Basic Working Principle of Hydraulic Rock Breakers

1.3 Basic Working Principle of Hydraulic Rock Breakers

A hydraulic rock breaker is an impact machine that converts hydraulic energy into mechanical energy. It contains two basic moving components — a piston and a distribution valve spool — which mutually feedback-control each other: the reciprocating motion of the valve spool controls piston commutation, and the piston in turn, at the start and end of each stroke, opens or closes the control oil passage of the valve, realising valve commutation — cycling in this way … The basic working principle of a hydraulic rock breaker is: through this piston-spool feedback control, the piston rapidly reciprocates under hydraulic (or gas) force and strikes the chisel to do work on the outside.

Hydraulic rock breakers come in many types and forms, which will be described in detail in later chapters. Below, the front-chamber constant-pressure rear-chamber variable-pressure hydraulic rock breaker is taken as an example to describe its working principle:

As shown in the diagram, when the return stroke begins, high-pressure oil enters the piston front chamber through oil port 1 and simultaneously acts on the lower end of the directional valve spool, keeping the spool stably in the state shown in diagram (a). At this time the piston front chamber has high-pressure oil; the rear chamber is connected to return T through oil port 4. Driven by the front-chamber oil pressure, the piston accelerates on the return stroke and compresses the nitrogen stored in the nitrogen chamber (except pure hydraulic type); the accumulator stores oil. When the piston's return stroke reaches control port 2, high-pressure oil reaches the upper end of the valve spool. At this point both the upper and lower ends of the spool are connected to high-pressure oil; because in the design the effective area of the upper end of the spool is greater than the effective area of the lower end, the spool switches to the state of diagram (b) under the action of the high-pressure oil. At this time both the piston front and rear chambers are connected to high-pressure oil; the accumulator discharges oil to supplement the system. Under the action of composite force F_q, the piston accelerates on the power stroke, strikes the chisel, and outputs impact energy. When the piston passes the impact point, control ports 2 and 3 are connected and linked to return oil T; the upper-end oil pressure of the valve spool drops; under the lower-end oil pressure the valve spool quickly switches back to the state of diagram (a). Returning to the original state, the piston starts the return stroke, entering the next striking cycle, and so on cyclically. In this process, the linkage relationship between piston and valve spool is shown in Fig. 1-2.

From Fig. 1-1 it can be seen that during the power stroke, ignoring piston gravity and friction resistance, the force F_q driving the piston impact work mainly includes hydraulic pressure and nitrogen gas pressure, i.e. F_q = π/4 · p_N · d₁² + π/4 · p · [(d₃² − d₁²) − (d₃² − d₂²)]. The driving force F_q is related to the front-rear chamber effective area difference, oil pressure p, and nitrogen chamber pressure p_N. Based on the different ratios of oil work to gas work, three working forms can be formed: pure hydraulic, hydraulic-pneumatic combined, and nitrogen-explosive.

Pure hydraulic: p_N = 0. In this form, the hydraulic rock breaker has no nitrogen chamber and the piston is completely driven by the upper-lower chamber oil pressure differential. F_q = π/4 · p · [(d₃² − d₁²) − (d₃² − d₂²)]. This form is the earliest form when hydraulic rock breakers first appeared.

Hydraulic-pneumatic combined: In this form d₁ < d₂, and simultaneously a nitrogen chamber is added at the piston tail, introducing nitrogen to do work, p_N > 0. F_q is mainly composed of two parts: the front-rear chamber oil pressure differential and the nitrogen compression-expansion force. F_q = π/4 · p_N · d₁² + π/4 · p · [(d₃² − d₁²) − (d₃² − d₂²)]. This form is currently the most common form of hydraulic rock breaker. Based on the different proportions of oil and gas work in the total driving force, i.e. different gas-to-liquid work ratios, products with different performance can be formed.

Nitrogen-explosive: In this form d₁ = d₂, p_N > 0. The upper-lower chamber hydraulic force is zero; the piston work during the power stroke is completely driven by nitrogen chamber gas pressure. F_q = π/4 · p_N · d₁². This form is the latest form of hydraulic rock breaker.

