Weight:
To facilitate a discussion on what constitutes the best rocker arm, it is important to establish a foundation regarding rocker arm weight. While many may advocate for reducing rocker arm weight at all costs, this problem is complex and influenced by several factors. The primary consideration revolves around the trade-off between valvetrain weight and attributes such as stiffness, toughness, and possibly frequency dampening. Striving towards either extreme presents challenges.
Drastically reducing weight can lead to an inadequate rocker arm, resulting in permanent deformation of the rocker and a gradual increase in valvetrain lash. Ultimately, this can cause catastrophic failure of the rocker arm body. On the other hand, a significant increase in rocker arm weight can induce valvetrain loft at lower RPM. Striking the right balance is crucial.
Here at Manton, we employ specific approaches when designing rocker arms. Firstly, we consider the application. For instance, if the rocker arm is likely to be used in a scenario with low spring pressure, it needs to be lighter compared to a scenario with high spring pressures. Secondly, we strategically add material only where it provides the most benefit. Finally, whenever possible, we aim to enhance the strength of the rocker arm without adding extra weight. This is typically achieved through heat treatment or material selection.
When discussing rocker arm weight, it is also important to note that what you observe on the gram scale is not necessarily what the valvetrain experiences during motion. While rotational inertia is indeed a crucial factor, it represents only one piece of the puzzle. Another significant aspect is the fulcrum length of the rocker arm. A longer fulcrum length reduces the angle change required for the rocker arm to lift the valve by the same amount. This reduces acceleration and minimizes the perceived "rocker arm weight." Moreover, a longer fulcrum length enables the spring to exert greater torque on the rocker arm, further reducing the effective "rocker arm weight." By combining these factors, the equivalent valve weight of a rocker arm is determined by one part rotational inertia and two parts fulcrum length.
Stiffness:
Having a stiff rocker arm is vital for the proper functioning and performance of an engine. Responsible for transferring motion from the camshaft to the valves, the rocker arm controls the crucial intake and exhaust processes. minimizing flex and deflection during operation is essential as any deflection in the rocker arm can result in inaccuracies in valve timing, leading to poor engine performance, reduced power output, and even potential damage to valves and other components. By maintaining a stiff rocker arm, you can achieve optimal valve control, improved efficiency, and enhanced overall engine reliability and longevity.
The selection of materials and proper heat treatment techniques are key factors in increasing the stiffness of a rocker arm. Material choice is critical as it determines the inherent strength and rigidity of the rocker arm. Steel alloys, known for their high tensile strength, are often preferred due to their ability to withstand the stresses and loads encountered during engine operation. Furthermore, heat treatment methods can further enhance the material's properties, including hardness, stiffness, durability and resistance to wear. These factors collectively contribute to enhanced engine performance and longevity.
In addition to material selection and heat treatment, the cross-sectional design of the rocker arm also plays a significant role in increasing stiffness. An I-shaped cross section is often utilized to enhance the rigidity of any structural member. The I shape provides material where it is needed most, which resists bending and twisting forces more effectively compared to other cross-sectional shapes. This increased resistance to deformation ensures minimal flexing or deflection during operation, resulting in all the benefits usually seen from minimizing deflection.
Roller Tip:
A roller tip on a rocker arm provides multiple advantages that enhance engine performance and durability. Firstly, it reduces friction between the rocker arm and the valve stem, resulting in decreased wear, heat generation, and increased valve tip life. Additionally, the roller tip rocker arm reduces side loading and scrubbing forces, promoting smoother valve operation and minimizing the risk of valve guide and stem wear. Secondly, the roller tip enables more aggressive camshaft profiles and higher valve lift without the concern of nose riding, which is common with shoe rocker arms. Lastly, roller tip rocker arms offer superior durability compared to conventional rocker arms. Their roller tip distributes the load evenly across the tip, reducing stress on the rocker arm body. This enhanced durability translates to longer component life, reduced maintenance, and improved overall reliability of the valve train system.
