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R-4360 Cam Disc

 
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mksmith



Joined: 12 Dec 2016
Posts: 35
Location: Columbus, Ga

PostPosted: Fri Sep 03, 2021 14:59    Post subject: R-4360 Cam Disc Reply with quote

I've recent acquired a copy of Graham White's book, "R-4360 Pratt's Major Miracle". I consider this book to be the ultimate reference for this engine and the authority on the engine generally.

That said, in reading the book, it seems that the early series engines used 3 lobed cams that rotated in the direction opposite that of the crankshaft while the C Series engines utilized 4 lobed cams that rotated in the same direction as the crankshaft.

I understand that cam disc speed, relative to the crankshaft is dependent on the number of lobes, what I do not understand is what determines which direction the cam disc rotates?

Is it possible that the earlier 3 lobed disc could rotate in the same direction as the crankshaft? And is it possible that the later 4 lobed cam disc could rotate in the opposite direction as the crankshaft?

Questions answered by two cutaway engines at Robins AFB Museum.
Early 3-Lobe Cam

Later 4-Lobe Cam
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kmccutcheon



Joined: 13 Jul 2003
Posts: 298
Location: Huntsville, Alabama USA

PostPosted: Fri Sep 03, 2021 16:40    Post subject: Reply with quote

This was discussed in the early days of this Bulletin Board, but the discussion was not fully developed. Please see
https://www.enginehistory.org/phpbb/viewtopic.php?t=61

Joseph Liston's book Aircraft Engine Design covers this on pages 243 - 244. The short answer is that you can design the cam to rotate in either direction relative to the crankshaft, but it is usually desirable for the cam to rotate opposite the crank in order to reduce the number of lobes.

If X is the number of lobes, Y is the number of cylinders and R the speed ratio, then:
For cam rings rotating in the opposite direction, X = (Y - 1) / 2 and 1 / R = 1 / (Y - 1).
For cam rings rotating in the same direction, X ' = (Y + 1) / 2 and 1 / R' = 1 / (Y + 1).

I do not recall ever seeing an explanation for why P&W changed its mind with the R-4360.
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Kimble D. McCutcheon


Last edited by kmccutcheon on Tue Mar 12, 2024 07:18; edited 1 time in total
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rwahlgren



Joined: 15 Aug 2003
Posts: 324

PostPosted: Mon Mar 11, 2024 16:53    Post subject: Reply with quote

http://www.enginehistory.org/Piston/P&W/R-4360/r-4360.shtml

Scroll down to the cam drive info.

One reason for the cam drive change, I think, is the slower cam ring speed.
Slower surface speed for the lifter roller cam interface, as well as the ring support bearing.
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kmccutcheon



Joined: 13 Jul 2003
Posts: 298
Location: Huntsville, Alabama USA

PostPosted: Tue Mar 12, 2024 07:21    Post subject: Reply with quote

rwahlgren wrote:

One reason for the cam drive change, I think, is the slower cam ring speed.
Slower surface speed for the lifter roller cam interface, as well as the ring support bearing.


I agree with this explanation. However, I have never seen a P&W explanation for the change.
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