All three forms have their advantages and disadvantages, but their overall performance improves from one generation to the next. The pure hydraulic type, as the earliest form product when hydraulic rock breakers first appeared, has a simple structure and reliable operation with no initial push force required, but has a low energy utilisation rate and is not suitable for manufacturing large-size products. The hydraulic-pneumatic combined type is a major breakthrough over the pure hydraulic type: by adding a nitrogen chamber at the piston tail, it effectively utilises the return-stroke energy and greatly improves impact force; but the structure is complex and initial push force is needed to work. The nitrogen-explosive hydraulic rock breaker, from an energy perspective, needs no oil work during the power stroke and is thus more energy-saving; simultaneously the piston front and rear chamber diameters are equal, which can effectively solve the difficulty of insufficient instantaneous oil supply during piston power stroke. However, due to the high initial nitrogen charging pressure, the push force required is larger.

1.4 Basic Structure and Classification of Hydraulic Rock Breakers

1.4.1 Basic Structure of Hydraulic Rock Breakers

Although hydraulic rock breakers come in many varieties, they share common structural characteristics. The basic composition of a hydraulic rock breaker includes: cylinder body, piston, distribution valve, accumulator, nitrogen chamber, chisel seat, chisel, high-strength bolts, and sealing systems. Different types of hydraulic rock breakers differ slightly in structure, but every rock breaker contains 2 basic moving components — piston and valve spool. Its basic structure is shown in Fig. 1-3.

(1) Impact mechanism

A hydraulic rock breaker has a relatively long and slender piston, which is the most important component. Based on the stress wave transmission theory, to maximally transmit the piston's impact energy, the diameter of the impact piston is generally basically equal to or close to the end diameter of the chisel tail, ensuring complete contact at the striking face and achieving the purpose of efficiently transmitting energy. The mating clearance between the impact piston and the cylinder body or liner sleeve is a very important technical parameter. If the clearance is too large, very large internal leakage will be produced, making the impact force insufficient and even causing the rock breaker to fail to operate normally; if the clearance is too small, piston motion may be sluggish or galling may occur, simultaneously causing manufacturing costs to rise sharply.

(2) Distribution mechanism

A hydraulic rock breaker generally has a distribution valve that changes the direction of hydraulic oil flow, through which it controls and drives the reciprocating motion of the impact piston. Distribution valve structural forms are many; they can generally be divided into two major categories: spool valves and sleeve valves. Spool valves are generally light in weight, low oil consumption, smaller in diameter, and have smaller mating clearance and leakage, but mostly have step-shaped structure, relatively poor structural machinability, and larger throttling losses. Sleeve valves are heavier, larger in diameter, and the mating clearance and leakage are also relatively larger; but their structural machinability is good, the opening area gradient is large, and throttling losses are small. The mating clearance between the valve spool and valve body or valve sleeve is another important technical parameter in hydraulic rock breaker manufacturing; clearances that are too large or too small will both cause the valve to be unable to function normally.

(3) Accumulator pressure-stabilising mechanism

Most hydraulic rock breakers have one or more accumulators, which play the role of energy storage and pressure stabilisation. A hydraulic rock breaker only does work externally during the power stroke; the return stroke is preparation for the power stroke. When the piston returns, hydraulic oil enters the accumulator at a pressure greater than the charging chamber pressure, and is stored as potential energy of the oil in the accumulator. It is released during the piston power stroke, converting most of the return-stroke energy into impact energy. In this way the accumulator plays the role of improving the system working efficiency, while also reducing pressure shocks and flow pulsations caused by the distribution valve spool switching.

(4) Actuating mechanism

The chisel is the actuating component of the hydraulic rock breaker that does external work, acting directly on the working object; it is a wear part requiring good abrasion resistance, hard on the outside and tough inside, and with hardness changing gradually from outside to inside. To adapt to various working conditions and working objects, chisels come in pointed, square, spade, and flat-head forms.

