Fluid handling – Systems – Multi-way valve unit
Reexamination Certificate
1999-11-23
2001-03-13
Michalsky, Gerald A. (Department: 3753)
Fluid handling
Systems
Multi-way valve unit
C251S129030, C251S129120
Reexamination Certificate
active
06199588
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the field of motor-driven valves. More particularly, the invention is a direct-drive servovalve in which a drive motor is employed to cause a substantially backlash-free shifting of the valve's spool. This is accomplished through the use of a uniquely-shaped tip of the motor's output shaft and a shaped groove in the valve's spool that receives said tip. To enhance engagement between the tip and groove, the shaft is spring-biased toward the spool. Furthermore, the geometric relationship between the tip and groove is also responsible for limiting the rotation of the motor.
BACKGROUND OF THE INVENTION
It is well known to use an electric motor to cause a shifting of a servovalve's spool. This is usually accomplished through a mechanical link that converts the rotary motion of the motor's output shaft into a linearly-directed force that acts on the valve's spool. One example of such a mechanical link is an offset tip of the motor's shaft engaging a groove/aperture in the spool. In this manner, rotation of the shaft causes the tip to move in an arc, thereby applying a force on the spool that is at least partially directed along the spool's longitudinal axis.
One problem with a mechanical link that employs an offset tip of the motor's shaft is that there can be significant backlash in the connection between the tip and the valve. This is usually due to the tip having a single linear contact with the shaped groove/aperture in the valve's spool. When the rotation of the motor's shaft is reversed, any play whatsoever between the tip and the sides of the spool's groove/aperture will allow the tip to move without a concomitant movement of the spool.
One method used in the prior art to overcome the above-noted problem is to fully retain the shaft's tip within a bushing located in the spool's receiver. This is taught by Spurbeck in U.S. Pat. No. 4,573,494. However, this is only a temporary solution since backlash will arise as soon as the bushing wears. In addition, the extra parts increase the valve's cost and maintenance requirements.
A second problem with prior art direct-drive valves is that it is both necessary and extremely difficult to precisely limit the amount of rotational movement of the drive motor's shaft. When a valve's spool is shifted due to a rotational movement of a drive motor's shaft, the amount of rotation determines the length of the valve's stroke (translation of the spool). If the motor's shaft rotates to a lesser or greater extent than is required, the spool may not shift a full stroke, or will shift too far, or may even shift a full stroke and then reverse direction and partially retrace its path. Therefore, precisely limiting the motor's rotation is absolutely critical to proper valve function.
There have been a number of methods employed in the prior art to limit the amount of rotation of the motor's output shaft. Most commonly, the motor includes internal stops that stop the rotor's movement. However, the stops can break or wear, resulting in improper rotation of the motor's shaft.
Another method for limiting the rotation of the motor's output shaft is taught by Hair et al in U.S. Pat. No. 5,040,568. The patent teaches the use of a shaped cam plate that is attached to the tip portion of the motor's shaft. When the motor is attached to the valve body, the plate is received within a specially-shaped cavity in the valve body. As the tip rotates, it causes the plate to shift within the cavity. The tip movement, and hence the motor's rotation, is stopped when a side of the plate abuts a sidewall of the cavity. While this is an effective method for limiting the rotation of the motor, it requires the use of a cam plate that must be precisely machined and secured to the shaft in a slip-free manner. Furthermore, the body of the valve must include a precisely machined cavity for receiving the plate. Once the plate is within the cavity, the cavity must remain free of corrosion and dirt, since any foreign material on the contact surfaces would adversely affect proper operation of the valve. In addition, any wear of the cam plate, of the connection between the plate and shaft, or of the cavity's sidewalls will result in inaccurate movement of the valve's spool. Additionally, the added parts and precise machining increase the valve's cost and its maintenance requirements.
SUMMARY OF THE INVENTION
The invention is a direct-drive servovalve that employs a unique method to convert the rotation of the drive motor's output shaft into a linear translation of the valve's spool. The method involves a trapezoidally-shaped tip of the output shaft engaging opposite sidewalls of a shaped groove in the valve's spool. The resultant geometric relation enables the conversion of a rotational movement of the tip into a linear movement of the spool. Furthermore, the geometry of the contacting surfaces also acts to limit the rotation of the motor's shaft.
The motor is preferably of the type commonly known as a torque motor and has a conventional stator and rotor. However, the tip portion of the motor's shaft has a trapezoidally-shaped cross-section and tapers down to a truncated end. Unlike the offset tips of the prior art, the longitudinal axis of the shaft extends through the center of the tip. The motor's shaft is allowed some longitudinal play, and a spring member or mechanism is employed to continually urge the shaft's tip toward the valve.
While the invention can be used with any type of valve in which operation of the valve requires a linear translation of a portion of the valve, the invention is preferably employed with a conventional spool valve. The spool is modified whereby it has a receiver designed to inwardly-receive at least a portion of the trapezoidal tip. In the preferred embodiment, the receiver is in the form of a circumferential groove that has tapered, flat sidewalls and a depth capable of receiving at least a portion of the trapezoidal tip. The taper of the groove's sidewalls is complementary to the taper of the tip whereby opposite sides of the tip can engage opposite sidewalls of the groove.
When the trapezoidal tip of the motor's shaft is received within the spool's groove, an area of contact is continuously maintained along opposite sides of the tip. This is due to the geometry of the contacting parts, and is enhanced by the spring in the motor that urges the shaft toward the spool. As a result, a substantially zero-backlash engagement between the two components is maintained throughout any operational movements of the valve's spool.
Once the tip and spool are engaged, rotation of the motor's shaft will cause the tip to press on the groove's sidewalls in a manner that causes a translation of the spool. The spool will shift an amount related to the angle of the tip's sides relative to the groove's sidewalls.
The translation will continue until an entire face of one side of the tip is parallel to and abuts the adjacent sidewall of the groove. Once this occurs, the tip cannot rotate any further, thereby stopping the rotation of the motor at a precise and predetermined point. This avoids the need for any additional structure to accomplish a limiting of the motor's rotation.
Therefore, the geometry of contact between the tip of the motor's shaft and the receiver in the spool provides a backlash-free connection and negates the need for any additional structure to limit the rotation of the motor. This results in a direct-drive servovalve that is low in cost and requires only a minimum of maintenance.
REFERENCES:
patent: 1910876 (1933-05-01), Appel
patent: 2797706 (1957-07-01), Harrison
patent: 3550631 (1970-12-01), Vanderlaan et al.
patent: 4503888 (1985-03-01), Brovold
patent: 4573494 (1986-03-01), Spurbeck
patent: 4672992 (1987-06-01), Vanderlaan et al.
patent: 4742322 (1988-05-01), Johnson et al.
Delaware Capital Formation Inc.
Gubernick Franklin
Michalsky Gerald A.
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