Need ideas on how to create a strong vibration in a medical instrument

[titansfk]

Hello,

I am trying to create a strong vibration inside of a small cylinder. It is for a torque wrench and will be subjected to autoclaving and will not be lubricated.

The way it currently works is to twist a torsion beam to set the correct torque on the fastener. Picture a cylindrical tube and inside is a flat disc with a curved cantilever beam off of the disc. This disc rotates with respect to the cylinder.

When the wrench is twisted (angle of twist is about 20 degrees) clockwise the cantilever beam is radially compressed inward as it engages a bump on the inside of the cylinder. It snaps past the bump and re-contacts the inside of the cylinder, making a nice click and some vibration. However the vibration is not strong enough.

The stress in the cantilever beam is at it’s max (in-fact it lives for only 400 cycles and I need 1200.) It is stainless steel carpenter custom 455 with an h900 heat treat. So increasing the beam deflection does not seem possible. The cylinder is made of the same material and hardness is about HRC44.

I am thinking about making a small mechanism of a pivoting hammer arm with an extension spring. This would essentially be the same as the cantilever beam, but I could increase the defection and kinetic energy. I also though about thinning the cylinder wall thickness in the hopes of transferring more vibration through the rest of the wrench.

Any ideas are greatly appreciated.

Sincerely,

Steve

 

[GregLocock]

What temperature does the autoclave run at? how are you driving the torque wrench at the moment, ie does it have external power?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[titansfk]

The temperature is 275 F in a steam environment and held for 15 min. The wrench is hand operated by twisting a handle on one end of the wrench.
Thanks.

 

[GregLocock]

So you could have an external drive? all you need is way of generating the vibration itself?

Do you have room to change the dimensions of the beam?

Why is the beam stainless?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[titansfk]

Greg,
I guess I am not sure what you mean by an external drive.

This is a hand-operated instrument. It gets sterilized in an autoclave after surgery.

Picture a long shaft within a hollow shaft. At one end is a hex driver, and at the other end is a t-handle to twist the wrench (apply torque). The solid shaft in the center of the wrench experiences angular displacement (angle of twist) during the tightening of the fastener.

Inside the wrench is a spring disc that engages a bump (or tooth) that deflects the cantilever beam on the spring disc. When the correct torque is reached (known angle of twist) the beam snaps past the bump and vibrates the wrench. Right now the vibration is more sound (a ringing) than a tactile feeling.

I want to increase the amplitude of the vibration and also increase the life of the beam.

Thanks for the help.
Steve

 

[GregLocock]

Sorry, I thought you wanted to operate it inside the autoclave.

OK, to make the beam live three times longer you need to reduce the stresses by 20-30%, typically, or change the material.

To make it more tactile, you'll need to increase the mass of the striker, or make it hit harder.

You have a cantilever in bending, so it is very easy to work out the stress. I assume it breaks at the root of the beam?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BobM3]

Can you acoustically tune the chamber to amplify the given vibration?

 

[titansfk]

I don't think I can tune it very much. I am locked into certain materials and size constraints. I can vary the diameter, wall thickness and height of the chamber small amounts. I might be able to add small geometric features such as grooves and holes also.

So maybe I can. Any suggestions based on these contraints?

I am also looking at Gregs suggestions in stress reduction and putting a large mass at the end of the cantilever beam where it strikes the cylinder (chamber) inner wall.
Thanks,
Steve Krause