MBD Simulation on a Piston Assembly

AIM: To do multibody dynamics (MBD) simulation on piston assembly using SOLIDWORKS

OBJECTIVE:

• To design parts like piston head, wrist pin, connecting rod, end cap, and crank.
• To make an assembly using all the parts.
• To do motion analysis in three cases mentioned below.
• To obtain plot for linear displacement of piston head and export it in excel sheet.
• To superimpose the plots in excel obtained from following cases:

Sl. No.	Wrist pin Offset	Crank speed
1.	0 mm	2000 rpm
2.	10 mm Positive	2000 rpm
3.	10 mm Negative	2000 rpm

THEORY:

A piston assembly is an inversion of a single slider crank mechanism. A piston assembly consists of a piston head, a wrist pin, a connecting rod, an end cap, and a crank.

A slider crank mechanism is designed to convert straight line motion to rotary motion, as in a reciprocating piston engine, or to convert rotary motion to a straight line motion, as in a reciprocating piston pump.

A slider crank linkage is a four link mechanism with three revolute joints and one sliding joint. The rotation of the crank drives the linear movement the slider, or the expansion of gases against a sliding piston in a cylinder can drive the rotation of the crank.

There are two types of slider cranks:

1. In-line: An in-line slider crank has its slider positioned so the line of travel of the hinged joint of the slider passes through the base joint of the crank. This creates a symmetric slider movement back and forth as the crank rotates.
2. Offset: If the line of travel of the hinged joint of the slider does not pass through the base pivot of the crank, the slider movement is not symmetric. It moves faster in one direction than the other. This is called a quick return mechanism.

Motion studies are graphical simulations of motion for assembly models. Motion studies do not change an assembly model or properties. They simulate and animate the motion we prescribe for a model. By using Solidworks mates to restrict the motion of components in an assembly when we model motion. From motion studies, motion analysis can also be done for accurately simulating and analyzing the effects of motion elements. It can also be used to plot simulation results for further analysis.

PROCEDURE:

• Create 3D models of piston head, wrist pin, connecting rod, end cap, and crank.

• After creating all the parts separately, assembly is created using all the parts. After exporting all the parts in Solidworks, the first part is made floating by right clicking on the first part and then selecting float option. Then select temporary axis visible option and then use appropriate mate option to constrain a part.
• The first assembly created is with piston head having no offset. Now do the motion analysis of this case of assembly, by proving a motor at the axis of the crank having a constant speed of 2000 rpm. Then give the direction of gravity, in this case in y-direction. Now make contact appropriately keeping all parts having same material, i.e., Steel (dry). After this set the frames to 7200 fps and also use the precise contact option. After this get a plot on linear displacement of piston head, then export it to excel sheet, also create an animation.

• Now the second assembly created is with piston head having 10 mm +ve offset. Now do the motion analysis of this case of assembly, by proving a motor at the axis of the crank having a constant speed of 2000 rpm. Then give the direction of gravity, in this case in y-direction. Now make contact appropriately keeping all parts having same material, i.e., Steel (dry). After this set the frames to 7200 fps and also use the precise contact option. After this get a plot on linear displacement of piston head, then export it to excel sheet, also create an animation.

• The third assembly created is with piston head having 10 mm -ve offset. Now do the motion analysis of this case of assembly, by proving a motor at the axis of the crank having a constant speed of 2000 rpm. Then give the direction of gravity, in this case in y-direction. Now make contact appropriately keeping all parts having same material, i.e., Steel (dry). After this set the frames to 7200 fps and also use the precise contact option. After this get a plot on linear displacement of piston head, then export it to excel sheet, also create an animation.

• Now superimpose all the three graphs into a single graph.

PLOTS AND ANIMATIONS:

• Piston head with No offset

ANIMATION: Piston head with No offset

• Piston head with 10 mm Positive offset

ANIMATION: Piston head with 10 mm positive offset

• Piston head with 10 mm Negative offset

ANIMATION: Piston head with 10 mm negative offset

• Superimposed graphs

DRIVE: Files - Piston assembly

CONCLUSION:

• The motion analysis helps us to analyze the linear displacement of the piston head in three different cases.
• As the frame rate is set to 7200 fps which helps us to get a smooth plot in spite of having a speed of 2000 rpm.
• After superimposing all the three graphs we can see that 10 mm offset doesn’t create a much significance difference in the linear displacement of the piston head.