enginehistory.org

[mksmith]

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
_________________
Mike Smith

[kmccutcheon]

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.
_________________
Kimble D. McCutcheon

[rwahlgren]

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.

[kmccutcheon]