The spacer is a mild steel tube with an outside diameter of 49mm, a wall thickness of 1.25mm, and a length of 38mm. Mild steel is linearly elastic which means it behaves according to Hooke's law. For calculation purposes, we assume the material is both homogeneous and isotropic. In other words, the material has the same physical composition throughout, and its mechanical properties are the same in all directions. The collapsible spacer that I removed from one of my rear hubs some while ago had suffered compressive deformation, with a change in length of 1mm. If I assume that the yield strength of the material is 250MPa and its elastic modulus is 200GPa, then the axial force applied to the spacer is 962kN. You might be wondering what does a force of 962kN feel like? If you place two 50g snickers on the palm of your hand, then the force that you feel pushing on your hand is one Newton. The applied axial force is 962,000 times that . When you apply a torque to the nut, an axial force perpendicular to the lever arm is applied to the spacer. The spacer is captured between the bearing cones with no axial force applied directly to the bearings as the nut is tightened. When the applied axial stress (stress = the axial force divided by the cross-sectional surface area) passes the yield point of the tube, compressive deformation takes place. Prior to reaching this stress, the tube was behaving elastically. After the yield point, the tube behaves plastically. This means that it has sustained permanent deformation, which is precisely what it has been designed to do. In reducing the length of the spacer, the frictional force experienced by the bearings increases. This ensures that the right amount of friction is experienced by the bearings. Too little or too much will shorten their life.
The reduction in length of the spacer is called strain, which is a unitless measurement of deformation. Up until the yield point, both the applied stress and strain were in a linear relationship. If the load had been removed during this time, the spacer would return precisely to its original length. After the yield point, this no longer happens. The strain remains and any further axial force applied will shorten the spacer with a lower level of applied axial force relative to the strain. The stress and strain are no longer proportional, with the strain increasing at a much greater rate. It is for this reason that the spacer is a one-use-only device. Do not confuse compressive deformation with buckling, they are quite distinct. Buckling is a phenomenon that impacts slender tubes, with slenderness ratio a function of tube length and radius of gyration. The images illustrate the placement of the spacer and a comparison between a new and old spacer. The compressive deformation is quite obvious.
Ron.
The reduction in length of the spacer is called strain, which is a unitless measurement of deformation. Up until the yield point, both the applied stress and strain were in a linear relationship. If the load had been removed during this time, the spacer would return precisely to its original length. After the yield point, this no longer happens. The strain remains and any further axial force applied will shorten the spacer with a lower level of applied axial force relative to the strain. The stress and strain are no longer proportional, with the strain increasing at a much greater rate. It is for this reason that the spacer is a one-use-only device. Do not confuse compressive deformation with buckling, they are quite distinct. Buckling is a phenomenon that impacts slender tubes, with slenderness ratio a function of tube length and radius of gyration. The images illustrate the placement of the spacer and a comparison between a new and old spacer. The compressive deformation is quite obvious.
Ron.