GrabCAD and Tikitreiler will be paying someone that nice fee for designing the best new trailer shackle assembly . While only one can win the money, everyone else can benefit from a few of my headaches during the design process.
Mastered Top Down Multi-body Design
One of the sweetest features to come along in Inventor is the ability to model multiple solid bodies in one part. Mark Flayler probably explained this best last year in his Top Down Design labs when he said multi-body modeling is best employed when multiple parts are design to work and fit together. This hinged suspension is a really great candidate.
I used one layout part file to run the design from. It contains all the key components within the overall design, including all the parameters. From here all parts are developed together and farmed out to their final form using the Make Component / Make Part tools.
Sketch Blocks
Sketch Blocks are the bomb when it comes to components that are part of the initial design strategy. Take for example this competition, where 3 component assemblies were furnished and had to be used: a parabolic leaf spring, a hub, and a damper. I went into each part and measured a cross section that I reproduced in the layout part’s MainLayout sketch, and created sketch blocks from these. You should be able to see the MainLayout sketch in the model browser, and cascaded below that, the sketch blocks used in the 2D sketch.
Once in place, these act like AutoCAD blocks whose entities are all packaged together like a single object. Moreover, this behavior allows layout such as this one to act like an assembly model, without the overhead. A really great planning tool for 2D movement behavior.
Main Constrained concept sketch and child
All the sketch blocks and main design limitations were constrained in the MainLayout sketch. No other solid body sketching was added in this sketch. This left a main sketch that was free of anything except direction for other sketches. Subsequent sketches for main components were developed on a centered plane, and contained projected sketch features from the sketch blocks and key items. When positioning changes were required during the design, the sketch blocks in the main layout were adjusted, and the remaining parts updated accordingly.
Design on the half shell
I’m not a huge fan of oysters, but I do like working with Inventor models in their half form, and mirroring them when done. This provides the stablest from of modeling a symmetrical body, and offers another great benefit. You can easily visualize more of the design by having the part always in half section.
A few points need to be adhered to prior to moving forward with this. First is the Mirror Solid tool. This bypasses the individual features that often don’t want to mirror well. It mirrors the entire solid body, and is a really great feature.
Next is some pre-thought out organization. You need to get everything that is intended to be symmetric developed prior to the mirror. Otherwise you will end up needing to mirror more individual features, and that will make the part quite difficult to navigate. Another thing to consider is whether it is best to model and mirror each part to completion before moving to the next, or to model all parts, and then mirror at the end. This really depends on the amount of symmetry in the design
Over using a sketch
I did go overboard on a few sketches, and should have pulled out a few things into other sketches. There’s a fine line that lies between the organizational ease of fewer sketches, and the pain that results when too many sketched features have to update from a single modified sketch.
Make components
This tool is probably the best compliment to the Multi-body design process. Make components develops individual parts from the separate solid bodies in the layout part file. It then grounds them automatically in an assembly created during the process. Utterly brilliant.
Not checking each piece separately
Multi-body modeling does reduce some situational awareness. Seems like it does just the opposite, huh? It does make the meshing of components easier to handle by far. However the Make Component command handling all the assembly work for you, takes some hidden feature inspection time that you might normally see during normal assembly. One of my parts was not developed properly, and I could not see it because it was always meshed together with the rest of the design.
Check each part individually before going into the assembly work.
As it was, the odd feature problem took all of 60 seconds to fix in the Layout part file, and updated automatically in the assembly.
Some Analysis
My wife says I over engineer everything. I take it as a compliment, since everything I build is designed to do Mach 2+.
It’s easy to see things in a static sense, and usually judge correctly what will be required for any particular 1-dimensional requirement.
I do still like some reinforcement when it comes to dynamic reaction forces in moving masses. Since Inventor can only simulate stresses in linear static models, I chose to run the design through Autodesk Simulation Multiphysics 2012 (formerly Algor), which will permit me to perform Mechanical Event Simulations (MES).
I verified my findings from Inventor, which in a static load condition reveals a minimum safety factor of 3, when loaded twice the design requirement. Yes, over engineered. However, what happens when we run over a pot hole and a 6 inch tall curb at 60 MPH? Well, I can tell you from real world experience that the tires, wheels, and tie rods on the truck pulling the trailer will have to be replaced, as will as the tires and wheels on the trailer too.
Simulation reports that if you do decide to drive 2 tons of load over a 6 inch curb at 60 mph, you are pushing your luck, but may likely be ok, but may need some new suspension parts. This one handled things pretty well, especially since little damping was considered for the air in the rubber tires, and was simulated more like having rocks for tires. More substantial analysis needs to be performed before taking this thing to a demolition derby. Standard road conditions however seem to be just fine.
