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Old 05-08-2011, 10:21 PM   #1
Charles Lamont
 
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Default Westbury 'Seagull'

Hello people. This is my first post in this neck of the woods. I live in the UK, West Midlands area. I have been doing model engineering for many years, but am currently tackling my first petrol engine.

It's a Westbury designed 10cc twin side-valve job, castings from Hemingway Kits 1.

You can see my progress (not very up-to-date atm, sorry) here: 2.

The latest problem I have with the design is the cam profile. As designed, the tappets are too small in diameter for
the cams, inlet in particular. As shown by plugging the dimensions into this brilliant resource 3, for a brief part of the motion, the cam is in contact with the edge of the tappet face, which seems unlikely to be good for either, or for noise (quiet is one of my criteria of success). Design dimensions are the same as the 'Seal': Base circle radius: 11/64", flank radius: 9/16", lift: 5/64" opening angle: 120°. I probably can make the tappets bigger, but ... any advice?

(1) http://www.hemingwaykits.com
(2) http://www.charleslamont.me.uk/Seagull
(3) http://modelenginenews.org/design/CamTable.php



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Old 05-08-2011, 11:29 PM   #2
stevehuckss396
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Default Re: Westbury 'Seagull'

Well you have 3 options

(1) Make a larger diameter lifter. Great if you have room.

(2) Reduce the lift of the lobe. This will change the shape of the lobe and less diameter will be needed.

(3) Make lifters with a curved surface. If the surface of the lifter that contacts the lobe has a ball end or even any kind of radius you can run a smaller diameter.

If I think of anything else I'll get back here but thats all i have right now.

Cheers



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Old 05-09-2011, 02:50 AM   #3
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Default Re: Westbury 'Seagull'

Hi Charles, very clear construction diary. How come everything looks so clean? I can tell this is going to be a very well made model by the time it is complete. Any ideas what you will use it for yet?
Also, where in the west midlands are you??

Rob.

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Old 05-30-2011, 08:27 PM   #4
Charles Lamont
 
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Default Re: Westbury 'Seagull'

Quote:
(1) Make a larger diameter lifter.
(2) Reduce the lift of the lobe.
(3) Make lifters with a curved surface.
Thanks. After further exploration, there is also the possibility of a larger cam angle, allowing a 'softer' cam.
I have decided I will probably increase the tappets to 9/32", which I can just get in, even with a bush.
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Old 05-30-2011, 08:39 PM   #5
Charles Lamont
 
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Default Re: Westbury 'Seagull'

Quote:
Originally Posted by RManley
Hi Charles, very clear construction diary.
Thanks, Rob
Quote:
How come everything looks so clean?
Does it? I clean parts before taking pics, but try to leave a bit of swarf around when showing setups.
Quote:
I can tell this is going to be a very well made model by the time it is complete. Any ideas what you will use it for yet?
Thanks again. No - no idea. I will need some kind of load to try it against, though.
Quote:
Also, where in the west midlands are you??
Shropshire
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Old 09-21-2011, 11:06 PM   #6
Charles Lamont
 
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Default Lapping Timing Gears

Does anyone have advice to offer on lapping the timing gears together?

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Old 09-21-2011, 11:47 PM   #7
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Default Re: Westbury 'Seagull'

In theory you can’t lap straight involute gears because they mesh with a rolling action, therefore there is no relative movement between teeth for lapping to take place.

In practice if the gears are mounted so that one can move from side to side you can sometimes improve things. With an idler setup like the seagull you would need to lap all three gears at once (moving the idler from side to side) so unless they really need it I wouldn’t bother.

Les



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