ENGINE REBUILDERS PTY LTD
7 BARRETT STREET, FLEMINGTON VIC 3031,
TEL: (03) 93723111, FAX: (03)93723122
ACN; 006 798 492 ABN: 74 006 798 492
05-12-00
T ECHNICAL NEWSLETTER: VALVE SPRINGS
The design engineers involved in selecting the conect valve spring for their engine have a number of different factors to select from before their final decision. Some of these factors are discussed below:
The task of the valve spring is to control the position of the valve relative to the mating components of the camshaft at one end of the valve and to the valve seat at the other end of the valve. Add the variable of engine revolutions per minute to the problem and it is obvious that the subject is quite complex. ln fact, the engineers use an experimental test rig rather than mathematics to select their spring. Why this is done can be appreciated from an understanding of the variables involved.
On-seat pressure: This term describes the pressure required to hold the valve onto the valve seat under the conditions expected to occur over the whole rev. range of the engine. At idle speed the engine will be running at 600 to 750 rpm for the normal engine design. A valve weight of l00grams for the intake and 90 grams for the exhaust valve shows the first variable. Generally, the engineer uses the heavier of the valves from which to establish a common spring for the engine. At idle speed, the task of the spring is to return the valve to it’s seat and hold it there until the camshaft comes around to open the valve again. Now consider the forces involved when the engine runs up to 4000rpm. The extra 10 grams at the inlet valve becomes energy that wants to bounce the valve back off the valve seat. The on-seat pressure has to control this energy and keep the valve on the seat.
Full lift pressure: This term describes the valve spring pressure on the valve when it is at full lift. With the 600 to 4000 rpm range it is easy to comprehend the job that the spring has to do. The successful selection has a spring that follows the camshaft under all conditions without bouncing off {not enough pressure} and doesn’t break through the oil film protecting the camshaft and destroying the camshaft. {too much pressure)
Coil bind: This term describes the condition that is a serious danger to the life of an engine. It occurs when the valve is at full lift and a wrong spring selection has the spring coils fully closed and touching each other. The spring will not close further. The danger is in the energy within the valve that refuses to stop suddenly. The valve will actually stretch as the heavy valve head tries to continue and the valve collet tries to stop the movement. Unfortunately this valve head is going to leave the valve collet behind and plunge into the piston. A valve breakage is not too common but can be caused by coil bind. Another, less common cause, is to have weak springs that don’t cause the valve to follow the camshaft and at high revs cause the valve to be thrown off the camshaft, passed the designed full lift and to a coil bind condition. Again , the valve doesn’t want to stop suddenly and a breakage can occur. The design engineer generally leaves 1,5mm before coil bind as a limit. This allows for a safe deacceleration of the valve and safe dispersal of the energy in the moving components.