Thus, the Ball Screw under a preload is displaced by a 0 when an axial load (F t) approximately three times greater than the preload is provided from outside. As a result, the displacement of the Ball Screw under a preload is half the displacement (2a 0 ) of the Ball Screw without a preload. As stated above, since the preloading is effective up to approximately three times the applied preload, the optimum preload is one third of the maximum axial load. Note that an excessive preload adversely affects the service life and heat generation. The maximum preload should be set at 10% of the basic dynamic load rating (Ca) in the axial direction.
With the Ball Screw, it is necessary to select a screw shaft so that it will not buckle when the maximum compressive load is applied in the axial direction. Fig.12 on A15-31 shows the relationship between the screw shaft diameter and a buckling load. If determining a buckling load by calculation, it can be obtained from the equation (5) below. Note that in this equation, a safety factor of 0.5 is multiplied to the result.
If an axial load is applied to the Ball Screw, it is necessary to take into account not only the buckling load but also the permissible tensile compressive load in relation to the yielding stress on the screw shaft. The permissible tensile compressive load is obtained from the equation.
When the rotational speed reaches a high magnitude, the Ball Screw may resonate and eventually become unable to operate due to the screw shaft’s natural frequency. Therefore, it is necessary to select a model so that it is used below the resonance point (dangerous speed). Fig.13on A15-34 shows the relationship between the screw shaft diameter and a dangerous speed. If determining a dangerous speed by calculation, it can be obtained from the equation (7) below. Note that in this equation, a safety factor of 0.8 is multiplied to the result.
