SPOOL FOR A SPOOL VALVE AND METHOD OF PRODUCING SAME

Abstract: The invention provides a spool for a spool valve, the spool (1) comprising at least one annular land (2) having at least one notch (3) therein which is part conical (4). The invention also provides a method of producing such a spool.

Description: This invention relates to spool valves and more particularly to a spool for a spool valve and a method of producing same. The spool of a spool valve normally comprises at least one annular land which cooperates with at least one annular land on the surface of a passage in a valve body in which the spool is slidable to control flow through the valve. In order to meter the fluid flow through the valve it is known to provide notches in a side surface of said land which extend inwardly from the periphery thereof. For reasons of cost, such notches are conventionally produced using a cylindrical or parallel sided milling cutter introduced radially of the spool at two or more positions around the circumference of the spool. Such spools suffer from the disadvantage that, due to the position of the inlet and outlet ports within the valve body, the flow rate through the valve can vary with the angular position of the spool given a constant differential pressure between the inlet and outlet ports of the valve, with the sensitivity to angular position of the spool increasing with increasing flow rate. Another disadvantage is that in order to maximise the flow rate large notches must be used and that when these are produced using a cylindrical cutter a segment of spool material is left between each pair of adjacent notches which is both mechanically weak and can easily become over hardened by heat treatment. Conventionally, to overcome this, a notch diameter is selected which leaves an acceptable wall thickness between the notches at the expense of flow rate. Advantageously, the notches should be designed so that as the spool moves from an initial position at which flow is wholly through the notches there is a gradual increase in the percentage of the circumference of the spool contained within the notches to provide a smooth transition to a spool position at which flow can occur around the full circumference of the spool. Thereafter, with increasing spool displacement, metering should remain at full circumference of the spool to reduce the susceptibilty of the flow to the angular position of the spool. With the known parallel-sided notch form it is not possible to blend the notches together to give the required smooth transition without either sacrificing flow rate or producing an unacceptable shape in the remaining spool material. The present invention provides a spool for a spool valve, the spool comprising at least one annular land having at least one part conical notch in a side surface thereof and extending inwardly from the outher periphery thereof. The invention also provides a method of producing a spool for a spool valve comprising the steps of obtaining a spool having at least one annular land thereon and forming in a side surface of said land at least one part conical notch which extends inwardly from the outer periphery thereof. The axis of said part conical notch may extend substantially radially of the spool, to which end the notch may be formed by a tool, e.g., a milling cutter having a frusto-conical cutting end portion, introduced substantially radially of the spool. Preferably a plurality of said notches are formed in said land and are angularly spaced therearound. Said plurality of notches may be of equal size and be evenly arranged around said land. Preferably, the inclusive conical angle of each of said notches is ± 30° where N equals the number of notches. Alternatively, said notches may be of two or more different sizes and be evenly arranged around said land, with the angular distance between the axis of any two adjacent notches preferably being equal to half the sum of the inclusive conical angles of the two adjacent notches. Whether the plurality of notches are of equal size or of two or more different sizes, the arrangement is preferably such that adjacent notches are seperated by an edge which lies in a plane extending longitudinally and radially of the spool. This will not only maximise the flow rate through a spool valve comprising the spool but, because of the conical form of the notches, said edge will be buttressed and therefore mechanically strong and less susceptible to over hardening as by heat treatment. If the or each notch is made part conical over the whole of its radial extent then diameter finishing operations, such as final grinding, could result in a change in the notch opening position along the length of the spool. To overcome this problem a portion of the or each notch at the outer periphery of said land may be part cylindrical. According to a preferred embodiment, the or each said notch comprises a first outer conical portion having a first inclusive conical angle and a radially inner blend form such as a radiussed portion or a conical portion having an inclusive conical angle larger than said first inclusive conical angle to blend the notch into the body of the spool. Thus, the or each notch may comprise a part cylindrical portion at the outer periphery of said land, a first outer conical portion and a radially inner conical or other portion as aforesaid. It has been found that if the milling cutter or other tool used to produce the or each notch is hollow or is recessed at its cutting end so as to produce a horseshoe-like cut at the radially inner end of the notch adjacent the tool body then flow through a valve incorporating the spool appears to be improved. The spool may have a plurality of lands thereon each comprising at least one said