CATI Tech Notes

How do you avoid Vibrations?

Vibrations are ubiquitous in our life. Their importance cannot be ignored even if we want to. e.g we do not want the vibrations from tire movement of a moving car, travel to steering wheel. (Not that our road conditions make it easy to ride…) The washer dryer in the house, are usually kept in the basement to transmit all the vibrations to the floor.  Have you ever tried using the blender on a plastic mount?   

 The general adage is: Sensitive objects need to be protected from Vibrations. Isolators are used in machine tools, photolithographic equipment, metrology equipment like coordinate measuring machines. All external vibrations generate internal vibrations in the working zone, for instance, between cutting tool and workpiece, or CMM’s measuring stylus and part being measured.

One can open any design book and get a standard equation for vibration.

f_n = ω_n / 2 π = (1 / 2 π) * √(k/m)

where k is system stiffness, m is the mass, and f is natural frequency.

The above equation leads to the conclusion

(a)  Softer isolaters transmit less vibration from support to object. Which means isolators should be made of metal springs or lightly filled rubber.

(b)  The object becomes sensitive to minor excitations. The practical aspect being, machine tool tables reversing directions or repositioning of photolithography tools.

Hence, it becomes important to make the isolating system dynamic. One approach is to simply attach a stiff block onto isolator. But this is an expensive approach, requiring floor space and lot of effort to move.

This leads to need for a dynamic damper, such that damping offered is relative to applied frequency and stiffness of the structure. E.g:  door dampers, or dampers used in race cars.

Vibrations in these two cases are unidirectional (axial). For more complex systems,(CMM machine, machine tools, photolithographic equipment ) designers need to consider multi-directional vibrations.

SolidWorks Simulations can help design dampers. Users can also uses standard built in functions to simulate the effect of a damper. SolidWorks Simulation has the following damping models.

Modal Damping : Modal damping is defined as a ratio of the critical damping C_cr for each mode. Critical damping C_cr is the least amount of damping that causes a system to return to its equilibrium position without oscillating.

The modal damping ratio can be determined accurately with proper field tests. The ratio varies from 0.01 for lightly damped systems to 0.15 or more for highly damped systems.

When experimental data is not available, use data from a similar class of systems to determine the damping properties. Smaller ratios are more conservative since higher ratios reduce vibration amplitudes. In general, neglecting damping leads to a conservative estimate of the system's response. The software has damping ratios for standard systems.

If designer knows the stiffness of damping material. He can use a more realistic model (Rayleigh Damping) to compute effective damping using combination of the mass and stiffness matrices.

Rajat Trehan

Product Manager - Design Validation

Computer Aided Technology Inc.

Compute Free Body Forces

Users need to know reaction forces on locations other than where restraints are applied. This is typically useful while trying to quickly validate the output of simulation results. The reaction force will be equal and opposite to the applied load. It’s only then that user can claim the object being simulated is in equilibrium. Another use of this feature is finding the load to create a certain amount of displacement.

To activate this feature, right click on a name of a test and select properties. Simply check the radio button for Compute Free Body force. After running a study with this flag on, right-click the Results folder and select List Free Body Force to list forces that act on faces, edges, and vertices. The forces can come from contact, external loads, restrains, or connectors.

 

To test or not to Test…

While analyzing a system users are tasked with getting rid of all the unwanted parts. Typically a SolidWorks User would create a configuration and manually go about suppressing parts/features not required. With the release of Simulation 2009, users now have the ability to choose parts they want to exclude from analysis.

If you are tasked with designing the handle of this case, there is no need to include the case and the pins that hold the handle in place in your analysis. You do need these parts to put together a drawing or show how the case is assembled.

Once the analysis has been defined users can then select the parts to exclude from the test. All they have to do is hit the control key on the keyboard and select parts from the analysis tree. Users then can right click on any selected part and select “Exclude from analysis”. This feature saves a lot of time and effort in selecting parts that are not needed in a test. 

 

Rajat Trehan

Product Manager – Design Validation

Computer Aided Technology Inc.

Losing Some Weight in 2009 – Lightweight Assembly Drawings

When you open your first assembly drawing in SolidWorks 2009, you will notice that the referenced files load as Lightweight.  This is expected and is a new change in SolidWorks 2009 to improve performance and memory usage.  Unfortunately, there is no way to turn this off. 

While the drawing is opening, you could receive a message that SolidWorks detects that the units of the drawing differ from the components in the assembly, which can affect the display of notes, cells, and custom properties.  And you are prompted to resolve those components or not. 

This means that there have been changes to some of the models referenced by the drawing.  SolidWorks needs to fully resolve those changed models, so that everything in the drawing is up to date and accurate.  Only the appropriate models will be fully resolved, the rest will load lightweight.  Note, if you select “No” to this message box, and also check “Don’t ask me again”, you will no longer see this message box, changed components will not fully resolve, and your drawings may be continually out of date.  To bring the message box back, go to Tools>Options>Advanced> and check the dismissed message box.  It will then appear next time you open a drawing that needs to resolve some components. 

 

If you do not want to work with the drawing while having some files loaded in Lightweight, then the easiest thing to do is to right click on the Sheet(s) from the FeatureManager and select Set Lightweight to Resolved.  This will load all components on that sheet as fully resolved.  

 

Design Better!

Leslie Lougheed

Getting the Overall Length of a Bent Tube

Numerous times, customers have asked me how to get the overall length of a tube created in SolidWorks.  If you have a seat of SolidWorks Office Premium, the Routing capability will give you the overall length of a tube/wire.  But many of us just have a SolidWorks Core or SolidWorks Office Professional seat, so we lack that capability that routing provides.  So here is a Redneck Workaround to get the overall length of a bent tube: 

The tube I will use for this example was made with the Sweep command using a profile and path.  I need to obtain the overall length of the tube and my driving dimensions do not capture that.  So what I will do is edit the path sketch and add some reference dimensions to it.  I will add two dimensions to determine the lengths of the arcs (arc length dimensions are made by selecting the two end points of the arc and then the arc itself), and I will add one dimension for the vertical straight length in my model. 

   

Next, I need to add global variable to sum up the length of the tube segments through the equations tool.   My equation will be Overall Length = the sum of the tube segment lengths.  The equation would be “Overall Length” = 5.0 + 0.785 + 1.5 + 0.785 + 4.0 for this example.  But I do not want to key in those values in the equation.  I want to select the dimensions from the sketch to ensure that the Overall Length updates if any of the dimensions update.

Lastly, I will add a Custom Property from File>Properties and link it to the global variable Overall Length.  This custom property can be displayed in a note in a drawing or in the BOM.

Design Better!

Leslie Lougheed