Analysis of How a Hydraulic Rock Breaker Works

2.2 Analysis of How a Hydraulic Rock Breaker Works

A hydraulic rock breaker has many structural forms. Starting from the working principle, the authors abstract and summarize the most fundamental, most critical ideas of a hydraulic rock breaker, and reduce them to three basic working modes: pure hydraulic, hydraulic-pneumatic combined, and nitrogen-explosive.

2.2.1 Pure Hydraulic Working Principle

The pure hydraulic working principle has three implementation forms: front-chamber constant pressure / rear-chamber variable pressure (abbreviated 'front-chamber constant-pressure principle'), rear-chamber constant pressure / front-chamber variable pressure (abbreviated 'rear-chamber constant-pressure principle'), and front-and-rear-chamber variable pressure (abbreviated 'variable-pressure principle').

(1) Front-chamber constant-pressure principle

This was the working principle first adopted at the start of hydraulic rock breaker development; all subsequent technical advances have built on it. The front-chamber constant-pressure hydraulic rock breaker is shown in Fig. 2-1.

From Fig. 2-1, the system consists of a cylinder body, piston, control valve, and oil passages. The cylinder body and piston make up the impact mechanism. The piston moves back and forth inside the cylinder body driven by the hydraulic oil, outputting impact energy externally and applying large impact force to the target, producing a hammer effect. The function of the control valve is to reverse the oil driving the piston, achieving periodic reciprocating motion of the piston.

The hydraulic rock breaker shown in Fig. 2-1 has its piston at the impact point; the valve spool is at the position where it has just completed switching from power stroke to return stroke. At this moment, high-pressure oil enters the cylinder's constant high-pressure chamber (chamber a) through the valve's constant high-pressure port, driving the piston on the return stroke (to the right). The oil in the piston's variable-pressure chamber (chamber b) is returned to tank through port 4 and the valve's variable-pressure / return-oil port. When the piston moves back until its front shoulder passes port 2 on the cylinder body, high-pressure oil is directed into push-valve port 5, causing the valve to switch (to the left). Because the valve's constant high-pressure chamber now connects to the intermediate variable-pressure chamber, high-pressure oil enters the piston's rear chamber b through port 4. Both sides of the piston are now under high-pressure oil, but because the pressure-bearing area of rear chamber b is greater than that of front chamber a, the piston starts to decelerate on the return stroke, its speed drops to zero, and it begins the power stroke (to the left). When the piston's central recess connects ports 2 and 3, the piston has just reached the impact point, completing one cycle; at the same time, push-valve port 5 connects to the return-oil line, so the spool switches to the right, returning to the position shown in Fig. 2-1, completing one full cycle and preparing for the piston's next return stroke. In this way, the piston achieves continuous impact, continuously outputting impact energy. Air chamber c in this working principle is vented to atmosphere.

(2) Rear-chamber constant-pressure principle

It should be pointed out that this working principle can only be realized on the condition that the pressure-bearing area of piston front chamber a is greater than that of rear chamber b, i.e. the front-chamber diameter of the piston is smaller than the rear-chamber diameter (d₁ > d₂).

Fig. 2-2 shows the schematic of a rear-chamber constant-pressure / front-chamber variable-pressure hydraulic rock breaker.

Compared with Fig. 2-1, the only difference is that port 1 on the cylinder body is connected to the valve's variable-pressure chamber instead of the constant-pressure (high-pressure) chamber; port 4 connects directly to the valve's constant-pressure chamber; all other oil passages are the same. Fig. 2-2 shows the moment the piston power stroke has just ended and the valve has already switched — the system is at the instant the return stroke starts.

The working characteristic of this principle is that the hydraulic rock breaker does not discharge oil during the return stroke, but discharges oil during the power stroke; and the pressure-bearing area of front chamber a is greater than that of rear chamber b. Because the power stroke discharge time is short and the flow is large, the hydraulic pressure losses of this principle are greater than those of the front-chamber constant-pressure principle. At present, most hydraulic rock breakers do not use this principle.

(3) Front-and-rear-chamber variable-pressure principle

The front-and-rear-chamber variable-pressure principle is shown in Fig. 2-3. From this schematic it is easy to see that this type of hydraulic impact device has a complex structure with many passages, which increases manufacturing costs. Therefore, it is not used in hydraulic rock breakers today; it is still used on some brands of hydraulic rock drills.