notch. The invention also provides a spool valve comprising a valve body having a passage therein and a spool according to the present invention slidable in said passage. Said passage may have at least one annular land on the surface thereof which cooperates with the at least one annular land on said spool. The invention will be more particularly described with reference to the accompanying drawings, in which:- Figure 1 is a side elevation of a spool for a spool valve according to the present invention, Figure 2 is a sectional elevation showing one of the lands of the spool of Figure 1, Figures 3 and 4 are detail views of the form of the notches formed in the lands of the spool, Figure 5 is a detail of the cutting end portion of a milling cutter for forming the notches, Figures 6(A) and 6(B) illustrate known notch configurations whilst Figure 6(C) illustrates the notch configuration of a spool according to the invention, and Figure 7 is a flow diagram illustrating the difference in flow characteristics provided by the known notch configurations and the notch configuration of a spool of the present invention. Referring to Figure 1, the spool 1 has annular lands 2 in known manner. Each of the lands 2 has a plurality of notches 3 which are in a side surface thereof and which extend radially inwards thereof. Each of the notches 3 is of equal size and comprises an outer part conical portion 4 the inclusive conical angle of which is substantially where N equals the number of notches, and a radially inner part conical portion 5 the inclusive conical angle of which is greater than that of the part conical portion 4. The portions 4 of adjacent notches 3 are separated by an edge 6 which lies in a plane extending longitudinally and radially of the spool, as will best be seen from Figure 2. The inclusive angle 7 between the notches 3 is such that the edges 6 are well buttressed by the material remaining between the notches. With the inclusive conical angle of the notches being , the inclusive angle 7 will remain constant between the points 8 and 9 shown in Figures 2 and 4. In addition, each of the notches 3 has a part cylindrical portion 10 at the outer periphery of the land 2. This part cylindrical portion 10 is contained between the points 11 and 12 as shown in Figure 2 so that it does not interfere with the geometry of the notch configurations at the edges 6. The part cylindrical portion 10 of each notch serves the function of preventing the opening point of the spool being shifted axially when the spool is subjected to diameter finishing operations such as grinding. The notches 3 can be formed in any suitable manner, such as by spark erosion or the like. However, a preferred method of producing the notches 3 is to use a milling cutter 13 as shown in Figure 5 the cutting end portion of which comprises a cylindrical portion 14 for producing the notch portion 10, a frusto-conical portion 15 for producing the part conical notch portion 4 and a frusto-conical portion 16 for producing the part conical notch portion 5. The inclusive conical angle 17 of the portion 15 is plus or minus 30°. The notches 3 are produced by introducing the milling cutter 13 radially of the spool 1 so that the axis of the part conical portions 4 of the notches 3 extend radially of the spool 1. Reference will now be made to Figure 6 in which like parts have been given like reference numerals and in which in each case the spool 1 is shown cooperating with an annular land 18 within a passage 19 of a valve body 20. Referring first to Figure 6A it will be seen that the known notch configuration 3 is part-cylindrical as shown. With such a known notch configuration it is not possible to arrange the notches too close together otherwise the material remaining between the notches will be mechanically weak and liable to over hardening during subsequent heat treatment or the like. Thus, due to the necessity to leave sufficient material between adjacent notches it is not possible to maximise the flow rate through a spool valve comprising such a spool. Moreover, spool valves with such a notch configuration suffer from the further disadvantage that, due to the position of the inlet and outlet ports within the valve body, the flow rate through the valve can vary with the angular position of the spool given a constant differential pressure between the inlet and outlet ports of the valve, with this sensitivity to angular position increasing with increasing flow rate. Thus, a notch configuration as shown in Figure 6A gives a flow rate versus spool displacement curve as shown at A in Figure 7. The notch configuration of Figure 6A is sometimes modified as shown in Figure 6B by blending the notches together. As shown by the curve B in Figure 7, such a modification of the notch configuration produces an improvement in the flow rate versus spool displacement but suffers from the disadvantage that there is a rapid change in the area available for fluid flow as the notches 3 emerge from within the land 18 as shown by the line B in Figure 7. Ideally the flow rate versus spool displacement curve should start off as a steep curve and there should then be a smooth transition to a straight line tangent to the original curve. It can readily be seen from the line B in Figure 7 that this is not achieved by the notch configuration of Figure 6B. It will also be seen from line C in Figure 7 that the notch configuration of the present invention as diagrammatically illustrated in Figure 6C not only maximises fluid flow through a spool valve comprising a spool according to the present invention but that the notch configuration also provides a smooth transition from partial flow through the notches 3 only to full flow around the full circumference of the spool.

