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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Failure of a pedestal piston and proposing three ways to strengthen the design.

Identifier  ExFiles\Box 66\1\  scan0092
Date  22th April 1926
  
X. 8040

To R.{Sir Henry Royce} from E.{Mr Elliott - Chief Engineer}

c. to BJ. Hs.{Lord Ernest Hives - Chair} By.{R.W. Bailey - Chief Engineer}

E1/M22.4.26.

RE. PEDESTAL PISTON.

We think it is clear why the pedestal piston has broken away at the skirt.

The piston is controlled in the cylinder by the skirt alone, all other surfaces being ground clear of the bore. Hence all side thrust whatsoever from the piston pin has to be transmitted through the pedestals to the head, and then to the skirt via the locally weakened section between the head and skirt.

We do not believe the stress in this section in combined tension and bending due to the offset weight of the skirt is sufficient alone to cause the trouble.

We calculate the latter to be about 900 lbs. per sq.in. at 2,800 r.p.m.

As this piston possesses decided merits in overcoming ordinary production knocks, we propose to carry on with tis development by strengthening the broken section in three ways :-

(1) Reducing the depth of the horizontal sawcut in the piston The amount we propose will not greatly affect its flexibility.

(2) Interrupting the oil collector groove locally below the scraper ring so as to avoid weakening the section where the sawcut finishes.

(3) Adding to the thickness locally on the inside where the sawcut finishes.

We propose by these means to put up the mudulus of the broken section to twice it is now.

E.{Mr Elliott - Chief Engineer}
  
  


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