Calculating Shear Force between a Moving Film and a Cylinder

[MartinSa]

TL;DR Summary

I have a cylinder covered in a thin film (think a catheter, covered by a thin sheath), how should I go about calculating the shear force it takes to unsheathe the sheathe from the catheter?

If I have a catheter covered in a thin sheathe (think a cylinder, covered by a thin plastic film), how should I go about calculating the shear force it takes to unsheathe the catheter from the sheath (no torque here, just the catheter moving in and out)? I've thought about strain gauges (but wouldn't know where to place them, since the shear is in between the sheath and the catheter where a strain gauge won't fit).

 

[JBA]

What you are basically dealing with come more under the category of interface friction between the two surfaces; and, this is dependent upon the selected materials. There tables of friction factors for multiple material combinations so you might do some googling to see if there one that includes your two materials. Once that is determined then the next factor is the contact force to use with the friction factor between the sleeve I.D. and catheter O.D. and this requires a calculation based upon the elastic modulus of the two materials and the fit of the the two, i.e. do you need a very low sliding resistance or a bit higher one to retain the sleeve before or during the use of the product. All of this taken into consideration, one other issue is the external force that will be applied to the O.D of the sleeve during removal; because, any additional gripping force on the outside of the sleeve will increase the total amount of friction resistance between the two items. Another issue is that the friction factor between two materials is also affected by the surrounding environmental conditions, i.e. wet vs dry, differential thermal expansions for the two items that can either reduce the friction grip or increase it due to a differential in temperature between the assembly environment and that at the application environment.

Based upon all of the above, I would suggest you first determine all of the above that apply to your application; and, based upon that information and then it is time to start some prototype testing under those varying conditions to determine what is or is not an actual issue for your application. This testing can be done using a sensitive load measuring device connected to one end of the sleeve that is extended beyond the end of the internal item and measuring the force required to pull the sleeve along the "catheter".
The next step is to then determine how close the manufactured item can be controlled to match your ideal by doing the same testing based upon "best case/worse case" manufacturing dimensional controls.