Basic Technical Parameters

2.1 Basic Technical Parameters

2.1.1 Parameters of a Hydraulic Rock Breaker

(1) Performance parameters

W and impact frequency f are the performance parameters that describe a hydraulic rock breaker. W defines the working capacity of the breaker; f defines its working rate.

The output power of a hydraulic rock breaker can be expressed as:

N = W × f                                          (2.1)

Because the two parameters that describe performance — impact energy and impact frequency — are mutually coupled, when designing a hydraulic rock breaker the ratio of W to f must be carefully balanced. Under the condition of minimum installed capacity, maximum working efficiency should be achieved. For a hydraulic rock breaker, large impact energy W is required and impact frequency f should be appropriately reduced, to meet the need for high impact force and good breaking effect. For a hydraulic rock drill, although it is also a hydraulic impact mechanism, it requires small impact energy W and as high an impact frequency f as possible, to meet the need for high-speed drilling.

(2) Working parameters

Maximum piston impact velocity v_m, working flow Q, working pressure p, and optimal push force F_T are the working parameters of a hydraulic rock breaker.

● Maximum piston impact velocity v_m: this is the instantaneous contact velocity when the piston strikes the tail of the chisel. The corresponding kinetic energy of the piston is defined as the hydraulic hammer impact energy W. When the piston's kinetic energy is completely transferred to the target, the hydraulic hammer impact energy is:

W = ½ mv²_m                                           (2.2)

where: m — piston mass.

From Eq. (2.2), the higher the piston impact velocity, the higher the impact energy.

However, increasing v_m is limited by two factors:

1) Material property limits of piston and chisel. Impact end velocity v_m is related to contact stress σ; the higher σ, the more it affects piston and chisel service life. Under the allowable contact stress σ, the typical selection is v_m = 9 to 12 m/s. As materials science advances, the value of v_m can be further increased.

2) Frequency limit of the impact mechanism. Because piston structure and stroke are limited, with a fixed piston stroke, accelerating to the required v_m takes very little time. Obviously, the larger v_m, the shorter the acceleration time required.

A low frequency means the piston's cycle time and stroke time are both long, while a high v_m necessarily leads to shorter stroke and cycle time — i.e. high impact frequency — which cannot meet low-frequency design requirements.

● Working flow Q: the flow delivered to the hydraulic rock breaker by the hydraulic pump during operation; it is an independent variable. The behaviour and performance parameters of the hydraulic rock breaker are all closely related to the working flow and are functions of the working flow; they change as the working flow changes.

● Working pressure p: the pressure the hydraulic system requires when the hydraulic rock breaker is operating — the system pressure needed to achieve its performance parameters. Working pressure p is a dependent variable; it changes as input flow Q and structural parameters change. During operation, when all other parameters remain fixed, the pressure p cannot be actively changed. Working pressure p and input flow Q satisfy the basic principle of hydraulic technology: the system pressure is determined by the external load. Based on this principle, hydraulic rock breaker design means using structural parameters and working flow to ensure that the system working pressure p is achieved.

● Push force F_T: when the hydraulic rock breaker operates, the acceleration of the piston on the power stroke causes the machine body to recoil, which causes the chisel to lose contact with the target and prevents the impact from working normally. To overcome this recoil, a force must be applied along the axis of the breaker body — called the push force. The push force must be large enough to keep the chisel in firm contact with the object being struck. The push force must be optimal. In other words, there is an optimal push force problem, which is closely related to the size class of the carrier machine. If the carrier is too small, the push force it can supply is insufficient; if it is too large, although the push force requirement is met, the investment cost of the carrier increases, which is also undesirable. In hydraulic rock breaker design, achieving high impact energy with a small push force has always been an optimization goal. This makes it possible to match a high-impact-energy hydraulic rock breaker with a smaller carrier, forming an efficient working combination and reducing operating costs.

(3) Structural parameters

The three piston diameters d₁, d₂, and d₃, working mass m, and working stroke S are the structural parameters of a hydraulic rock breaker. The structural parameters determine its performance parameters. Designing a hydraulic rock breaker is essentially determining the structural parameters d₁, d₂, d₃, m, and S that will ensure the required performance parameters are achieved. Once the structural parameters are fixed, all performance parameters and working parameters change with input flow and are functions of input flow.

2.1.2 Working Oil Pressure and Rated Pressure

(Rated pressure is denoted p_H throughout this section)

When the hydraulic rock breaker operates, the hydraulic oil pressure drives the piston into motion, and the pattern of piston motion is determined by the pattern of change in this oil driving force — this is piston kinematics and dynamics.

Considering piston mass m, acceleration a, and the piston's inertia force F_K, Newton's second law gives:

F_K = ma                                             (2.3)

The driving force F equals F_K in magnitude but is opposite in direction. The driving force F acting on the piston is generated by the oil pressure p in the chamber, and can be expressed as:

p = F_K / A = ma / A = (m / A) · dv / dt            (2.4)

where: m — piston mass, constant;

 A — piston pressure-bearing area, constant;

 v — piston velocity; the instantaneous flow q driving piston motion satisfies:

Av = q                                              (2.5)

Since v and q in Eq. (2.5) are functions of time, differentiating v and q with respect to time gives:

A dv / dt = dq / dt                                 (2.6)

Substituting Eq. (2.6) into Eq. (2.4) gives:

p = (m / A²) · dq / dt                             (2.7)

In Eq. (2.7), m / A² is a constant; dq / dt represents the rate of change of system flow.

From Eqs. (2.3) – (2.7), system pressure is established on the basis of changing input flow to the oil chamber. In other words, the change in hydraulic oil flow builds up piston acceleration and inertia force, which in turn forms the oil chamber pressure p.

System oil pressure p is proportional to piston mass m and flow rate-of-change dq/dt, and inversely proportional to the square of the piston pressure-bearing area A. To reduce system oil pressure p, increasing the piston pressure-bearing area A is the most effective method, but it also makes the machine body larger, so both factors must be considered in design.

System oil pressure p is a function of flow and is a dependent variable; it cannot be changed actively during operation, only changing as input flow changes. Because the oil flowing into the oil chamber is a function of time when the hydraulic rock breaker operates, oil pressure p also varies with time and has no constant value. The oil pressure given on a product data sheet, which the authors call the rated oil pressure, is denoted p_H. At this pressure, the performance parameters of the hydraulic rock breaker reach their rated values. p_H is a virtual parameter — it does not actually exist — but it is extremely important in the design and use of a hydraulic rock breaker. In design, p_H is used as the basis for calculating performance parameters, working parameters, and structural parameters, and for selecting hydraulic system components. In the field, it becomes an important reference for the operator to judge whether the system is working normally or not. The parameter p_H will be discussed further in later chapters.