As far as crush size we decided to go with 1.5" x 1.5" x 1". This brought our actual forces down to about 3200 lbs, which after a 1.5 safety factor brings our target to 5000 lbs. We account for the safety factor because we don't want to design a machine that works so closely to it's requirements. Not only is there the increased chance of failure, the machine would then be working at it's max all the time. In short, having the safety factor will lengthen the life of the machine and it's parts.
As for the more robust design, here is a video that covers the new model:
As you can see in the video we have moved from direct rack and pinion operation to using lead screws. This combined with a gear ratio of 3:1 in the sprockets we plan to use allows us to multiply the forces provided by our motors by a very large amount.
For instance, our final crush which we are designing to be 5000 lbs gets multiplied by .073 to give us the torque needed at lead screw. This is because lead screws operate like a very long and shallow incline, and to lift 1 lb, we only need .073 in-lbs of torque. So that gives us 365 in-lbs at the lead screw. Then our gear ratio of 3:1 divides the torque needed at the motor by 3. This gives us a grand total of 122 in-lbs of torque at the largest motor.
There is a down side to multiplying the torque like this. Our speed or RPM of the motor is drastically reduced. So while powerful, we sacrifice speed. Since our project doesn't need to be fast, however, this is an acceptable sacrifice.
For anyone interested, I will link the work in progress E-Drawing of the computer model we created in Solidworks, a 3D modeling and engineering software. To view the model, you will need to download the Solidworks viewer first,
E-drawings viewer: Solidworks E-drawing Viewer
The assembly E-Drawing: Crusher Assembly (mediafire download)
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