Cub Mower Identification needed

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Jim Becker
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Re: Cub Mower Identification needed

Postby Jim Becker » Sat Jul 13, 2013 9:41 am

Smokeycub wrote:Again I must be missing something. Mechanically, I can't see how it can change it's speed, even within one revolution.

Yes, that is exactly what happens. The 2x difference happens at 60 degrees, which is a pretty sharp angle and beyond the practical operating range of most joints. It can be worse, imagine turning it to 90 degrees. One end turns freely through a limited angle while the other end is blocked from moving at all.

This is a lot easier to understand by playing with a joint that isn't attached to anything.

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Re: Cub Mower Identification needed

Postby Smokeycub » Sun Jul 14, 2013 1:09 pm

Well, thanks Jim and Matt for trying to clarify this for me.

The following are quotes from Wikipedia about U-joints and prop shafts under Jims link about CV Joints.

"Early front wheel drive systems such as those used on the Citroën Traction Avant and the front axles of Land Rover and similar four wheel drive vehicles used universal joints, where a cross-shaped metal pivot sits between two forked carriers. These are not CV joints as, except for specific configurations, they result in a variation of the angular velocity. They are simple to make and can be tremendously strong, and are still used to provide a flexible coupling in some propshafts, where there is not very much movement. However, they become "notchy" and difficult to turn when operated at extreme angles."

By "notchy" I guess they mean rotational variation or NOT constant velocity, something one might feel as a vibration.

"The fact that it failed to maintain constant velocity during rotation was recognized by Robert Hooke in the 17th century, who proposed the first constant velocity joint, consisting of two Cardan joints offset by 90 degrees, so as to cancel out the velocity variations."

Matt Kirsch wrote:The speed varies during the rotation. It slows down and speeds up as it turns.

So, if a shaft is perfectly straight, in perfect alignment with whatever it's driving, this phenomenon doesn't apply? ...but if the shaft is at, let's say, a 15 degree angle, according to the chart, it will deviate its speed from approx. 1.1 turns to .9 turns within each revolution? Is that what it's saying? If so, then I must be conceptually inept because I still don't see it. Am I the only one who doesn't? Also, why would adding an extra u-joint (using a CV joint) make any difference, or is it just minimizing the effect. I'm sure, however, that the concept is valid, otherwise there wouldn't be a need for CV joints and the like. Seems the key is angular velocity. I understand what it's saying, I guess I just don't agree, even though Mr. Hooke figured it out in the 17th century.

Jim Becker wrote:This is a lot easier to understand by playing with a joint that isn't attached to anything.

A picture is worth a thousand words so I'll have to try playing with a pto shaft.
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Re: Cub Mower Identification needed

Postby Redclip » Sun Jul 14, 2013 7:33 pm

Ray, Here is another Wikipedia article that may explain it better-

The PTO shaft in question is known as a Double Cardan Shaft-

"A configuration known as a double Cardan joint drive shaft partially overcomes the problem of jerky rotation. This configuration uses two U-joints joined by an intermediate shaft, with the second U-joint phased in relation to the first U-joint to cancel the changing angular velocity. In this configuration, the angular velocity of the driven shaft will match that of the driving shaft, provided that both the driving shaft and the driven shaft are at equal angles with respect to the intermediate shaft (but not necessarily in the same plane) and that the two universal joints are 90 degrees out of phase. This assembly is commonly employed in rear wheel drive vehicles, where it is known as a drive shaft or propeller (prop) shaft."

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