Engineering Corner

Modelling Pushability

As pushability is related to tube longitudinal stiffness, it is calculated to be:


Where

  • klong is the longitudinal spring constant,
  • E is the modulus of elasticity,
  • A is the cross-sectional area
  • L is the length of the catheter shaft.

To maximize pushability klong must be maximised.
This can be achieved in various ways:

  • By maximizing the cross-sectional area of the tubing.
  • By maximizing the modulus of elasticity by using a stiffer material.
  • By decreasing the overall part length.

 

Modelling Torqueability

Torqueability is a measure of the tubes torsional stiffness and can be modelled using:


Where

  • ktorq is the torsional spring constant
  • G is the shear modulus
  • J is the polar moment of inertia
  • L is the length of the catheter shaft

Maximizing torqueability is achieved by maximizing the torsional stiffness value, ktorq, through actions that include:

  • Maximizing the polar moment of inertia. For a tube, the governing equation for J is:

In order to maximize J, the outside diameter and the wall thickness should be maximised.

  • By maximizing the shear modulus using a stiffer material.
  • By decreasing the overall part length.

Modelling Flexibility

The flexibility or flexural stiffness of a tube is determined by:


Where

  • kflexural is the flexural spring constant
  • E is the modulus of elasticity
  • I is the moment of inertia
  • L is the length of the catheter shaft.

To improve the flexibility of the tube the flexural stiffness value, kflexural, the must be minimized through actions that include:
Minimizing the moment of inertia. For a tube, the governing equation for I:


In order to minimize I, the wall thickness must be at a minimum:

  • By minimizing the modulus of elasticity by using a soft material.
  • By increasing the overall part length.

Flexibility is not however a measurement of kink resistance. Experimental Data has shown that tubes with a small ID/OD ratio perform better than those with higher ID/OD ratios. That is those with thick walls have better kink performance than those with thinner walls.

Engineer's Calculator

The Convertor calculator is an invaluable tool for any engineer – it easily converts measurements of area, force, pressure, speed, power, light etc.

Download this useful calculator from the link below
Convertor calculator download

French Catheter Scale

Please find below the French Catheter Scale, a standard scale used in the measurement and designing of catheters.
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