Hydraulic Accumulators

Hydraulic accumulators are essential components in modern hydraulic systems, designed to store potential energy in the form of pressurized fluid. They play a critical role in maintaining system stability, absorbing shocks, and providing additional fluid when demand peaks. The proper selection and maintenance of accumulators, including the use of appropriate fluids like universal tractor hydraulic oil, directly impacts system efficiency and longevity.

This guide explores the three primary types of hydraulic accumulators, their working principles, capacity calculations, and installation best practices. Understanding these fundamentals is crucial for engineers, technicians, and anyone involved in the design, operation, or maintenance of hydraulic systems.

Types of Hydraulic Accumulators

1. Weighted (Gravitational) Accumulators

The weighted accumulator, as illustrated in Figure 6-6, stores and releases energy through the positional change of a heavy weight. This design features a weight (1) that acts upon hydraulic oil (3) through a piston (2), creating pressure within the system. When storing energy, fluid enters the accumulator through port a via a check valve, pushing the piston and raising the weight. When releasing energy, the piston and weight descend together, forcing fluid out through port b.

These accumulators are valued for their simple structure and stable pressure output. However, they are limited by their small capacity relative to their large physical size, lack of flexibility in response, and potential for leakage. Proper maintenance, including regular checks of fluid levels and using high-quality universal tractor hydraulic oil, can help mitigate some of these issues.

Today, weighted accumulators are primarily found in a limited number of large-scale stationary hydraulic systems where their specific characteristics are beneficial despite their drawbacks. The compatibility with universal tractor hydraulic oil makes them suitable for certain agricultural and heavy machinery applications.

Figure 6-6: Weighted Accumulator

1 - Weight | 2 - Piston | 3 - Hydraulic Oil (typically universal tractor hydraulic oil)

2. Spring-Type Accumulators

Figure 6-7 depicts the structure of a spring-type accumulator, which utilizes the expansion and contraction of a spring to store and release energy. The force from spring (1) acts upon hydraulic oil (3) through a piston (2). The pressure of the hydraulic oil is determined by the spring's preload and the piston's surface area.

A notable characteristic of spring-type accumulators is that pressure varies with spring deflection. As the spring expands and contracts, its force changes, resulting in corresponding changes in oil pressure. To minimize these pressure fluctuations, springs are typically designed with low stiffness and limited travel, which restricts the working pressure range of these accumulators.

These accumulators are commonly used in low-pressure, small-capacity systems, particularly for buffering in hydraulic circuits. They offer the advantages of simple construction and relatively quick response times. However, their capacity is limited, and they operate at lower pressures compared to other designs.

When maintaining spring-type accumulators, it's important to use the recommended fluid, often universal tractor hydraulic oil, to ensure proper lubrication of moving parts and prevent premature wear of the piston seals and spring components.

Figure 6-7: Spring-Type Accumulator

1 - Spring | 2 - Piston | 3 - Hydraulic Oil (compatible with universal tractor hydraulic oil)

3. Gas-Charged Accumulators

Gas-charged accumulators store and release energy through the compression and expansion of gas. For safety reasons, inert gases or nitrogen are typically used, as they are non-reactive and reduce the risk of combustion. The most common types of gas-charged accumulators are piston-type and bladder-type designs, as shown in Figure 6-8. These accumulators are particularly effective when using high-quality fluids like universal tractor hydraulic oil, which maintains its properties even under varying pressure conditions.

(a) Piston-Type Gas Accumulator

Figure 6-8a shows the structure of a piston-type gas accumulator. Hydraulic oil enters through port a, pushing the piston and compressing the gas in the upper chamber to store energy. When system pressure drops below the accumulator's internal pressure, the gas expands, pushing the piston and releasing hydraulic oil to meet system demands.

This design offers simplicity, reliability, and ease of maintenance. However, it requires high-precision cylinder machining and is prone to piston seal wear. The inertia and friction of the piston result in higher costs, potential leakage issues, and reduced response sensitivity. Regular fluid changes with appropriate universal tractor hydraulic oil can help maintain seal integrity and overall performance.

(b) Bladder-Type Gas Accumulator

Figure 6-8b illustrates a bladder-type gas accumulator, which uses a flexible bladder to separate the gas and hydraulic fluid. The bladder (2) is fitted inside the housing (3) and connected to a charging valve (1). A mushroom-shaped stop valve (4) prevents the bladder from being extruded through the fluid port during gas expansion.

Bladder-type accumulators offer several advantages, including minimal fluid contact with the housing, reduced friction, and faster response times. The bladder design prevents gas from mixing with the hydraulic fluid, ensuring system cleanliness and efficiency. These accumulators work exceptionally well with universal tractor hydraulic oil, maintaining consistent performance across a wide range of operating conditions.

Figure 6-8: Gas-Charged Accumulators

1 - Charging Valve | 2 - Bladder | 3 - Housing | 4 - Mushroom-shaped Stop Valve
Note: Both designs require compatible hydraulic fluids, with universal tractor hydraulic oil being a preferred choice for many agricultural and industrial applications due to its versatile properties.

Accumulator Capacity Calculation

The capacity of an accumulator is one of the primary indicators when selecting the appropriate unit for a specific application. Different types of accumulators require different calculation methods. Here, we focus on the capacity calculation for bladder-type gas accumulators, the most widely used design, particularly when used as energy storage devices or auxiliary power sources. Proper capacity calculation ensures optimal performance, especially when using fluids like universal tractor hydraulic oil that have specific volume characteristics under pressure.

Bladder-type gas accumulators require pre-charging before operation. After charging, the bladder occupies the entire volume of the accumulator housing. Let's assume the volume inside the bladder at this stage is V₀ with pressure p₀. During operation, hydraulic oil enters the accumulator, compressing the bladder. At this working state, the gas volume inside the bladder is V₁ at pressure p₁. When hydraulic oil is released, the bladder expands to volume V₂ at pressure p₂, as shown in Figure 6-9.

According to Boyle's Law for gases, which describes the relationship between pressure and volume of a gas at constant temperature:

p₀V₀ⁿ = p₁V₁ⁿ = p₂V₂ⁿ = constant

(Equation 6-1)

Where:

• p₀, V₀ = Pre-charge pressure and volume of gas in the bladder with no hydraulic oil input
• p₁, V₁ = Pressure and volume of gas in the compressed bladder during operation
• p₂, V₂ = Pressure and volume of gas in the expanded bladder after energy release
• n = Polytropic exponent determined by the accumulator's operating conditions

Figure 6-9: Operating States of a Bladder-Type Gas Accumulator

When the accumulator releases energy slowly, such as for pressure maintaining or leakage compensation, the gas can be considered to operate under isothermal conditions, and n = 1 is used. For rapid energy release, such as in large oil supply applications, adiabatic conditions are assumed, and n = 1.4 is appropriate.

The choice of hydraulic fluid, such as universal tractor hydraulic oil, can influence these calculations due to variations in compressibility and thermal properties.

Let V represent the maximum volume of oil stored in the accumulator. This can be expressed as:

By combining Equation (6-2) with Equation (6-1), we can derive:

Accumulator Installation and Usage

The installation position of an accumulator in a hydraulic system is determined by its intended function. Proper installation and maintenance, including using the correct fluid such as universal tractor hydraulic oil, are crucial for ensuring optimal performance, safety, and longevity of both the accumulator and the entire hydraulic system.