A common challenge engineers face in the application of counterbalance or overcenter valves is machines with a high degree of load dynamics. Vehicles with slender booms, multiple booms, wear pads with varying frictional forces or pneumatic tires are all subject to higher levels of load dynamics. Concrete pumps are a common example as they require multiple long, slender booms to deliver concrete to the work site. Rough terrain forklifts or telescopic handlers with pneumatic tires are other examples.
In a telehandler, the long cylinder can act as a capacitor and store energy when fully extended. The pressure within the cylinder will rise to system pressure at the end of the stroke, and a counterbalance valve will reseat and lock system pressure in the cylinder irrespective of the load-induced pressure.
When the operator begins to lower the load, this stored energy gives the counterbalance valve the message that a heavy load is on the boom, and less pilot pressure is required to open the valve. The counterbalance valve opens very quickly and allows the stored energy to dissipate, causing a momentary runaway condition or prompting the valve to overreact. The consequence of this is an initial instability as the boom is retracted.
There are two main types of counterbalance valves on the market, known as direct acting and differential area (figure 1). With both designs, the valve has a single poppet to meter flow from the cylinder to the directional valve, and the load pressure works against this poppet. The difference in the two designs, which is critical to boom stability, relates to the spring force required to reseat the poppet.
In direct-acting valves, the load pressure acts on the full area of the poppet. The larger area requires greater spring force to reseat the poppet. This direct-acting design is common among the millions of types of relief valves applied every year in standard hydraulic circuits.
In contrast, a differential-area valve works by applying load pressure to a differential area between the poppet and the seat. This creates a smaller area for the load pressure to act upon, so less spring force is required to reseat the poppet. This results in a valve that can meter large amounts of flow very rapidly. While this can be beneficial when dealing with very high-flow applications, it can lead to instability and squealing in typical moderate-flow hydraulic applications.
Direct-acting valves are more stable because the heavier spring makes the poppet less reactive to small fluctuations in load pressure. This steeper relief characteristic, which prevents small pilot pressure changes from opening the valve quickly, provides a stable and controlled metered flow.
Differential-area valves, on the other hand, react more abruptly to changes in the pilot pressure. It takes only a small change in pilot pressure to increase the metered flow by a large amount, thus making the valve more unstable.
Addressing boom instability
For machines with high dynamic loads, designers must look beyond standard valves. Many vehicle engineers use restrictive and semi-restrictive valves for highly dynamic vehicles. These valves operate by restricting the opening, so the oil has to be driven across an orifice. While the restriction may effectively reduce instability by limiting flow, it is inherently inefficient in that it generates heat and makes it difficult to control the actuator speed.
A more efficient alternative to restrictive valves is a two-stage valve, such as Eaton’s 1CEL design. The two-stage valve creates an initial restriction that is removed as the valve stabilizes and the pilot pressure increases. The two-stage valve maintains an initial counterbalance pressure when the valve is opened to prevent total decay of the stored energy within the cylinder. This is done by maintaining the counterbalance pressure through the center poppet and inner spring. Once this energy is dissipated, the outer poppet and spring will behave the same way the pressure setting does in a standard counterbalance valve and reseat the poppet if the load begins to run away or overreact.
By having a fixed outer spring and an adjustable inner spring, the two-stage valve allows the designer to establish a range of acceptable pressure settings that fit the specific application. For example, the valve could be set at 200 bar (3,000 psi) with a counterbalance pressure between 35 and 70 bar (500 to 1,000 psi).
Two-stage valves are an efficient solution for highly dynamic machines as they reduce sudden instability or judder at the beginning of a cylinder’s movement with an initial counterbalance pressure.
Field testing proves effectiveness
Eaton conducted a series of field tests on a telehandler with an 8,000-lb (3,630-kg) lift capacity to demonstrate how a two-stage valve can eliminate boom instability. In the testing, load-reactive valves were replaced with Eaton two-stage valves on the machine’s base lift cylinder.
The machine was instrumented with string potentiometers on the extend and lift cylinders. Pressure gauges were placed on the manifold from the control valve to the overcenter valve to monitor the lift, extend and tilt functions. The gauges were placed on both the base and rod side of the cylinder. An inertial measurement unit (IMU) sensor was placed at the end of the boom to measure movement.
To compare the performance of the different types of valves and to determine the conditions in which oscillation occurs and those in which it is most pronounced, a series of tests was conducted at multiple engine speeds (1,040 rpm, 1,500 rpm and 2,300 rpm) with the telehandler’s forks both loaded and unloaded. The load was a 1,000-lb (454-kg) weight.
The testing established three boom up-down situations in which the load-reactive valves exhibited instability, resulting in machine oscillation:
• Raising and lowering the boom from a stationary position. The effect is most significant when the forks are loaded and the engine is idling (1,040 rpm). A significant difference is also seen at 2,300 rpm with the forks loaded. Differences are also apparent at all speeds with the forks unloaded.
• Raising, retracting then lowering the boom from a stationary position. The effect is significant at all three engine speeds when loaded. When unloaded, the impact is highest at engine idle, but still present at 1,500 and 2,300 rpm.
• Raising and lowering the boom while driving. The further the boom is extended, the greater the oscillation and the more pronounced.
In comparison, the two-stage valves increased stability, practically eliminating machine oscillation. Figures 2-5 show acceleration and pressure data for the raise-retract-lower tests. As shown in figure 2, the boom experienced two spikes in acceleration with both the two-stage and load-reactive valves: one when the boom reaches its maximum height, and one when the boom is commanded down. The main difference occurs after the second spike, when the boom outfitted with the load-reactive valves continued to produce significant fluctuations in acceleration, indicating instability and oscillation of the machine.
Pressure data also provide a strong indicator of valve performance. Figure 4 shows how system pressure can be damped with the proper valve. In the pressure boom tilt down graph in figure 5, pressure fluctuations can be observed that resonate through the system, leading to instability of the load on the forks. The reduced pressure fluctuations seen with the two-stage valves indicate less oscillation and greater stability.
Designers and owners of machines with long, unstable booms have considered boom oscillation a nuisance, but the results of this testing show that it can be eliminated.
Maurice Ashmore, Eaton’s global chief engineering manager, SiCV & MCD, wrote this article for SAE Truck & Off-Highway Engineering's IFPE 2020 show coverage. Eaton is exhibiting in South Hall 3, S80231.Continue reading »