(5) Blank-firing prevention mechanism

Because a hydraulic rock breaker has large impact energy, if the piston is allowed to directly strike the cylinder body it will seriously damage the rock breaker body — producing blank-firing. The blank-firing prevention structure consists of adding a hydraulic buffer chamber at the front of the cylinder body. When the chisel has not contacted the rock and moves forward, the impact piston enters the buffer chamber, compressing the oil inside and absorbing impact energy, realising cushioned protection of the machine body. At the same time the front-chamber oil inlet is closed, so that under gravity and rear-part nitrogen action the piston cannot retreat; only when the chisel re-contacts the rock and pushes back with greater arm pressure does the impact piston push out the buffer chamber and high-pressure oil can then enter the front chamber, thereby continuing normal operation. As shown in Fig. 1-4, after the hydraulic rock breaker has broken through the object being broken, the piston can blank-fire at most 1 to 2 times before stopping. The operator needs to re-select the impact point, press the chisel tightly, apply pressure, and the chisel pushes the piston away from the lower chamber oil inlet, and work can start again.

(6) Other mechanisms

Other mechanisms of the hydraulic rock breaker include: connecting frame, vibration damping mechanism, sealing system, automatic lubrication system, etc.

1.4.2 Classification of Hydraulic Rock Breakers

There are many types of hydraulic rock breakers and many classification methods. The main classification methods are as follows:

(1) Classification by operating method

Hydraulic rock breakers are classified by operating method into carrier-mounted and handheld. Handheld types are small rock breakers, also called hydraulic chisels; mass is generally below 30 kg, operated by hand, powered by a dedicated hydraulic pump station, and can widely replace pneumatic chisel operations. Carrier-mounted types are medium and large rock breakers, directly installed on the boom of hydraulic excavators, loaders, and other hydraulic carrier machines, using the carrier machine's power system, hydraulic system, and boom motion system to perform operations.

(2) Classification by working medium

Hydraulic rock breakers are classified by working medium into pure hydraulic, hydraulic-pneumatic combined, and nitrogen-explosive three major categories. Pure hydraulic types rely entirely on hydraulic oil pressure to drive the piston to work; hydraulic-pneumatic combined types rely on hydraulic oil and rear-part compressed nitrogen simultaneously to drive the piston to work; nitrogen-explosive types rely entirely on the instant expansion of nitrogen in the rear nitrogen chamber to push the piston to do work.

(3) Classification by feedback method

Hydraulic rock breakers are classified by feedback method into stroke feedback and pressure feedback. The difference lies in the way the feedback signal is collected for distribution valve commutation. Stroke feedback hydraulic rock breakers rely on the piston opening and closing high-pressure oil feedback holes in the stroke to control distribution valve commutation; the positions of the feedback holes can only be rigidly set, and limited by structural conditions, feedback holes can be set at most 3; therefore stroke feedback hydraulic rock breakers cannot achieve stepless adjustment of impact frequency. Pressure feedback hydraulic rock breakers rely on collecting system pressure or nitrogen chamber pressure at the piston tail to control distribution valve commutation; as the piston enters the nitrogen chamber, the nitrogen chamber pressure continuously changes, and when the pressure sensor installed in the chamber detects a preset pressure, the valve commutates through microcomputer control; since the commutation pressure can be set arbitrarily, pressure feedback hydraulic rock breakers can achieve stepless adjustment.

(4) Classification by distribution method

Based on the distribution valve form, they can be classified into 3-way valve single-face return oil and 4-way valve double-face return oil two major categories. Single-face return oil structural forms have the advantages of simple oil passages and easy control; in practice they are relatively commonly used. Single-face return oil can be divided into front-chamber return oil and rear-chamber return oil types; of these, front-chamber return oil forms have the shortcomings of large suction and return oil resistance, so the current most common form is the front-chamber constant-pressure, rear-chamber return oil form. 4-way valve double-face return oil is also called the double-acting type; its characteristic is no constant pressure chamber, with the front and rear chamber pressures alternately high and low; but due to the complex oil passages of the double-face return oil structural form, it is uncommon.

(5) Classification by distribution valve layout

Based on distribution valve layout, they can be classified into internal-mount and external-mount two types. Internal-mount type can further be classified into spool type and sleeve type. Internal-mount distribution valves are integrated with the cylinder body in one, with compact structure; external-mount distribution valves are independent outside the cylinder body, with simple structure and convenient maintenance and replacement.

In addition, based on noise level they can be classified into low-noise and standard types; based on external housing form they can be classified into triangular, tower-shaped, and enclosed rock breakers, etc. The various classification methods are summarised in Fig. 1-5.