The current paper considers servo valve torque motors having four air-gaps. An ideal for these types of torque motors is to have four equal air-gaps at free state. However, real servo valve torque motors have unequal air-gaps due to production errors. Because the influence of the inequality of air-gaps is not clear, servo valve manufacturers have made great efforts to establish the equality of the air-gaps. The current paper gives a prospect on how an imbalance of air-gaps changes the flux densities in the air-gaps and the resultant torque characteristic. An important result of the paper is that some of the air-gap imbalance induces a torque change of magnitude of second-or higher-powers of the imbalance. The imbalance of the air-gaps due to different pole heights results in an offset proportional to the magnitude of the imbalance. These results can be used for setting a tolerance limit for air-gap thickness.

This paper describes a theory and experiments for the conversion of electric current to mechanical torque by a torque motor in servo valves. The feature of the torque motor treated in this paper is the use of a permanent magnet. These kinds of torque motors are most commonly used in modern servo valves. For this type of torque motor, the theory developed by Merritt is most notable among fluid power engineers and scientists. However, his theory does not agree very well with experiments because he ignored the magnetic reluctance and leakage flux of permanent magnets. This paper corrects these points and offers some formulae that agree well with experiments.

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