Adjusting screw:
The rocker arm adjusting screw serves several important functions in the valvetrain system of an internal combustion engine. Its primary purpose is to adjust the valve lash, ensuring proper valve timing and smooth engine operation. Additionally, the adjusting screw acts as a structural component, bearing surface, and facilitates oil channeling. To ensure optimal performance, several measures are taken during the manufacturing process. First, the adjusting screws are made from a hot working tool steel, which provides increased heat tolerance and reduces the risk of failure. They are also carefully heat- treated to achieve the ideal hardness, preventing distortion without compromising strength. Threading is another critical aspect, as all adjusting screws undergo thread rolling for maximum strength. The bearing surfaces are CNC turned to ensure high sphericity, enhancing their bearing characteristics. You may notice that our rocker arms with adjusting screws feature a ball on the adjuster and a cup on the pushrod. This design serves two purposes: firstly, the cup catches oil when the engine is turned off, ensuring lubrication during the next start; secondly, the ball adjuster provides better load support, utilizing its full cross-section, compared to a cup adjuster, which is only supported at its outer edge. This design minimizes the likelihood of adjuster failure. However, if you are looking for a cup adjusting screw we make those also.
Rocker shaft:
The reliability of the rocker arm heavily relies on the rocker arm shaft, which performs two critical tasks: providing excellent bearing characteristics and withstanding the load of the valvetrain system. Often overlooked is the fact that both the pushrod and the valve both exert upward pressure on the rocker arm, with only the rocker arm shaft holding it down. As a result, the rocker arm shaft, and consequently the underside of the rocker arm body, some of the highest loads in the valvetrain. To address this, we prioritize the strength of our rocker shafts by using a reasonably large diameter and ensuring a thick wall thickness when constructing them from tubing. Additionally, all our rocker arm shafts undergo a heat treating process to enhance stiffness and durability. To ensure optimal bearing performance, we focus on creating a smooth and cylindrical surface for the rocker arm shaft. This is accomplished through centerless grinding of each shaft after heat treatment. Lastly, we employ surface treatments that minimize friction, wear, and the likelihood of material transfer from the rocker arm shaft to the rocker arm, tailoring the approach to each application.
Bushing:
Ensuring optimal performance of the rocker arm relies on getting the bushing just right, despite the majority of bearing loads being supported by the oil film layer between the rocker arm shaft and the bushing, the bushing performs several crucial tasks. Firstly, the clearance between the bushing and rocker shaft determines the rate of oil leakage and in some cases affects the oil flow through the hole valvetrain. Secondly, the bushing plays a vital role in dispersing the oil evenly within the rocker arm shaft and bushing interface. Thirdly, the roundness and smoothness of the bushing are essential as they directly impact the total load that the oil film layer between the bushing and rocker arm shaft can handle. However, the most critical function of the bushing is to be prepared to bear the load of the rocker arm if, for any reason, the oil film layer experiences a momentary collapse. This capability ensures the continuous and reliable operation of the rocker arm, maintaining its peak performance.
Rockerarm Geometry:
Rockerarm geometry encompasses a set of crucial measurements that collectively determine how the mechanical signal transmitted from the camshaft is modified by the rockerarm before it reaches the valve tip. Additionally, it establishes the positioning of the pushrod and rockerarm shaft in relation to the valve tip. The three primary rockerarm geometry measurements are the fulcrum length, offset, and rockerarm ratio.
The fulcrum length denotes the distance between the center of the rockerarm shaft and the center of the roller. Although it is predominantly influenced by the valve and pushrod port hole locations, it impacts a couple of aspects. Firstly, it affects the effective rockerarm weight, as previously discussed in the “weight” section. Secondly, a longer fulcrum length increases the likelihood of the rockerarm flexing.
The offset represents the extent to which the pushrod or roller is shifted relative to the center of the bushing. Introducing offset is typically done to enhance pushrod clearance, should be avoided if possible as it also subjects the rockerarm to twisting loads, making it more prone to flexing it also places additional stress on the bushing oil film layer.
