Running a Centrifugal Clutch with No Springs?

[JCRacing]

Hello,
I am building a racing lawn mower and I need a clutch before and after a transmission. The clutch from the engine to the input on the transmission will be controlled by belt tension. On the output side to the axle I would like to run a centrifugal clutch, but I need one that only disengages when I press the clutch in. I would like the clutch to engage instantly on within 250rpm but no one makes springs that low (that I can find). Is there some trick to get it lower on could I just run without a spring?

Thank you

 

[landuse]

Will you be able to still have your engine running at 250RPM, or is it gearing that makes it so low

 

[JCRacing]

Well, it will be like a normal riding mower or a car where when you clutch the engine is essentially disconnected from the transmission, so the transmission could be stopped while the engine is at 3500rpm. The reason for the second clutch on the output shaft would be so while the mower is moving to stop the rotation of the transmission completely. I would like to have this capability so that I can shift on the fly with a transmission that has no synchros.

 

[itsid]

wouldn't it be easier to just add a one way bearing to the axle sprocket?
That way the turning axle would NOT turn the output of the transmission,
but you'd loose reverse gear (if you have a reverse gear of course)

Also... if you have the engines governor removed, wouldn't it rev out the moment you release the load? Is that a good idea?

'sid

 

[JCRacing]

I have never seen one of those except for on bicycles, do you know a place where I could find one? No, when ever I shift I will be letting off of the throttle, just like in a car.

 

[itsid]

uhm.. A Honda Recon TRX250 or yamaha raptor uses one for the starter clutch afaik;
R/C Helis use them,
www.renold.com has some light-medium and medium-heavy useable listed...

But I actually have no idea if there is one that you can easily put in a sprockethub or something.

'sid

 

[mckutzy]

I think you might be to make a dog clutch on the output shaft to final drive.
Can use two lovejoy couplers as the dog. Just need to have a sliding coupler with a spline, or use a keyed shaft, where the coupler has a inset key(key wont come out, welded to coupler) to engage shaft.
I have been working on this idea for a while now. An idea for the moving side, weld a small v pulley to the moving coupler for a surface to engage the shift fork. It will free wheel easily.
Large old logging,mining and ship building machines/cranes used something similar to this as a transmission.

 

[JCRacing]

http://www.surpluscenter.com/sort.asp?keyword=PSF3&catname=powerTrans&PAGELEN=20&PageNo=2

Are these what you are talking about?

 

[mckutzy]

Yes exactly what I mean.

 

[Doc Sprocket]

It will be interesting to see how this works out. Most guys (AFAIK) begin to accelerate and then *gently* upshift into their racing gear and stay there during the race. I was wondering this myself, as I am building a race tractor for the first time. I tore my tranny down several times while I re-arranged ratios and experimented with it, and drew the same conclusion- you do NOT want to be shifting all over the place for fear of eating the shift keys or even the gears. (For anybody reading this, not familiar with the mechanics involved- the shift keys are basically sliding keystock within the transmission- there is no synchro mechanism. The gears are indexed every 90* for the keys, and shifting can be rather abrupt, shall we say).

Have you talked to other racers to see what they do about this issue? As I see it, it looks like this: Your overall race ratio is pretty steep (5:1 or greater). We all know that it's too high for starting without roasting your belt or clutch, so you need to start off in a lower gear, then upshift.

I have thought about chamfering the leading edges of the shift keys, and/or the indexes in the gears, but removing too much material is spooky. Hardening the keys is out, because the keys will lose their spring temper, which is dead important. So what to do? Just shift gently, like some racers have told me?

By the way- have you checked with your local racing organization to see if this secondary clutch idea will even be legal?

EDIT- one further note- I do know that a lot of guys upgrade to kevlar belts because they are a lot more tolerant of excess heat/slippage.

 

[mckutzy]

Hey doc, not to hijack I'd like to see some pics of your trans apart, On another thread or pm would be nice.

 

[Doc Sprocket]

Hey doc, not to hijack I'd like to see some pics of your trans apart, On another thread or pm would be nice.

Can do. I believe I shot a few when I was re-arranging the internals.