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Bristol Cherub

 
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sithomas



Joined: 01 Jan 2019
Posts: 28
PostPosted: Sun Mar 12, 2023 14:14    Post subject: Bristol Cherub Reply with quote
Thanks for the article on the Bristol Cherub. Seems fitting that Bristol would make such a complicated valve drive mechanism!

I am still trying to figure out how it works. From what I can figure from the photos, the cam is the usual style cam. The torsion rods (central part of photo BC3-42) have fingers that I think run on the cam, thus twisting the rod. I doubt they would run directly on the cam, but I can't see anything that fits between the cam and finger.

The other end of the rod has a 4 spline fitting. This would connect to the cylindrical thing in the top left of photo BC3-46. I believe this is the device that would push down on the valve. You can see another part of it in photo BC3-04.

Is that even close to being right?

Photo BC3-71 appears to be upside down.
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kmccutcheon



Joined: 13 Jul 2003
Posts: 298
Location: Huntsville, Alabama USA
PostPosted: Sun Mar 12, 2023 16:17    Post subject: Re: Bristol Cherub Reply with quote
sithomas wrote:
Thanks for the article on the Bristol Cherub. Seems fitting that Bristol would make such a complicated valve drive mechanism!


Actually, it is quite simple... Think about the parts count. There are no tappets or push rods. You are correct that the fingers run against the cam lobes and transmit movement via the torsion shafts to the rockers, which act directly on the valve stem ends. So rather than the cam, tappet, push rod, rocker arm, valve of the standard mechanism, we have finger/torsion rod, rocker, valve!

sithomas wrote:
I am still trying to figure out how it works. From what I can figure from the photos, the cam is the usual style cam. The torsion rods (central part of photo BC3-42) have fingers that I think run on the cam, thus twisting the rod. I doubt they would run directly on the cam, but I can't see anything that fits between the cam and finger.


You have it.

sithomas wrote:
The other end of the rod has a 4 spline fitting. This would connect to the cylindrical thing in the top left of photo BC3-46. I believe this is the device that would push down on the valve. You can see another part of it in photo BC3-04.

Is that even close to being right?


Again, you have it.

sithomas wrote:
Photo BC3-71 appears to be upside down.


You are correct. The images have now been flipped.
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sithomas



Joined: 01 Jan 2019
Posts: 28
PostPosted: Tue Mar 14, 2023 10:42    Post subject: Reply with quote
Looking back, you are correct about the valve drive mechanism. "Unorthodox" would be more appropriate rather than "complicated".

Do any other aero engines use torsion drive for the valve mechanism? I wonder how the power draws compare between a pushrod and a torsion rod?

Crazy idea for the day: By making a slot in the side of the cam profile, a desmodromic valve system would not be that different to the current layout. Comments?
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