TECH5 is a regular feature where EngineLabs asks industry leaders five technical questions. This week’s guest is Billy Godbold, valvetrain engineering group manager with COMP Cams.
EngineLabs: What design theory drove the development of the conical spring?
Billy Godbold: Common practice in modeling any spring is to treat each coil as an element in a mass-spring-damper system. If you start to look at springs in this way, it becomes more obvious why you really do not want each coil to have the same frequency. Otherwise, the whole system goes into resonance together at a certain rpm, and all ends badly. In conventional cylindrical springs, the only way to change frequency is to change spacing. Unfortunately, a progressive spacing will also focus the stress on the coil with max spacing. Hence, the common approach is to use dual (or triple) valve springs in racing with considerable interference to quickly dampen coil resonance. The downside to that is the heat, wear and galling introduced by the interference. The math behind the conical springs has been known for well over a hundred years and there were attempts to take advantage of the designs in automotive applications dating back to at least the ’20s. The issues were always limits in the manufacturing accuracy and material properties.
EngineLabs: What sets this design apart from a beehive spring?
Billy Godbold: The beehive springs were COMP’s first attempt to take advantage of a conical “type” design working within the material and manufacturing limits allowed at their introduction. If you imagine starting at the bottom of the spring at max diameter and working up, as you reduce the diameter you also reduce the load that can be achieved when constrained by the material strength (UTS). Hence, we used Ovate wire to effectively spread the stress and put more wire into a given space (Installed Height), then stayed at max OD most of the way up to achieve the required load. Today, newer material is available, that can achieve higher loads with the progressive reduction in diameter. Also, the manufacturing process of a conical is can be very difficult. The whole geometry moves during stress relieving much more so than a beehive. However, with state-of-the-art CNC coiling machines, we can now run a few setup pieces to optimize the “as coiled” diameter and pitch profiles so that the spring achieves the optimum geometry after all processing.
EngineLabs: Please explain “progressive frequency” in more detail?
Billy Godbold: If you go back to the mass-spring-damper individual coil idea, the idea is to have each part of the spring have a different natural frequency. Hence, no standing wave can be established in the system and the coil movement after valve closing quickly dampens between events. With a traditional spring, the whole system may oscillate quite violently 3-6+ times between valve closing and the next opening. These cycles both reduce spring life (as to the coil those seem almost the same as valve events or engine cycles) and the available load to control the next cycle.
EngineLabs: Should racers take a different approach in selecting a conical spring rate for their application?
Billy Godbold: Because of the greatly reduced mass and the self-dampening, the engine builder can step back considerably in load. However, many engine builders are already well aware of this tendency with the availability of smaller diameter dual valve springs used in both circle track and drag racing. The trend is exactly the same, but you may be able to step back even further on loads.
EngineLabs: Are new materials or processes used in the construction of the conical spring?
Billy Godbold: Yes, new wire has become available that allows COMP Cams to do more in terms of load, rate and frequency that we would have thought possible just a few years ago. These new materials are very difficult to form into the ovate shape used on our beehive springs, but fortunately conical designs work just as well or better with the round profiles available. I really can’t stress how critical new materials were to the development of these valve springs. With both new materials and new manufacturing machines and techniques, these designs would not have been possible.