LO 9-5 Identify and describe the purpose, construction, and operation of components used in the valve train

Valve Springs

Valve springs hold the valve closed on the seat and must maintain tension on the valve train when the valve is open. If inertia of the valve train is great enough, weak valve springs will allow the valve to stay open after the cam rotates to its inner base circle, which should normally allow the valve to close. This condition is known as valve float . Valve springs with more tension may seem to be a good solution; however, excessive spring tension will slam the valve against the seat. High spring tension will also prematurely wear the valve face and seat, potentially stretch the stem, and wear cam lobes and other valve train components due to high valve opening pressures. HD diesel engines do not rotate fast and generally require only a single valve spring. High-speed automotive diesels will use two valve springs. An inner spring, on some high-speed diesels, helps prevent valve float Figure 38. During service, valve springs should be carefully inspected for free length, cracks, nicks, pitting, straightness, and spring tension. A valve spring testing apparatus, which operates like a weigh scale, is used to check spring tension. The spring is first compressed to a height specified by the manufacturer. Then the spring scale reports the valve spring’s tension in units of pounds, ounces, or grams weight. For example, if a spring is compressed to 1" (2.5 cm), a corresponding manufacturer’s specifications would indicate that an acceptable valve spring pressure of 150 lb (68 kg) or greater is required. If spring tension falls below this pressure (e.g., 145 lb [66 kg]), the valve spring must be discarded Figure 39.

FIGURE 09-38: Parts of a valve mechanism. This valve from a high-speed diesel uses dual springs to prevent valve float. Most low-speed, HD diesels use only one.

FIGURE 09-39: A valve spring tester measures a spring’s free length and compressed tension.