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Issue 21
FALL 1999

STARTING TORQUE! Itís not just the motor to consider...
One of the application problems we see in the area of selecting a motor, is adequate starting torque. The choice of the motor is only half the problem, one also must consider the hydraulic circuit.

THE MOTOR - It is only possible to accelerate a load smoothly away from rest, if the motor design is such that the flow through the motor and output torque remains constant, regardless of angular shaft position. A flow control cannot control rotation smoothly when the motor flow is pulsating. The pulsation results in a pressure ripple, which creates shaft cogging. The motor should have high starting torque efficiency, otherwise the breakaway frictional forces will accentuate the cogging motion.

To accelerate a load smoothly from rest, one has to initially overcome the breakaway frictional forces of the motor and load, plus the load. We recommend the circuit shown in fig 1.

This circuit shown is superior to a meter in/meter out directional control valve because the valve metering lands are generally of a fixed nature with no latitude for error. This results in an unbalanced flow through the motor, subsequent cogging of the motor shaft. The valve is not pressure compensated and any change in spool position changes the pressure drop whilst also changing the inlet flow and outlet flow. As the motor rotates, the frictional forces diminish reducing the pressure drop, which can cause the motor to start with a jolt or cog.

With the meter out circuit, the inlet flow and pressure is on demand, such that there is adequate supply to meet the motor’s needs, including internal leakage, under all conditions for the specific speed, acceleration or torque. Energize the directional valve and the motor will see full pressure across the motor, the pressure compensated proportional flow control valve starts to ramp open, causing a change in the pressure drop. When the pressure drop is sufficient to generate the torque required, (overcoming friction and the load) the motor starts to rotate. In the recommended circuit, the outlet flow of the motor is always controlled and as long as the ramp rate is not too fast then the motor will start positively and run smoothly.

Should the load vary, then the internal leakage will vary causing the motor speed to change. This can be compensated for by a closed loop control, but this adds complexity. The NUTRON motor’s integral pressure feed back system will stabilize the speed within 1% despite a varying load, without this complexity.


– High Pressure Pumps for water-based fluids

The high pressure (up to 12600 psi) type XW hydraulic pump is suitable for a wide range of fluids, including water and water-based fluids. Separation of the lubricating oil and the pumped fluid is achieved by installing a cavity between the cylinder block and the case. Bypass, from the pistons, is collected in this cavity and returns to the inlet side of the pump.

The pumps are available with either three or six pistons. The six piston version can be supplied with duplex outlets, which may be used for simultaneous deliveries or for high and low pressure applications, when using an automatic unloading valve. There are nine displacement sizes available:  1.5 gpm @ 1760 rpm, max pressure 12600 psi up to 11 gpm @ 1760 rpm, max pressure 2500 psi


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Engineering and Sales: 56 Summersea Rd., Mashpee, MA 02649
508-717-5441, fax 508-681-8068
Manufacturing and Service: 1520 Alfred Rd., Lyman, ME 04002
Phone: (207) 439-5272 Fax: (207) 439-8611
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