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Wet multiple plate clutches consist of friction plates, on which a friction material is bonded, and mating plates that are plain metal plates. Since the frequency and the range of load in the field of forklift trucks vary widely and are more severe than those for passenger cars, the wet multiple plate clutches on forklift trucks are often damaged. Damaged clutches that were returned from the field typically had 3 types of symptoms: 1.Only the friction material was damaged, 2.Only the mating plates were deformed, 3.Both symptoms were observed. It was clear that the cause of these symptoms depended on the difference of the operating application and the strength criteria of each part. This showed that a design guide for wet multiple plate clutches considering the strength balance between the two parts according to the work application was required. The relevant flow chart of this design process was proposed.

A method of life calculation for wet multiple-plate clutches has not been established; they are tribological parts and the damage of such parts depends on many intertwining parameters such as operating condition and lubrication condition. The focus of this study is on the clutch plate surface temperature, T, that represents many parameters. A T-N curve was developed for the friction material that is similar to the S-N curve for metal fatigue. Load frequency was measured from actual operating conditions in the field, and from this data the frequency of T was predicted. Field life could then be calculated for wet multiple plate clutches from the aforementioned T-N curve, the field frequency of T, and Miner's law.