Operational Theory

Force on Crosshead Guides

 

 

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.

 

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

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

35.2

2547.857

898.2

50

29.8

2156.992

820.5

55

25.6

1852.987

751.6

60

22.2

1606.887

687.5

65

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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