Variable Force Tensioner
1, the small plunger high pressure oil chamber filling speed is fast, can choose a small preload, low stiffness plunger spring (usually design plunger spring, need to consider the lowest oil pressure, chain wear limit state, ordinary single plunger tensioner, in order to high-speed conditions of the support effect, usually choose a larger plunger diameter. This will make the engine starting process, the tensioner is difficult to quickly build pressure to produce hydraulic damping force, the need for a larger spring force for pre-tightening, small plunger can reduce the pretension and stiffness of the plunger spring demand. The preloading force of ordinary tensioner plunger spring is 30-40N, the preloading force of VFT small plunger spring is 2~4N, and the preloading force of large plunger spring is 7~10N, and the improvement is obvious.
2, the large plunger support is stable, strong vibration absorption ability, can cope with high speed and large incentive conditions;
3. The check valve with large plunger head can effectively limit the maximum plunger force level and reduce the force of the chain.
In summary, the VFT is a combination of both fish and bear PAWS optimization solution, on the one hand to take advantage of the rapid response of the small plunger, on the other hand, the system has the stability of the use of large plungers. In a sense, the VFT realizes the division of labor between the two plungers, and the regulation of spring Force is placed solely on the small plunger + spring pin structure, which is why BorgWarner engineers explain that this type of Tensioner is named Variable Force Tensioner: Two separate independent chambers to control variable force of spring and hydraulic force.
According to the working principle, it is not difficult to infer the shape of the damping characteristic curve of VFT. Liquid pressure unloading in the reset section, low preload spring scheme feedback force is closer to the zero line; In the initial stage of compression, the small plunger responds quickly and the feedback force climbs rapidly. During the compression process, the two plungers discharge oil at the same time, the liquid pressure is superimposed, and the slope is steep. The maximum feedback force is controlled by the pressure relief valve to ensure that the chain force of the timing system is within the safety level of the fatigue limit. With rapid response, VFT can achieve large curved area under the condition of small feedback force, and the damping characteristics are closer to the ideal state.
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