View source for Steam Engine Reviews/Piston Head Stress

The following review is provided by Mike Bastel.

Stress Calculations<br>
OSE Steam engine<br>
Piston bolt<br>

calculation in metric for bending piston force: F = A * p<br>
A = 3,75” * 3,75” * 0,785 = 11,04 sq.inch = 71,2 cm² = 7120 mm²<br>
p = 250 psi = 250 lbs / (25,4 * 25,4) = 0,387 lbs / mm² = 1,76 N / mm² <br>

--> F = 7120 mm² * 1,76 N/mm² = 12531 N

assumed loose fit of the bolt in both parts

Mmax = F * (l + 2 * s) / 8 = 12531 N * (0,3” + 2* 0,1”) / 8 <br>
= 12531 N * 12,7 mm / 8 = 19893 Nmm

σ = M / W = M / 0,1 / d³ = 19893 / 0,1 / 6,35³ = 777 N / mm²

for mild steel St 37-2 allowable σ : 330 N/mm²   STATIC<br>
for tempered high grade steel 30 CrNiMo 8 allowable σ: 1440 N / mm² STATIC<br>
only very high grade steel tempered will work, if just calculating static load

Dynamic calculation<br>
σzul = σD * b1 * b2 / βk / ν<br>
b1 : 0,88 for St 37-2 ( fine finished surface Ra 6,3)<br>
b1 : 0,72 for 30CrNiMo 8 <br>
b2 : 1 (diameter below 10 mm)<br>
βk : 1,3 for St37-2; 1,4 for 30CrNiMo8<br>
ν : 1,5 average value<br>
σDSch =  290 N/mm² for St37-2; σDSch =  1070 N/mm² for 30CrNiMo8

--> for St37-2: σzul = 131 N/mm²<br>
--> for 30CrNiMo 8 σzul = 367 N/mm² 

no steel will stand 777N/mm² longtime (fatigue)

Calculating area pressure<br>
Aproj = d * l = 6,35mm * 7,62mm = 48,4mm²    in stem<br>
p = 12531N / 48,4mm² = 259 N/mm² in stem fit<br>
Aproj = d * s * 2 = 6,35 mm * 5,04mm = 32mm² rod connector<br>
p = 12531 / 32mm² = 392 N/mm² in fit to rod connector

tolerable pressure for non-sliding surfaces, mild steel, dynamic load: 75N/mm²<br>
tolerable pressure for non-sliding surfaces,high grade steel, dynamic load: 150N/mm²

tolerable pressure for sliding surfaces, mild steel on steel: none. high rate of wear !!!<br>
tolerable pressure for sliding surfaces, steel on bronze: 25 N/mm²