Hello,
I am trying to model a gear train in CosmosMotion 2008. There are four spur gears with 6, 7, 8, and 9 teeth. The gear teeth use a radius instead of a true involute shape. The gears have about a 2.00 inch pitch diameter. The fastest gear is driven by a motor at 10 rpm. The motion study property manager has "use precise contact" checked. I've tried frame rates uo to 800 frames per second, accuracy from .0001 to .000000001, Integrator types Gstiff, Wstiff, and S12_GStiff.
No matter what I've tried: The assembly starts with clearance between the teeth but after about a quarter of a revolution the teeth penetrate each other about .010 inch (as seen in the graphic display).
Thanks for any suggestions or advice?
Dave
I am trying to model a gear train in CosmosMotion 2008. There are four spur gears with 6, 7, 8, and 9 teeth. The gear teeth use a radius instead of a true involute shape. The gears have about a 2.00 inch pitch diameter. The fastest gear is driven by a motor at 10 rpm. The motion study property manager has "use precise contact" checked. I've tried frame rates uo to 800 frames per second, accuracy from .0001 to .000000001, Integrator types Gstiff, Wstiff, and S12_GStiff.
No matter what I've tried: The assembly starts with clearance between the teeth but after about a quarter of a revolution the teeth penetrate each other about .010 inch (as seen in the graphic display).
Thanks for any suggestions or advice?
Dave
3D contact is based on penetration. If the parts penetrate, the program applies a force at the centroid of the interfering volume based on the contact properties to push it back out.
You can think of the contact properties as a nonlinear spring. The stiffness and exponent define the displacement based force component with penetration distance and the damping and penetration distance determien the velocity based force response. In reality, the velocity based force is a representation of energy dissipation during the collision/contact. When parts just touch, the damping is zero. The damping then ramps up to the maximum damping value you specify over the penetration distance. This eliminates a mathematical singularity of sorts.
Anyway, the values for the stiffness and damping are the fundamental factors to modify. They are really a function of the material and size of the geometry contacting. For gears, use the default steel materials in the contact dialog. As a starting point, I would uncheck the select material once you do this, and bump up the damping to 500 to 1000 (if units are mm and N). I aim for a damping value 0.1 to 1 % the stiffness value.
You can obtain more accurate stiffness characteristics by performing a contact analysis in SolidWorks Simulation and look at the variaction of force with compression distance between the two bodies where they contact.
It's important to remember what we're doing here. We have a rigid body mechanism and we are emulating part stiffness are localized areas of contact. This is an approximation when compared to tru FEA type simulations, but it can provide very usef information when it comes to understanding the dynamics of a system when there is large displacement.
I hope this helps give you some insight into contacts.
For the solver settings, if you are rotating at high rpm, focus on the maximum time step and reduce that to a couple of orders of magnitude less than value for results.
For integrationed, GSTIFF is the most robust integration. SI2 can occasionally help with contact models, but to be honest in all my years of using the ADAMS solver, I've been most sucessful just with GSTIFF.
Cheers,
Ian