OHV rocker arm pad radius?

[kitabel]

I've been back and forth on this, and found not only conflicting data and answers, but strange geometric reasoning.
1. "the pad radius is the long (valve-side) lever arm length", but (if true) why are there so many shorter exceptions?
2. "the radius is the valve lift" (no reason given)
3. "as long as it doesn't present an angle to the stem it's not important"
Clearly, the radius must present at whatever arc position of the lever during the full range of lift. If the radius is too small, the stem edge strikes the rocker lever at zero &/or full lift, rather than the pad. If the radius is too large, rolling motion is reduced and scrub increases.
AFAIK, since there is both rolling and scrubbing motion involved, is the radius an attempt to reduce scrub to the absolute minimum?
I've tried to plot changes in ratio, but it's all over the place...
Almost all of the better on-line tech stuff is aircraft, locomotive, diesel, etc. with roller tips and useless.

Anyone?
Links?

 

[YvesLLewelyn]

Kit - Rather than a circular curvature I think the ideal shape would be of spiral form - either an involute or a logarithmic spiral. Both probably would have the valve-side lever length as the base circle diameter of the spiral.
Involutes have the property of always being (and pushing) at right angles to the line-of-action (the valve stem). Logarithmic spirals can be made to have a true rolling motion.
On the rare occasions that I have had to make a rocker I have made up a ball-and-socket arrangement between the end of the rocker and valve stem which has worked quite well. Ball-and-sockets are used on a few production engines - notably the Oz Ford SOHC 4-litre sixes. Stan Sainty (in Oz) also uses the same arrangement on his Top Fuel engines.

 

[YvesLLewelyn]

Base circle radius I think that should be, not diameter.

 

[tbuelna]

kitabel,

The ideal shape for a rocker tip pad would be a profile whose form is derived from a conjugate motion that gives the least amount of relative sliding at the line of contact with the valve stem tip, minimizes the effective pressure angle of the contact geometry, and also minimizes the maximum offset of the contact line throughout the rocker travel. The goal is to minimize the amount of side load and bending applied to the valve stem, and to minimize the friction losses due to sliding.

Also, a rocker tip profile with a large radius of curvature at the point of max lift (and usually the point of max force the rocker tip will experience) will minimize the contact stress at the valve stem tip and rocker pad.

Good luck.
Terry

 

[GregLocock]

What physical effect causes the valve to rotate?

You don't want to eliminate that entirely, or your valves will stop rotating, and then you will be very unhappy.

Cheers

Greg Locock


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[YvesLLewelyn]

GL - Just what does cause the valve plus spring etc. to rotate? - Surely not the off-centre wiping action of the rocker pad on the end of the valve stem? Wouldn't it be more likely that the helical spring undergoes a rotating force when compressed and released?

TB - Your ideal shape requirements seem OK - but what is the type of curve required to give these ideals and how could it be found?

 

[tbuelna]

YvesLLewelyn,

All sliding contacts in a valetrain are usually designed to produce rotation in the axisymmetric parts, in order to equalize wear. Surfaces like tappet faces usually have some degree of crowning and the mating cam profiles have a slight axial taper. This combination ensures a slightly off center, but consistent, contact condition which continuously rotates the tappet.

Achieving valve rotation is not so important in modern engines that use hardened valve seat. In some older engines, like old small block Chevy's with the exhaust valve seats machined directly into an induction hardened area of the cylinder head, valve seat recession was a serious problem. So these Chevy engines would use a "rotator" built into the exhaust valve spring retainer. The rotator was a type of ratcheting device that would index the valve slightly during each spring compression. It was the torsional motion of the spring wire as the helical valve spring was compressed and relaxed that drove the rotator around its axis.

As for your question regarding the definition of the ideal rocker arm tip profile, as I noted it's some form of conjugate action. I can describe what the curve should look like, but unfortunately I'm not smart enough to tell you how to mathematically derive such a shape. Sorry!

Regards,
Terry

 

[YvesLLewelyn]

TB - Thank you for your comments. I thought GL seemed to be implying that the valve was rotated by the wiping action of the rocker tip. I couldn't see how this could develop enough torque to turn the valve. If you look at the various videos on Youtube (the one below being a good example)

http://www.youtube.com/watch?v=6Vz4mjPKXJ0&feature=related

the valve (or is it just the spring) seems to rotate quite vigorously even with a roller tip. In fact in the above example the valve spring rotates even when the rocker is not pushing down on it - vibration maybe?
I have read that the taper on a cam lobe (on a V-8) is also to pull the cam into the engine - not being mechanically located in the outwards direction.
As for the correct shape of the rocker tip - with varying valve clearances and valve heights etc. and over such small distances I doubt if a "correct" shape would make any difference. This is possibly why roller tips are used - they are probably a lot more tolerant of varying clearances etc.

 

[YvesLLewelyn]

This is another interesting video.

http://www.youtube.com/watch?v=_REQ1PUM0rY&feature=related

 

[patprimmer]

Whether or not you can see it, the valves and if flat tappets, the followers do rotate in all engines I have ever seen.

You can see the effect in the wear pattern or with an adjustable strobe light.

Regards
Pat
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