Fig. 2-3 shows the position at the end of the piston power stroke, start of the return stroke. When the return stroke begins, high-pressure oil from the valve's intermediate chamber enters piston front chamber a through the left chamber and cylinder port 1, pushing the piston to the right. The oil in rear chamber b is discharged into the oil tank through cylinder port 5 and the valve's right chamber. During the return stroke, when the piston's left shoulder passes port 2 on the cylinder body, high-pressure oil through port 7 pushes the valve spool to switch to the right; the valve spool instantaneously switches the supply and discharge oil paths of the cylinder body — cylinder port 5 goes to high pressure and cylinder port 1 goes to tank return — so the piston starts to decelerate, its speed quickly drops to zero, and it switches to power-stroke acceleration. When the piston power stroke reaches the impact point, the piston's central recess connects cylinder ports 2 and 3, ports 4 and 5 connect, the valve spool's left side connects through port 7 with ports 2 and 3 to return oil, and the valve spool's right side port 6 connects through ports 4 and 5, the valve's right side and intermediate chamber, to high pressure, causing the spool to switch to the left, changing the cylinder's supply and discharge oil paths, and completing one working cycle of the piston. The hydraulic impact device's piston and spool return to the state shown in Fig. 2-3 — the start of the return stroke. In this way, the hydraulic rock breaker, through the piston's continuous reciprocating motion, continuously outputs impact energy externally, effectively completing the impact work.

All three pure hydraulic working principles described above are currently used in hydraulic rock drills, hydraulic rock breakers, and other hydraulic impact mechanisms, but hydraulic rock breakers still more commonly use the hydraulic-pneumatic combined working principle.

2.2.2 Hydraulic-Pneumatic Combined Working Principle

From the analysis of the pure hydraulic working principle, we can see that all of the impact energy of a pure hydraulic impact mechanism is supplied by hydraulics. However, as use of pure hydraulic rock breakers increased and research advanced, it was found that hydraulic losses were quite large, which limited further efficiency improvement. Oil flowing through the passages inside the cylinder body must rub against the tube walls, and the hydraulic losses caused by bends, diameter changes, and flow direction changes are considerable; the larger the flow, the greater the losses, and this is especially severe during the power stroke.

At present, the hydraulic-pneumatic combined working principle is mainly used for hydraulic rock breakers requiring large impact energy and low frequency, and for hydraulic pile drivers.

To improve efficiency, after extensive research, people found a simple and effective method: using gas and oil together to supply the impact energy of the hydraulic rock breaker. This reduces the flow required during the power stroke — reducing hydraulic losses and improving working efficiency — hence the hydraulic-pneumatic combined hydraulic rock breaker.

The structural principle of the hydraulic-pneumatic combined hydraulic rock breaker is very simple: just charge the air chamber c in the three pure hydraulic principles mentioned above with nitrogen at a certain pressure. Because nitrogen is now present, when the piston makes the return stroke, the nitrogen is compressed and energy is stored; when the power stroke occurs, this energy is released together with the oil to drive the piston, achieving kinetic energy at the impact point, and converting it into impact energy. Clearly, the role of nitrogen necessarily reduces the amount of oil used during the power stroke, reducing oil consumption and thus achieving lower hydraulic losses and higher efficiency.

Compared with a pure hydraulic rock breaker, the effective pressure-bearing area of piston rear chamber b in a hydraulic-pneumatic combined hydraulic rock breaker is reduced. This reduction in effective pressure-bearing area means less oil consumption during the power stroke and lower hydraulic losses — this is the key reason why hydraulic-pneumatic combined hydraulic rock breakers have developed rapidly in recent years. Hydraulic-pneumatic combined hydraulic rock breakers almost all use the front-chamber constant-pressure working principle; this is also a key feature of the hydraulic-pneumatic combined type.

2.2.3 Nitrogen-Explosive Working Principle

The working principle of a nitrogen-explosive hydraulic rock breaker is not fundamentally different from that of a hydraulic-pneumatic combined hydraulic rock breaker; the structural parameters of the piston simply differ. The key difference is that the front and rear piston diameters are equal, i.e. d₂ = d₁, and all impact energy is supplied by nitrogen.

Equal front and rear piston diameters is the main feature of the nitrogen-explosive hydraulic rock breaker. During the power stroke, the rear chamber does not consume oil, and all impact energy can be supplied by nitrogen. Of course, the nitrogen's stored energy is supplied by hydraulics during the return stroke and converted into the kinetic energy of the power stroke. Therefore, in the final analysis, it is still hydraulic energy that is converted — but through gas medium compression and energy storage, the stored nitrogen energy is released during the power stroke and converted into the piston's mechanical energy.

It should be pointed out that only the front-chamber constant-pressure principle can be applied to the nitrogen-explosive hydraulic rock breaker; neither the rear-chamber constant-pressure principle nor the front-and-rear-chamber variable-pressure principle can be applied to a nitrogen-type hydraulic rock breaker. The reason is clear once you understand the piston feature that d₂ = d₁.