier 10th Edition, whilst similar, has at least one difference and maybe an interesting point:

1. The word nearly is added as follows:

“The hub and spokes of the flywheel nearly remained in place”

2. Goes on to state:

“This sudden failure of the rim casing was unexpected, as it was thought the the flange bolts were the parts to give way first. The tensile strength of the wheel rim at the point of fracture was about four times the strength of the wheel rim at a solid section.”

 

[Author]

Posts