This paper presents the vibratory characteristics of new types of hydraulic o scillatoran oscillator without a restoring force having a dead zone and an oscillator with low-suppressed restoring torque at a reference angle. These take advantage of the generally undesireable valve instabilityp henomenona, n d moreoverp ossessl ower frequencyc haracteristicst han other oscillators.
At first, we derived the nondimensionalizeed quations of motion of oscillators corres ponding to the vibratory displacement ranges. Next, formulas on the exact vibratory characteristics of the oscillator with a restoring torque having a dead zone were obtained by solving the sectionally linear nondimensionalizedi fferential equations given reasonable connecting conditions. Then, available approximate formulas of the frequency characteristics were presented.
For the oscillator with low-suppressedr estoring torque at a reference angle, the vibratory characteristics were investigated by analogue simulations computed using the nondimensionalized equations of motion for which nonlinear restoring torque terms were experimentally measured.

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Complaintsa rose from some of the working fields that workpiecesp roduced by a hydraulic copying system show intolerable deviation in their dimensions.
In this paper the authors firstly revealed that such a deviation in dimensionsc ame from thermal expansion of structural elements of the hydraulic copying system. And the major part of the thermal drift is caused by the temperature variation of hydraulic fluid.
Secondary experimental result and theoretical result of the drift were compared and two methods to reduce the thermal drift were proposed; the first method is to arrange tool tip and stylus tip of pilot valve in the same plane normal to the copying direction; the second method is to place an alminium subplate between the copying cylinder and the pilot valve. Effect of these two methods were verified in experiments.

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