MBD Simulation on a Piston Assembly Using SolidWorks
Aim:
To the model parts of Piston Crank assembly and to perform Multibody Dynamics Simulation using solidworks on the piston crank assembly under three different offset conditions.
Objective:
- To model parts of the piston crank.
- Then to assemble the Piston crank Assembly with proper constraints.
- To perform motion analysis at different wrist pin offset values.
- Finally to analyze the resulting plots and to superimpose the resulted plots.
Theory:
A crank is an arm attached at a right angle to a rotating shaft by which circular motion is imparted to or received from the shaft. When combined with a connecting rod, it can be used to convert circular motion into reciprocating motion, or vice versa. The arm may be a bent portion of the shaft, or a separate arm or disk attached to it. Attached to the end of the crank by a pivot is a rod, usually called a connecting rod (conrod).
The term often refers to a human-powered crank which is used to manually turn an axle, as in a bicycle crankset or a brace and bit drill. In this case a person's arm or leg serves as the connecting rod, applying reciprocating force to the crank. There is usually a bar perpendicular to the other end of the arm, often with a freely rotatable handle or pedal attached.
The Piston Assembly comprises of the following parts:
1. Piston Head
2. Grudgeon Pin (Wrist pin)
3. Connecting rod
4. End Cap
5. Crank
Software Used:
Solidworks
Procedure & Outputs:
As per the dimensions provided in the video the parts of the piston crank are drawn as follows,
1.Piston:
To create it Revolve,Extrude,Extrude cut,Revolve cut,Fillet and several other 2D sketch tool(line,circle,arc,etc) commands are used.
2.Wrist pin:
To create it Extrude,Chamfer and several other 2D sketch tool(circle) commands are used.
3.Connecting Rod:
To create it Extrude,Fillet,Extrude cut and several other 2D sketch tool(circle,line,mirror,etc) commands are used.
4.End Cap:
To create it Extrude,Fillet,Extrude cut and several other 2D sketch tool(circle,line,mirror,etc) commands are used.
5.Crank:
To create it Extrude,Extrude cut,Mirror and several other 2D sketch tool(circle,3 point arc,line,etc) commands are used.
6.Assembly:
To assemble the components each parts are brought one by one into the assembly window and constrained properly using the mate options to achieve the desired motion.
7.Motion Analysis:
To continue with the motion analysis the wrist pin is offsetted in three different ways and its effect on the linear displacement of the piston is analysed.
The position of the wrist pin is going to be varied as follows. But the centre of the axis of the piston head and the crank remains on the same plane in all the following three cases.
| Sl.No |
Wrist Pin Offset |
Crank Speed (Rpm) |
| 1 | "0" mm | 2000 |
| 2 | "10" mm Positive | 2000 |
| 3 | "10" mm Negative | 2000 |
Youtube Link:
7.1. "0" mm Offset:
In this case the front plane of wrist pin and Rotation axis of Crank coincides with each other in the same plane("0" offset) by using the coincident constraint.
The linear displacement of piston starts between the 2.0000E+02 and 3.0000E+02 and then follows a uniform curve. This smooth constant curve indicates the linear displacement of piston with respect to time is constant in this case.
7.2. "10" mm Positive Offset:
In this case, '10' mm offset in the positive direction between the plane of wrist pin and the crank plane while keeping the Rotation axis of Crank unchanged. The crank rotation is in the clockwise direction.
The linear displacement of piston starts a little above 2.0000E+02, Which caused due to the positive offset provided. This case also provides a uniform smooth curve,this smooth constant curve indicates that the linear displacement of piston with respect to time is constant.
7.3. "10" mm Negative Offset:
In this Case, we provided "10" mm offset in the negative direction between the plane of wrist pin and the crank plane while keeping the Rotation axis of Crank unchanged. The crank rotation is in the clockwise direction.Using the flip direction command offset in the negative direction is achieved.
The linear displacement of piston starts a little below 3.0000E+02, Which is caused due to the negative offset provided. Similar to the previous cases the smooth constant curve indicates that the linear displacement of piston with respect to time is constant.
7.4. Superimposed Graph:
From the above three cases a smooth plot is achieved for all the cases of offsets.This above superimposed graph is used to compare the results of all the three plots resulted from the three different offset conditions.Thus a superimposed graph is one which can be used to merge 2 or more plots into one to provide better understand and compare the various results obtained from different cases.
The Superimposed Graph the above three offset conditions is,
Parts and Assembly:
https://drive.google.com/file/d/15Zc12HrY_4lSvjPNmtjTzXSScGaqcqD3/view?usp=sharing
(The Parts and assembly files are put into Single zip file)
References:
1. https://en.wikipedia.org/wiki/Crank_(mechanism)
3.https://www.quora.com/What-is-a-piston-pin-offset-Why-is-it-provided
Conclusion:
Piston pin offset refers to the piston wrist pin being a bit offset to one side of the piston i.e. for it to not be in the centre of the piston.For instane when the piston pin is in the centre of the piston, now when the piston reaches top or bottom dead centre then, in this kind of setup the rod is straight up & down placing huge load on the rod.
The crank has to rotate past TDC or BDC in order to get the piston moving again. This robs a lot of both power and RPMs from the engine.Now with an offset pin, the piston wrist pin is a little offset to one side of the piston therefore the rod is not straight up and down when the piston is at TDC or BDC. This in turn allows the crank to rotate with much less resistance giving the engine more power and speed.
Having offset pin relieves the stress that is caused in the rod when it reaches top or bottom dead centre. For the rod does not have to slam the piston up and then back down. The rod travels more in an arc.