How does a Motorcycle Sprag Clutch work?

[GeneA]

I have posted this on several motorcycle forums and have not received any answers. So I thought I would ask the experts of Mechanical Engineering.
I wish to know the operation of a strag clutch, what engages, and disengages? How does it do that? Is there a place that has drawings and a white paper I could be pointed too? Can someone lead me through the theory of operation?
I was having a problem on a starter on my RoadStar and took it apart to examine a component that several people had pointed me too on the motorcycle forums, however there was nothing wrong I could find. So, I wanted to trace through the operation of the starting system and was stopped at the starter clutch gear. The problem was the starting motor would spin but the engine would not but then, for no apparent reason it started to work again. I didn't see any parts like a bendix gear, or a fork that pushed a gear out into a flywheel. How, how does this work?
Thanks for any response.

 

[Paulcet]

Try

http://faq.f650.com/FAQs/SpragClutchFAQ.htm

 

[GeneA]

Paulcet,
I found that site in my search for an answer. I was hoping for something a little more in depth. Thank you for your response.

 

[jbartos]

Suggestion: Visit

http://www.f650.com/Forums/User/UserProfile.aspx?UserName=KEM

etc. for more info

 

[jbartos]

Small encore: Visit

http://home.mira.net/~iwd/2x2x2/tech/2wdoverview.html

etc. for more info

 

[GeneA]

Thank you J,

I read the post on the sprag clutch from the link you gave me. I got a really good response from Jay over at Cucamonga Yamaha. However, I am still in search of a mechanical drawing of the sprag clutch, so it can all make sense to me. This must be a tough question. When I get the answer I am going to post it everywhere just in case there is someone out there like me that needs to know how something works. I guess that is a nice way of saying a dummy, like myself. I may post this over to the mechanical group too.
Again, thanks

 

[Scoobystu]

Try posting the question in the Motorcycle engineering forum. There are a couple of guys there who know their stuff too.

 

[GeneA]

Will do Scoobystu, Thanks!

 

[60hzhmmm]

GeneA,
A sprag clutch is a clever machine that uses the inclined plane to work.

Picture a precisely machined star. This star is surrounded by a cylinder so the tips of the star barely clear it. Picture a daisy laying on a table inside a ring.

Now between each point of the star there is a steel ball bearing free to roll around anywhere in the area defined by the ring and the neighboring point. If you were to rotate the star the ball bearing would ultimately be hearded to the outer perimeter defined by the cylinder.
The bearing would just be pushed around and around by the tip trailing it as the star turns.

NOW the trick here is that the stars points are NOT straight but are precisely machined into graceful arcs.

Picture the ring surrounding the daisy shrinking.... as the daisy revolves... the petals would begin to bend in the direction they are rotating FROM. Right? Well the machined star points are machined into gracefully bent arcs in the same manner.

Why? because as the balls are hearded forward and outward by their star points they will eventually wedge SOLID between the star arc and the outer cylinder. When this happens the cylinder is forced to follow the rotating star. The clutch just engaged...
If the cylinder now being driven actually started spinning faster than the star the ball bearings would un-jam.., disengage.

In a sprague there are usually about six star points and of course 6 ball bearings. Sprague clutches are fairly weak as the entire drive funnels through the ball bearing point contacts with the cylinder. As the cylinder gets warn less balls hook up properly causing the others to recieve higher loading and uneven forces, the star being shoved off center slightly causing the whole system to malfunction. They are cheap and simple. All those turn styles at the grocery stores are sprague clutches. Also too much or to little or the wrong type of lubricant can spell disaster.

Hope this helped.. I once was baffled too and had the same exact problem on a Honda CB600. Good luck.

 

[GeneA]

Thank you 60hzhummmm. The explanation was great and extremely helpful. So, what you are saying is the starter gear turns the starter clutch gear, which cause the starter clutch to esentially lock on to the shaft, right? Once the engine starts the rotation of the shaft spins faster and disengages the starter clutch, which now freewheel. Right?

 

[60hzhmmm]

Exactly! The starter turns the star the engine is connected to the outer cylinder. Only the star can drive the cylinder. A sprague clutch will disengage almost instantly and with little stress as the disengagement occurs just as the torque passes through zero as the cylinder is accelerated by the engine. But! When engaging, the acceleration of the star by the starter motor is huge and the time before engagment is the many degrees of star rotation before the balls make it to "jamming land" the resulting lockup is rather violent. Hence the regular failure of sprague clutches used in starter systems. Sprague clutches are perfect for turn styles!