Jieve
Mechanical
- Jul 16, 2011
- 131
We have an application on a test rig where we are applying a load to rotating shaft through a bearing via a linear actuator. The load is applied through a spring setup, similar to a hanging vibration isolator. The max load capability needs to be around 2000lbf, with a total spring constant around 750lbf/in.
My understanding is that I should be concerned with both the natural frequencies of the load subsystem, (which as I see it, in its simplest form boils mostly down to a 1 DOF mass/spring system), and the natural frequencies of the springs themselves.
The rotating shaft has variable speed 0-60Hz and mostly negligible vibration; but my concern (and I’m not sure how valid this is), is that most springs I’ve found both individual and when using multiple springs in parallel to reach the max load with a smaller spring, all typically have critical frequencies in the range of 50-100Hz (k/m doesn’t deviate all that much when using smaller springs to reach the same load capacity and spring constant).
To mitigate any potential issue, I’m trying to think of simple ways to dampen the system and prevent any spring surge that may occur. But I’m not even entirely sure that this is a concern.
So not sure if that is sufficient information, but my questions (maybe very basic):
1)If the excitation source is mainly through shaft vibration mounted in spherical roller bearings with minimal imbalance, is spring surge in this application something that I really need to worry about?
2)Say I use 4 or 8 springs in parallel instead of a single larger spring (assume compression springs between two parallel plates). Each spring still has its individual natural frequency, am I correct that there is nothing different about this system that makes the concern about surge any less concerning (assuming all other factors being equal)?
3)The design I’m thinking about is a spring (or multiple) in a cylindrical housing; imagine a piston being pulled down to compress the spring(s). Is there a simple way to dampen potential vibrations? I’ve thought through using friction material in contact with the springs to automotive shock absorber type dashpots, to submerging the spring in oil… just wondering if anyone has any different ideas I haven’t thought of that might be simpler. And am I correct that adding this damping between the plates would eliminate the surge concerns? Or would I still really need to find another way around operation so close to the spring’s natural frequency?
4)Finally on a related note, Shigley describes the equation for the fundamental critical frequency for springs between 2 plates as f = 0.5 sqrt(k/m), where m is the mass of the active coils of the spring. However, the general equation for system natural frequencies is f = 1/2pi sqrt(k/m). I noticed the Roymech site has a better derivation than Shigley, but just wondering why that pi factor drops out in the spring case (I haven’t worked through the derivation). But I’ve seen elsewhere on the web where this is discussed as being a typo in the Shigley case, (but Mark’s and Roymech have this too) so figured I’d ask.
My understanding is that I should be concerned with both the natural frequencies of the load subsystem, (which as I see it, in its simplest form boils mostly down to a 1 DOF mass/spring system), and the natural frequencies of the springs themselves.
The rotating shaft has variable speed 0-60Hz and mostly negligible vibration; but my concern (and I’m not sure how valid this is), is that most springs I’ve found both individual and when using multiple springs in parallel to reach the max load with a smaller spring, all typically have critical frequencies in the range of 50-100Hz (k/m doesn’t deviate all that much when using smaller springs to reach the same load capacity and spring constant).
To mitigate any potential issue, I’m trying to think of simple ways to dampen the system and prevent any spring surge that may occur. But I’m not even entirely sure that this is a concern.
So not sure if that is sufficient information, but my questions (maybe very basic):
1)If the excitation source is mainly through shaft vibration mounted in spherical roller bearings with minimal imbalance, is spring surge in this application something that I really need to worry about?
2)Say I use 4 or 8 springs in parallel instead of a single larger spring (assume compression springs between two parallel plates). Each spring still has its individual natural frequency, am I correct that there is nothing different about this system that makes the concern about surge any less concerning (assuming all other factors being equal)?
3)The design I’m thinking about is a spring (or multiple) in a cylindrical housing; imagine a piston being pulled down to compress the spring(s). Is there a simple way to dampen potential vibrations? I’ve thought through using friction material in contact with the springs to automotive shock absorber type dashpots, to submerging the spring in oil… just wondering if anyone has any different ideas I haven’t thought of that might be simpler. And am I correct that adding this damping between the plates would eliminate the surge concerns? Or would I still really need to find another way around operation so close to the spring’s natural frequency?
4)Finally on a related note, Shigley describes the equation for the fundamental critical frequency for springs between 2 plates as f = 0.5 sqrt(k/m), where m is the mass of the active coils of the spring. However, the general equation for system natural frequencies is f = 1/2pi sqrt(k/m). I noticed the Roymech site has a better derivation than Shigley, but just wondering why that pi factor drops out in the spring case (I haven’t worked through the derivation). But I’ve seen elsewhere on the web where this is discussed as being a typo in the Shigley case, (but Mark’s and Roymech have this too) so figured I’d ask.
