In this example the task was to recreate a cam profile for an automotive camshaft given actual lifter data taken with a dial indicator as a function of the crankshaft degrees. Eventually this cam characteristic will be used to develop a valve train design and verify clearances between valves and clearances between valve and piston as a function of typical design values such as lobe-lobe separation, crankshaft phasing, valve stem length, valve diameter, etc.
A few brief comments about the cam design process using the methods previously developed are as follows in the image captions:
- This a clip from an Excel file where the actual tappet lift data were recorded and plotted as a function of the cam degrees (5 deg increments) using the Excel Chart tool. The data were taken from a flat faced tappet.
Three different plots of the data were generated for the entrance, rise and fall, and the exit of the profile. The data was broken into three sets in order to better fit the profile with polynomial equations using Chart Trendlines.
The lift profile for the intake and exhaust cams are the same in this example.
The polynomials can be used to calculate the lift at any crank or cam angle of rotation specifically for use in the cam sketch layouts and as assembly constraints as well as Assembly4 animation.
. - Screenshot 2022-11-19 141340.jpg (252.47 KiB) Viewed 2624 times
- This is simply a polar plot of the lift data using the cam angle as a variable. If the follower or tappet was a knife edge the cam profile would be the same as this polar plot.
In our case, however, the tappet is a flat face follower which requires that a sketch layout be created to establish the cam profile B-spline using the process developed in the earlier posts. As you will see in the next images, the profile is quite different than the polar plot of the lift.
. - Screenshot 2022-11-19 142722.jpg (58.32 KiB) Viewed 2624 times
- For this example we introduce two new variables for the camshaft design: The crankshaft angle and the lobe-lobe separation angle in cam degrees between the intake and exhaust cams. In a four stroke engine, the camshaft revolves once for every two revolutions of the crankshaft.
The dependent variable b_cam_angle is calculated from the independent ac_crank_angle variable as shown in the image. Variable bb is the intake cam angle and bb_2 is the exhaust cam which lags the intake by the lobe-lobe separation of 110 deg.
Note that a conditional statement is required to fully define the exhaust cam angle because the crank angle varies between 0 and 720 deg for one rotation of the camshaft.
The polynomials from the Excel file are used to calculate the y6_lift and y6_lift_2 (intake and exhaust) for each portion of the cam profiles. Using three polynomials greatly complicates the process as the multiple nested conditional statements attest. But it was necessary to get the required accuracy of curve fits for all cam angles. The calculated lift (+/-0.02 mm) is well within the dial indicator accuracy and manufacturing tolerances.
. - Screenshot 2022-11-19 135854.jpg (342.16 KiB) Viewed 2624 times
- The calculated lift is used to constrain the tappet and cam assembly. The lift variable releases the vertical degree of freedom of the tappet as a function of the crank shaft angle using the Attachment Offset properties.
The background image is the lift profile as a function of the crankshaft angle (-360 to 360 deg) for the exhaust and intake tappets showing the lobe-lobe separation of 220 deg (110 deg camshaft angle).
The cam rotation degree of freedom is released by the variables bb and bb_2 not shown in this image using the Attachment Offset Angle and Y-axis properties.
. - Screenshot 2022-11-19 140456.jpg (286.52 KiB) Viewed 2624 times
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The following video to demonstrate the lobe separation and phase angle with the crankshaft was generated using the Save feature of the Assembly4 animator as a function of the crankshaft angle 0-720 deg looped four times:
The Variables object in the Assembly4 workbench is a valuable tool for these kinds of tasks, much more convenient and responsive than a spreadsheet. These variables can also be easily set up as global variables using an Assembly4 master file for use in sketches, expressions, and placement or attachment properties by inserting the master file link in an assembly Model.
Stay tuned for Update#2 describing the development of a parametric sketch schematic for valve train design including crank, cam, tappet, push rod, rocker, and valve...
Code: Select all
OS: Windows 10 Version 2009
Word size of FreeCAD: 64-bit
Version: 0.21.30922 (Git)
Build type: Release
Branch: master
Hash: 8ec1279ea8ee32a36fae683b42b5cfc5821734b5
Python 3.10.6, Qt 5.15.4, Coin 4.0.0, Vtk 9.1.0, OCC 7.6.3
Locale: English/United States (en_US)
Installed mods:
* Assembly4 0.12.4
* fasteners 0.4.15
* freecad.gears 1.0.0
* QuickMeasure 2022.10.28