Valve Springs

GC porting, development, valve and seat work, combustion chambers, cams, head construction, etc
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Guy Croft
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Valve Springs

Post by Guy Croft » Sat Mar 28, 2009 3:38 pm

Some tech info on valve springs and definitions that may be helpful. I have included some basic definitions partly for the benefit of those for whom English is not their native langauge.


Spring cap: This is the part that holds the spring in place near the valve tip. It is retained to the valve stem by collets that may have one or two grooves.

Spring platform: This is the circular machined region where the spring base sits.

Spring base: (also called spring seat). This is the part that the spring sits on. It has the dual function of locating the spring concentric to the guide to stop it moving around and also preventing the spring from cutting into the platform.
Spring fitted/installed length The fitted length refers to the length of the outer spring when fitted. It is the measurement from the underside of the spring cap where the outer spring locates to the spring base. It determines the available space for the spring pack. It depends on: VSH (valve setup or tip height), collet position relative to valve tip, spring cap design, spring base thickness.

Spring coil binding: Spring binding (also known as springs ‘boxed’) occurs when the closest coil set in the pack closes up fully. It must always be avoided. Springs must always 2mm between the closest coils at full lift. If the springs just close up lightly, eventually they will fracture. Severe binding – where the spring presents a solid pack to the valvetrain even before full lift - can break the cambelt, peen over the valve tip, wear out the tappet and cam profile too and cause rapid spring fracture. Note that a flat spring base changes the installed height but does not affect the ‘boxed’ height. A stepped spring base changes both.

Valve float/bounce: Float: the valve and tappet come off the cam profile and assume uncontrolled motion in the cylinder. Bounce: the valve bounces on the valve seat instead of closing under control. They are almost inseparable in their effect. Float can cause valves to hit each other- or the piston - and can cause bounce. Bounce degrades the valve/seat and can set off alarming vibration thru the spring pack. Vibration travels up and down the pack just like a pressure wave and the coils collide erratically with each other. Bounce leads to float as well. When that freakish spring motion develops, leading to power loss and valve impact, the culprit can be the spring pack or the cam profile. The spring must certainly have a natural frequency when installed (a very different value from the free state) that is outside any excitation frequency the spring is likely to see in service. The problem comes when unexpected high frequency excitations develop ‘for no particular reason’ – sometimes referred to as ‘harmonics’ and actual spring behaviour under very high-speed conditions is so hard to predict with certainty that even well-designed springs are now known to exhibit some ‘oddities’ during the valve event. The effect is much like how car suspension behaves with faulty shock absorbers. Float and bounce can be induced by over-revving beyond the capacity of the spring pack, springs of insufficient stiffness for the mass of the valvetrain and by use of cams with poorly designed profiles. Use of some ‘reprofiled’ cams can certainly put the valvetrain at risk.

Spring rate: The stiffness or rate of a spring pack, be it a single, dual or triple array is typically stated in lbf per inch (pounds for short) and is governed by the number of springs in the pack (3VSR have 3) number of coils, coil material, coil thickness. The force exerted by the spring on the cam determines the contact stress on the tappet and cam nose, valve tip and it depends on the spring rate, free length and installed length. Some springs have ‘rising rate’ induced by variable coil thickness or variable pitch between them and their poundage increases non-linearly with lift. The longer the spring in the free state the more force (commonly called ‘poundage’) it will exert on the camshaft when lift starts.

What the spring does: The spring’s job is to keep the valvetrain (valve, cap, tappet, collets – and a percentage of its own mass) under the proper control of the camshaft, without at the same time, being so stiff it wears everything out. In valve opening the cam would project the valve straight into the piston were it not for the spring restraining it. Within the rpm band, the valvetrain inertia (function of mass x cam acceleration) in the valve opening phase must always be less – by a good margin – than the restraining force exerted by the spring. Assessment of spring suitability is based on ‘installed poundage’ and the ‘poundage at full lift’. If the installed poundage is too high there will be early and pronounced wear on the valve tip and tappet, and if too low, control of the valve will be lost immediately, which, given the proximity of both valves to the piston near TDC is rather a critical situation. If the poundage is too low at full lift the valve will not stop – it will keep going until the spring ‘boxes’ or it hits something (usually the piston). On valve closure the spring has to be stiff enough to pull the valve back onto its seat and keep the valvetrain on the cam profile throughout that phase. On high boost turbocharged engines, especially those with launch control and flat-shift the cylinder pressures acting on the valves may well call for unusually stiff springs.

Damping: There is little inherent damping in a valvetrain and conventional ‘undamped’ springs are often designed far stiffer than they need to be to simply overcome valvetrain inertia. This isto ensure their natural frequency (at which they will vibrate/resonate in an uncontrolled fashion) is far higher than any working frequency they will encounter in normal operation. Damping can only be achieved by use of interference springs – where the coils are a tight fit inside each other and it is a highly effective way of spurious coil movement and thus eliminating float and bounce.


GC
Attachments
2005_0318Image0007.JPG
Valve spring available lift can easily be checked in situ in a mill or drill press ..
2005_0318Image0007.JPG (150.11 KiB) Viewed 7069 times
spring boxed height check.JPG
or compressed in a vice. Here the stepped spring base and spring cap are fitted. I compress, measure the spring installed length and then allow 2mm clearance between closest coils. That determines the max lift the spring should be subjected to.
spring boxed height check.JPG (13.79 KiB) Viewed 7066 times
valve cap std to guide_02.JPG
Here is the spring cap fitted and I'm checking it does not hit the guide at full lift. I have to allow for the stem seal too.
valve cap std to guide_02.JPG (10.79 KiB) Viewed 7065 times
Valvetrain parts.JPG
Some of the parts from definitions above
Valvetrain parts.JPG (64.97 KiB) Viewed 7065 times
TC VSH.JPG
This is what I call 'VSH' or valve setup height
TC VSH.JPG (58.9 KiB) Viewed 7085 times
Collets and top hats_names.jpg
more naming_of_parts
Collets and top hats_names.jpg (57.81 KiB) Viewed 7069 times
Springs.doc
Article above plus installation info on GC race springs 3VSR
(43.5 KiB) Downloaded 638 times

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