The rockerarm ratio signifies the ratio between the fulcrum length and the distance from the center of the rockerarm shaft to the center of the ball on the adjusting screw. This ratio roughly determines the multiplication of camshaft lift to obtain the expected valve lift. However, it is essential to be aware that some rockerarm manufacturers may not provide accurate information about their rockerarms' ratio for various reasons. One reason is to conceal the extent of rockerarm flex, compensating for the lost lift and maintaining the illusion of rigidity. This can lead to issues such as potential valve-piston interference and reduced stability in the valvetrain system, limiting achievable RPMs.
Another crucial consideration is that the actual ratio between valve lift and camshaft lift is influenced by factors beyond just the rockerarm ratio, including changes in angles between the valve, rockerarm, and pushrod during the rockerarms motion. Although these changes can impact lift multiplication, they are usually minimal compared to the effects of the rockerarm ratio. In summary, rockerarm geometry involves vital measurements that govern the modification of the mechanical signal transmitted from the camshaft to the valve tip. The fulcrum length, offset, and rockerarm ratio play essential roles in determining the valve lift and overall performance of the valvetrain system. Understanding the true rockerarm ratio and its interactions with other factors is crucial for achieving optimal engine performance and avoiding potential issues.
Oiling:
The importance of the oiling system in a rocker arm is often underestimated, as it performs two crucial functions. Firstly, it efficiently dissipates the heat generated by the rocker arm system. Secondly, it provides lubrication to each bearing interface. Bearing lubrication is a complex issue that requires careful consideration, with the main objective being to reduce wear and friction between components, particularly in the heavily loaded areas.
To achieve this goal, three techniques are employed. The first technique is the implementation of a hydrostatic wedge. In this method, the pressure from the oil pump separates the components from each other creating a gap so that oil can leak out. The second technique, called the hydrodynamic wedge, occurs when oil clings to the bushing while the rocker arm rotates. As the rocker arm turns, the oil moves with it, pulling oil down towards the underside of the rocker shaft, which then forces the shaft and bushing apart.
The last technique involves a "squeeze film." When the valvetrain is at rest between cycles, the rocker arm settles, enabling oil to fill the gap between the underside of the rocker arm shaft and the bushing. This primes the area for the next cycle. When the rocker arm is under load, it takes time for the oil to be squeezed out from between the shaft and bushing, giving time to create a hydrodynamic wedge. All three techniques are evident at both the pushrod and rocker arm shaft interface, and hydrodynamic and squeeze film wedging can also be observed at the roller tip.
Relation to the system:
When considering a rocker arm, it's crucial to bear in mind that it operates in conjunction with the entire valvetrain. A well-designed rocker arm not only fulfills its own function effectively but also enhances the performance of the surrounding components. To achieve this, we prioritize ensuring the rocker arm's rigidity. Some might assume that a flexible rocker arm could be advantageous, but there are two main reasons why this is not the case.
Firstly, when the rocker arm is under load, it behaves like a spring, storing energy that is later released. Unfortunately, this released energy is mostly transferred to neighboring components, leading to accelerated wear. To counter this, we focus on making the rocker arm stiffer to reduce the energy stored within it.
Secondly, a flexible rocker arm can cause premature valve seating. As the valvetrain nears the end of its lift cycle the rocker arm is loaded and tends to flex. Consequently, the valve will be closer to the seat than intended, and in some cases, it might even hit the valve seat before reaching the seating ramp of the camshaft. This forceful impact can cause damage to the valve, seat, and valve spring.
Another way the rocker arm contributes to the optimal functioning of the entire valvetrain is by allowing for the use of a more rigid pushrod. This is done by correctly positioning the pushrod. We aim to accommodate the largest possible pushrod diameter because a pushrod's strength largely depends on its diameter. This will help to avoid similar issues that are caused by a flexing rocker arm.