Claims: 1. A spool (1) for a spool valve, the spool comprising at least one annular land (2) having at least one part conical notch (3) in a side surface thereof and extending inwardly from the outer periphery thereof. 2. A spool according to claim 1, wherein the axis of said part conical notch (3) extends substantially radially of the spool. 3. A spool according to claim 1 or 2, wherein said land (2) has a plurality of notches (3) angularly spaced therearound. 4. A spool according to claim 3, wherein said notches (3) are of equal size and are evenly arranged around said land (2). 5. A spool according to claim 4, wherein the inclusive conical angle of each of said notches (3) is ± 30° where N equals the number of notches. 6. A spool according to claim 3, wherein said notches (3) are of two or more different sizes and are evenly arranged around said land. 7. A spool according to claim 6, wherein the angular distance between the axis of any two adjacent notches (3) is equal to half the sum of the inclusive conical angles of the two adjacent notches. 8. A spool according to any one of claims 4 to 7, wherein adjacent notches (3) are separated by an edge (6) which lies in a plane extending longitudinally and radially of the spool (1). 9. A spool according to any one of the preceding claims, wherein the or each said notch (3) comprises a part cylindrical portion (10) at the outer periphery of said land (2). 10. A spool according to any one of the preceding claims, wherein the or each said notch (3) comprises a blend form for blending said part conical notch portion (10) with the body of the spool (1). 11. A spool according to claim 10, wherein the or each said notch (3) comprises a first outer conical portion (4) having a first inclusive conical angle and a radially inner radiussed portion or conical portion (5) having an inclusive conical angle larger than said first inclusive conical angle. 12. A spool according to any one of the preceding claims, wherein the spool (1) has a plurality of lands (2) thereon each comprising at least one said notch (3). 13. A spool valve comprising a valve body having a passage therein and a spool (1) according to any one of the preceding claims slidable in said passage. 14. A spool valve according to claim 13, wherein said passage has at least one annular land on the surface thereof which cooperates with the at least one annular land (2) on said spool (1). 15. A method of producing a spool for a spool valve comprising the steps of obtaining a spool (1) having at least one annular land (2) thereon and forming in a side surface of said land at least one part conical notch (3) which extends inwardly from the outer periphery thereof. 16. A method according to claim 15, wherein said notch (3) is formed by a tool (13) introduced substantially radially of the spool (1). 17. A method according to claim 16, wherein said tool is a milling cutter (13) having a frusto-conical cutting end portion (15, 16). 18. A method according to claim 16 or 17, wherein said tool (13) is hollow or has a recessed cutting end portion. 19. A method according to any one of claims 15 to 18, wherein said spool (1) is subjected to a final grind after the forming of said at least one notch (3). 20. A method according to any one of claims 15 to 19, wherein said spool (1) is subjected to a hardening treatment after the forming of said at least one notch (3). 21. A spool for a spool valve when formed by the method of any one of claims 15 to 20.

Inventors: CLEASBY, KENNETH GEORGE
Applicants: VICKERS SYSTEMS LIMITED
Application year: 1988
Main classification: F16K11
IPC codes: F16K11, F15B13