Điều này có vẻ quá tốt là đúng (và nó là). Tăng tốc không có nghĩa là không động lực. Cam Điều này dường như không có lực lượng năng động hoặc căng thẳng trong đó! Hình 8-8 cho thấy những gì thực sự xảy ra ở đây. Nếu chúng ta quay trở lại chức năng chuyển và đồ họa phân biệt nó hai lần, chúng tôi sẽ quan sát, từ định nghĩa của đạo hàm như chức năng ofthe độ dốc tức thời, | DESIGN OF MACHINERY CHAPTER 8 This seems too good to be true and it is . Zero acceleration means zero dynamic force. This cam appears to have no dynamic forces or stresses in it Figure 8-8 shows what is really happening here. If we return to the displacement function and graphically differentiate it twice we will observe that from the definition of the derivative as the instantaneous slope ofthe function the acceleration is in fact zero during the interval. But at the boundaries of the interval where rise meets low dwell on one side and high dwell on the other note that the velocity function is multivalued. There are discontinuities at these boundaries. The effect of these discontinuities is to create a portion of the velocity curve which has infinite slope and zero duration. This results in the infinite spikes of acceleration shown at those points. These spikes are more properly called Dirac delta functions. Infinite acceleration cannot really be obtained as it requires infinite force. Clearly the dynamic forces will be very large at these boundaries and will create high stresses and rapid wear. In fact if this carn were built and run at any significant speeds the sharp comers on the displacement diagram which are creating these theoretical infinite accelerations would be quickly worn to a smoother contour by the unsustainable stresses generated in the materials. This is an unacceptable design. The unacceptability of this design is reinforced by the jerk diagram which shows theoretical values of infinity squared at the discontinuities. The problem has been engendered by an inappropriate choice of displacement function. In fact the cam designer should not be as concerned with the displacement function as with its higher derivatives. The Fundamental law of Cam Design Any cam designed for operation at other than very low speeds must be designed with the following constraints The cam function must be continuous through the first and second derivatives of displacement
