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Operational Theory
Force on
Crosshead Guides
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The force on the
crosshead guides varies during the stroke of the piston. At Top
Dead Centre when the gas load on the piston acts vertically
downwards, there is no side component of thrust. However as the
piston starts to move downwards and the angle the connecting rod
makes with the vertical increases, so the component of side
thrust increases. Although as a percentage of piston force, the
guide force is at a maximum at 90° ATDC, the gas force on the
piston at this point is relatively low. By using a spreadsheet
the guide force component can be calculated for any crank angle.
The point at which it is a maximum will depend on a number of
factors, primarily crank angle at which max cylinder pressure
occurs, and con rod/crank length ratio, but is usually between
20 - 25° ATDC.
Note: for 100%
accuracy the inertia load on the piston should also be taken
into account. This is subtracted from the gas load on the piston
as the piston is accelerating downwards and added as the piston
de-accelerates. This however would complicate the calculation.
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The table shows
the thrust on the guides for a 960mm bore engine with a
stroke of 2.5 metres and a conrod length of 2.88
metres.
Maximum guide
force occurs at 23° ATDC
|
Cylinder Press (Bar) |
Piston Force (kN) |
Thrust on Guides (kN) |
Crank Angle ° |
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106.4 |
7701.476 |
0.0 |
0 |
|
107.8 |
7802.811 |
59.1 |
1 |
|
109.8 |
7947.576 |
120.4 |
2 |
|
111.8 |
8092.341 |
183.9 |
3 |
|
114.2 |
8266.058 |
250.4 |
4 |
|
116.9 |
8461.49 |
320.3 |
5 |
|
119.7 |
8664.161 |
393.5 |
6 |
|
122.5 |
8866.831 |
469.7 |
7 |
|
125.2 |
9062.263 |
548.4 |
8 |
|
128.1 |
9272.172 |
631.0 |
9 |
|
130.4 |
9438.651 |
713.4 |
10 |
|
131.7 |
9532.748 |
792.2 |
11 |
|
132.6 |
9597.892 |
869.7 |
12 |
|
132.6 |
9597.892 |
941.6 |
13 |
|
131.4 |
9511.034 |
1004.2 |
14 |
|
129.8 |
9395.222 |
1062.1 |
15 |
|
127.7 |
9243.219 |
1113.8 |
16 |
|
124.7 |
9026.072 |
1154.7 |
17 |
|
121.7 |
8808.925 |
1192.2 |
18 |
|
118.6 |
8584.54 |
1225.3 |
19 |
|
114.8 |
8309.487 |
1247.3 |
20 |
|
111.2 |
8048.911 |
1267.4 |
21 |
|
107.4 |
7773.858 |
1281.0 |
22 |
|
103.5 |
7491.568 |
1289.2 |
23 |
|
98.5 |
7129.656 |
1278.7 |
24 |
|
94.6 |
6847.365 |
1277.7 |
25 |
|
91.9 |
6651.933 |
1289.2 |
26 |
|
88.1 |
6376.88 |
1281.7 |
27 |
|
84.4 |
6109.066 |
1271.5 |
28 |
|
80.9 |
5855.728 |
1260.4 |
29 |
|
77.7 |
5624.104 |
1250.3 |
30 |
|
62.9 |
4552.846 |
1170.3 |
35 |
|
51.5 |
3727.688 |
1083.0 |
40 |
|
42.3 |
3061.771 |
987.3 |
45 |
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35.2 |
2547.857 |
898.2 |
50 |
|
29.8 |
2156.992 |
820.5 |
55 |
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25.6 |
1852.987 |
751.6 |
60 |
|
22.2 |
1606.887 |
687.5 |
65 |
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19.6 |
1418.693 |
633.7 |
70 |
|
17.5 |
1266.69 |
584.9 |
75 |
|
15.8 |
1143.64 |
540.7 |
80 |
|
14.5 |
1049.543 |
503.3 |
85 |
|
13.3 |
962.6845 |
463.8 |
90 |
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