The present invention relates to a mechanical detent for holding a valve spool in a predetermined position along its longitudinal axis.
One application of a detent is to hold a spool that is spring-centered, i.e., the spool returns automatically to a neutral position when the detent is disengaged. Another application of a detent is to hold a spool that is not spring-centered, i.e, the spool is manually moved to all positions including neutral. In addition to physically holding the spool in a certain position ("stop" position), a mechanical detent may also be used for a "feel" position. To move the spool past a feel position requires the operator to exert a higher force, indicating that a particular stage of the spool stroke has been reached or passed through. However, the spool is not physically held in this position.
Maintaining a valve spool in a predetermined position by means of a mechanical detent has been attempted in many forms. Various forms of the detent include spring-loaded radial detents and C-shaped spring detents which surround the circumference of the valve spool. All of the above detents engage a groove or a step on the periphery of the valve spool.
The spring-loaded radial detents engage a groove on the spool stem, producing a radial force at a single point of contact. Some problems with the spring-loaded radial detents are that they require a number of parts, take up a large amount of space in the housing of the valve spool, and are costly both in part costs and assembly time. For example, in the case of a ball-type radial detent, the ball construction must display a radius approximately two or more times greater than the depth of the groove in order to provide a means for effectively moving the ball out of the groove. Thus, the deeper the groove is made, the larger the ball size must be. In trying to keep the ball size within an acceptable size range, while maintaining an acceptable groove depth, this required ratio between the ball and the groove results in a relatively shallow groove and a relatively large ball.
Due to the built-in requirement of this relatively shallow groove, this construction creates the need for heavy loading of balls by spring(s) in order to maintain the ball in the groove. Additionally, where the ball size must be at least two times the groove depth, the only way to minimize the overall size of the valve structure is to have close toleranced housing valve spool and engaging parts. Such close toleranced parts increase the overall cost of the valve structure.
In the case of a C-shaped spring detent, the C-shaped spring surrounds the circumference of the valve spool and radially extends around the grooveless surface of the valve spool. When the groove of the valve spool is moved past the C-shaped spring, it springs radially inward, engaging the groove. The C-shaped spring provides essentially continuous contact around the surface of the spool. As in the case of the ball design, the C-shaped spring must have a relatively large cross-sectional diameter with respect to the groove depth since it must extend well beyond the surface of the spool to be effectively moved out of the groove. This required dimension of the groove engaging element creates the need for a relatively shallow groove to keep the C-ring size in an acceptable range and a corresponding heavy loading to maintain the element in the groove.
In addition to the constraint of requiring a relatively large groove engaging element, a relatively shallow groove, and a corresponding heavy load, the structure of the C-shaped spring and the ball-type detent also requires large spring size in order to have as low a rated spring (load/deflection) as possible. A low rated spring is necessary in order to have a maximum groove depth. Having a groove that is as deep as possible is desireable for manufacturing and tolerancing purposes. A low rated spring is necessary in the case of a maximum groove depth in order to move the element out of the groove. Without a low rated spring, a large load is required to move the element out of the groove. However, due to the construction of the above-mentioned detents it is necessary to further increase the overall size of the spring to achieve a low rated spring.
For example, to have a low rate C-shaped spring, the overall diameter D of the spring (the full length of the "C") must be large with respect to the diameter d of the wire of the spring, further increasing the size of the structure. The rate of the C-shaped spring is related to the ratio d/D. The lower this ratio, the lower the spring rate. Since the diameter d of the wire must be at least two times the groove depth, the numerator of this rate, d, has a somewhat "fixed" minimum value. Thus, to have a lower spring rate, the denominator, D, the overall size of the spring, must be large. Where a spool size is predetermined, modification of the spool may be necessary in order to accomodate a large, low rate spring.
In the case of the ball-type detent, having a coil spring having a wire size d and an overall spring length D, the spring rate is also related to the ratio d/D. To obtain a low spring rate, the length of the spring, D, must be large, making the overall size of the spring very large. The length of the spring especially affects the size of the total valve structure due to its orientation. In the ball-type detent, the coil spring is positioned radially outward from the ball. Thus, each increase in spring length further extends the radial distance between the spool and the housing required to accomodate the ball and spring. Although the surrounding parts can be constructed to be close-fitting to minimize the size, this is an expensive process.
Another disadvantage of the spring-loaded radial detents and the C-shaped spring detents is that, since the radius of the groove engaging element must be substantially greater than the depth of the groove, an enlarged load angle results. The load angle is defined as the angle between a horizontal axis through the groove engaging element and the axis traveling through the point of contact of the engaging element against the side wall of the groove and the center of the element. The load angle is equal to the angle of the side wall of the groove, or the groove angle. Due to the large load required in these types of detents, as well as the correspondingly high loading rates involved, these systems usually employ an edge contact point, whereby the outer diameter edge of the spool stem at the point where the groove begins contacts the wire or ball. This construction provides poor load angle control and high point contact stresses.
Examples of the spring-loaded radial detents are found in U.S. Pat. Nos. 3,986,701 to Hopkins, 4,040,675 to Richmond et al. and 4,260,132 to Habiger. Examples of the C-shaped detent are shown in U.S. Pat. Nos. 4,413,805 to Green et al. and 4,185,661 to Gill et al.
It is an objective of the invention to create a detent having a groove engaging element having a radius which is independent of the groove size; the radius may be larger than, equal to or smaller than the depth of the groove.
It is another objective of this invention to create a detent having a groove engaging element which provides improved "feel" and "stop" characteristics with a reduced diameter.
It is another objective of the invention to create a detent which does not require a large number of parts and which requires reduced space in the valve spool assembly.
Another objective of this invention is to create a detent having improved load angle control and reduced contact stresses.
Another objective of this invention is to create a detent which provides improved "feel" and "stop" characteristics upon engagement and disengagement of the groove.
Another objective of the invention is to create a detent such that the clearance between the spool and the engaging elements is not as critical.
Another objective of the invention is to be able to place a detent anywhere on the spool and to avoid having to modify the spool to accomodate a low rate spring.
It is a further objective of the invention to create a detent which is easy to assemble and disassemble.
These objectives are achieved by a detent having a linear element spring having extensions which extend a sufficient distance beyond the surface of a spool to be engageable with the engaging element which moves the spring out of circumferential grooves on, the spool. Thus, the wire diameter of the spring is not critical, and may, for example, be reduced since the groove engaging portion of the wire is not the primary contact surface used to force the spring out of the groove.
The spring may be formed from a single wire having three or more linear segments interconnected at elbows. Alternatively, one or more linear springs may be positioned by a retainer. The extensions should extend a distance at least twice the diameter of the spring wire. Since the radius of the spring wire is not dependent on the particular desired groove depth, the radius of the spring wire can be greater than, equal to or less than the depth of the circumferential groove. Additionally, since there is no minimum required value for the spring wire in relation to the groove depth, a low rate spring may be designed without requiring a large spring size. Also, since the spring wire need not be at least two times the size of the groove depth, the point of contact between the groove engaging element and the side wall of the groove is below the edge point where the groove meets the outer diameter of the spool.
Further objects, features, and advantages of the present invention will become more apparent from the following description when considered in conjunction with the accompanying drawings which show, for purposes of illustration only, embodiments in accordance with the present invention.